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Keywords:

  • Myzus persicae;
  • dispersal;
  • behaviour;
  • resistance;
  • neonicotinoid;
  • thiamethoxam

Abstract

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

BACKGROUND

The peach potato aphid Myzus persicae is a major agricultural pest capable of transmitting over 100 plant viruses to a wide range of crops. Control relies largely upon treatment with neonicotinoid insecticides such as thiamethoxam (TMX). In 2009, a strain denoted FRC, which exhibits between 255- and 1679-fold resistance to current neonicotinoids previously linked to metabolic and target site resistance, was discovered in France. Dispersal behaviour may potentially further enhance the resistance of this strain. This study investigated this possibility and is the first to compare the dispersal behaviour of aphid clones of the same species with differing levels of neonicotinoid resistance.

RESULTS

Comparing the dispersal behaviour of the FRC strain with that of a clone of lower neonicotinoid resistance (5191A), and a susceptible clone (US1L) highlighted several differences. Most importantly, the FRC strain exhibited an increased ability to locate untreated areas when presented with an environment consisting of both TMX-treated and untreated plant tissue.

CONCLUSION

The altered dispersal behaviour of the FRC may partially account for the high level of neonicotinoid resistance exhibited by this strain in the field. Since the dispersal of aphid vectors is key to the transmission of viruses across crop fields this has implications for current crop protection practice. © 2013 Society of Chemical Industry

1 INTRODUCTION

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

The recently discovered FRC strain of the peach potato aphid Myzus persicae (Sulzer) exhibits a resistance factor (RF) of 1679 to the neonicotinoid imidacloprid and 225 to thiamethoxam in topical bioassays.[1] Resistance in this population appears to be due to a combination of two separate mechanisms, a target-site mutation within the nicotinic acetylcholine receptor (nAChR) and enhanced detoxification of the insecticide by cytochrome P450 monooxygenases. While a mixed population and not a clone, all FRC individuals carry the same mutation within the nAChR and exhibit the same levels of neonicotinoid resistance, hence for the purposes of this study they will be treated as a clone. Neonicotinoid resistance is also achieved by the upregulation of P450 genes in the Greek M. persicae clone 5191A.[2] 5191A, however, exhibits a 60-fold RF to imidacloprid in topical bioassays[3] hence this clone's resistance, unlike that of FRC, is only detectable within the laboratory. Myzus persicae is already resistant to three other classes of insecticide by four other distinct mutations.[4] On the basis that resistance to one neonicotinoid chemical generally confers resistance across a whole insecticide class[5] and that the major method of control of this pest is by the application of insecticides the discovery of the FRC resistant strain presents a serious agricultural threat.

Resistance to neonicotinoids in insect pests is commonly achieved by upregulation of metabolic pathways.[6] It has been shown that, compared with susceptible strains, neonicotinoid resistant strains of Bemisia tabaci (Genn.) produce larger amounts of the primary metabolite 5-hydroxy-imidacloprid, which has a lower binding affinity for the nAChR than imidacloprid and would be subjected to secondary metabolism through inactivating conjugation reactions. This upregulated metabolism appears to be due to increased expression of P450s, with monooxygenase activity correlating strongly with neonicotinoid resistance in this species. Other species which show upregulation of P450s correlated with resistance to neonicotinoids include Nilaparvarta lugens (Stal),[7] Drosophila melanogaster (Meigen)[8] and Musca domestica (L.).[9] In terms of neonicotinoid resistance, metabolic changes occur more frequently than target site mutations. Alteration of the nAChR target site has occurred in N. lugens[10] but in this case it arose in an artificially selected laboratory strain. It has also been suggested that target site resistance may occur in the Colorado potato beetle Leptinotarsa decemlineata (Say), but the mutation remains unproven.[11] Other than the M. persicae FRC population there is therefore little evidence of target site based neonicotinoid resistance arising in the field.

