Facultative Mutualism between the Navel Orangeworm Amyelois Transitella (lepidoptera: Pyralidae) and Aspergillus Flavus By

The navel orangeworm, Amyelois transitella Walker (Lepidoptera: Pyralidae), is an economic pest of considerable importance by virtue of its habit of attacking damaged or overripe tree nuts and fruits in Californian orchards. Its economic impact is increased by its common association with the highly toxigenic fungus Aspergillus flavus Link. Despite the capacity of this fungus-insect association to damage a wide variety of tree crops, relatively little is known about the ecology of the interaction. I examined three aspects of this association to examine the possibility that the interaction represents a facultative mutualism. These studies are here presented in three chapters. First, to determine whether associating with the fungus allows the navel orangeworm to utilize its hostplants more efficiently, I conducted a series of laboratory bioassays to test if the presence of A. flavus decreases the toxicity of hostplant phytochemicals. Growth rates, mortality rates, and pupal weight for navel orangeworms were measured on an artificial potato dextrose agar (PDA) diet containing almond meal, in the presence and absence of the furanocoumarins xanthotoxin and bergapten, which occur in some fruit hostplant species. In the absence of furanocoumarins, larvae reached adulthood in 29.6 d on average in the presence of the fungus as opposed to 43.8 d in the absence of A. flavus (t = 16.06; df = 3; P < 0.001). Female and male pupae had on average five or six g more mass, respectively, when the larvae were fed with a diet containing fungus (t = 5.56; df = 3; P = 0.011 and t = 7.16; df = 3; P = 0.006, respectively). By contrast, the presence of furanocoumarins at natural concentrations in the diet (at concentrations determined by prior LC50 experiments) in the absence of the fungus extended the development time of the navel orangeworm by 14 to 22 d on average for bergapten