Venturia canescens parasitizing Galleria mellonella and Anagasta kuehniella: is the parasitoid a conformer or regulator?
Harvey, J.A.
The effects of parasitism by Venturia canescens (Hymenoptera: Ichneumonidae) on the growth of Galleria mellonella and Anagasta kuehniella (Lepidoptera: Pyralidae) were examined by measuring the weight gain of hosts parasitized from the late second (LL2) to the fifth (L5) instar. Unparasitized G. mellonella are much larger than A. kuehniella in all corresponding stages of larval development. The development of V. canescens was more successful in A. kuehniella than G. mellonella, because late final instar parasitoids were usually unable to egress from the cuticle of C. mellonella. Parasitized G. mellonella larvae (a) show a marked reduction in weight gain, which was more apparent the earlier that parasitism occurred, and (b) left their feeding chambers to spin precocious cocoons just prior to being destroyed by the parasitoid. Weight reduction was also shown by parasitized L2-L4 A. kuehniella compared with controls, although the relationship between host size at parasitism and subsequent weight reduction was not as clear as in G. mellonella. V. canescens was therefore partially able to control host weight gain, which varied between host species, and this control was probably mediated by the parasitoid larva. Although parasitized hosts continued to grow after parasitism, V. canescens clearly arrested the development of the larger host species, G. mellonella at an earlier stage than the smaller host species, A. kuehniella. The results suggest that V. canescens, and perhaps other so-called 'conformer' koinobiont endoparasitoids, that exhibit flexible larval growth patterns, may also regulate host growth in response to their own nutritional and physiological requirements.
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