Why do Spodoptera exigua multiple nucleopolyhedrovirus occlusion bodies lose insecticidal activity on amaranth (Amaranthus hypocondriacus L.)?

Amaranth (Amaranthus hypocondriacus L.) is a pseudocereal that is widely consumed in Mexico and other tropical regions, but which is attacked by Spodoptera exigua (Hübner) larvae. Products based on Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) can be highly effective insecticides, but when applied to amaranth viral occlusion bodies (OBs) lost 50% of their activity in a 6 h period. A series of experiments was performed to examine the role of solar radiation, leaf oxidative enzymes, leaf pH and insect feeding behavior, all of which failed to explain the reduced prevalence of virus infection in S. exigua larvae on amaranth. The concentration of the flavonoid rutin on amaranth leaves was >200-fold higher than on another preferred host plant, sweet pepper (Capsicum annuum). However, in laboratory assays, rutin generally resulted in an increased prevalence of infection when administered in mixtures with semi-synthetic diet and OBs. We conclude that the cause of OB inactivation on amaranth remains elusive, although understanding how this will influence its efficacy as a biological insecticide requires field studies.