Adaptive Behaviors And Strategies For Survival And Reproduction Of Cotesia Vestalis

In nature,the relationship between a parasite and its host is a fascinating issue in co-evolution.Parasites adapt to selection pressures from bio-factors such as its host and predators.As a result,parasites evolved many adaptive strategies to cope with these pressures in the process of natural selection.When a parasitoid encounters selection pressures from host defences it may employ many strategies such as behavior manipulation to adapt to its host.Similarly,competition from other parasitoids and predators may also drive counter adaptations.In the present study,the parasitoid wasp Cotesia vestalis(Hymenoptera:Braconidae)was used to investigate adaptability to external pressures from its host and a predator,as well as internal pressures from superparasitism and inbreeding.The aim of the studies was to investigate the ecological mechanisms for success of the parasitoid and to facilitate the better used of this parasitoid in bio-control of the pest Plutella xylostella(Lepidoptera:Plutellidae).In the study of parasitoid adaptation to the predator,parasitism by C.vestalis was associated with behavioural changes in its larval host(P.xylostella)that reduce risk of intra-guild predation(IGP).Compared with unparasitised caterpillars,parasitised P.xylostella exploited fewer feeding patches in 24 hours(unparasitised:2.65 ± 0.22/individual larva;parasitized:1.50 ± 0.11/individual larva)and exploited fewer leaves in 3 days(unparasitised:2.14 ± 0.18/individual larva;parasitized:1.45 ± 0.14/individual larva).This phenomenon of moving less frequently to new feeding patches on plants was consistent with lower probability of falling from the plant when they were exposed to artificial wind(unparasitised>76%,parasitized>53%)or forging C.vestalis females(unparasitised>90%,parasitized>50%).Further,the ground forging predator Pardosa pseudoannulata(Araneae:Lycosidae)killed significantly fewer parasitised larvae when larvae were exposed to artificial wind(unparasitised>58%,parasitized>38%).Though the reduced movement gave reduced risk of IGP,the parasitised larvae fed less than unparasitised larvae and developed more slowly than unparasitised larvae.These results suggest trade-off between IGP avoidance and nutrient intake by the host.In the study of parasitoid adaptation to the host,when exposed sequentially to 3 hosts,C.vestalis females reject significantly more parasitised hosts(third encounter>38%)than unparasitised host(third encounter>11%),suggesting that females can discriminate parasitised larvae from unparasitised larvae.C.vestalis did exhibit such discrimination when encountering unparasitised larvae daubed with parasitized or unparasitised larvae hemolymph,suggesting volatiles from the hemolymph were not the reason for parasitized host recognition.However,when host larvae were injected with parasitised larvae hemolymph or venom extract,parasitoids showed significantly higher rejection suggesting parasitoids detected a modification in the parasitised larvae's hemolymph by the ovipositor.Though parasitised host recognition exists in C.vestalis,superparasitism is very common in the laboratory.Further experiments showed that superparasitism did not result in disadvantageous influences on progeny production and bionomics of C.vestalis.A method for parasitoid offspring analysis using microsatellites was then used to investigate whether a superparasitised eggcould develop to adulthood.Results showed that when the time interval between parasitism and superparasitism is short(10 minutes,2 hours or 6 hours),development is unaffected by superparasitism.While when the time interval prolong to 12 hours,the rate of superparasitised egg develop to adulthood is significant lower than parasitized egg,only 5 from total 30 offsprings were the progenies of superparasitised parasitoids.In the following study,C.vestalis was used to test whether inbreeding depression occurs and whether it threatens a laboratory population.Results showed that,sib mated parasitoids exhibited a significantly female biased sex ratio(37.6%)in generation 2 compared with non-sib mated parasitoids(52.3%).But this was reversed in generations 7-9 and there were no effects of sib-mating on survival or emergence in C.vestalis.Host larvae parasitised by generation 7 sib mated parasitoids or non sib mated parasitoids did not show difference in variables such as parasitism rate,cocoon emerge rate,or host death rate.Male bias was,however,significantly stronger among progeny from sib mated parasitoids.These phenomena were evident also when 300 host larvae were exposed to one pair of generation 8 sib mated parasitoids.These results showed that C.vestalis is not immune to inbreeding depression at more than seven generations,and inbreeding depression was determined by sibling mating generation level.Overall,the findings of this series of studies strongly support the prediction that this parasitoid employs ecological strategies to adapt to its host and for its reproductive success.These results suggest avoidance of high level inbreeding in laboratory-reared population of C.vestalis,and give evidence for better used of parasitoid C.vestalis in a host-parasitoid-predator system.