Although less commonly cited, behavioural changes may also contribute to insecticide resistance. For example, an imidacloprid resistant strain of Myzus nicotianae Blackman only exhibited such tolerance when the insecticide was applied in an oral ingestion assay rather than topically. This increased tolerance appeared to be linked to an alteration in feeding behaviour shown by the resistant strain compared with a susceptible strain.[12] A resistant strain of the German cockroach Blattella germanica (L.) showed increased aversion to agar containing fructose, glucose, maltose and sucrose, all ingredients commonly contained within commercial gel baits, than a susceptible strain.[13] Topical application of abamectin or fipronil reduced the difference in mortality between resistant and susceptible strains, indicating that this change in feeding behaviour contributed significantly to the resistance.

In theory, dispersal behaviour might also contribute to resistance. An individual which shows an increased tendency to quickly disperse from an insecticide-treated to an untreated plant may ingest a lower dose of insecticide and so be more likely to survive. In this way a resistant strain could potentially arise. This has happened in the Colorado potato beetle Leptinotarsa decemlineata (Say), where a Bt resistant strain shows increased flight activity compared with a susceptible strain upon ingestion of Bacillus thuringiensis (Bt) toxin.[14] Larvae from Bt resistant strains have also been shown to be more behaviourally responsive to the presence of Bt than susceptible larvae, moving away from foliage treated with the toxin.[15] While previous work has linked enhanced dispersal behaviour to increased levels of pyrethroid resistance in two resistant M. persicae clones,[16] this is the first study to investigate the possibility of a behavioural component to neonicotinoid resistance in aphids by comparing the dispersal behaviours of neonicotinoid resistant and susceptible M. persicae clones. This study reports the novel finding of a behavioural component to the high levels of neonicotinoid-resistance exhibited by the M. persicae FRC population.

2 METHODS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

2.1 Insects and plant material

The susceptible M. persicae clone US1L was obtained from Imperial College London, while 5191A and FRC were obtained from Syngenta, with the permission of Rothamsted Research, UK. All three clones were reared on Chinese cabbage (Brassica chinensis L. cv. Apex) in separate environmentally controlled chambers (22 ± 1 °C, 16:8 h L:D photoperiod, 50% relative humidity). Chinese cabbage plants for the FRC culture were sprayed to run off with 10 mg L−1 thiamethoxam[17] (TMX) (Actara®, provided by Syngenta Crop Protection UK) diluted with Milli-Q water. Such pressuring was required to retain the resistance of the population. To obtain adults of known age, apterous aphids were removed from the main culture and reared on leaf discs. Discs with a diameter of 3.5 cm were cut from the tissue either side of the central vein of 2-week-old true Chinese cabbage leaves, inverted, and set into 2% agar within the wells of a six-well plate. Five adult aphids were placed on the disc and allowed to reproduce for 24 h before removal. The remaining nymphs were raised to the first day of adulthood before being used in experiments. Aphids were starved in a Petri dish for 1.5 h prior to use.

Chinese cabbage plants were grown individually in 3-inch pots filled with compost (Levington F2 + sand) in a growth room (20 ± 1 °C, 16:8 h L:D photoperiod and 50% relative humidity). Plants for all experiments were used at the seedling stage, when two to three true leaves had formed.

2.2 Sub-lethal dose work

Dose–response experiments were carried out prior to the dispersal experiment, in order to calculate a sub-lethal dose potentially capable of altering the behaviour of M. persicae without causing a high rate of mortality. Literature often cites the LD15, the dose which causes the mortality of 15% of the population, as a suitable concentration for this purpose.[18-20] Due to their differing levels of resistance, the three clones required different concentrations of TMX to reach this LD15 dose.

Chinese cabbage plants as described earlier, were not watered for 24 h to allow the compost to dry. They were then root drenched with 50 mL of TMX and left for a further 24 h to allow uptake of the insecticide. Ten adult aphids of the same clone were then placed on each plant, which was covered with a 1240 cm3 cylindrical plastic ventilated cage and placed into an environmentally controlled chamber under conditions of 22 ± 1 °C, 16:8 h L:D photoperiod, 50% relative humidity. After a further 96 h mortality of the aphids was assessed.

