Effect of Adult Experience on in-Flight Orientation to Plant and Plant–Host Complex Volatiles inAphidius erviHaliday (Hymenoptera, Braconidae)

The effect of adult experience on in-flight orientation to plant–host complex volatiles byAphidius erviHaliday was studied in a wind tunnel bioassay, usingAcyrthosiphon pisum(Harris), maintained on broad bean plants (Vicia faba) as host. A short oviposition experience (15 s) on the plant–host complex (PHC) was sufficient to induce a drastic decrease of flight propensity and stimulated a foraging behavior characterized by intense walking activity. However, flight activity resumed to normal levels 1 h after the oviposition experience on the PHC occurred. For parasitoids conditioned on the PHC for at least 1 min the recorded proportion making oriented flights to the PHC was significantly higher than that for naive females. In contrast, oviposition experience in the absence of plant material did not influence theA. erviflight response. Oviposition attempts on aphid dummies without egg release did not reduce flight activity. WhenA. ervifemales were exposed to glass beads coated withAc. pisumcornicle secretion, a priming effect was observed, resulting, compared with naive females, in a significantly higher rate of oriented flights to the PHC. In contrast, oviposition attempts visually induced by colored aphid dummies did not influence flight behavior. A strong reaction to volatile cues from uninfested plants was induced by oviposition experience on newly infested broad bean plants. This appears to be a case of associative learning. In fact, uninfested broad bean plants are basically unattractive to naiveA. ervifemales. The results demonstrate that adult experience has a considerable influence onA. ervibehavior and may have important implications for biological control of natural pest aphid populations.     

Population level effects of cadmium and the insecticide imidacloprid to the parasitoid, Aphidius ervi after exposure through its host, the pea aphid, Acyrthosiphon pisum (Harris)

The objective of this study was to assess the influence of toxic substances with different modes of action on a two-species system: an aphid-specific parasitoid, Aphidius ervi Haliday, feeding on the pea aphid, Acyrthosiphon pisum (Harris). The instantaneous rate of population increase (r_i) was used as a measure of population level toxic effect in this study. The toxicants evaluated were imidacloprid, a nonpersistent neurotoxic insecticide, and cadmium, a chronic pollutant with a tendency to accumulate. We evaluated the effects of cadmium and imidacloprid on A. pisum and A. ervi because both toxicants can occur together in polluted areas where crops are grown. Cadmium (200 or 400 mg kg⁻¹ dry weight soil) and imidacloprid (4 or 40 g a.i. ha⁻¹) were applied to soil contained in plastic pots in which broad bean plants, Vicia faba L., were grown. Results of this study indicated that cadmium at the concentrations tested, reduced population growth rate of the pea aphid. Imidacloprid also reduced aphid growth rate, but only at the highest concentration tested (40 g a.i. ha⁻¹). Combinations of cadmium and imidacloprid had the greatest impact on aphid growth rate. Imidacloprid alone had no effect on population growth rate of the parasitoid. However, cadmium alone or in combination with imidacloprid had a negative impact on A. ervi by reducing population growth rate 77%. These results indicate that negative impacts on parasitoids may occur in areas where cadmium contamination is present and imidacloprid is used to control aphids.     

<Emphasis Type="Italic">Aphidius ervi</Emphasis> Preferentially Attacks the Green Morph of the Pea Aphid, <Emphasis Type="Italic">Acyrthosiphon pisum</Emphasis>

The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae) is found in red and green color morphs. Previous work has suggested that the aphidiine parasitoid Aphidius ervi Haliday preferentially attacks green pea aphids in the field. It is not clear whether these results reflect a real preference, or some unknown clonal difference, such as in immunity, between the aphids used in the previous studies. We used three susceptibility-matched pairs of red and green morph pea aphid clones to test for preferences. In a no-choice situation, the parasitoids attacked equal proportions of each color morph. When provided with a choice, A. ervi was significantly more likely to oviposit into colonies formed from green morphs when the neighboring colony was formed from red morph aphids. In contrast, red morphs were less likely to be attacked when their neighboring colony was of the green morph. By preferentially attacking green colonies, A. ervi may reduce the likelihood of intraguild predation, as it is suggested that visually foraging predators preferentially attack red aphid colonies. Furthermore, if this host choice behavior is replicated in the field, we speculate that color morphs of the pea aphid may interact indirectly through their shared natural enemies, leading to intraspecific apparent competition.     

