Cues of Predation Risk Induce Instar‐ and Genotype‐Specific Changes in Pea Aphid Colony Spatial Structure

Deploying collective antipredator behaviors during periods of increased predation risk is a major determinant of individual fitness for most animal groups. Pea aphids, Acyrthosiphon pisum, which live in aggregations of genetically identical individuals produced via asexual reproduction warn nearby conspecifics of pending attack by secreting a volatile alarm pheromone. This alarm pheromone allows clone‐mates to evade predation by walking away or dropping off the host plant. Here, we test how a single alarm pheromone emission influences colony structure and defensive behavior in this species. Relative to control colonies, groups exposed to alarm pheromone exhibited pronounced escape behavior where many individuals relocated to adjacent leaves on the host plant. Alarm pheromone reception, however, also had subtle instar‐specific effects: The proportion of 1st instars feeding nearest the leaf petiole decreased as these individuals moved to adjacent leaves, while the proportion of 2nd–3rd instars feeding nearest the leaf petiole remained constant. Fourth instars also dispersed to neighboring leaves after pheromone exposure. Lastly, alarm pheromone reception caused maternal aphids to alter their preferred feeding sites in a genotype‐specific manner: Maternal aphids of the green genotype fed further from the petiole, while maternal aphids of the pink genotype fed closer to the petiole. Together, our results suggest that aphid colony responses to alarm pheromone constitute a diversity of nuanced instar‐ and genotype‐specific effects. These behavioral responses can dramatically change the spatial organization of colonies and their collective defensive behavior.     

Alate production in an aphid in relation to ant tending and alarm pheromone

1. Winged dispersal is vital for aphids as predation pressure and host plant conditions fluctuate. 2. Ant‐tended aphids also need to disperse, but this may represent a cost for the ants, resulting in an evolutionary conflict of interest over aphid dispersal. 3. The combined effects of aphid alarm pheromone, indicating predation risk, and ant attendance on the production of winged aphids were examined in an experiment with Aphis fabae (Homoptera: Aphididae) (Scopoli 1763) aphids and Lasius niger (Formicidae: Formicinae) (Linné, 1758) ants. 4. This study is the first to investigate the joint effects of alarm pheromone and ant attendance, and also the first to detect an influence of alarm pheromone on the production of winged morphs in A. fabae. 5. After a period of 2 weeks, it was found that aphid colonies exposed to intermittent doses of alarm pheromone produced more winged individuals, whereas ant tending had the opposite effect. The effects were additive on a log scale, and ant attendance had a greater proportional influence than exposure to alarm pheromone. A tentative conclusion is that ants have gained the upper hand in an evolutionary conflict about aphid dispersal.     

Has the Attraction of Predatory Coccinellids to Cornicle Droplets Constrained Aphid Alarm Signaling Behavior?

When attacked by a predator, aphids of many species secrete cornicle droplets, containing an alarm pheromone, that results in the dispersal of nearby conspecifics. As females are parthenogenetic, alarm signaling functions to enhance the survival of clone-mates. Enigmatically, however, aphids are physically able to, but usually do not emit alarm pheromone when initially detecting a predator, but rather signal only when captured by a predator. We hypothesized that cornicle droplets may be attractive to natural enemies and result in an increased risk of predation for the signaler, thereby selecting for prudent alarm signalers. We tested this hypothesis by investigating the olfactory cues that the multicolored Asian ladybird beetle, Harmonia axyridis Pallas, uses to locate pea aphids, Acyrthosiphon pisum. In choice tests, H. axyridis were attracted to odors from pea aphid colonies, whether feeding or not feeding on a host plant leaf, but were not attracted to cornicle droplets containing alarm pheromone. Further, individual pea aphids emitting cornicle droplets were not located more often or in a shorter period of time by beetles than aphids not emitting cornicle droplets. Thus, the cost of emitting early alarm signals is not prohibitively high in regards to the attraction of predators such as H. axyridis.     

