Consequences of interspecific competition on the virulence and genetic composition of a nucleopolyhedrovirus in Spodoptera frugiperda larvae parasitized by Chelonus insularis

Nucleopolyhedroviruses (Baculoviridae) are virulent insect pathogens that g enerally show a high degree of host specificity and have recognized potenti al as biological insecticides. Whenever viruses are applied for pest contro l, a proportion of the infected insects will also be parasitized by hymenop teran or dipteran parasitoids and interspecific competition for host resour ces will occur; the severity of such competition is likely to be modulated to a large degree by the virulence of each type of parasite. We examined th e impact of parasitism by the solitary egg-larval endoparasitoid Chelonus i nsularis (Hymenoptera: Braconidae) on the speed of kill of nucleopolyhedrov irus-infected Spodoptera frugiperda (Lepidoptera: Noctuidae) larvae and the pattern of host growth and virus production in infected and/or parasitized hosts. We also examined the effect of parasitism on the virulence, infecti vity and genetic composition of serially passaged virus. Both parasitism an d viral infection resulted in a marked reduction in host growth. When third instar larvae were dually parasitized and virus-infected, the growth rate was even more severely affected compared to parasitized larvae. There was a significant increase in virus production in larvae infected at later insta rs. Interspecific competition resulted in a substantial decrease in pathoge n production in parasitized larvae infected at the fourth instar, but not i n parasitized larvae infected at earlier instars. The serial passage experi ment resulted in the appearance of four distinct genetic isolates of the vi rus detected by restriction endonuclease analysis. Of the three isolates th at appeared in nonparasitized larvae, two showed increased virulence, expre ssed by mean time to death, and for one of these the infectivity, expressed as LC50, was reduced. One isolate that appeared in parasitized larvae (iso late D) had increased virulence and infectivity. Southern blot analysis ind icated that virus isolate D was most likely generated by point mutation of a restriction site or by alterations such as duplications, deletions or by recombination of two or more genotypic variants present in the wild-type nu cleopolyhedrovirus isolate. Our study provides clear evidence of interspeci fic competition within the host, since, depending on the timing of inoculat ion, adverse effects were observed upon both the parasitoid and the virus.