Clayton C. Beegle

Effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni (Lepidoptera: Noctuidae) and the parasite, Hyposoter exiguae (hymenoptera: Ichneumonidae)

Abstract The effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni and the parasite, Hyposoter exiguae , was investigated to determine if the virus could invade and multiply in the tissues of the parasites, if parasites which emerged from virus-infected T. ni larvae had normal emergence, fecundity, and longevity, and if the parasite could serve as a vector for the virus. Light microscopy revealed particles which appeared to be polyhedra within the lumen of the midgut of parasite larvae from virus-infected hosts. Transmission electron microscopy confirmed the presence of polyhedra and free virions within the midgut of the larvae. Polyhedra or free virions were never found within any parasite tissues. Parasite larvae within hosts exposed to virus before parasitization perished when their hosts died of virus infection. Parasite larvae in hosts exposed to virus after parasitization completed their development before their hosts died of virus infection. The proportion of parasites which survived increased as the time between host parasitization and host virus exposure increased. Parasite larvae which developed in hosts exposed to the virus soon after parasitization spent significantly less time in their hosts than did parasites which developed in noninfected hosts. There was no significant difference in time spent in the pupal stage, percent adult emergence, adult longevity with and without food and water, and fecundity of parasites which developed in virus-infected hosts and those which developed in noninfected hosts. Female parasites laid as many eggs in virus-infected hosts as they did in noninfected hosts. Sixty percent of the female parasites which oviposited in virus-infected hosts vectored infective doses of virus to an average of 6% of the healthy hosts subsequently exposed to them. None of the healthy host larvae exposed to male parasites which had been exposed to virus-infected host larvae became infected with the virus. Forty percent of the female parasites which developed in virus-infected hosts transmitted infective doses of the virus to an average of 65% of the healthy host larvae exposed to them. Ninety percent of the male parasites which developed in virus-infected hosts transferred infective doses of the virus to an average of 21% of the healthy host larvae exposed to them.

Differential susceptibility of parasitized and nonparasitized larvae of Trichoplusia ni to a nuclear polyhedrosis virus.

Abstract Nonparasitized second-instar larvae of Trichoplusia ni were twice as susceptible (at the LD 50 level) to the singly enveloped T. ni nuclear polyhedrosis virus as those parasitized by Hyposoter exiguae (Hymenoptera: Ichneumonidae). The LD 50 values for nonparasitized and parasitized larvae were 1.58 × 10 3 and 3.16 × 10 3 polyhedra/ml of diet, respectively. The LD 95 value for parasitized larvae was approximateely 5 times higher than that for nonparasitized larvae. The slopes ( b values) were 1.2 for parasitized larvae and 1.7 for nonparasitized larvae. The LT 50 values for parasitized larvae also were significantly longer than those for nonparasitized larvae. No significant difference was found between the food consumption of parasitized and nonparasitized T. ni larvae.