Effects of deltamethrin on walking, flight and potato virus Y-transmission by pyrethroid-resistant Myzus persicae

In laboratory tests with moderately pyrethroid-resistant (R₁) Myzus persicae, more walked or flew from excised leaves pre-treated with the pyrethroid deltamethrin, or its aqueous formulation ‘Decis’ at 10 mg a.i. litre^-1 than from untreated leaves. Neither a carbamate (pirimicarb) nor an organophosphate (demeton-S-methyl) induced flight. R₁ alatae and apterae which had dispersed from deltamethrin-treated leaves rarely transmitted potato virus Y (PVY); however transmission by the more pyrethroid-resistant R₂ alatae was only halved and that by R₂ apterae undiminished. Treatment with deltamethrin decreased spread of PVY both in flight chamber and field experiments.     

The effect of Pymetrozine, a feeding inhibitor of Homoptera, in preventing transmission of cauliflower mosaic caulimovirus by the aphid species Myzus persicae (Sulzer)

Mature turnip plants, mechanically infected as seedlings with the semi-persistent, aphid transmitted caulimovirus, cauliflower mosaic (CaMV), were treated by spraying with either a solution of Pymetrozine plus adjuvant oil, adjuvant oil or water only. At the same time turnip seedlings were sprayed for each of the three treatments. Two h after spraying, Myzus persicae were caged onto an infected turnip plant for each of the three treatments. Twenty four h later, groups of 20 aphids were transferred from the infected plants, to seedlings from each of the three treatments. After 24 h, these were removed and seedlings were later recorded for infection. This acquisition/transmission assay was repeated at 3, 7, 14 and 21 days from treatment. Only aphids exposed to the Pymetrozine treated source plants were shown to move off the plant and failed to transmit CaMV effectively to treated or control seedlings during the 0 and 3 day assays. The majority soon died when transferred to test seedlings. Progressively, more aphids were found to survive and transmit CaMV during the 7 day and 14 day assays. By 21 days no significant effect could be recorded between treatments and controls. Aphids transferred from control treated source plants to Pymetrozine treated seedlings were able to transmit CaMV within all the assays, although higher mortality was recorded in the day 0 assessment when compared to those transferred to control treated seedlings. We conclude from this trial, that a single foliar treatment of 100 mg litre¹ Pymetrozine to CaMV infected turnip plants, effectively reduces the vectoring capability of M. persicae, that feed on these plants, for up to 7 days. However, Pymetrozine failed to stop virus transmission to treated seedlings from the ingress of viruliferous aphids. Pymetrozine was not shown to cause any phytotoxic responses to plants used in this trial.     

Alarm pheromone mediates production of winged dispersal morphs in aphids

The aphid alarm pheromone ( E )- β -farnesene (EBF) is the major example of defence communication in the insect world. Released when aphids are attacked by predators such as ladybirds or lacewing larvae, aphid alarm pheromone causes behavioural reactions such as walking or dropping off the host plant. In this paper, we show that the exposure to alarm pheromone also induces aphids to give birth to winged dispersal morphs that leave their host plants. We first demonstrate that the alarm pheromone is the only volatile compound emitted from aphid colonies under predator attack and that emission is proportional to predator activity. We then show that artificial alarm pheromone induces groups of aphids but not single individuals to produce a higher proportion of winged morphs among their offspring. Furthermore, aphids react more strongly to the frequency of pheromone release than the amount of pheromone delivered. We suggest that EBF leads to a 'pseudo crowding' effect whereby alarm pheromone perception causes increased walking behaviour in aphids resulting in an increase in the number of physical contacts between individuals, similar to what happens when aphids are crowded. As many plants also produce EBF, our finding suggests that aphids could be manipulated by plants into leaving their hosts, but they also show that the context-dependence of EBF-induced wing formation may hinder such an exploitation of intraspecific signalling by plants.     