Plants were treated with a range of at least five different TMX concentrations, diluted with Milli-Q water. Control mortality was assessed from aphids on plants root drenched with pure Milli-Q water. The dose–response tests were repeated three times and the results plotted and fitted in Microsoft Excel to produce dose–response curves for each clone, in order to calculate LD15 doses.[21] This provided a suitable sub-lethal concentration at which to assess the dispersal behaviour of the three clones. For a more comprehensive comparison of the resistance factors of the 5191A and FRC strains see Bass et al.[1]

2.3 Dispersal test

Chinese cabbage plants at the seedling stage with two to three true leaves were left to dry for 24 h and then root drenched with 50 mL TMX. Plants were drenched with a TMX concentration within 0.1 ppm of the LD15 value of the appropriate clone (Table 1). Control plants were root drenched with 50 mL Milli-Q water. All plants were left to stand for 24 h at conditions of 22 ± 1 °C, 16:8 h L:D photoperiod, 50% relative humidity to take up the TMX or Milli-Q water.

Table 1. The LD15 concentrations of thiamethoxam for three Myzus persicae clones with differing levels of neonicotinoid resistance, as calculated from dose–response tests
CloneRepLD15 ± SELD50 ± SESlope ± SEP-valueMean LD15 (mg L−1)Drench concentration (mg L−1)
  1. SE, standard error.

US1L10.02 ± 0.010.08 ± 0.011.16 ± 0.160.9970.10.1
20.18 ± 0.062.20 ± 0.630.69 ± 0.140.875
30.11 ± 0.053.20 ± 1.240.51 ± 0.100.991
5191A10.92 ± 0.083.65 ± 0.171.26 ± 0.070.8610.80.9
20.66 ± 0.062.17 ± 0.101.45 ± 0.080.901
30.81 ± 0.041.90 ± 0.062.04 ± 0.100.605
FRC141.20 ± 3.60166 ± 7.801.24 ± 0.070.83942.442.5
241.60 ± 2.00105.30 ± 3.201.87 ± 0.080.178
344.40 ± 3.70125.90 ± 6.201.66 ± 0.110.682

A pipette was used to transfer 20 mL of 2% agar into a 5 cm diameter Petri dish. A 3.5 cm diameter hole was then punched into the middle of a leaf, freshly removed from the plant. The leaf was placed abaxial surface uppermost within the agar before it fully set, ensuring that the outer edges protruded underneath the agar to prevent any aphids from accessing the lower surface. A 3.5 cm diameter disc punched from the same location on a different leaf was also placed into the partially set agar, positioned within the vacant hole left in the original leaf (Fig. 1). The agar was then left for 1 h to set. Ten adult apterous M. persicae of the same clone were placed onto the centre of the disc. A second Petri dish in which holes had been punched and covered by netting was inverted and placed over the first Petri dish to form a ventilated lid, and the edges sealed together using Parafilm ‘M’.

image

Figure 1. The set-up of the arenas used to test aphid dispersal as viewed from above. Both the leaf and the leaf disc were inverted before being set into agar. Aphids were therefore exposed to the undersides of the leaves.

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The set-up was then placed on a light box underneath the lens of a time lapse camera. A total of 12 microcosms could be recorded under the camera at the same time, each representing a different treatment/clone combination. Four different combinations of disc and leaf TMX treatments were offered separately to each aphid clone. These were both the leaf and disc coming from separate control plants treated with Milli-Q water, both the leaf and the disc coming from plants treated with TMX, the disc treated with TMX and the leaf from a control plant and finally the disc from a control plant and the leaf treated with TMX. In each case, when an arena contained either a leaf or a leaf disc treated with TMX, the plant had been treated with a dose of TMX equivalent to the LD15 value for the corresponding aphid clone contained within the arena. The camera was set up to record an image of all 12 dishes every 5 min for a total of 24 h. The position of the treatments under the camera was randomised for each of the ten repeats.

