1. Introduction
Cotton aphid
Aphis gossypii Glover (Hemiptera: Aphididae) has a wide range of hosts and is harmful to cotton, potato, and melons, among others [
1]. Its damage includes direct feeding injury to foliage and stems and mission transmission of plant viruses [
2], including cotton leafroll dwarf virus [
3,
4]. Wingless aphids develop faster than winged aphids. Wingless aphids have adaptations that maximize fecundity, making them more difficult to control [
5]. Insecticides are still the main method for control of
A. gossypii in most agricultural settings. However, past frequent use of insecticides has resulted in
A. gossypii populations resistant to all the major insecticide groups [
6]. Nauen et al. showed that field populations of
A. gossypii and
Myzus persicae Sulzer in several European countries were highly resistant to pirimicarb and oxydemeton-methyl [
7]. Patima et al. demonstrated that
A. gossypii populations in cotton fields in several regions of Xinjiang, China, have developed very high levels of resistance to deltamethrin, cypermethrin, and omethoate [
8]. Insecticide use may also harm populations of important natural enemies of pest insect [
9], contributing in this case to resurgence of aphid populations. Therefore, research and development of non-chemical pesticides for cotton aphid control are needed.
Plant essential oils (EOs) are volatile, naturally occurring, complex compounds found in aromatic plants, where they act as secondary metabolites. EOs intended for commercial uses can be collected by distillation, mechanical pressure, or extraction [
10]. EOs have potential as alternatives to commercial synthetic pesticides for use in green agriculture. Karamanoli demonstrated that EOs of spearmint and peppermint in soil declined about 90% within 30 days [
11]. Because of such rapid degradation of EOs in nature, they do not accumulate in the environment and thus leave no polluting residues. Some botanical insecticides such as pyrethrum and neem are already well established as commercial pest control products [
12], and are recommended for control of insects such as aphids, mirid bugs and whiteflies on field crops, as well as pests of glasshouse crops, house plants, and stored products [
13]. Today, more than 100 commercial neem formulations are used worldwide [
14]. Insecticides from the oils of citrus peel, containing limonene, have recently had moderate commercial success in North America and Europe [
15]. Following neem oil, orange oil was the most frequently used botanical insecticide in California in 2016, with 7.0 tons of active ingredient being applied [
16]. Additionally, the EOs of
Chenopodium ambrosioides L. are used for insect pest management [
17], with an average of 8000 kg sold per year in California, United States [
15].
Plant EOs have great potential in the control of aphids. A number of articles have been published both at home and abroad reporting the fumigant and contact activities of EOs in the Lamiaceae and Asteraceae families against aphids. The EOs of
Mentha piperita L.,
Mentha pulegium L., and
Ocimum basilicum L. showed excellent efficacy in both contact and fumigation tests against
Lipaphis pseudobrassicae Davis,
M. persicae, and
A. gossypii [
18].
Lavandula angustifolia Bubani oils had fumigant activity against
Acyrthosiphon pisum Harris [
19].
Mentha longifolia L. oils had fumigant and contact activities against
Aphis craccivora Koch, and affected the development, survival, and reproduction of
A. craccivora [
20].
Artemisia dracunculus L. had high fumigant toxicity to
A. gossypii [
21]. The EOs
Santolina chamaecyparissus L. and
Tagetes patula L. showed contact activity against
Myzus persicae Sulzer and
Rhopalosiphum padi L., while sublethal concentrations of the two essential oils reduced aphid fecundity [
22].
At present, the EOs of Lamiaceae and Asteraceae have been reported to have antifeedant activity against a variety of aphids. Abualfia et al. discovered that the EOs of
Salvia officinalis L.,
Lavandula spica L. and
Mentha spica L. caused different degrees of interference with aphid feeding [
23]. Zhang et al. found that the ethanol extract of
Xanthium strumarium L. had stronger antifeedant activity against aphids feeding on
Lycium barbarum L., with a plant rejection rate of more than 82% [
24]. Zhou demonstrated that
Flaveria bidentis L. oil had different levels of antifeedant activity on
Lipaphis erysimi Kaltenbach and
Rhopalosiphum maidis Fitch [
25]. However, research on the antifeedant activity of plant EOs on aphids is relatively scarce. We sought to determine whether the EOs from Lamiaceae and Asteraceae plants exhibit antifeedant effects on cotton aphids, which will provide a theoretical basis for the development of subsequent antifeedant.
