Mixed-genotype infections of Trichoplusia ni larvae with Autographa californica multicapsid nucleopolyhedrovirus: Speed of action and persistence of a recombinant in serial passage

Fast-acting recombinant baculoviruses have potential for improved insect pest suppression. However, the ecological impact of using such viruses must be given careful consideration. One strategy for mitigating risks might be simultaneous release of a wild-type baculovirus, so as to facilitate rapid displacement of the recombinant baculovirus by a wild-type. However, at what ratio must the two baculoviruses be released? An optimum release ratio must ensure both fast action, and the eventual competitive displacement of the recombinant virus and fixation of the wild-type baculovirus in the insect population. Here we challenged Trichoplusia ni larvae with different ratios of wild-type Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and a derived recombinant, vEGTDEL, which has the endogenous egt gene (coding for ecdysteroid UDP-glucosyltransferase) deleted. Time to death increased with the proportion wild-type virus in the inoculum mixture, although a 1:10 ratio (wild-type: recombinant) resulted in equally rapid insecticidal action as vEGTDEL alone. Five serial passages of three different occlusion body (OB) mixtures of the two viruses were also performed. OBs from 10 larval cadavers were pooled and used to initiate the following passage. Although the wild-type baculovirus was maintained over five passages, it did not go to fixation in most replicates of the serial passage experiment (SPE), and there was no good evidence for selection against the recombinant. Long-term maintenance of a recombinant in serial passage suggests an ecosystem safety risk. We conclude that for assessing ecological impact of recombinant viruses, SPEs in single and multiple larvae are relevant because of potential modulating effects at the between-host level.