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Article

Preliminary Evidence of Foliar Spray Effectiveness Against the Invasive Cactus Cylindropuntia pallida (Rose), F.M. Knuth in South Africa

by
Keletso Makaota
1,
Thabiso Michael Mokotjomela
2,*,
Caswell Munyai
3,
Thembelihle Joyce Mbele
4 and
Nontembeko Dube
1
1
Center for Invasion Biology, Department of Zoology and Entomology, University of Free State, Bloemfontein 9310, South Africa
2
Centre for Invasion Biology, School of Life Sciences, University of KwaZulu-Natal, Pietermaritzburg 3200, South Africa
3
School of Life Sciences, University of KwaZulu-Natal, Pietermaritzburg 3200, South Africa
4
South African National Biodiversity Institute, Free State National Botanical Garden, Bloemfontein 9310, South Africa
*
Author to whom correspondence should be addressed.
Int. J. Plant Biol. 2025, 16(4), 113; https://doi.org/10.3390/ijpb16040113
Submission received: 30 July 2025 / Revised: 3 September 2025 / Accepted: 8 September 2025 / Published: 25 September 2025
(This article belongs to the Topic Plant Invasion)
Browse Figures
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Abstract

Using the biometric differences (i.e., plant physical characteristics) between the C. pallida populations previously treated with herbicide and those of the untreated populations, we tested the prediction that herbicidal treatment suppresses C. pallida plant growth in South Africa, where invasive cacti display high resilience to herbicide treatment. We also determined whether the surrounding communities knew of C. pallida invasion and whether they experienced any negative impacts. Overall, biometric analyses supported the study’s prediction because C. pallida plant height in treated populations was significantly lower than in untreated populations and before treatment. The average plant height of populations not treated with herbicide (i.e., experimental control) and those measured before treatment were not significantly different, but both were significantly greater than the heights of populations previously treated with herbicide. Similarly, the mean number of fruits, cladodes and juveniles per C. pallida plant treated with herbicide was significantly lower than in the untreated populations. We found a significant positive correlation between all measured parameters and C. pallida plant height. Out of 39 participants, 16 (41.0%) confirmed C. pallida presence in and around their properties, with 21 (53.9%) participants having experienced negative impacts directly and indirectly through fatal injuries on pets and livestock in the sampled sites. We confirmed the effectiveness of herbicide in suppressing the invasion of C. pallida and that people experience substantial negative impacts in areas where the species has established.

1. Introduction

Cactus invasion is a significant problem in South Africa [1,2,3,4]. Cacti species are among the most widespread species globally, and South Africa has approximately 35 taxa [5]. Historically, more than 200 cacti species have been introduced into South Africa for various socio-economic purposes [4,6]. In particular, the multiple introductions through activities such as the horticulture trade and via commercial and subsistence agriculture could have partly increased the propagule pressure and invasion success of cacti species [1,7,8]. In addition, cacti species generally have a high tolerance to extreme conditions of temperatures ranging from −6 to 55 °C [9], and have prolific reproduction rates, which increase the chances of population growth and range expansion [8,10,11]. Consequently, in combination with the high diversity of local environmental conditions [12], this has led to most cacti species forming large, dense infestations in South Africa that reduce the quality of grazing land [11,13].
Because many alien plant species are associated with human life as ornamentals, food, and medicine [8,14,15,16,17], many studies have argued that creating an awareness of their negative impacts could have positive repercussions for their effective management [18,19,20,21]. Specifically, investigating the local perceptions of the affected area could ensure greater awareness and a systematic understanding of the local community’s needs and how they relate to and interact with target alien species [18,20,22]. The awareness of alien species is important for detecting new populations of emerging species, and so encouraging a rapid management response [16,21,23,24]. Community awareness has contributed positively to many alien control projects through positive participation, planning, and decision-making [21]. However, creating an effective level of awareness may be challenging due to the limited understanding of the scientific content and of species identification by citizen scientists, resulting in delayed response action because of lengthy verification processes.
Since 1995, the management of plant invasions, including those of cacti species, has been prioritised to reduce their negative impacts in South Africa, and this has led to the establishment of clearing programmes, such as Working for Water [5,25,26,27]. Also, a new unit was developed in 2009 that is dedicated to clearing the emerging alien populations in order to avoid the high costs and the accumulated negative impacts associated with established large populations [16,23]. Furthermore, South Africa has developed and enacted legislation and policy in the form of the National Environmental Management: Biodiversity Act (NEM:BA, Act 10 of 2004) and the Alien and Invasive Species Regulations (hereafter called NEM:BA-A&IS Regulations; Department of Environmental Affairs, 2014; revised in 2020). According to the NEM:BA-A&IS Regulations of 2020, ten cacti species are listed under category 1a of alien species, including Cylindropuntia pallida (Rose), F.M. Knuth, and they are earmarked as targets in the national eradication programme [27,28].
Cylindropuntia pallida (Rose), F.M. Knuth, commonly known as the pink-flowered thistle cholla, is an emerging alien invader in the cactus family [29,30]. Cylindropuntia pallida reproduces vegetatively, with easily detachable spiny segments (cladodes) and fruits that can root within 20 days after being in contact with the soil, even without being watered [31]. The main dispersal strategy is through the movement of animals and humans by attaching to fur, skin, clothing, and even vehicle tyres [11,31]. Other dispersal agents are flood waters and heavy rain [11,32]. Cylindropuntia pallida is currently in the spreading phase of the invasion process in South Africa [33,34]. It can grow in a great variety of habitats, including grasslands, shrublands, and arid regions [32,35], and in South Africa, it prefers to invade the dry savanna and karoo biomes [29]. It can also resist drought and survive short periods of frost [32].
Different control methods have been considered to manage the invasion, including physical removal [31], chemical control [30], and the most effective biological control [30,36,37]. Although expensive when applied over vast areas, a foliar spray method is currently used to eradicate C. pallida populations in South Africa, with a success rate of over 90% [34,35]. The herbicide consists of triclopyr as the active ingredient, which is mixed with a dye to ensure that no plant parts are missed [38]. Triclopyr is a systemic herbicide that acts by disrupting the cell growth of a plant after absorption, leading to the death of the C. pallida plant within three months after application [35,39]. Since 1995 to this end, the South African government has spent about ZAR 15 billion on alien plant control operations. The budget has risen exponentially since 2010, with the annual spending in 2019 being around ZAR 2 billion [4]. However, a piece of highly desirable information—especially given the lack of an account of adequate progress and field evidence of success and what would be the return on investment for so many millions of rands spent on the containment and control of biological invasions in South Africa [21,27,35,40,41].
This study seeks to contribute knowledge on the effectiveness of the management approach in eradicating C. pallida in South Africa. Although the cactus invasions are being effectively cleared with a foliar spray and followed with post-treatment monitoring [35], there are recovering populations in some of the sites [35]. Although less understood, C. pallida dispersal mainly occurs through landscaping human activities [15,16], and domestic animal interaction [11,32,34]. There are ongoing studies on reproductive and dispersal strategies both in the wild and captivity to determine the feasibility of eradication and the role of humans in driving the invasion processes for C. pallida in South Africa [42].
Against this background, we aimed to provide evidence of effective management by comparing the biometric differences between the C. pallida populations previously treated with herbicide and those of the untreated populations using plant height as a proxy for growth. Since cactus reproduces both sexually and asexually, we compared the numbers of cladodes, fruits and juveniles in the C. pallida populations as a reproduction proxy between the two populations. We predicted that herbicidal treatment would suppress the growth and reproduction of the C. pallida plants. We also determined whether the surrounding communities knew of C. pallida’s presence and whether they had experienced any negative impacts.