The effect of artificial diet on the production of alarm pheromone by Myzus persicae

The cornicle secretion of Myzus persicae reared on artificial diet only elicits an alarm response in plant‐reared conspecifics after the young aphids have been transferred to plants for 7 days. Acetate in the form of 0.32% sodium acetate has been added to the diet as an early step in synthesis of the alarm pheromone, (E)‐β‐farnesene (EBF). The cornicle secretion of diet‐reared aphids then elicits an alarm response. However, there is no difference in internal EBF concentration between plant‐ and diet‐reared aphids. Puncturing aphids, either plant‐ or diet‐reared, with a pin shows that both can elicit an alarm response, whereas it is reduced by half with diet‐reared individuals. Although there is no significant difference in the concentration of EBF produced, the total amount in diet‐reared aphids is increased by acetate in the diet to a level similar to that in plant‐reared individuals: the size of aphids reared on an acetate‐supplemented diet is increased and comparable with the size of those that are plant‐reared. Bioassays with a range of EBF concentrations show a high threshold for the alarm response. It is concluded that the different size of aphids reared on plants and standard diet results in them secreting, respectively, above and below the response threshold.     

Alarm pheromone emission by pea aphid, Acyrthosiphon pisum, clones under predation by lacewing larvae

Genetic variation in anti-predator traits has been shown for a variety of species. Aphid alarm pheromone, ( E )-β-farnesene, is released by attacked aphids and causes a variety of behavioral defense reactions in the signal receivers. In pea aphids, Acyrthosiphon pisum Harris (Homoptera: Aphididae), ( E )-β-farnesene mediates the production of winged offspring in the presence of natural enemies. While variation in the propensity for pea aphids to produce winged offspring is well-documented, little quantitative information is available about clonal differences in ( E )-β-farnesene emission or the amount of alarm pheromone released in aphid colonies. We tested the wing induction response of four clones when attacked by a predatory lacewing larva, Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae), and found that three of the four clones increased the proportion of winged offspring under predator attack. We then investigated the emission of aphid alarm pheromone of these clones of pea aphid under attack. Alarm pheromone emission in aphid colonies of initially 25 adults varied from 81.2 to 10 851.0 ng per aphid colony over 24 h. There were no differences between clones in total emission or in emission per consumption event. These results show that there is substantial variability in alarm pheromone emission within clones and that the propensity to produce winged offspring in some clones is not a simple function of the propensity of alarm pheromone production in these clones.     

Infection with an insect virus affects olfactory behaviour and interactions with host plant and natural enemies in an aphid

Aphid ecology and population dynamics are affected by a series of factors including behavioural responses to ecologically relevant chemical cues, capacity for population growth, and interactions with host plants and natural enemies. Using the aphid Rhopalosiphum padi (L.) (Homoptera: Aphididae), we showed that these factors were affected by infection with Rhopalosiphum padi virus (RhPV). Uninfected aphids were attracted to odour of uninfected aphids on the host plant, an aggregation mechanism. However, infected aphids were not attracted, and neither infected nor uninfected aphids were attracted to infected aphids on the plant. Infected aphids did not respond to methyl salicylate, a cue denoting host suitability. Infected aphids were more behaviourally sensitive to aphid alarm pheromone, and left the host plant more readily in response to it. RhPV reduced the lifespan and population growth rate of the aphid. The predacious ladybird, Coccinella septempunctata (L.) (Coleoptera: Coccinellidae), consumed more infected aphids than uninfected aphids in a 24‐h period, and the aphid parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) attacked more infected than uninfected aphids. However, the proportion of mummies formed was lower with infected aphids. The results represent further evidence that associated organisms can affect the behaviour and ecology of their aphid hosts.     

<Emphasis Type="Italic">S</Emphasis>-isorobinal as the female sex pheromone from an alarm pheromone emitting mite, <Emphasis Type="Italic">Rhizoglyphus setosus</Emphasis>