2.4 Statistical analysis

Image analysis was carried out using a macro written in the program ImageJ (Rasband, http://imagej.nih.gov/ij/) which was capable of calculating the change in pixels between two successive images. Since leaf shrinkage over a 5 min period was minimal, change in pixels was considered equivalent to aphid movement between frames. The macro was capable of detecting the change in pixels within two different regions of interest in the dish, with the central disc forming one region and the leaf/agar/dish sides forming the other. The number of aphids located on the leaf, disc and agar were manually recorded for each treatment at hourly intervals from the still images. Dividing the change in number of pixels in a region of the dish by the number of aphids located there gave the rate of aphid movement. The rate of movement across the entire dish area for treatments where all plant tissue was either control or TMX treated were determined. The proportion of aphids on the disc out of the number found on both the leaf and disc in each treatment and the proportion of aphids on the agar out of the total number of aphids in the dish were also calculated.

Data were analysed by a series of t-tests on pre-planned treatment comparisons at each hourly time point following an initial F-test for an overall treatment effect. Differences in the proportion of aphids located on the disc and the proportion of aphids located on the agar were compared between dishes containing only control treated plant tissue and the other three possible treatments. Differences in the rate of movement were compared between dishes containing completely control treated and completely TMX treated plant tissue. Proportions of aphids upon the disc and agar were arcsine transformed and the rates of movement log transformed prior to analysis. All analyses were carried out using SAS software, version 9.2.

3 RESULTS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

3.1 Rate of movement

When under completely control treated conditions, the resistant clone 5191A exhibited significantly higher rates of movement over the entire arena than the susceptible clone US1L at 18 out of the 24 hourly time points (Fig. 2). There was no evidence of any consistent significant pattern of difference in rate of movement between the 5191A and FRC strains in a completely control treated environment, or a completely TMX treated environment. There was a slight decrease in the rate of movement of US1L in the completely thiamethoxam treated environment towards the end of the experiment (Fig. 2).

image

Figure 2. The rate of movement of the three Myzus persicae clones, US1L, 5191A and FRC, within arenas containing either completely control treated or completely thiamethoxam treated plant material.

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3.2 Aphid location

There were no consistent patterns of significant differences in the proportions of US1L on the disc between any treatments (Fig. 3a). The proportions of 5191A upon the disc were significantly greater in the control disc TMX leaf treatment than the control leaf and disc treatment for 11 time points (Fig. 3b), and in the control disc TMX leaf treatment rather than the TMX leaf and disc treatment for 19 time points, with greater proportions of 5191A upon the disc in the control disc TMX leaf treatment than all other possible treatments for the last 3 h of the experiment. There were significantly greater proportions of FRC upon the disc in the control disk TMX leaf treatment than all three other treatments for a total of 18 time points, more than any other clone (Fig. 3c). This significant difference between treatments began 4 h into the experiment and lasted until the end.

image

Figure 3. The proportion of (a) US1L, (b) 5191A, and (c) FRC on the disc out of the total number of Myzus persicae on leaf tissue in Petri dish arenas containing four different possible thiamethoxam (TMX) treatments, control treated leaf and disc, TMX-treated leaf and disc, TMX-treated leaf and control-treated disc and control-treated leaf and TMX-treated disc. Asterisks indicate that the proportion of aphids on the disc in the control disc TMX leaf treatment was significantly greater than that for all other treatments at that time point. Error bars represent ± the standard error.

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The proportions of 5191A on the agar were significantly greater than those of both FRC and US1L for 22 time points in the control disc and leaf treatment (Fig. 4a) and the last seven time points of the TMX leaf control disc treatment (Fig. 4b). There were no other consistent patterns of significant difference in the proportions of aphids on the agar between clones for any other treatments (Fig. 4c and d).

image

Figure 4. The proportion of three Myzus persicae clones on the agar in a Petri dish arena where (a) all plant tissue is control treated with Milli-Q water, (b) all plant tissue is treated with thiamethoxam (TMX), (c) the disc is control-treated and leaf TMX-treated, and (d) the disc is TMX-treated and leaf is control-treated. Asterisks indicate that the proportion of 5191A on the agar was significantly greater than all other clones at that time point. Error bars represent ± the standard error.

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3.3 Aphid mortality and reproduction

No mortality of aphids occurred during the experiment, although there was a low level of reproduction. Since newborn nymphs move very little initially this was not judged to significantly affect the change in pixels between images as detected by the software.