2. Materials and Methods

2.1. Study Species

The study species (i.e., C. pallida) is a typical deciduous succulent with white spines and produces pink flowers in summer (Figure 1). These features also distinguish it from other cactus species [11,31].

2.2. Study Sites

Overall, the sites were located in the Northern Cape (NC), Free State (FS) and Eastern Cape (EC) provinces and were named according to the nearest town as a reference. The nine sites (i.e., Maphiniki, Barkley East, Edenburg, Philippolis, Bethulie, Colesberg, Richmond, Cookhouse and Cradock) located in areas in which C. pallida occurs and were identified as targets for clearing with the collaboration of the South African National Biodiversity Institute’s regional scientists (Figure 2). All the sites selected for this study similar in that the primary land use was livestock farming and grass was the dominant vegetation [43]. Because of the erratic distribution of the populations of C. pallida as an emerging alien cactus species in South Africa [35], the sites were far from each other and did not allow full replication within the site. Each site comprised a reasonable C. pallida population (i.e., >50 mature plants in a continuous patch; [35]).
The sites were classified into two groups. The first group comprised all nine sites, which had measurements of height as the proxy for C. pallida plant size taken independently, where there was no herbicide treatment (i.e., experimental control), before and after treatment. The height of C. pallida plants was measured between November 2018 and March 2023 in different sites with established populations (see Table S1).
Another group entailed sites with measurements of quantifying attributes indicating growth and reproduction of C. pallida plants (i.e., number of cladodes, fruits and juveniles), where there is no prior and after herbicide treatment. The site grouping was mainly dependent on the consistency of the variables measured. For example, Cookhouse (EC) and Richmond (NC) had C. pallida populations that had not been previously cleared or treated with herbicides, while Colesberg (FS) and Edenburg (FS) were sites where C. pallida populations had been treated once with a herbicidal foliar spray in 2018. However, the number of cladodes, fruits and juveniles was sampled from November to March 2023 in the four sites above.

2.3. Management of Cactus in South Africa

In South Africa, the alien cactus is treated with the foliar spray method with a 2% concentration of herbicide active ingredients fluroxypyr (Pyridyloxy compound 320 g/L); and a triclopyr (Pyridyloxy 480 g/L) following Mokotjomela et al. [35] and this study. The application rate was 50–75 mL Garlon/50 mL mineral oil/10 litres of water by a certified applicator. The herbicide was mixed with spray oil, and blue dye was applied using the 20 L knapsack sprayer with calibrated nozzles to allow the coarse spraying of the whole shoots of each plant until drip-off occurred [31]. The spraying was performed during cool times when air temperatures were less than 30 °C to avoid extreme conditions leading to plant dormancy and stomatal closure [45], and during active growing seasons: November–March 2017 and 2018.