Variation in selected life-history parameters of the parasitoid wasp,Aphidius ervi: Influence of host developmental stage

We determined the age-specific fecundity and survival of the solitary parasstoid wasp,Aphidius ervi Haliday (Hymenoptera: Aphidiidae), under constant laboratory conditions. Wasps were reared in each of the four nymphal instars of apterous pea aphids,Acyrthosiphon pisum (Harris) (Homoptera: Aphididae): L₁ (age 24h), L₂ (48 h), L₃ (72 h), and L₄ (120 h). Age-specific survival (l_x) and fecundity (m_x) differed between parasitoids developing in different aphid instars. The wasps’ life-time reproductive success, as indexed by the intrinsic rate of population increase (r_m), varied non-linearly with adult biomass and host size at parasitization. A close agreement between larval growth rates in different host instars and adult reproductive performance suggests that, inA. ervi, fitness correlates may be significantly influenced by larval ontogeny and trade-offs in resource allocation.     

Learning by the parasitoid wasp, Aphidius ervi (Hymenoptera: Braconidae), alters individual fixed preferences for pea aphid color morphs

Learning, defined as changes in behavior that occur due to past experience, has been well documented for nearly 20 species of hymenopterous parasitoids. Few studies, however, have explored the influence of learning on population-level patterns of host use by parasitoids in field populations. Our study explores learning in the parasitoid Aphidius ervi Haliday that attacks pea aphids, Acyrthosiphon pisum (Harris). We used a sequence of laboratory experiments to investigate whether there is a learned component in the selection of red or green aphid color morphs. We then used the results of these experiments to parameterize a model that examines whether learned behaviors can explain the changes in the rates of parasitism observed in field populations in South-central Wisconsin, USA. In the first of two experiments, we measured the sequence of host choice by A. ervi on pea aphid color morphs and analyzed this sequence for patterns in biased host selection. Parasitoids exhibited an inherent preference for green aphid morphs, but this preference was malleable; initial encounters with red aphids led to a greater chance of subsequent orientation towards red aphids than predicted by chance. In a second experiment, we found no evidence that parasitoids specialize on red or green morphs; for the same parasitoids tested in trials separated by 2 h, color preference in the first trial did not predict color preference in the second, as would be expected if they differed in fixed preferences or exhibited long-term (> 2 h) learning. Using data from the two experiments, we parameterized a population dynamics model and found that learning of the magnitude observed in our experiments leads to biased parasitism towards the most common color morph. This bias is sufficient to explain changes in the ratio of aphid color morphs observed in field sites over multiple years. Our study suggests that for even relatively simple organisms, learned behaviors may be important for explaining the population dynamics of their hosts.     

The temperature-dependent duration of development and parasitism of three cereal aphid parasitoids, Aphidius ervi, A. rhopalosiphi, and Praon volucre

Temperature dependencies were established for the egg-to-mummy and mummy-to-adult phases, for mummy mortality, and for parasitism of Aphidius ervi Haliday, Aphidius rhopalosiphi De Stefani-Perez, and Praon volucre (Haliday) (Hymenoptera, Aphidiidae), three parasitoids of Sitobion avenae (Fabricius) (Homoptera, Aphididae), at 8°C, 12°C, 16°C, 20°C, and 25°C on winter wheat (cv. Haven). A physiological model described temperature-dependent development over the full temperature range, whereas a linear model was fitted for data above 8°C and used to estimate the lower temperature thresholds and day-degrees (° D) required for development. The thresholds for A. ervi were 2.2°C for egg-mummy development and 6.6°C for mummy-adult development, those for A. rhopalosiphi were 4.5°C and 7.2°C, and those for P. volucre were 3.8°C and 5.5°C. The time to develop into mummies and adults differed significantly between the three species: A. ervi development into mummies required an average of 159 ° D, while development into adults took an average of 73 ° D. The corresponding average times required for A. rhopalosiphi and P. volucre to develop mummies were 124° D and 126° D, while their development into adults required an average of 70° D and 150° D, respectively. Mummy mortality was 25–35% at 8°C and less at the higher temperatures tested, but began to increase again at 25°C, showing a quadratic relationship between mortality and temperature. Parasitization was very low or, in the case of P. volucre, absent up to 12°C and thereafter increased with increasing temperature. The relationship between parasitization, recorded as percent aphids mummified, and temperature was linear at the temperatures tested and depended on species. A. ervisuperparasitized 11.1% aphids at 20°C and 16.6% aphids at 25°C, whereas superparasitism was low in A. rhopalosiphi and absent in P. volucre. From 16°C to 25°C the P. volucre sex ratio increased. For A. ervi and A. rhopalosiphi there was no trend with temperature, but at 20°C and 25°C it was close to even. Field data for 1996 and 1997 allowed for a comparison of actual and expected emergence of overwintering mummies. In both years, parasitoids were predicted to have emerged from overwintering mummies well in advance of the onset of aphid infestation, and more than a month earlier than the first parasitized aphids were found in winter wheat. Observations from trap plants in other crops supported the predictions of the models. Other factors that can affect biological control by cereal aphid parasitoids are discussed.     