The female sex pheromone of Rhizoglyphus setosus Manson (Astigmata: Acaridae) was identified as S-isorobinal (4S-4-isopropenyl-3-oxo-1-cyclohexene-1-carboxyaldehyde), which stimulated males sexually and enhanced the frequency of the male’s tapping and mounting behavior. Although the female hexane extract indicated no sign of sex pheromone activity against tested males, possibly due to the presence of the alarm pheromone neryl formate, an SiO₂ column fraction containing isorobinal elicited sex pheromone activity at a dose of one female equivalent. The stereochemistry of natural isorobinal was identified as S by an HPLC using a chiral column. Both S- and R-isorobinals exhibited maximum activity at the same dose of 1 and 10 ng with a convex dose–response relationship. Amounts of S-isorobinal were determined to be 11.7 ± 1.0 ng per female and 6.4 ± 1.3 ng per male by GLC. This is the second example of two pheromones (the alarm pheromone neryl formate, and the sex pheromone S-isorobinal) demonstrated to be components of the same opisthonotal gland secretion.Chemical ecology of astigmatid mites. LXXVIII     

Insecticidal control of aphids and the spread of potato leafroll virus in potato crops in eastern Scotland

Field experiments were carried out in eastern Scotland in 1976-78 to test the ability of granular insecticides, applied to soil at planting, and of insecticide sprays applied to the foliage, to control aphids and spread of potato leafroll virus (PLRV) in potatoes. The three years provided contrasting opportunities for virus spread. In 1976, the main vector of PLRV, Myzus persicae, arrived in early June and multiplied rapidly in untreated plots, and PLRV spread extensively. In 1977, M. persicae arrived 4–6 wk later than in 1976 and most spread of PLRV, which was less than in 1976, occurred after the end of July. In 1978, few M. persicae were recorded but the potato aphid, Macrosiphum euphorbiae, arrived early and very large populations developed in untreated plots. However, little spread of PLRV occurred in 1978, supporting other evidence that M. euphorbiae is an inefficient vector of PLRV in field conditions. In each year, granular insecticides decreased PLRV spread to a quarter or less of that in control plots. Thiofanox gave somewhat better and longer-lasting control of aphid populations than disulfoton, especially of M. persicae, but did not give greater control of PLRV spread. Application of three (1976) or five (1977) sprays of demeton-S-methyl to plots treated with granular insecticides further improved the control of M. euphorbiae but had less or no effect on M. persicae, especially where organophosphorus resistant aphids (R1 strain) were found. These supplementary sprays of insecticide did not further improve the control of PLRV but, in 1978, four sprays of demephion or pirimicarb to plots not treated with granular insecticide decreased PLRV spread. These data, together with previous findings, indicate that the amount of virus spread depends on the date of arrival and rate of multiplication of M. persicae in relation to the timing and effectiveness of removal of PLRV sources in crops. It is concluded that in Scotland insecticide granules should be used routinely only in crops of the highest grade of seed potato. Their use for other grades need be considered only in years following mild winters, when aphids can be expected to enter crops earlier and in larger numbers.     

The control of Aphis fabae Scop, on spring-sown field beans (Vicia faba L.)

Disulfoton or phorate granules or demeton-S-methyl, menazon and vamidothion sprays, applied once in early June as preventive treatments before heavy aphid colonies developed, gave good control of Aphis fabae Scop, on field beans. Phosalone gave relatively poor results and DDT was ineffective. Applications in June to crops sown in February and early March were made with minimal wheel damage to the crop and are known to be less harmful to bees than sprays at flowering time. Eradicant treatments with demeton-S-methyl and dimethoate sprays or with disulfoton or phorate granules on heavily infested plants during flowering were also effective, but menazon was less satisfactory. These eradicant sprays are likely to be harmful to bees, and wheel damage in late June reduced yield by 1–2 cwt/acre (125–250 kg/ha). Peak populations of 3000 aphids/plant in early July reduced yield by 6 cwt/acre (750 kg/ha) in one trial.     

S-isorobinal as the female sex pheromone from an alarm pheromone emitting mite, Rhizoglyphus setosus

The female sex pheromone of Rhizoglyphus setosus Manson (Astigmata: Acaridae) was identified as S-isorobinal (4S-4-isopropenyl-3-oxo-1-cyclohexene-1-carboxyaldehyde), which stimulated males sexually and enhanced the frequency of the male’s tapping and mounting behavior. Although the female hexane extract indicated no sign of sex pheromone activity against tested males, possibly due to the presence of the alarm pheromone neryl formate, an SiO₂ column fraction containing isorobinal elicited sex pheromone activity at a dose of one female equivalent. The stereochemistry of natural isorobinal was identified as S by an HPLC using a chiral column. Both S- and R-isorobinals exhibited maximum activity at the same dose of 1 and 10 ng with a convex dose–response relationship. Amounts of S-isorobinal were determined to be 11.7 ± 1.0 ng per female and 6.4 ± 1.3 ng per male by GLC. This is the second example of two pheromones (the alarm pheromone neryl formate, and the sex pheromone S-isorobinal) demonstrated to be components of the same opisthonotal gland secretion.     