2.4. Plant Biometry Data Collection

Data collection was performed concurrently with eradication during the summer season of 2018 to 2022 (in February and March). To determine the effect of herbicide treatment, the height of each C. pallida plant was measured with a measuring tape held against a person standing near each stand. The plants’ height was measured vertically, starting at the ground surface. The measurements were taken as the baseline before herbicide application, and sites were revisited for monitoring only after three years for this study. The height measurements were taken in C. pallida populations being cleared and the newly identified and targeted for future clearing but not included in the annual operations plan. Such height measurements were patchy and restricted to only nine sites. We included every plant taller than 0.25 m.
To determine the growth and reproduction of C. pallida, in a 2 m2 plot marked at every 10 m interval of each transect, we counted and recorded the number of cladodes, and fruits in each mature plant. We marked four transects in each site as replicates, and each transect was at least 100 m long and 2 m wide. This method was repeated across four sites [46]. The locations of all sampled C. pallida plants were recorded using a Garmin Oregon 750 model device. The number of cladodes was estimated by counting the exact number of cladodes in a portion of the canopy and then multiplying it by the total number of equal portions for each plant canopy [47]. The same approach was used to determine the number of fruits, and adjustments were made based on the density of fruits per portion.
To determine the possibility of the further invasion and population expansion of C. pallida populations in each site, the number of juvenile plants within a 2 m2 quadrat was counted around each mature plant in each transect per site at every 10 m interval of each transect following the procedure for choice of quadrat sizes based on vegetation cover type and size as outlined in Hill [46].
The transects were used as within-site replicates because there were not enough sites with large populations (i.e., >50 plants) and sites for replication. Also, since Mokotjomela et al. [35] found no correlation between C. pallida plant size and the toxicity of the herbicide, the assumption is that the C. pallida plants at the sprayed and unsprayed sites had the same size/density before treatment in two of the four sites.

2.5. Interviews with Communities Affected by C. pallida

Interviews were conducted with participants residing in the Cradock (EC) and Richmond (NC) towns that were invaded by C. pallida. Cradock (now Nxuba), a town in Eastern Cape Province, was an additional site where interviews were conducted because the two towns had human populations around them, which could give better insight and supply reliable samples to reveal people’s perceptions about C. pallida. The other field sites (Cookhouse, Edenburg, and Colesberg) had C. pallida populations located on private farms with a limited number of human participants in the vicinity.
The snowballing method was used to select the interviewees after those living closest to C. pallida stands had initially been identified (see [20,48]). We investigated the local context in which engagement was sought to ensure awareness following Shackleton et al. [20] and systematically asked about the needs of the community affected by C. pallida. For ethical considerations, face-to-face interviews were conducted with informed consent, and questions were translated into each of the interviewees’ home languages (including Afrikaans, isiXhosa, Sesotho and Setswana when necessary [20]. The questions were structured to seek information about the presence of C. pallida and when the communities had started noticing it and its negative impacts on people and animals, and to find out whether the species was abundant, dispersing, and expanding the population, and whether any action was being taken to control the species. The questionnaire comprised a set of twenty questions, which were a mixture of open-ended and closed questions. However, only eight main questions relevant to understanding the awareness and presence of C. pallida and its negative impacts were considered for analyses in the current study.
The interviews were conducted in July 2023; each was about 15 min, and 39 participants were interviewed. The study’s ethical clearance was obtained from the University of Free State, with Ethics Clearance number UFS-HSD2023/1208.

3. Data Analysis

3.1. Comparisons of C. pallida Biometrics Between Sites

To determine the herbicidal effect on the C. pallida plant height between the populations treated with herbicide and after treatment, a univariate general linear model analysis of variance was applied to analyse the data in SPSS version 29. The C. pallida plant height data for nine sites was classified into three site conditions namely: “No Treat”—independent measurements of height where no herbicide treatment is applied; “Before Treat”—measurements taken in each site prior to application of herbicide, and “After Treat”—the measurements taken after at least three years of site monitoring. The C. pallida plant height was fitted as the dependent variable; the site conditions (i.e., “No Treat”, “Before Treat” and “After Treat”) were independent variables, while site was treated as a random effect. Dunnett’s post hoc test determined significant differences between the mean C. pallida plant height in different site conditions, with “No Treat” considered as the experimental control at a significance of p < 0.05.
The number of cladodes and the fruits and the juvenile plants were compared to determine if plant growth and reproduction, respectively, are similar in populations previously treated with herbicide and those not treated. We applied a generalised linear model to analyse the counts’ data using SPSS version 29. To build a model, the counts of the cladodes per C. pallida plant, its fruits, and the juveniles were specified as the response variables and their classifications (i.e., “After Treat” and “No Treat”) as well as the individual study sites were defined as the predictor variables. We compared the variance coefficients for the Generalised Linear Models fitted with Poisson and negative binomial error distributions to select the best-fitting regression model. Model selection relied on the Akaike Information Criterion (AIC) and checks of the data dispersion residuals.
Finally, the non-parametric Partial correlation was performed to investigate whether there was a relationship between the C. pallida plant height and its cladodes, fruits, and juveniles to infer if large and uncontrolled plants can encourage further spread and invasions. Additionally, height was used as a proxy for environmental gradients and was included as a control variable.