Recognition of heterospecific parasitism: Competition between Aphidiid (Aphidius ervi) and Aphelinid (Aphelinus asychis) parasitoids of Aphids (Hymenoptera: Aphidiidae; Aphelinidae)

The solitary parasitoids Aphidius erviHaliday (Hymenoptera: Aphidiidae) and Aphelinus asychisWalker (Hymenoptera: Aphelinidae) attacked but generally did not oviposit in pea aphids parasitized by the other species. Wasps selectively oviposited in unparasitized hosts when given a choice. Host discrimination depended on the recognition of internal cues. Females of A. asychiseither could not recognize or ignored A. ervi'sexternal host marking pheromone. Under most conditions, A. ervisurvived in superparasitized hosts, killing competing A. asychislarvae by physical attack and possibly physiological suppression. The outcome of larval competition was not affected by oviposition sequence or age difference between larvae; A. asychissurvived only when it had substantially completed larval development before the host was superparasitized by A. ervi.It is suggested that competition for host resources incurs a cost, for the winner in terms of reduced size or increased development time and for the loser in terms of lost progeny and searching time. Consequently, heterospecific host discrimination can be functional. Internal, and probably general, cues enable wasps to recognize and avoid oviposition in hosts already parasitized by an unrelated species.     

The effects of two species of hymenopterous parasitoid on the reproductive system of the pea aphid, Acyrthosiphon pisum

Two cultures of first instar pea aphids, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae), were subjected to parasitization by Ephedrus plagiator (Nees) and Aphidius ervi Haliday (Hymenoptera: Braconidae) respectively, and a third culture, of fourth instar pea aphids, was subjected to parasitization by E. plagiator. Significant parasitogenic effects on the reproductive system occurred within 24 h of exposure to parasitoids in the form of reductions in both embryo number and the size of the largest embryo, and degenerative changes in the embryos themselves. No evidence was found of direct feeding by the larvae on the host's tissues, until just before larval pupation. The largest embryos of some fourth instar aphids escaped the effects of parasitization probably because they had developed a resistant cuticle by the time of oviposition by the parasitoid. Results suggested that embryonic degeneration was an indirect effect of parasitization due to starvation, interference with the aphid's hormone levels, or both.

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Résumé Trois élevages de 100 à 200 premiers stades d'A. pisum, nés à 24 heures les uns des autres, ont été divisés chacun en un lot témoin et un lot expérimental. Deux des élevages expérimentaux ont alors été soumis au parasitisme d'Ephedrus plagiator ou de Aphidius ervi, le troisième élevage a pu se développer jusqu'au 4è stade avant d'être exposé à E. plagiator. Des échantillons de 10 pucerons ont été retirés de chaque lot après 24 heures et ensuite toutes les 48 heures pour être disséqués dans du liquide physiologique. Les nombres d'ovarioles et d'embryons et la taille du plus gros embryon ont été notés. Des cellules développées, issue de la séreuse de l'oeuf de l'hyménoptère se sont disséminées dans l'hémolymphe de l'hôte et ont servi d'aliment aux larves en développement. Seul le dernier stade larvaire des parasitoïdes s'est alimenté sur les tissus de l'hôte et ceci avait lieu juste avant la nymphose. Des effets significatifs se sont traduits après 24 heures par la réduction du nombre des embryons, par la diminution de la taille du plus grand de ceux-ci et par des altérations dans les embryons eux-mêmes.