In this study, we examined the effects of EOs from seven plants (Ocimum sanctum L., Ocimum basilicum L., Ocimum gratissimum L., Mentha piperita L., Mentha arvensis L., Tagetes erecta L., and Lavandula angustifolia Mill.) on the growth, development and fecundity of A. gossypii. In addition, we analyzed the feeding behaviors of A. gossypii using the EPG technique. Lastly, we measured honeydew excretion by aphids feeding on cotton leaves treated with EOs of seven plant species, using honeydew excretion as a measure of aphid feeding.
3. Discussion
We examined the effects of seven plant EOs (O. sanctum, O. basilicum, O. gratissimum, M. piperita, M. arvensis, T. erecta, and L. angustifolia) on the growth, development, feeding behavior, and honeydew excretion of A. gossypii. All seven EOs had varying effects on A. gossypii, with the most effective from the point of view of aphid control being the EOs of O. sanctum, M. piperita, and T. erecta.
In general, it is the case that plant EOs can adversely affect the growth and development of aphids. For example, the EO of
Tagetes minuta L. significantly reduced the fecundity of
Acyrthosiphon pisum Harris,
M. persicae, and
Aulacorthum solani Kaltenbach [
26]. The EOs of
Origanum majorana L.,
Mentha pulegium L., and
Melissa officinalis L. significantly reduced the longevity and fecundity of
M. persicae [
27]. We showed that the adult longevity of
A. gossypii was significantly shortened and its fecundity significantly reduced by the EOs of six of our seven test plants (
O. sanctum,
O. gratissimum,
M. piperita,
M. arvensis,
T. erecta, and
L. angustifolia). Also, the net reproductive rate (
R0), intrinsic rate of increase (
rm), and finite rate of increase (
λ) of
A. gossypii were all significantly reduced, and the mean generation time (
T) was significantly increased after treatment with EOs of
O. sanctum,
M. piperita and
L. angustifolia. The growth, development, and fecundity of
A. gossypii were all diminished, possibly due to a reduction in time spent in phloem and xylem feeding, reducing the level of nutrition available for growth [
28].
Insects assess whether a plant is suitable for feeding by sampling plant parts for their nutrients [
29]. Aphid probing and prolonged feeding are closely associated with the level of plant tissue damage and plant virus transmission [
30].
A. gossypii obtains the nutrients it needs for growth and development mainly by feeding on the phloem sap [
31]. Aphids promote the success of such feeding by secreting water-soluble saliva, which can prevent plant phloem proteins from clogging sieve tubes [
32]. In our experiment, we observed that secretion of saliva (E1) and phloem feeding (E2 waves) by
A. gossypii were both reduced after exposure to EOs, leading to less successful feeding by
A. gossypii. Similarly, for pea aphids (
A. pisum) treating its host plants with kaempferol led to an increase in the time spent in non-probing behaviors [
33]. When plants infested with
A. gossypii were treated with imidacloprid,
A. gossypii spent significantly longer time on intercellular apoplastic stylet pathway activities, which are non-production behaviors taking time away from feeding [
34]. For
Diaphorina citri Kuwayama non-probing behaviors (Np) were prolonged after exposure to the EOs of guava (
Psidium guajava L.) [
35]. Similarly, in our study,
A. gossypii spent more time in non-probing behaviors after exposure to the plant EOs examined here. Also, the duration and number of events of non-vascular probing (C waves) increased in
A. gossypii following exposure to our test EOs, indicating that more probing occurred between the plant epidermis and the microtubule bundles, requiring more time to find a suitable feeding site. Zhou et al. observed plants’ aqueous extracts of
Nerium indicum Mill.,
Cinnamomum camphora L.,
Ginkgo biloba L., and
Lycopersicon esculentum Mill. significantly prolonged the duration of non-vascular probing in
A. gossypii [
36]. In contrast to phloem ingestion, xylem ingestion supplies water but not nutrition, and increased time spent in xylem feeding may be linked to a decrease in phloem feeding [
37]. Aphids spend more time xylem feeding when under stress [
38]. We found the same behavior in our experiment, in which the xylem feeding was prolonged after treatment with EOs, presumably to maintain the internal water equilibrium [
39].