3.2. Interviews: Knowledge of and Community Interactions with C. pallida

The data, in the form of responses from the different participants, were pooled before analysis because we had a limited sample, partly due to the local people’s reluctance to be interviewed, and there were very few human settlements near the C. pallida populations. Overall, 39 individuals were interviewed on eight key questions that focused on community awareness and known interactions with C. pallida; 11 were from Cradock (Eastern Cape), and 28 were from Richmond (Northern Cape).
The responses to the questions were captured verbatim in an Excel spreadsheet and later were transcribed, and each response was coded to generate countable data. Then, we applied the generalised linear model regressions to compare the grouped responses per question between towns, using the Pearson chi-square analysis at the significance of p < 0.05 in SPSS version 29. The questions were fitted into the model as response variables, while the towns were considered predictor variables. The questions and their responses were then classified into two groups, each containing four questions: the presence of C. pallida in the local area, the period of presence and negative impacts on people and animals, the behaviour of C. pallida—its abundance, spread/dispersal, and population—and actions taken against C. pallida.

4. Results

4.1. Cylindropuntia pallida Biometry Comparisons

Overall, biometric analyses supported the study’s prediction because C. pallida plant height in treated populations was significantly lower than in untreated populations and before treatment (F = 152.15; df = 1; p < 0.001). The treatments were also significantly different (F = 38.68; df = 2; p < 0.001). Dunnett’s post hoc revealed that the average plant height for C. pallida populations not treated with herbicide (Mean ± SE: 56.8 ± 2.2; N = 105) and that one measured before treatment (Mean ± SE: 52.9 ± 1.7; N = 170) were not significantly different (p = 0.168), but both were significantly greater height (p < 0.001) than populations previously treated with herbicide (Mean ± SE: 40.2 ± 1.7; N = 163; Figure 3). Similarly, the sites displayed significantly different average height of C. pallida plants (F = 21.01; df = 2; p < 0.001).
Regarding the number of cladodes, the populations treated with foliar spray generally had significantly lower numbers than those not treated with herbicides (Wald χ2 = 73.3; df = 1; p < 0.001; Table 1). Similarly, the mean number of fruits per C. pallida plant in populations treated with herbicide was significantly lower (Wald χ2 = 624.4; df = 1; p < 0.001; Figure 3), and average number of juveniles around adult plants than the untreated populations (Wald χ2 = 102.4; df = 1; p < 0.001; Figure 3) than the populations not treated with herbicide.
The various sites also showed significantly different variation in the numbers of cladodes (Wald χ2 = 99.7; df = 3; p < 0.001; Table 1), fruits (Wald χ2 = 428.5; df = 3; p < 0.001; Table 1), and juveniles (Wald χ2 = 37.7; df = 3; p < 0.001; Table 1). The Bonferroni post hoc test showed that while Richmond had significantly higher numbers of cladodes, fruits, and juveniles than the other sites, the numbers of cladodes and fruits were significantly lower than those in Cookhouse (p < 0.05; Table 1).
Partial correlation analysis revealed that there was a significant relationship between all the variables measured and the height of the plants (Table 2). The strongest correlation was observed between plant height and the number of cladodes, followed by plant height and the number of fruits present. However, when height was used as a control variable for environmental gradients, there was no significant correlation to the number of fruits recorded in C. pallida plants, suggesting the strong influence of the environment on plant growth (see Table 2).

4.2. Community Interviews on Their Knowledge of and Interactions with C. pallida

A total of 39 participants were included in the study; 28 (71.8%) lived in Richmond and 11 (28.2%) in Cradock. C. pallida was present in both towns. When the participants were questioned about whether the plant was present on their private properties, 16 (41.0%) confirmed that the plant was present on their properties. The other 23 participants (59.0%) responded that no C. pallida plants were present on their properties. When asked how long the plant had been present in the local area and towns, the Cradock participants had been aware of the plant for only three years or less. However, the participants in Richmond had been aware of the presence of the plant for as long as five years (Figure 4a). No significant difference was observed between the responses in the two towns (Pearson χ2 = 2.5; df = 5; p = 0.776).
Regarding the abundance of C. pallida plants on the participants’ properties, only two of the participants in Cradock had C. pallida plants, and they estimated them to be only a few plants (Figure 4a,b). In Richmond, an equal ratio of participants had plants either present or absent on their properties, and of the 14 participants who had plants present on their properties, 12 defined their number as relatively few (1–25 individual plants); two small-scale farmers reported having a moderate number of plants (25–50 individual plants) on their properties. No significant difference was observed between the towns with respect to the presence (Pearson χ2 = 3.3; df = 1; p = 0.059) and abundance (Pearson χ2 = 3.5; df = 1; p = 0.175; Figure 4a) of C. pallida plants on their private properties.
Regarding the negative impacts of C. pallida on human livelihoods, 21 (53.9%, N = 39) participants claimed that they or their household members had previously been pricked and injured by the plants (Figure 5). Of the 39 participants, 21 had experienced the direct negative impacts of C. pallida (Figure 5c). Some participants had pets and livestock that had experienced those negative impacts (Figure 5d) including six who stated that their pets had been injured. The other three said that their livestock (goats/sheep) had been wounded by C. pallida plants (Figure 5e). The participants who owned livestock said that the sheep would not stay still or in place after being pricked and that the injured goats could not go out to graze while they were limping.
When questioned about the dispersal of the plant (whether the propagules attached to their apparel, tools, or animals), most of the participants answered ‘No’ and added that they generally stayed away from the plant; however, 17 of the participants (44%, N = 39) answered ‘Yes’ (Figure 5b), and some added that propagules had sometimes attached to their clothing without their noticing. One of the farmers also said that he had seen propagules attached to some of his animals, posing a risk of further spread. No significant difference was observed between the two towns regarding the dispersal of C. pallida (Pearson χ2 = 0.9; df = 1; p = 0.654; Figure 5).
Regarding the management of C. pallida plants, participants were asked whether and, if so, how the plants in their hometown were being cleared. Most of the participants (82.1%; N = 39) in both towns said ‘No’ because they were not aware of any clearing being performed, while the 18.0% who answered ‘Yes’ indicated that individuals cleared C. pallida on their properties (Figure 6c). When asked whether the plant was spreading further in their properties, all the participants in Cradock answered ‘No’; therefore, 11 (82%) local participants did not have C. pallida in their properties (see Figure 6a). In Richmond, 28 (36.0%) participants responded ‘Yes’ to the question (Figure 6d); this was significantly greater than the observation in Cradock (Pearson χ2 = 14.6; df = 2; p < 0.001; Figure 6).