     

Heterospecific larval competition and host discrimination in two species of aphid parasitoids: Aphidius ervi and Aphidius smithi

Females of the solitary aphid parasitoids Aphidius ervi Haliday and A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae) discriminated between unparasitized pea aphids and those parasitized by the other species. Oviposition restraint varied with the attack sequence and the length of the interval between successive attacks. The tendency to reject a previously parasitized host increased with interval length; A. smithi females rarely oviposited in aphids that had been parasitized 30 h earlier by A. ervi. Early first-instar larvae of A. ervi physically attacked and killed older A. smithi larvae, and older A. ervi larvae killed younger A. smithi, possibly by physiological suppression. Neither species appeared to have a competitive advantage when their eggs hatched at the same time. The evolution of heterospecific host discrimination in A. ervi and A. smithi is discussed. It is suggested that avoidance of multiparasitism is adaptive for both parasitoid species: for A. smithi because it is the inferior larval competitor, and for A. ervi because immatures develop more slowly in multiparasitized than in initially unparasitized hosts.

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Compétition interspécifique et discrimination des hôtes chez deux parasitoïdes de pucerons: Aphidius ervi et A. smithi

Résumé Les femelles des parasitoïdes de pucerons: Aphidius ervi Haliday et A. smithi Sharma & Subba Rao (Hyméno. Aphidiidae) distinguent les pucerons du pois sains des parasités par d'autres espèces. La rétention de la ponte dépend de la séquence de l'attaque et du temps écoulé entre des attaques successives. La tendance au rejet d'un puceron précédemment parasité augmente avec l'importance du délai; A. smithi a rarement pondu dans des pucerons qui avaient été parasités 30 h avant par A. ervi. Les jeunes larves de premier stade de A. ervi ont attaqué physiquement et tué les larves plus âgées de A. smithi, et les larves plus âgées de A. ervi ont tué des larves plus jeunes de A. smithi par élimination physiologique. Aucune espèce ne semble avoir un avantage quand les oeufs ont éclos en même temps. L'évolution de la discrimination interspécifique de l'hôte chez A. ervi et A. smithi est discutée. On estime que la tendance à éviter le multiparasitisme est adaptative chez les 2 espèces: pour A. smithi parce qu'il est dominé dans la compétition larvaire et pour A. ervi parce que les larves se développent plus lentement dans un hôte multiparasité que dans un hôte initialement sain.

     

Excised or intact inflorescences? Methodological effects on parasitoid wasp longevity

The development of accurate and repeatable experimental techniques is a cornerstone of any research program. Indeed, the first stage in developing a conservation biological control program typically involves ranking the suitability of various plant species as food resources for the target species of natural enemy in the laboratory or glasshouse. Herein the choice of flower presentation method is a highly relevant consideration. It is unclear whether excised flowers with their peduncles submerged in water will generate similar effects on the life history traits of a natural enemy compared with those using flowers remaining intact on a rooted plant. Either method has been used in 86 previous studies, yet none has quantified this effect. It is possible that both plant nectar content and production are altered as a result of changes in the physiological condition of the excised flowers. A laboratory test was designed to assess the influence of flower presentation method (excised or intact inflorescences) and different types of nectar (artificial and natural) on the longevity of the wasp Aphidius ervi, an important parasitoid of aphids. Distinct differences were revealed in the suitability of the nine flower species and three control treatments on parasitoid wasp longevity, with buckwheat being the most suitable plant. However, apart from coriander, flower presentation method and wasp gender generally did not affect parasitoid longevity for the set of species tested. As there was little evidence that parasitoid wasp longevity would be altered on excised flowers, and because of reasons pertaining to improved logistical and experimental requirements, the use of excised flowers is cautiously recommended to researchers for further laboratory evaluations of the effects of nectar provision on parasitoid fitness.     

Survival time and resistance to desiccation of diapause and non-diapause eggs of temperate Aedes (Stegomyia) mosquitoes

Time-related changes in the patterns of host discrimination may have a complex form depending on whether or not hosts are marked following first parasitisation, the persistence of chemicals in these marks, and changes in the host as a result of being parasitised. Here we present an experimental design capable of detecting temporal patterns in host discrimination of solitary parasitoids. This method is tested against experimental data from Aphidius ervi attacking the aphid, Acyrthosiphon kondoi. The results demonstrate that there are statistically significant changes in the patterns of egg distribution in aphid hosts held for intervals between 1 and 48 h between parasitisation. These changes appear to be attributable to increasing oviposition restraint by the superparasitising female as the time interval between attacks is increased. The form of this relationship is not significantly different to a sigmoid function and the rate of change of restraint was greatest between 6–8 h after the first oviposition.     

