1. Introduction
Southeastern states, including Georgia and Florida, are among the top producers of fresh market yellow and zucchini squash (
Cucurbita pepo L.) in the United States, with a market valued at USD 37.6 million in 2019 [
1]. The majority of Georgia’s yellow and zucchini squash (hereafter referred to as squash) are grown in the fall season, during which pressure from pests and pathogens is typically higher than in the spring [
2,
3,
4]. The sweetpotato whitefly,
Bemisia tabaci Gennadius MEAM1 (Hemiptera: Aleyrodidae), is arguably one of the most important pests of squash. Feeding by whitefly nymphs results in squash silverleaf disorder (SSL), a reversible physiological disorder that reduces photosynthesis, stunts plants, and diminishes fruit quality [
5,
6,
7,
8]. Feeding by whiteflies also results in the transmission of plant viruses, which can be severely yield-limiting. Yield losses from
B. tabaci and whitefly-transmitted viruses, combined with management actions, cost growers tens of millions of dollars each year [
9,
10,
11].
A complex of at least two whitefly-transmitted viruses impacts squash in Georgia, viz., cucurbit leaf crumple virus (CuLCrV) and cucurbit yellow stunting disorder virus (CYSDV) [
4]. Both viruses are relatively new to Georgia. CuLCrV was first documented in the U.S. in 1998–1999 in Arizona, Texas, and California, and it was found in Florida and Georgia in 2006 and 2009, respectively [
12,
13,
14,
15]. CYSDV was first observed in California and Arizona in 2006 and in Georgia in 2016 [
16,
17]. Cucurbit leaf crumple virus is in the genus
Begomovirus and family
Geminiviridae, and Cucurbit yellow stunting disorder virus is in the genus
Crinivirus and family
Closteroviridae [
18,
19]. CuLCrV is transmitted in a persistent and circulative manner by whiteflies [
20,
21,
22,
23]. The acquisition access period (AAP), inoculation access period (IAP), and latent period each take hours to days [
24]. CYSDV is transmitted by whiteflies in a semi-persistent and non-circulative manner [
25,
26]. Criniviruses, such as CYSDV, have shorter AAPs and IAPs (minutes to hours) than begomoviruses, with no latent period before they can be inoculated to a susceptible host [
25,
27,
28]. Whiteflies can acquire and transmit multiple viruses at once, and thus, both viruses are often observed as mixed infection in squash, resulting in more severe symptoms [
4,
24].
Squash seedling/transplant production in open greenhouses is a common practice that often leaves plants vulnerable to virus infection at their most susceptible stage. This can result in the establishment of vector reservoirs and virus inoculum sources once infected seedlings are transplanted into the field [
29,
30,
31,
32,
33]. Resistant crop varieties offer the best protection against whiteflies and viruses, but there are no commercially available squash varieties with resistance to whiteflies and/or CuLCrV and CYSDV [
3,
34,
35]. Insecticide use, often multiple applications per week, is the norm for whitefly and virus management, indirectly [
35,
36]. However, most insecticides, despite suppressing whiteflies, do not reduce virus transmission, as one or a few viruliferous whiteflies past the latent period can readily inoculate a susceptible squash plant within minutes of feeding [
37,
38].
The current management strategy of prophylactic and frequent insecticide applications can also increase risks of insecticide resistance development.
Bemisia tabaci MEAM1 populations around the world have developed resistance to nearly all insecticide classes used [
37,
39,
40,
41,
42]. Such indiscriminate insecticide applications pose risks to applicators, non-target organisms, and the environment, and can negatively impact the biological control of pests by natural enemies. Reflective mulch can disorient whiteflies and prevent landing on transplanted seedlings by reflecting visible and UV light [
43,
44,
45,
46]. Living mulches, such as buckwheat, clover, perennial peanut, yellow mustard, cowpea, and sorghum have also been shown to effectively reduce whitefly abundance, SSL intensity, and virus incidence in crops [
32,
47,
48,
49]. In addition, row covers and other methods of physical exclusion have been shown to be extremely effective in protecting greenhouse seedlings and direct-seeded plants in the first few weeks of the growing season [
33,
46,
50,
51,
52,
53]. Despite the effectiveness of reflective mulch and row covers, they are seldom used in commercial squash production in the southeastern U.S.