5. Discussion

Our study provides evidence supporting the study’s prediction that herbicidal foliar spray substantially suppresses C. pallida growth—this was attested by a significantly lower plant height, number of cladodes, fruits and juveniles in the population treated with herbicide compared to the untreated populations. It is worthwhile to note that, fewer plants observed in the treated sites were individuals that recovered from cleared populations following herbicide application. Corroborating the previous studies elsewhere [31], we show that thwarted growth and few fruits, as well as the cladodes, suggest that herbicidal treatment may reduce C. pallida’s future invasion, as shown for South Africa. Using structured interviews and field observations, we also showed that the local communities and animals experienced the negative impacts of C. pallida in selected areas where it has established.
It is contended that the observed growth suppression is a likely effect of the herbicidal treatment administered. It has been shown that the efficiency of herbicides differs on the physiological principles involved during the herbicidal action on the target plant, with systemic herbicides killing plants after their absorption by either accelerating or retarding the metabolic activities of weed plants [49,50]. Systemic herbicides are known to create oxidative stress and damage critical physiological processes [50], and thus, we argue that triclopyr might have a similar effect in C. pallida plants. It has been shown that the triclopyr effects are identical to excessive auxins, leading to increased uncontrolled plant growth [39], and ester formulations against root and stem sprouting species while remaining persistent in plants until they die [39], i.e., 1–5 months depending on the C. pallida plant size [31]. Noting that the impact of herbicides may not always manifest in visible injuries [50,51], we ascribe the retarded C. pallida plant growth to the effect of the herbicide that may have inhibited the critical photosynthetic and other metabolic processes, thus restricting the availability of resources for plant growth [50]. Similarly, effective foliar spray control using triclopyr herbicide has been reported in Spain and Australia, where C. pallida is invasive, and this was only observed where the whole plant was sprayed [31]. Further comparison of our results with other cactus management studies is limited because other countries have different environmental regulations regarding the use of herbicides. For example, some South African and Australian herbicides to control C. pallida are not allowed in some parts of Europe [45]. Thus, since C. pallida plants observed in all sites not previously treated with herbicides thrived, long-term monitoring is also required to observe further changes in plant growth, other attributable factors driving its invasion, and the timing of the changes to inform adjustments in the management strategy.
Our finding that the C. pallida plant height in populations treated with herbicide confirmed the effectiveness of the foliar spray method in containing its invasion. Consistently, the earlier studies [42,43,44,45,46,47,48,49,50,51,52,53,54,55], argued that plant characteristics can be used as reliable indicators of the impact that management has on alien plant species. In this study, the taller C. pallida plants had many cladodes, fruits and juveniles (in Richmond and Cookhouse), which supports the observation by previous studies (e.g., [42,56,57,58]) reporting the positive correlation between plant size, i.e., height and fecundity in alien plants. High fecundity is an essential driver of the spread of alien species and successful invasion by many species [54,59,60]. This observation suggests that if management intervention for C. pallida is stopped, further invasion may occur since we recorded many fruits and cladodes correlated with the number of juveniles. We also argue that the significantly greater number of C. pallida juveniles observed in Richmond was likely an effect of the presence of a larger human population around the study sites, which reportedly facilitates dispersal and invasions because of people’s multiple uses of the alien species [2,61,62]. Furthermore, C. pallida’s dispersal is stimulated by attachment to clothing, shoes, vehicle tyres, and domestic animals in South Africa [11,34,35]. It is possible that many plants in Cookhouse could be ascribed to the biophysical disturbance created by farm livestock and other human activity on the land [17], and this combined with the greater plant height, points out to higher chances of invasion than in populations treated with herbicide (i.e., Edenburg and Colesberg), which had fewer fruits (i.e., low fecundity). The suppression of growth and reproduction in C. pallida plants can also limit their competitive advantage over native plant species in consuming natural resources [60], reduce available dispersal propagules, i.e., cladodes and seeds in this study [31,34,59], and their overall density-based impacts [35,63,64]. Taller plants have also been associated with more significant negative impacts [18,65]; therefore, we suggest that C. pallida populations with taller plants be prioritised and targeted for management to reduce the risk of further invasion. Indeed, monitoring the effectiveness of implemented management efforts could guide decision-making for management’s investment in effective control methods [66], financial costs, and time [67].
Previous studies have shown that people’s perceptions of invasive alien species are influenced by factors such as the species’ history, the extent of invasion, benefits, and negative impacts [20,68,69,70]. For example, in our study, two participants consistently indicated that C. pallida was beneficial in keeping burglars away and had medicinal properties, as Balandran-Quintana et al. [71] reported. Our analysis also suggests that about half of the participants were aware of the C. pallida plants both in the wilderness and on their properties as ornamental plants [11,20]. This, taken together with (for example) other invasive alien cactus such as Opuntia fiscus indica, which has been declared to support people’s livelihoods [72], results in a conflict of interest in their management [20,72,73,74,75] reported that the community of eastern Free State typically opposed the clearing of Acacia mearnsii, arguing that it was a critical source of firewood. However, the finding that they were unaware that it was prohibited corroborates the report by Mhlongo et al. [70] on Lantana camara, which pointed to the need for more awareness campaigns about alien species and their negative impacts on the environment [16,21,75]. Jubase et al. [21] found that a lack of awareness about the negative impacts of invasive alien plants in small towns could result from limited public awareness and engagement activities. Formal education about invasive alien species could greatly influence people’s perceptions of the negative impacts of invasive alien species [76,77].
Our mixed (i.e., non-significant differences) results indicated that a proportion of the participants were still unaware of the negative impacts of C. pallida in the study areas, which could lead to the further unintentional and intentional spread of the species as ornamental plants [21]. Another possible explanation for the mixed results is the different extent of invasion between Richmond and Cradock, with most Cradock participants having few plants on their properties. However, that could change as the invasion intensifies because the negative impacts are thought to be directly proportional to the extent of the invasion [42,69,78]. The reported fatal injuries to people and animals in the study sites can be ascribed to the sharp and hooked spines of C. pallida. Previous studies also reported reduced grazing space for domestic livestock such as sheep and goats [11,31,34,35,37,79,80]. The negative impacts of alien cacti species on people and domestic livestock are well-known for O. stricta, which is widespread in South Africa [81] and in parts of Kenya [78]. A study in Spain has associated the manual clearing of C. pallida plants with the risk of injury and its further spread [31]. Our study reported fatalities to animals and injuries to humans caused by C. pallida, and such observation is also consistent with the report of Deltoro et al. [31] from Spain, highlighting the unreported loss of biodiversity. The observed fatal animal injuries may disrupt ecological services because it has been shown that different organisms, especially vertebrates, have distinct roles in maintaining habitat functionality [82]. Thus, more research would help explain the negative impacts of C. pallida. Our results also provide field evidence that could help management to reprioritise the limited resources to clear areas with heavy C. pallida infestations in South Africa.
About half of the participants (N = 39) in the current study indicated that C. pallida was present in low abundance on their properties and their mixed perceptions of whether the species was dispersing in the wilderness to a low level of awareness [20,21], and a lack of scientific knowledge of ecological processes in invasion biology [22,83,84]. It has been argued that the effective management of alien and invasive species is partly hindered by limited knowledge about the species, leading to wrong responsive action and planning [22,24,85,86]. For example, the use of wrong control methods was supported by the responses showing that the plant was spreading further, even after attempting to remove it. Although Korperlainen and Pietilainen [8] found that human activities cause the dispersal of most invasive alien species, it was apparent that citizens in Richmond and Cradock were unaware of how communities could be facilitating the dispersal of C. pallida. However, the participants pointed out that C. pallida disperses by attaching to human apparel, vehicle tyres, tools, and livestock, which is consistent with previous reports elsewhere in South Africa [11,34] and in Spain [31,32]. A raised awareness could guide people on how to interact with the plant in a way that prevents dispersal.
Regarding the management efforts that were in place, the few people in the current study who had C. pallida on their properties responded by attempting to clear it manually, using different methods such as mechanical means (with gardening tools) or burning, although the efforts were futile, possibly because of a lack of adequate knowledge of the species [22,83]. A recent study by Mhlongo et al. [70] pointed out that communities in Limpopo had attempted to clear invasive alien plants without the proper knowledge, leading to resprouting and further spread. This partly indicates that some people who have C. pallida in their gardens do not want too much of it, probably because of its negative impacts; however, they do not know about using suitable methods or that manual removal has a high risk of injury [31] and unintentional dispersal [32]. Therefore, we propose that a basic knowledge of clearing methods should be shared with community members during awareness campaigns to minimise the chances of unintentional spread during operations, as Novoa et al. [2] suggested. Several studies have consistently shown positive returns from regular and well-planned awareness campaigns in managing biological invasions [22].