4 DISCUSSION

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

Studies into the behavioural aspects of insecticide resistance in hemiptera have typically focused on differences in feeding behaviour[14, 18, 22, 23] rather than dispersal. In terms of differences in dispersal between resistant and susceptible clones of the same species no studies have yet been published using neonicotinoids. Differences in dispersal behaviour have, however, been shown to exist between M. persicae clones resistant to other insecticides. Clones carrying carboxylesterase resistance exhibit a reduced tendency to disperse from wilting leaves as compared with susceptible clones.[24] Differences in dispersal behaviour in response to parasitoid attack have also been shown between susceptible M. persicae clones and clones carrying either carboxylesterase or kdr resistance, with resistant clones less likely to disperse from leaves after exposure to parasitoids and subsequently more likely to be parasitised.[25, 26] Finally, clones carrying kdr and carboxylesterase resistance have been shown to exhibit a reduced dispersal response to aphid alarm pheromone compared with susceptible clones.[27-29] MACE resistant M. persicae clones, however, appear more responsive to aphid alarm pheromone than non-MACE forms and suffer lower levels of parasitism.[30] MACE resistant and sensitive forms also distribute differently upon the host plant, with larger numbers of MACE forms being found on the growing points of pepper plants. Pyrethroid-resistant M. persicae have been shown to walk off deltamethrin-treated excised potato leaves in greater numbers than a susceptible clone[16] but aphids were only observed for 10 min and were never offered a mix of treated and untreated leaf tissue. Najar-Rodríguez et al.[31] showed differences in landing and settling behaviour of two separate lineages of Aphis gossypii Glover, but the clones differed in preferred host plant species rather than insecticide resistance. Studies showing differences in dispersal between aphid species are more common, but do not link such behaviours with insecticide resistance.[32-34]

Two different dispersal behaviours were noted in 5191A and FRC, the two resistant clones or strains within this study. While 5191A showed an increased tendency to disperse away from all plant tissue regardless of insecticide treatment, the FRC population exhibited an increased ability to locate itself to areas of untreated plant tissue in an environment consisting of mixed insecticide treatment. While these changes in dispersal behaviour should increase the fitness of both clones under thiamethoxam-treated conditions and so contribute significantly to their neonicotinoid resistance, the enhanced ability of the FRC population to locate untreated plant tissue and therefore target its dispersal may be more relevant given the high level of resistance already exhibited by this strain.[1]

The differential dispersal behaviour exhibited by the neonicotinoid resistant FRC population in this study should not be confused with the general repellency effect of some insecticides, which may also cause changes in feeding and dispersal. These effects are often seen and considered beneficial in insect control.[35-38] For example, the large pine weevil Hylobius abietis (L.) will avoid feeding on Scots pine twigs treated with lambda-cyhalothrin but will feed from untreated areas on the same twig[39, 40] while exposure to imidacloprid-infused leaves has been shown to increase the general motility of M. persicae.[41] The study reported here, however, reports the discovery of a dispersal behaviour performed in response to TMX which differs between a resistant and susceptible clone of the same species, thus potentially linking this behaviour to the presence of neonicotinoid resistance in M. persicae.

The dispersal behaviour of the FRC population has significant implications for agricultural practises, particularly when spraying infested crops with neonicotinoids, as it is likely that the FRC, if present, will relocate to any untreated areas within a field. This finding therefore emphasises the importance of ensuring that an insecticide is applied evenly throughout a crop. The modified dispersal behaviour exhibited by the FRC population may also possibly be of benefit in the case of an unsprayed trap crop[42] as there is potential for the aphid to move into this untreated region for destruction by other means.