At this time, there is no single management tactic that is effective enough to suppress whiteflies and reduce the transmission of viruses in Georgia and other southeastern states. Therefore, an integrated pest management (IPM) program comprised of existing cultural and chemical tactics aimed at pre- and post-transplant protection is essential to limit yield losses and maintain sustainability. The first objective of this study was to evaluate the effect of insect exclusion netting (IEN), either alone or in combination with insecticides, in greenhouse production of squash seedlings. The second objective was to evaluate UV-reflective and live mulches, as well as insecticides in combination with row covers, under open field conditions. One greenhouse and two field trials were conducted over four field seasons (2018–2021) with the goal of developing a combination of reduced-risk tactics to mitigate yield loss and enhance sustainability in squash production.
4. Discussion
Bemisia tabaci MEAM1 and whitefly-transmitted viruses are the biggest limiting factors to profitable and sustainable yellow and zucchini squash production in Georgia, United States. In this study, several management tactics aimed at reducing the impacts of B. tabaci MEAM1 and whitefly-transmitted viruses were evaluated. This study demonstrated that adequate management measures in seedling greenhouses are key to minimizing virus inoculum sources in the field. Additionally, this study evaluated UV-reflective and live mulches, row covers, and insecticides in the field over four consecutive field seasons. The outcomes revealed that protection methods and insecticides helped manage whitefly populations and, to some extent, virus symptom severity. The benefits of these management tactics led to an increase in yield that was most prominent during the seasons when virus pressure was highest.
Whitefly-transmitted viruses are emerging worldwide [
55], and the southeastern United States is no exception to this global trend. Mixed infections of CuLCrV, CYSDV, and cucurbit chlorotic yellows virus (CCYV) were found to be widespread in cucurbit fields in Georgia in 2021 [
4,
56]. Squash silverleaf disorder (SSL), a physiological disorder resulting from whitefly feeding, also affects squash production [
5,
43]. Yellow and zucchini squash varieties with resistance or tolerance to whiteflies and/or whitefly-transmitted viruses are not commercially available [
34,
35]. Current management programs for whiteflies and whitefly-transmitted viruses rely heavily on insecticides [
37,
42,
57]. There is not a single management tactic available that can effectively reduce whitefly-mediated transmission of viruses to desirable levels. This study intended to offset this critically important need by evaluating a number of management tactics with the goal of developing IPM recommendations for yellow and zucchini squash production in the southeastern United States.
The first step to reducing virus pressure and the resulting reduction in yield is to avoid introducing virus-infected seedlings into the field. Squash seedlings are typically grown in open greenhouses without any insect exclusion materials. Consequently, virus-infected seedlings may be transplanted into fields, facilitating rapid secondary spread of viruses after planting. The IEN used in this study was effective in reducing whitefly abundance and virus incidence in the greenhouse study. The insecticides cyantraniliprole and terpene constituents of
Chenopodium ambrosioides near
ambrosioides extract also reduced the number of whitefly adults on seedlings, although cyantraniliprole alone reduced virus incidence. Acibenzolar-S-methyl, a salicylic acid analog, does not have insecticidal properties but is known to induce plant defenses against viruses by activating the salicylic acid pathway [
58,
59,
60]. While fewer adult whiteflies were observed in the acibenzolar-S-methyl treatment compared with the non-treated control in our study, the former did not reduce virus incidence in squash seedlings. It is possible that the timing of our acibenzolar-S-methyl application could be optimized to improve virus protection [
59], or that multiple applications may be necessary for suppressing CuLCrV-induced symptoms on squash seedlings [
60]. The greenhouse experiment in this study demonstrated that seedling infection in open greenhouses could be minimized with the use of IEN in conjunction with insecticides.
In addition to transplant seedling protection, an effective IPM program should include tactics that reduce whitefly abundance and virus inoculum in the transplanted field. Previous studies have shown that different mulch types can have varying effects on whiteflies and virus transmission [
38,
43,
45,
49,
61,
62]. Summers and Stapleton [
43] observed fewer whitefly adults and nymphs in squash plots with UV-reflective mulch than in bare-ground plots and found that reflective mulch was as effective for whitefly management as imidacloprid. In our study, the use of UV-reflective plastic mulch reduced the number of immature and adult whiteflies and significantly reduced SSL intensity (
Figure 4). Marketable yield was doubled in plots with UV-reflective mulch compared with live mulch and white plastic. Reflective mulch may have encouraged yield increases by several complementary mechanisms: (1) by repelling whiteflies and reducing direct feeding injury and SSL intensity [
43]; (2) by reducing virus transmission due to reduced feeding [
38,
45,
61]; and (3) by radiating additional light energy onto plants, which augments photosynthesis and growth [
45,
63,
64,
65]. Although UV-reflective mulch has been shown to reduce virus pressure in squash [
66,
67], reduced virus severity was only detected in one year of our study. This was likely due to explosive whitefly populations in the experimental area and the fact that a single viruliferous whitefly can transmit the virus to a susceptible host plant [
68,
69,
70].