6. Conclusions

Overall, we found that the foliar spray management used to treat C. pallida in South Africa effectively suppressed plant growth, reproduction, and possibly further invasions. Treated populations had shorter plants with fewer cladodes and fruits than untreated sites. We also found that taller plants are highly likely to reproduce, which may allow the invasion to thrive. We recommend continuing the herbicide treatment, prioritising taller C. pallida plants to prevent further spread in South Africa, as C. pallida spreads mainly through human activities and animal movements. However, this recommendation must apply to small populations since foliar spray methods have dire financial implications for large populations. Additional comparative studies on the vegetation and soil characteristics are required to improve the knowledge of the impacts of C. pallida. We also noted that some local people were familiar with C. pallida plants in the study sites, but their short history with the invasion of the species and the limited extent of the invasion could be the reason for their mixed perceptions of C. pallida’s impacts, dispersal, and further spread on their properties. It is important to note that the recorded negative consequences of C. pallida and the pertinent recommendations apply only to the sampled sites. Therefore, we recommend increasing awareness campaigns in communities affected by C. pallida to educate residents about its impacts, dispersal pathways, and strategies for preventing further invasions. Given the ability of this species to recover, management of dispersal pathways is highly recommended to effectively contain further invasion [87], and future studies should focus on integrating foliar spray with other management strategies such as biological control.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/ijpb16040113/s1. Supplementary Material Table S1: R-C-E-C sites.

Author Contributions

T.M.M., N.D., K.M. and T.J.M. conceptualised the study, developed the field methods, and collected the data; T.M.M. and K.M. analysed the field data, drew the study map, and wrote the manuscript; C.M., N.D. and T.J.M. read and edited the draft of the manuscript and provided important insights into how to improve the study. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Centre for Invasion Biology, and the APC was funded by the University of Free State.

Institutional Review Board Statement

The study was conducted in accordance with the Declaration of Helsinki. The study interviews’ component was ethically cleared by the University of Free State under Ethics Clearance number UFS-HSD2023/1208.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

Available on request because the nationwide study is ongoing.

Acknowledgments

We thank the Centre for Invasion Biology (housed in Stellenbosch University) for funding the running costs of the study. We also thank the University of the Free State for providing transport for the fieldwork and Patrick Mohasi for assisting with the interviews. Loyd R. Vukeya drew the maps showing the study sites and vegetation types. We thank all the community members of Cradock and Richmond who agreed to participate in the study, as well as the property owners who allowed us access to collect data.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Cylindropuntia pallida plants/stands showing their pink flowers, which are typically used for identification, and spiny cladodes in Katu, Northern Cape (Photo credits: Dr Thabiso M Mokotjomela).
Figure 1. Cylindropuntia pallida plants/stands showing their pink flowers, which are typically used for identification, and spiny cladodes in Katu, Northern Cape (Photo credits: Dr Thabiso M Mokotjomela).
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Figure 2. The nine study sites (i.e., Maphiniki, Barkly East, Edenburg, Philippolis, Bethulie, Colesberg, Richmond, Cookhouse and Cradock) overlaid on the national vegetation map [44].
Figure 2. The nine study sites (i.e., Maphiniki, Barkly East, Edenburg, Philippolis, Bethulie, Colesberg, Richmond, Cookhouse and Cradock) overlaid on the national vegetation map [44].
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Figure 3. Variation in C. pallida plant height (m) in populations not treated with herbicide (No Treat), before treatment (Before Treat), and after treatment (After Treat) across nine sites.
Figure 3. Variation in C. pallida plant height (m) in populations not treated with herbicide (No Treat), before treatment (Before Treat), and after treatment (After Treat) across nine sites.
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Figure 4. Interview responses of participants in Cradock and Richmond. Questions: (a) Presence—Is the plant present on your property? (b) Time—How long has the participant been aware of the plant’s presence in their respective towns? (c) Impacts on people—Have the participants experienced any negative impacts from the cactus plants? (d) Impacts on animals—Have any of the pets/livestock of the participants experienced negative impacts?
Figure 4. Interview responses of participants in Cradock and Richmond. Questions: (a) Presence—Is the plant present on your property? (b) Time—How long has the participant been aware of the plant’s presence in their respective towns? (c) Impacts on people—Have the participants experienced any negative impacts from the cactus plants? (d) Impacts on animals—Have any of the pets/livestock of the participants experienced negative impacts?