Alterations in dispersal behaviour could lead to changes in the transmission of aphid-vectored plant viruses. Transmission of plant viruses is the major cause of yield loss and economic damage to crop systems by aphids.[43] Myzus persicae is a known vector of at least 100 plant viruses.[44] Aphids acquire plant viruses during feeding (for a review see Fereres and Moreno[45]). Virions enter and adhere to the aphid stylets or midgut[46] during either short probes of the mouthparts into the plant tissue in the case of non-persistent viruses,[47] or longer sustained phloem feeding in the case of persistent viruses.[48, 49] Transmission occurs when a viruliferous aphid disperses from a viral host plant and relocates to another, healthy, host plant. The inoculation of non-persistent viruses is associated with salivation during brief cell punctures by the stylets[50, 51] while that of persistent viruses is thought to occur during phloem salivation.[47] Previous studies have often shown the application of insecticides to be beneficial in reducing the spread of plant viruses.[20, 52-55] From this study it appears that M. persicae disperses at the same rate regardless of the presence or absence of neonicotinoid treatment, hence in theory, the application of neonicotinoids should exert either a beneficial (due to increased vector mortality) or else no effect on the rates at which plant viruses spread within a crop, supporting such findings.

Of more relevance from a control perspective, the FRC may be more difficult to remove from the field than other M. persicae clones due to this combination of behavioural, metabolic and target site resistance. Treating plants with neonicotinoids would probably cause the FRC to relocate to any untreated or partially sprayed plants. In the case of a persistent virus where feeding must occur for several hours in order for the virus to successfully infect a new host plant,[56] this could limit or slow the spread of plant viruses to untreated plants, with treated plants gaining fuller protection from plant viruses. A plant virus, however, may be more difficult to remove from a crop when resistant aphids are present due to the persistence of the vector.

Different aphid species have been shown to disperse at different rates[33, 34] as do alate individuals of the same clone that differ in ovariole number.[57, 58] This is the first study, however, to show differences in dispersal rates between different aphid clones of the same species with regard to apterous individuals. The greater rate of movement of 5191A compared with the other clones or strains, even under completely control-treated conditions, suggests that different clones may disperse across a field at different rates. It is therefore also possible that the rate of viral spread within a field may vary according to the different aphid clones present. In particular, the presence of 5191A and potentially other resistant clones may lead to the spread of crop disease at a greater rate, although the lack of an enhanced dispersal rate exhibited by the resistant FRC population should be noted. The effect of the enhanced dispersal of resistant M. persicae on the spread of plant viruses however remains theoretical and further work is urgently required to both determine whether this enhanced dispersal occurs under field conditions, and to link such changes in behaviour to any possible effects on vector ability.

It is difficult to elucidate the mechanisms behind the unique dispersal behaviour shown by the FRC in this study, and further work would be required to do so. Potentially it is possible that the FRC population responds to either olfactory cues from TMX-treated host plants, or else chemical signalling molecules ingested in the phloem of TMX-treated plants, in a manner different to that of the neonicotinoid susceptible US1L clone. TMX has been shown to induce upregulation of salicylic acid-associated plant defence and stress responses in treated plants.[59] Myzus persicae appears to use phloem constituents in host discrimination[60] so it is possible that when feeding on treated plant tissue FRC and 5191A detect such stress chemical markers, leading to the induction of the alternative dispersal behaviours shown in this study. US1L may be either incapable of detecting such chemical markers, or else may not respond to them behaviourally. Alternatively the resistant aphids may be responding to olfactory signals produced by the host plant or insecticide. Aphids are capable of detecting plant volatiles and appear to use them when locating host plants.[61] It is the blend of different host volatiles, rather than individual volatiles, which appears attractive to aphids[62] and that certain plant volatiles offered individually may even be repellent. It is therefore possible that treatment of plants with TMX masks or changes such mixes of host plant volatiles and makes them less attractive to resistant M. persicae, while the susceptible clone US1L makes no such distinction. Different lines of the cotton aphid A. gossypii show different behavioural responses to the volatiles of the same host plants;[31] hence it is possible for different clones of M. persicae to also show different behavioural responses to the same volatiles. Further olfactometry work would be needed to determine whether this is truly the case.

