Pathogenesis of the baculovirus AcMNPV in the soybean looper, Pseudoplusia includens

For sustainable organic soybean farming, inexpensive non-synthetic pesticides are needed to control lepidopteran pests such as Pseudoplusia includens. Farmers can use naturally occurring baculoviruses to limit P. includens infestations. However, baculovirus pathogenesis and mortality in P. includens must be evaluated to provide confidence in baculovirus efficacy against P. includens. To quantify mortality and pathogenesis of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) in P. includens, larvae were inoculated with an AcMNPV recombinant carrying the lacZ reporter gene (AcMNPV-hsp70lacZ). Newly-molted penultimate (fifth) instar P. includens inoculated orally with occlusion bodies (OB) and fifth instar larvae inoculated intrahemocoelically with budded virus (BV) were highly susceptible to fatal infection (LD50=40 OB and 25 BV, respectively). To study pathogenesis, larvae were inoculated orally with 60 OB and cells examined for LACZ signals over time. LACZ was first detected in midgut cells 4 hours post inoculation (hpi) and by 8 hpi LACZ was present in tracheal cells, indicating systemic infection. By 24 hpi, 88% of larvae expressed LACZ. To evaluate hemocyte susceptibility in P. includens after oral inoculation with AcMNPV OB, LACZ signaling in hemocytes and hemolymph BV titer was quantified around the onset of systemic infection; LACZ was first detected at 24 hpi, which correlated with BV circulating in hemolymph. Flow cytometry studies confirmed hemocyte susceptibility to AcMNPV. These results demonstrate that P. includens is permissive to fatal AcMNPV infection, suggesting that it may be effective for limiting economic damage caused by P. includens infestations in organic soybean crops.