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Figure 5. Digital field evidence of the impacts of C. pallida: (a–g) show direct impacts on people and domestic livestock; (h–j,m) show birds that died after being trapped in C. pallida; (k,l,n,o) (apparently a mongoose)and (q) (apparently an aardwolf) show unidentified mammal skeletons with C. pallida spines on them; (p) shows a striped mouse with C. pallida spine; and (r) shows a dead lizard.
Figure 5. Digital field evidence of the impacts of C. pallida: (a–g) show direct impacts on people and domestic livestock; (h–j,m) show birds that died after being trapped in C. pallida; (k,l,n,o) (apparently a mongoose)and (q) (apparently an aardwolf) show unidentified mammal skeletons with C. pallida spines on them; (p) shows a striped mouse with C. pallida spine; and (r) shows a dead lizard.
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Figure 6. Interview responses for participants in Cradock and Richmond. Questions: (a) Please estimate the abundance of the plant on your property. (none = no C. pallida; plants; few = 1–25 individual plants; moderate = 25–50 individual plants); (b) Dispersal—Is the plant further dispersing by attaching to apparel, tools, and animals? (c) Population spreading—Is the plant spreading further on your property? (NA = Not applicable; plant is not present on the property); (d) Control action—Are the plants being cleared and, if so, who is clearing the plants? (NA = Not applicable; not aware of any clearing). Individual = Individual members clearing their properties; Community = a small group of community members clearing the plants.
Figure 6. Interview responses for participants in Cradock and Richmond. Questions: (a) Please estimate the abundance of the plant on your property. (none = no C. pallida; plants; few = 1–25 individual plants; moderate = 25–50 individual plants); (b) Dispersal—Is the plant further dispersing by attaching to apparel, tools, and animals? (c) Population spreading—Is the plant spreading further on your property? (NA = Not applicable; plant is not present on the property); (d) Control action—Are the plants being cleared and, if so, who is clearing the plants? (NA = Not applicable; not aware of any clearing). Individual = Individual members clearing their properties; Community = a small group of community members clearing the plants.
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Table 1. Variation in mean number of cladodes, fruits and juveniles in four sites, and the GLM statistical comparisons. ‘a’ is the parameter used as a reference during multiple comparisons. Superscript letters indicate the significant difference among the average number of cladodes, fruits and juveniles.
Table 1. Variation in mean number of cladodes, fruits and juveniles in four sites, and the GLM statistical comparisons. ‘a’ is the parameter used as a reference during multiple comparisons. Superscript letters indicate the significant difference among the average number of cladodes, fruits and juveniles.
Number of Cladodes per plantGLM Comparison
SiteMean (N)SEχ2DfSig.
Overall (Treated vs. No Treatment)73.291<0.001
Colesberg76.3 (53) a15.315.2281<0.001
Cookhouse209.1 (57) b14.71.32810.249
Edenburg33.4 (45) c16.659.0061<0.001
Richmond166.7 (48) d16.1a--
Number of fruits per plantGLM comparison
SiteMean (N)SEχ2DfSig.
Overall (Treated vs. No Treatment)624.451<0.001
Colesberg0.6 (53) a18.51009.191<0.001
Cookhouse100.3 (57) b19.4365.9521<0.001
Edenburg0.6 (45) ac200.08310.773
Richmond94.8 (48) bd17.8a--
Number of juvenilesGLM comparison
SiteMean (N)SEχ2DfSig.
Overall (Treated vs. No Treatment) 27.281<0.001
Colesberg2.5 (53) a0.315.731<0.001
Cookhouse6.1 (57) b0.36.4710.011
Edenburg1.4 (45) ac0.334.6121<0.001
Richmond3.5 (48) bd0.3a--
Table 2. Partial correlation analysis between the height of the plants and the number of cladodes, number of fruits, and number of juveniles measured in four sites/populations for C. pallida and uses of plant height to control for environmental gradients (Correlation is significant at p < 0.001 level).
Table 2. Partial correlation analysis between the height of the plants and the number of cladodes, number of fruits, and number of juveniles measured in four sites/populations for C. pallida and uses of plant height to control for environmental gradients (Correlation is significant at p < 0.001 level).
Control VariablesGladodesFruitsJuvenilesHeight
-none-GladodesCorrelation1.0000.6980.6090.817
Significance (2-tailed) <0.001<0.001<0.001
Df0201201201
FruitsCorrelation0.6981.0000.5780.757
Significance (2-tailed)<0.001 <0.001<0.001
Df2010201201
JuvenilesCorrelation0.6090.5781.0000.479
Significance (2-tailed)<0.001<0.001 <0.001
Df2012010201
HeightCorrelation0.8170.7570.4791.000
Significance (2-tailed)<0.001<0.001<0.001
Df2012012010
HeightGladodesCorrelation1.0000.2110.430
Significance (2-tailed) 0.003<0.001
Df0200200
FruitsCorrelation0.2111.0000.377
Significance (2-tailed)0.003 <0.001
Df2000200
JuvenilesCorrelation0.4300.3771.000
Significance (2-tailed)<0.001<0.001
Df2002000
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MDPI and ACS Style

