Infectivity, speed of kill, and productivity of a baculovirus expressing the itch mite toxin txp-1 in second and fourth instar larvae of Trichoplusia ni

Abstract

A cDNA clone of the gene coding for the paralytic neurotoxin (tox34) from the female straw itch mite, Pyemotes tritici, was created by RT-PCR and inserted into the genome of the Autographa californica nucleopolyhedrovirus (AcMNPV) under the control of the AcMNPV p10 promoter. This recombinant virus, AcTOX34.4, caused a rigid paralysis in infected larvae. The infectivity of AcTOX34.4 was compared to the wild-type parent strain, AcMNPV-C6, in second and fourth instar larvae of the cabbage looper, Trichoplusia ni. There were no significant differences in LD(50) values between the recombinant virus and its wild-type parent strain but, as expected, the LD(50) was lower for second instar larvae. The mean time to death and yield of occlusion bodies were measured in second and fourth instar T. ni larvae at a high (100% mortality) and low (<50% mortality) doses of the virus. The mean time to death of recombinant infected larvae was reduced by 50-60% compared to larvae infected with the wild-type strain, depending on virus dose and instar, with these larvae becoming paralysed after approximately 60 h and dying 10-20 h later. This is among the fastest speeds of kill recorded for recombinant baculoviruses. Fourth instar larvae were found to succumb to the recombinant virus more quickly than the second instar larvae. The increase in the speed of kill of the recombinant virus was accompanied by a large reduction of approximately 95% in the yield of progeny virus. The yield of virus showed a highly significant relationship with time to death, but this relationship was complex and varied between the different viruses, concentrations, and instars. The yield per unit weight of the larvae was found to be constant at a low virus dose and increased over time at a high virus dose, irrespective of instar and virus. It is predicted that these changes in the performance of the recombinant virus would act toward reducing its fitness, leading to it being outcompeted by the wild type in field situations.