Decision to superparasitize based on larval survival: competition between aphid parasitoids Aphidius ervi and Aphidius smithi

Oviposition by a parasitoid wasp in an already parasitized host (= superparasitism) may result in larval competition and the loss of offspring. Consequently, in solitary species, the decision to superparasitize should be based on the probability of offspring survival. Females of Aphidius ervi Haliday and A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae), two parasitoids of the pea aphid, discriminated between conspecific-and heterospecific-parasitized hosts. Both species showed partial preference for aphids previously attacked by A. smithi, a result predicted from the fact that A. ervi is superior to A. smithi in larval competition under most conditions. Females of A. smithi selectively attacked conspecific-parasitized pea aphids when given a choice between these and self-parasitized hosts. Results indicated that females of both species responded to an external pheromone-like marker to discriminate between selfconspecific-and heterospecific-parasitized aphids. In addition, A. smithi may use internal cues to recognize, and avoid oviposition in, aphids previously parasitized by A. ervi.

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Résumé L'oviposition par une guêpe parasite dans un hôte qui a déjà été parasité (= superparasitisme) pourrait conduire à une compétition entre larves et à une perte de progéniture. Par conséquent, dans les espèces solitaires, la décision de superparasiter devrait être basée sur la probabilité de survie de la progéniture. Les femelles de Aphidius ervi Haliday et de A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae), deux parasites du puceron du pois, ont discriminé entre les hôtes parasités par des individus de la même espèce et ceux parasités par des individus d'une espèce différente. Les deux espèces de parasite ont montré une préférence partielle pour les pucerons préalablement parasités par A. smithi, résultat qui était prévisible du fait que, dans la majorité des cas, A. ervi est supérieur à A. smithi dans la compétition entre larves. Les femelles de A. smithi ont sélectivement attaqué les pucerons parasités par un individu de la même espèce lorsqu'il leur était donné de choisir parmi ceux-ci et ceux parasités par les même individu. Les résultats indiquaient que les femelles des deux espèces répondaient à une sorte de phéromone externe pour distinguer les pucerons parasités par le même individu de ceux parasités par un individu de la même espèce et ceux parasités d'une autre espèce. En plus, A. smithi pourrait utiliser des signaux internes pour reconnaître et éviter l'oviposition dans les pucerons préalablement parasités par A. ervi.

     

Chemically mediated associative learning in the host foraging behavior of the aphid parasitoidAphidius ervi (Hymenoptera: Braconidae)

The effect of experience on the responsiveness of the braconid parasitoidAphidius ervi to host(Acyrthosiphon pisum)-associated cues was investigated on bean plants(Vicia faba) using a wind tunnel bioassay. Oviposition experience on the plant-host complex significantly increased the oriented flight and landing responses ofA. ervi females to an undamaged plant and to a plant-host(A. pisum) complex. However, oviposition experience onA. pisum aphids when isolated from the plant did not change their responses to the intact plant and the complex. Searching on an unwashed plant which had been previously damaged byA. pisum also increased their response to an undamaged plant and a host-damaged plant, whereas the experience of searching on an undamaged plant did not significantly change their responses to undamaged plants. However, when parasitoids were allowed to search on an undamaged plant which had been sprayed withA. pisum honeydew, this significantly increased their response to an undamaged plant. Oviposition experience on the plant-host complex and foraging experience on a host-damaged plant or an undamaged plant sprayed with honeydew also significantly reduced the mean time taken by the parasitoids to respond in the wind tunnel. The behavioral changes associated with such experience were acquired within 30 min and persisted for at least 3 days. The results demonstrate the capacity ofA. ervi to learn associatively olfactory cues from plants, host-damaged plants, and plant-host complexes and confirm the role of aphid honeydew as a host recognition kairomone forA. ervi.     