After aphids feed on phloem, excess water, which still contains some sugar, is excreted as honeydew to concentrate sugars in the fluids retained for ingestion [
40]. In contrast, xylem feeding results in little or no honeydew production because most ingested xylem is retained to maintain water equilibrium [
41]. Honeydew excretion is, therefore, closely tied to the level of phloem feeding by an aphid [
42]. Honeydew often covers plant surfaces and affects photosynthesis and respiration [
43]. When an aphid’s level of phloem feeding is reduced, its rate of growth and development declines, and it excretes less honeydew. By quantifying the volume of honeydew excreted by cotton aphids, we were able to determine the degree to which plant essential oils had influenced the feeding of cotton aphids. Extracts of
Syngonium podophyllum Schott,
Xanthium sibiricum Petr.et Widd, and
Tephrosia vogelli Hook have been shown to reduce honeydew excretion by
M. persicae and
Lipaphis erysimi Kaltenbach [
44]. In our study, the daily rate of honeydew excretion by
A. gossypii declined significantly after treatment with plant EOs. This decline in honeydew production indicates that plant EOs have significant antifeedant effects on
A. gossypii, but the EOs of different plants varied in the levels of their antifeedant effects on
A. gossypii. We observed that the honeydew of
A. gossypii was dispersed on the filter paper, likely indicating that the EO-treated leaves were not acceptable for aphid feeding, and aphids kept moving and searching for better sites. This suggests that the honeydew production of
A. gossypii was reduced and no suitable sites were located, given that there was no overlapping of honeydew to from blotches on the filter paper.
From an integrated pest management point of view, when considering the effects of EOs on pest control, we also need to examine their effects on natural enemies. Chemical control not only leads to an increase in pest resistance but also kills large numbers of natural enemies, weakening their contribution to pest control, which may lead to pest outbreaks. It has, however, been demonstrated that many EOs are much less damaging to natural enemies than to the target pests. Papadimitriou et al. compared the biological activity of the EO of
M. pulegium against
A. gossypii,
Tetranychus urticae Koch, and the mirid
Nesidiocoris tenuis Reuter, and found that the plant’s EO was highly lethal to both
A. gossypii and
T. urticae at a concentration of 1000 μL/L, but had no toxic effects on
N. tenuis [
45]. In another study, the LC
50 values of the EOs of
M. piperita,
Mentha longifolia L.,
Salvia officinalis L., and
Salvia rosmarinus Spenn. were about four times higher against the coccinellid
Coccinella undecimpunctata L. than to the pest,
Aphis punicae Passerini [
46]. Ebadollahi et al. observed that the EO of
Satureja intermedia C.A.Mey showed significant contact activity against
Aphis nerii Kaltenbach, but was safe against its predator
Coccinella septempunctata L. [
47]. These findings suggest that insect natural enemies may be more tolerant to a wide range of plant EOs than are target pests.
Although this paper showed that the EOs of
O. sanctum,
O. basilicum,
O. gratissimum,
M. piperita,
M. arvensis,
T. erecta, and
L. angustifolia all had different antifeedant effects on
A. gossypii, in-depth studies are needed to determine their individual antifeedant mechanisms. We will identify the active ingredients in plant essential oils in subsequent research and further evaluate which components in the plant essential oils have antifeedant effects on cotton aphids.
A. gossypii, as a major pest of cotton, has developed a high level of resistance to conventional chemical control, and botanical antifeedants are a novel means for its control. Antifeedants appear to both reduce colonization of plants by reducing their attraction for pests and reduce feeding of any pest individuals that do begin to attack the host plant, slowing their population growth. EOs protect the host plant and also avoid harming natural enemies and other non-target insects [
48]. EOs can, therefore, be used in combination with biological control, natural enemy conservation, and other such techniques to better promote sustainable management of
A. gossypii in cotton fields.
Currently, botanical insect antifeedants are not yet used on a large scale due to costs in their mass production [
49]. Plant EOs are also limited by their relatively high volatility, which limits the duration of their efficacy. Technologies that promote the slow, controlled release of EOs by means of microencapsulation [
50] using polymer films have potential to increase the persistence of EOs, allowing for and less frequent applications.