Improving secondary pick up of insect fungal pathogen conidia by manipulating host behaviour

It is often assumed that efficient application of a mycoinsecticide involves hitting the target pest insect directly with a lethal dose of conidia. However, secondary pick‐up of conidia from surrounding vegetation may be a more important source of inoculum. We have investigated ways of increasing conidia acquisition by enhancing host movement. The aphid alarm pheromone, E‐β‐farnesene, significantly increased mortality among peach potato aphids. Myzus persicae Sulzer, that were exposed for 24 h to discs of green pepper leaf sprayed with conidia of Verticillium lecanii (Zimmerman) Viegas then transferred to fresh untreated discs to allow disease development. A more practical approach to increasing conidia pick‐up appears to be the use of sub‐lethal doses of the chloronicotinyl insecticide imidacloprid. One percent of the recommended dose, applied systemically, dramatically increased aphid movement; quantified by image analysis of videotaped aphid behaviour. This resulted in greater mortality from mycosis in experiments where aphids were exposed to insecticide‐treated leaf discs that had been sprayed with fungal conidia. A comparison with results from an experiment where conidia were sprayed directly onto aphids which were feeding on insecticide‐infused pepper discs established that synergy was due to an indirect effect of the insecticide, i.e. through increased movement, rather than a direct effect viz. predisposition of insecticide‐weakened insects to disease.     

Characterization of transgenic rice plants that express rgp1, the gene for a small GTP-binding protein from rice

 The rgp1 gene, which encodes a small GTP-binding protein from rice, was introduced into rice protoplasts by electroporation. Transformed protoplasts were cultured on liquid protoplast-culture medium for 1 month, and then cells that had proliferated were transferred to a selection medium that contained 50 mg/l hygromycin B. Among 50 colonies that were selected and transferred to regeneration medium, 3 colonies generated shoots. However, two of the three shoots failed to form roots and ceased growing. A single regenerated shoot that formed roots was planted in soil and transferred to a greenhouse. Southern hybridization showed that the regenerated plant harbored a single copy of the introduced gene. The transformant (T₀) plant was shorter than the controls, it developed three times as many tillers as controls, it developed three times as many tillers as control plants but it produced mostly sterile seeds. In a test of hygromycin resistances, viable seeds segregated into resistant and sensitive seedings at a ratio of approximately 1 : 3. The progeny (T₁) plants were short with many tillers, and some produced seeds normally. The T₂ seedlings grew more rapidly than control seedlings for the first 28 days after germination, but control plants subsequently outgrew the T₂ plants. Northern blotting analysis revealed that the rgp1 gene in T₂ plants was expressed consitutively throughout all developmental stages. The results suggest that the observed phenotypic changes were due to expression of the exogenous rgp1 gene. Received: 21 September 1997/Accepted: 31 March 1998     

Soybean Aphid Response to their Alarm Pheromone E-ß-Farnesene (EBF)