Asymmetric size effect of sexes on reproductive fitness in an aphid parasitoid Aphidius ervi (Hymenoptera: Aphidiidae)

Most studies on size–fitness relationships focus on females and neglect males. Here, we investigated how body size of both sexes of an aphid parasitoid, Aphidius ervi Haliday, affected the reproductive fitness. Reproductive fitness was generally positively correlated with body size for both sexes in this species. Large individuals of both sexes had greater longevity, large males fathered more progeny, and large females had higher fecundity, parasitism, and greater ability in host searching and handling. We demonstrated in this study that size effects of males and females were asymmetric on different reproductive fitness parameters. With increasing body size females gained more than males in longevity and fecundity while males gained more than females in the number of female progeny. Regardless of female size, large males sustained a female-biased population longer than small males. These results suggest that male body size should also be considered in the quality control of mass-rearing programs and the evaluation of parasitoid population growth.     

Effect of Bt-cotton expressing Cry1A(c) on the survival and fecundity of two hymenopteran parasitoids (Braconidae, Encyrtidae) in the laboratory

We examined the effect of Bt-cotton (Event 531) plants expressing the Bacillus thuringiensis δ-endotoxin CryIA(c) on two hymenopteran endoparasitoids, Cotesia marginiventris and Copidosoma floridanum. In the laboratory, parasitized and unparasitized Pseudoplusia includens larvae were reared on foliage from a conventional soybean cultivar (Pioneer 97B61), a conventional cotton cultivar (DPL 5415), or a Bt-cotton cultivar (NuCotn 33B). C. marginiventris developed significantly faster within P. includens larvae feeding on Pioneer 97B61 and DPL 5415 compared to those feeding on NuCotn 33B. C. marginiventris that developed inside P. includens larvae feeding on NuCotn 33B suffered reduced longevity, and females had fewer ova. NuCotn 33B also affected the growth and development of P. includens parasitized with C. floridanum and life history parameters of adult C. floridanum. Parasitized and unparasitized P. includens developed more slowly when they were fed NuCotn 33B and the prepupae weighed less. Survival of parasitized and unparasitized P. includens was lower when larvae were fed NuCotn 33B and some evidence points to higher susceptibility of parasitized caterpillars to intoxication by NuCotn 33B. Fewer C. floridanum adults emerged from hosts fed NuCotn 33B, but pupal weight and adult longevity were unaffected. Analysis comparing the two experiments conducted with C. floridanum suggests that older NuCotn 33B plants (90–120 days after planting) may affect parasitoid development and adult survival less than younger NuCotn 33B plants (60–90 days after planting). Feeding on NuCotn 33B by P. includens affected the survival and development of the two hymenopteran endoparasitoids studied here, and the degree of the effect was similar to that observed with natural resistance found in soybean plants. It remains to be determined if the effects demonstrated here are less than, equal to, or greater than the impact of conventional insecticide applications used in conventional, non-transgenic, cotton.     

Juvenile hormone effects on polymorphism in the pea aphid, Acyrthosiphon pisum

When reared in short days (LD 12:12) at 15°C, apterous Acyrthosiphon pisum gave birth to sexual females (oviparae) exclusively for the first eight days of larviposition. After this time they switched to the production of parthenogenetic females (viviparae). Topical application of juvenile hormones I, II and III to fourth instar or adult ovipara-producers induced the precocious appearance of parthenogenetic females in the progeny sequence. Various forms intermediate between oviparae and viviparae were also produced and repetitive JH-I treatments resulted in a few alatiform progeny. However, many of the JH induced apterous, parthenogenetic females appeared to be normal viviparae and were capble of reproduction. Thus, prenatal treatment of oviparous embryos with JH diverts development towards the viviparous form. JH-I treatment of long-day reared A. pisum had no effect on the type of progeny produced.

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Effets de l'hormone juvénile sur le polymorphisme d'Acythosiphon pisum

Résumé Quand il est élevé sous jours courts (LN 12/12) à 15°C, le type anglais vert d'Acyrthosiphon pisum ne donne naissance qu'à des femelles sexuées (ovipares) pendant la première partie de sa période de reproduction. Ensuite quelques types intermédiaires ovipares/vivipares peuvent apparaitre avant que les pucerons ne bifurquent spontanément vers la production de femelles parthénogénétiques (vivipares). L'application cutanée d'hormones juvéniles (JH I, II, et III) aux larves de quatrième stade ou à des adultes producteurs d'ovipares provoque l'apparition prématurée d'intermédiaires et de vivipares dans la descendance. Les différentes formes intermédiaires produites par des applications répétées de J.H. comprenaient des types ailés ou partiellement ailés. Cependant, les vivipares aptères induits par J.H. étaient morphologiquement normaux et beaucoup étaient capables de se reproduire. Des traitements semblables aux J.H. de vivipares élevées en jours longs (LN 16/8) n'ont pas eu d'effets sur le type de la descendance.On ne sait pas si l'action de JH exogène sur l'induction des vivipares est direct ou indirect. La reprogrammation des embryons, autrement destinés à se développer comme ovipares, est examinée en relation avec notre connaissance du contrôle endocrine du polymorphisme des pucerons.

