Effects of pea aphid secondary endosymbionts on aphid resistance and development of the aphid parasitoid Aphidius ervi: a correlative study

In order to reduce parasite‐induced mortality, hosts may be involved in mutualistic interactions in which the partner contributes to resistance against the parasite. The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), harbours secondary bacterial endosymbionts, some of which have been reported to confer resistance against aphid parasitoids. Although this resistance often results in death of the developing parasitoid larvae, some parasitoid individuals succeed in developing into adults. Whether these individuals suffer from fitness reduction compared to parasitoids developing in pea aphid clones without symbionts has not been tested so far. Using 30 pea aphid clones that differed in their endosymbiont complement, we studied the effects of these endosymbionts on aphid resistance against the parasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae: Aphidiinae), host–parasitoid physiological interactions, and fitness of emerging adult parasitoids. The number of symbiont species in an aphid clone was positively correlated with a number of resistance measurements but there were also clear symbiont‐specific effects on the host–parasitoid interaction. As in previous studies, pea aphid clones infected with Hamiltonella defensa Moran et al. showed resistance against the parasitoid. In addition, pea aphid clones infected with Regiella insecticola Moran et al. and co‐infections of H. defensa–Spiroplasma, R. insecticola–Spiroplasma, and R. insecticola–H. defensa showed reduced levels of parasitism and mummification. Parasitoids emerging from symbiont‐infected aphid clones often had a longer developmental time and reduced mass. The number of teratocytes was generally lower when parasitoids oviposited in aphid clones with a symbiont complement. Interestingly, unparasitized aphids infected with Serratia symbiotica Moran et al. and R. insecticola had a higher fecundity than unparasitized aphids of uninfected pea aphid clones. We conclude that in addition to conferring resistance, pea aphid symbionts also negatively affect parasitoids that successfully hatch from aphid mummies. Because of the link between aphid resistance and the number of teratocytes, the mechanism underlying resistance by symbiont infection may involve interference with teratocyte development.     

In-field production of parasitoids of Dysaphis plantaginea by using the rowan aphid Dysaphis sorbi as substitute host

A system was developed to provide the parasitic wasp Ephedrus persicae Froggatt (Hymenoptera: Braconidae: Aphidiinae), which attacks the rosy apple aphid Dysaphis plantaginea (Passerini) (Homoptera: Aphididae), with the alternative host Dysaphis sorbi Kaltenbach (Homoptera: Aphididae) in apple orchards. Rowan trees (Sorbus aucuparia L.) arranged along the side of an unsprayed orchard were artificially infested in late February 2002 with eggs of D. sorbi. Colonies of D. sorbi successfully developed from the introduced eggs and persisted on several trees until the end of June. The only primary parasitoid species emerging from a sample of mummified aphids collected in spring from the infested rowan trees was the braconid wasp species E. persicae. In a host-switching experiment, nymphs of D. plantaginea proved suitable for female parasitoids originating from mummified D. sorbi. A series of mummies collected from the rowan trees in early summer contained diapausing parasitoids and hyperparasitoids that only hatched in April of the following spring. These observations suggest the possibility of establishing a local population of E. persicae in apple orchards, so that D. plantaginea can be readily attacked by diapause-emerging parasitoids in early spring.     

Do differences in life‐history traits and the timing of peak mating activity between host‐associated populations of Chilo suppressalis have a genetic basis?

The development of host races, genetically distinct populations of the same species with different hosts, is considered to be the initial stage of ecological speciation. Ecological and biological differences consistent with host race formation have been reported between water‐oat and rice‐associated populations of Chilo suppressalis. In order to confirm whether these differences have a genetic basis, we conducted experiments to determine the extent to which various life‐history traits and the time of peak mating activity of these populations were influenced by the species of host plant larvae were raised on. Individuals from each population were reared for three consecutive generations on either water‐oat fruit pulp or rice seedlings. Descendants of both populations had higher larval survival rates, shorter larval developmental periods, higher pupal weight, and longer adult forewings, when reared on water‐oats than when reared on rice. The time of peak of mating activity differed between the descendants of each population, irrespective of whether they were raised on water‐oats or rice. These results indicate that although some life‐history traits of host‐associated populations of C. suppressalis are influenced by the host plant larvae are raised on, time of peak mating activity is not. Because it is a stable, objective, phenotypic trait, further research on difference in the time of peak mating activity between host‐associated populations of C. suppressalis should be conducted to clarify the mechanism responsible for host race formation in this species.