Host specificity in the host-seeking larva of the dipteran parasitoidMallophora ruficaudaand the influence of age on parasitism decisions

Abstract

AbstractLarvae of the robber flyMallophora ruficaudaare ectoparasitoids of white grubs and adults are an important apiculture pest in Argentina. Females oviposit on tall grasses and the second instar larva actively searches and locates hosts. There are nine potential hosts in the distribution area of this parasitoid andCyclocephala signaticollis(Coleoptera: Scarabaeidae) is the most parasitized in the field. However,M. ruficaudahas a certain degree of behavioural flexibility towards different host species, and not being a strict specialist. The conditions under which the parasitoid orientates and accepts different hosts’ species are unknown. We studied the host specificity ofM. ruficaudatowards three species ofCyclocephalagenus and we determined whether this specificity depends on larval age. We also evaluated whether larva orientation towardsCyclocephalaspecies changes with chemical cue concentration. We assessed host specificity measuring the orientation and acceptance behaviours towards kairomones extracts and live individuals ofCyclocephalaspecies usingM. ruficaudalarvae of low and high life expectancy (i.e., young and aged second instar larvae). We observed that young larvae orientated only towardsC. signaticollischemical stimulus, whereas aged larvae orientated also towardsC. modesta, and the same was observed with increasing stimuli's concentration. Both young and agedM. ruficaudalarvae orientate towards liveC. signaticollisandC. putridaspecies and rejectedC. modesta. Also, we found that larvae accepted allCyclocephalahosts. In conclusion, our results indicate that specificity in the laboratory, observed through host orientation and host acceptance behaviours, depends not only on the availability of host species, but also on the nature of the host's stimuli combined with parasitoid age.