Makaota, K.; Mokotjomela, T.M.; Munyai, C.; Mbele, T.J.; Dube, N. Preliminary Evidence of Foliar Spray Effectiveness Against the Invasive Cactus Cylindropuntia pallida (Rose), F.M. Knuth in South Africa. Int. J. Plant Biol. 2025, 16, 113. https://doi.org/10.3390/ijpb16040113

AMA Style

Makaota K, Mokotjomela TM, Munyai C, Mbele TJ, Dube N. Preliminary Evidence of Foliar Spray Effectiveness Against the Invasive Cactus Cylindropuntia pallida (Rose), F.M. Knuth in South Africa. International Journal of Plant Biology. 2025; 16(4):113. https://doi.org/10.3390/ijpb16040113

Chicago/Turabian Style

Makaota, Keletso, Thabiso Michael Mokotjomela, Caswell Munyai, Thembelihle Joyce Mbele, and Nontembeko Dube. 2025. "Preliminary Evidence of Foliar Spray Effectiveness Against the Invasive Cactus Cylindropuntia pallida (Rose), F.M. Knuth in South Africa" International Journal of Plant Biology 16, no. 4: 113. https://doi.org/10.3390/ijpb16040113

APA Style

Makaota, K., Mokotjomela, T. M., Munyai, C., Mbele, T. J., & Dube, N. (2025). Preliminary Evidence of Foliar Spray Effectiveness Against the Invasive Cactus Cylindropuntia pallida (Rose), F.M. Knuth in South Africa. International Journal of Plant Biology, 16(4), 113. https://doi.org/10.3390/ijpb16040113

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