When attacked by natural enemies some insect pests, including many aphid species, alert neighboring conspecifics with alarm pheromones. Cornicle secretions with pheromones benefit the attacked aphid but are costly to produce, while alarm pheromone benefits probably fall largely on alerted conspecifics. Given these variable benefits, the likelihood of a secretion may change depending on aphid density. Thus, we first hypothesized that the common alarm pheromone in aphids, E-ß-farnesene (EBF), was present in soybean aphid (Aphis glycines Matsumura) cornicle secretions and would elicit an alarm response in aphids exposed to it. Second, since aphids other than the secretor also benefit from cornicle secretions, we hypothesized that the likelihood of secretion would increase concurrently with the density of neighboring clonal conspecifics. Third, because alarm reaction behavior (e.g. feeding cessation) is probably costly, we hypothesized that alarm reaction behavior would decrease as conspecific density (i.e. alternative prey for an attacking natural enemy) increased. We found that soybean aphids 1) produce cornicle secretions using EBF as an alarm pheromone, 2) are less likely to release cornicle secretions when alone than in a small group (~10 individuals), but that the rate of secretion does not increase further with additional conspecific density, and 3) also exhibit alarm reaction behavior in response to cornicle secretions independent of aphid density. We show that soybean aphids can use their cornicle secretions to warn their neighbors of probable attack by natural enemies, but that both secretion and alarm reaction behavior does not change as density of nearby conspecifics rises above a few individuals.     

Laboratory and field investigations with organophosphorus resistant Tetranychus urticae on strawberries

Laboratory tests using a slide dip technique confirmed that strains of Tetranychus urticae resistant to demeton-S-methyl and dimethoate had developed on strawberries in south Hampshire. These pesticides had been used very successfully for many years for the combined control of aphids and red spider mite. In small plot screening trials dicofol, phenisobromolate, chloropropylate, quinomethionate and dinobuton gave consistently good results. Binapacryl was very effective but caused damage to the plants. A number of other acaricides including formetanate and methiocarb were shown to be worth further investigation. Tetradifon, also used for some years on strawberries, gave variable results.     

The effect of the granulovirus (PapyGV) on larval mortality and feeding behaviour of the Pandemis leafroller,Pandemis pyrusana (Lepidoptera: Tortricidae)

An indigenous betabaculovirus (PapyGV) of the Pandemis leafroller, Pandemis pyrusana (Kearfott), was studied in the laboratory and greenhouse to determine how the virus affected leafroller mortality and foliar damage. Probability of mortality increased with virus concentration as observed after 7 and 10 days of feeding on virus treated diet in neonates and second instar larvae. LC₅₀ estimates for neonates at 7 and 10 days was 2743 and 389 occlusion bodies (OBs)/mm². For second instars, LC₅₀ was 139,487 and 813 OBs/mm² at 7 and 10 days. There was no biologically significant mortality response to increasing virus concentrations by fourth instar larvae; however, when fourth instar larvae were infected with virus on diet and then fed apple leaves, the leaf area consumed declined up to 50% with higher virus concentrations. In a greenhouse study, neonate larvae that fed on seedlings treated with water showed >90% survival and 80% pupation rate of larvae after being transferred to diet. In contrast, larvae that fed on apple seedlings sprayed with 3×10⁶ OBs/ml showed poor survival when transferred to diet after acquiring the virus and failed to reach the pupal stage. This virus shows promise for population regulation and can produce reduction in feeding damage.     

Orco mediates olfactory behaviors and winged morph differentiation induced by alarm pheromone in the grain aphid,Sitobion avenae

Olfaction is crucial for short distance host location and pheromone detection by insects. Complexes of olfactory receptors (ORs) are composed of odor-specific ORs and OR co-receptors (Orco). Orcos are widely co-expressed with odor-specific ORs and are conserved across insect taxa. A number of Orco orthologs have been studied to date, although none has been identified in cereal aphids. In this study, an Orco gene ortholog was cloned from the grain aphid, Sitobion avenae, and named “SaveOrco”; RNA interference (RNAi) reduced the expression of SaveOrco to 34.11% in aphids, resulting in weaker EAG (electroantennogram) responses to plant volatiles (Z-3-hexene-1-ol; methyl salicylate, MeSA) and aphid alarm pheromone (E-β-farnesene, EBF). Aphid wing differentiation induced by EBF was investigated in both RNAi treated and untreated aphids. EBF induced production of winged aphids in both pre-natal and post-natal periods in untreated aphids, but no such induction was observed in the RNAi-treated aphids. We conclude that SaveOrco is crucial for the aphid's response to pheromones and other volatiles, and is involved in wing differentiation triggered by EBF.