Since resistance to one neonicotinoid compound usually confers resistance to all other compounds within that class[63-65] it is possible that the FRC and 5191A M. persicae clones will exhibit similar behavioural differences in response to all neonicotinoid compounds. Although the nature of the relationship between behavioural and physiological insecticide resistance is in debate[66-72] it is still potentially possible that the behavioural resistance exhibited by the FRC clone would occur in the presence of other neonicotinoid compounds, although this would require further work to confirm. Indeed, the resistance factor for the FRC population is substantially higher for imidacloprid than TMX. It should be noted that in this study the FRC aphids were treated with a dose greater than the conventional field dose. TMX is not generally applied to Chinese cabbage, but the maximum recommended application rate on potato is 400 mg L−1 ha−1.[73] Further study is therefore needed to determine whether the FRC would behave in a similar way at conventional field rates. US1L and 5191A would be unable to survive such rates, and so would present no problem under current control regimes. In addition, the dispersal behaviour of the FRC population should also be investigated at 0.1 mg L−1 and 0.9 mg L−1, to verify that any changes in behaviour are due to the resistance of the strain and not due to the increased concentration of TMX applied.

It is difficult to accurately predict the behaviour of insects in the field from laboratory results. Aphids in the field would be exposed to a range of temperature and light conditions, both of which have been shown to affect host-finding ability of the potato aphid Macrosiphum euphorbiae (Thomas)[74] as well as the effects of weather, predation and physical barriers to movement between host plants such as soil.[75, 76] While it has been shown that M. euphorbiae can travel up to 1.8 m over bare soil to successfully locate host plants[26] this experiment is not comparable to such conditions.

Aphids have been shown to disperse upwards from the older leaves at the bottom of the plant to the younger, newer leaves at the top.[77] It is suggested that this is a mechanism for finding suitable feeding spots upon a host as the population rapidly grows. While the leaf tissue in each treatment of this study always originated from two separate plants, it is possible that the dispersal behaviour exhibited by these clones is representative of intra-plant movement on the same host rather than inter-plant dispersal throughout a field. Understanding the differences in intra- and inter-plant dispersal between these clones would require further work in suitable field conditions.

Much dispersal of aphid populations is mediated by winged individuals, known as alates.[78] There is, however, evidence that dispersal of unwinged apterae can also cause substantially large infestations of plants.[32] While revealing several interesting implications, further work on alates is needed before a full understanding of the dispersal differences between these clones can be obtained. In addition, this study only compared dispersal behaviour between single examples of a susceptible, resistant and highly resistant clone or strain. Comparisons between 5191A and FRC and a different susceptible clone, such as 4106A, or further highly neonicotinoid resistant clones which may arise in the future, could reveal differences in behaviour not exhibited by the three clones included in this study. Nevertheless this study reveals significant differences in the dispersal behaviour of M. persicae clones of different neonicotinoid resistance levels, and it would not be unreasonable to suppose that similar behavioural differences might also occur between these clones under field conditions. The lack of literature on the behaviour of neonicotinoid resistant aphid clones makes this an especially interesting study. This may be the first step towards discovering the presence of such behavioural differences in the field between neonicotinoid resistant M. persicae.

In summary, this is the first characterisation of a behavioural resistance in aphids that actively disperse away from neonicotinoid treated leaves to untreated plant material. Together with the intrinsic biochemical and target site resistance it represents new challenges in the field for aphid control and the viruses they spread.

ACKNOWLEDGEMENTS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

We thank Rothamsted Research for the provision of the 5191A clone. Thanks go to Stephen Foster for identification of the US1L susceptible clone. This work was jointly funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and Syngenta Crop Protection.

REFERENCES

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  • 1
    Bass C, Puinean AM, Andrews M, Cutler P, Daniels M, Elias J, et al, Mutation of a nicotinic acetylcholine receptor β subunit is associated with resistance to neonicotinoid insecticides in the aphid Myzus persicae. BMC Neurosci 12:51 (2011).
  • 2
    Puinean AM, Foster SP, Oliphant L, Denholm I, Field LM, Millar NS, et al, Amplification of a cytochrome P450 gene is associated with resistance to neonicotinoid insecticides in the aphid Myzus persicae. PLoS Genet 6:111 (2010).
  • 3
    Philippou D, Field L and Moores G, Metabolic enzyme(s) confer imidacloprid resistance in a clone of Myzus persicae (Sulzer) (Hemiptera: Aphididae) from Greece. Pest Manag Sci 66:390395 (2010).
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