     

The influence of insecticide residues on primary parasitoid and hyperparasitoid foraging behaviour in the laboratory

The behaviour of Aphidius rhopalosiphi DeStefani-Perez (Hymenoptera: Braconidae) was recorded in laboratory bioassays in the presence and absence of both residues of honeydew from the English grain aphid, Sitobion avenae (F.) and the pyrethroid insecticide, deltamethrin. Insecticide concentrations ranged from the field recommended dose rate (6.25 g ai/ha in 2001 water) to 1/16th of field rate. Parasitoids responded strongly to patches of honeydew on filter papers, however the addition of increasing concentrations of deltamethrin caused increasingly early departure from the honeydew-treated areas. Parasitoids pre-exposed to field concentration residues for between 1 and 20 min showed shorter retention times and abnormal types of behaviour on honeydew-treated patches compared to control wasps. Recovery of normal behaviour patterns occurred over a 12 h period away from the insecticide source. Further laboratory studies examined the foraging of A. rhopalosiphi and one of its associated hyperparasitoids, Dendrocerus carpenteri (Curtis), on mature wheat plants treated with an artificial honeydew solution and deltamethrin. Wasps exposed to deltamethrin residues were observed to spend shorter visit times, to groom more frequently and to rest less frequently than those on insecticide-free plants. Differences were found between the distribution of parasitoids on insecticide-treated and untreated plants, with D. carpenteri showing greater movement down insecticide-treated plants accompanied with an increase in time spent on abaxial leaf surfaces compared to A. rhopalosiphi. The results are discussed in terms of the repellency of deltamethrin and the implications for integrated pest management of differing sub-lethal insecticide effects on primary parasitoid and hyperparasitoid behaviour.     

Ballooning activity of Chilo partellus larvae in relation to size of mother, egg batches, eggs and larvae and age of mother

In a study of male alary dimorphism, clones of pea aphid, Acyrthosiphon pisum (Harris), were collected five times over two seasons in one location and once each from three other latitudes in the Great Plains region of North America. All clones produced some males when subjected to a short photoperiod (L14: D10 or L13: D11) at 20 °C. The presence of three distinct classes of clones based on the morph of males they produced (apterous only; alate only; apterous and alate in equal proportions) provides evidence for a genetic basis for the dimorphism. The relative frequencies of clones producing only apterous males varied geographically, providing some evidence to support the hypothesis that populations are not panmictic over the Great Plains region.

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Résumé Le dimorphisme alaire mâle, — ou présence de mâle ailé et aptère —, s'observe à l'intérieur et en fonction des clones de A. pisum Harris. Notre objectif est de décrire la distribution et la fréquence de ces deux morphes. 143 clones de pucerons du pois ont été récoltés sur Medicago sativa dans quatre stations de la Grande Plaine d'Amérique du Nord, entre 44° 45 et 53° 12 N: à cinq reprises pendant deux saisons dans une station, et une fois dans chacune des 3 autres stations. Tous les clones, soumis à une photopériode courte: L14: D10 ou L13: D11 à 20 °C, ont produit quelques mâles. Les 8 échantillons ont donné à la fois des mâles aptères et ailés, et les observations dans les clones confortent les expériences.Trois classes de clones ont été formées à partir des morphes produits: mâles aptères seuls, mâles ailés seuls, ou quantités égales de chaque morphe. La fréquence par rapport à la distribution binomiale a été utilisée pour montrer que pour presque tous les clones produisant les deux morphes, les proportions étaient 1:1. Ces observations prouvent que la présence ou l'absence d'ailes est déterminée génétiquement, probablement par un gène diallélique lié au sexe. La fréquence des clones produisant uniquement des mâles aptères varie en fonction des populations examinées, ce qui laisse supposer une perte de panmixie à travers la Grande Plaine, quant au morphe des mâles.