Live mulch intercropped with white plastic mulch only reduced adult whitefly abundance as effectively as the UV-reflective mulch in the 2018 season. This effect did not extend to abundances of whitefly eggs and nymphs, virus symptom severity, SSL intensity, or yield, and no effects of live mulch were observed in 2019, 2020, or 2021. Similarly, Frank and Liburd [
48] did not observe reduced whitefly abundance or virus incidence in plots with live mulch. However, other studies have found that plantings with live mulch had comparable whitefly abundances, virus incidences, and yields to reflective mulch and even insecticide treatments [
49,
61]. Flowering live mulches can increase the abundances of natural enemies that attack whiteflies, such as hover flies (Diptera: Syrphidae), predatory wasps (Hymenoptera: Scoliidae, Sphecidae, Eumenidae, Vespidae), and lady beetles (Coleoptera: Coccinellidae [
47,
48,
71,
72,
73]. Poor germination and growth of buckwheat in at least two years of our study due to heavy rains may have negatively impacted natural enemy populations and, subsequently, whitefly populations.
Insect-proof row covers installed immediately after planting were equally or more effective than insecticides in reducing whitefly abundance, SSL intensity, and virus symptom severity in squash plants. Additionally, protecting young plants with row covers dramatically increased marketable yield, while the insecticide treatments alone had little to no effect. Webb and Linda [
33] and Costa et al. [
51] saw similar reductions in whitefly abundance and SSL intensity, as well as increases in yield when squash was protected using row covers. Other studies have also seen virus incidence reduction with the use of row covers [
50,
51]. Row covers also exclude other pests that can cause additional injury and yield reductions [
33,
50]. The use of row covers in squash production is relatively new in the southeastern United States but is already practiced for frost protection in crops such as strawberries [
74,
75].
Insecticides have effectively reduced whitefly abundance and SSL intensity in squash in other studies [
37,
61,
76]. Of the insecticides tested in our field trials, cyantraniliprole and flupyradifurone offered the best protection against adult whiteflies. Afidopyropen and spirotetramat plus pyriproxyfen also provided protection against whitefly nymphs. All four products have different modes of action (
Table 1) and represent alternatives to commonly used neonicotinoids, such as imidacloprid. Some insecticides can also help prevent virus infection under certain conditions, including cyantraniliprole and flupyradifurone [
36,
39,
53,
77,
78,
79,
80,
81,
82]. In this study, reduced virus symptom severity, but not virus incidence, was observed in plots treated with cyantraniliprole and flupyradifurone compared with the non-treated control. Virus infection was ubiquitous in the trials, often reaching 100%. Unlike many conventional insecticides that rely only on toxic activity against the vector, cyantraniliprole and flupyradifurone rapidly inhibit vector feeding, potentially limiting virus transmission [
37,
78,
81,
83,
84,
85,
86]. Thus, insecticide applications remain an important part of IPM programs for whiteflies and viruses. However, reliance on insecticides is not recommended, as most insecticides do not impact virus incidence in the long-term, and insecticide resistance in whitefly populations is a major concern [
37,
38]. There is already evidence that
B. tabaci MEAM1 populations in the southeastern United States have developed high levels of resistance to the commonly used neonicotinoids imidacloprid and thiamethoxam, with varying tolerances to pyrethroids, flupyradifurone, buprofezin, and dinotefuran [
41,
87,
88].
With no “silver bullet” for managing whitefly-transmitted viruses in squash, a combination of multiple tactics with additive effects is the most effective management approach at this time. Combining the use of row covers and UV-reflective plastic mulch with effective insecticides may help achieve optimum control and ensure profitable yields until virus-resistant squash varieties become available [
48,
49,
61,
79,
82]. Besides improving the management of whiteflies and viruses, integrating these cultural management practices can reduce reliance on insecticides and lessen resistance development risks [
62]. These practices can be implemented by both small- and large-scale growers and may also be effective in other crops, such as watermelon, where squash vein yellowing virus, another whitefly-transmitted virus causing vine decline in watermelon, is common [
79,
80,
89,
90].