The spread of the fungus Aspergillus flavus by the navel orangeworm (Amyelois transitella) via frass

The navel orangeworm, Amyelois transitella (Lepidoptera: Pyralidae), is a known vector of Aspergillus fungus species in Californian orchards. Given their overlapping host range, the competition between caterpillars and fungi for resources would seem to be a drawback of this partnership, as both organisms use the same plant materials for nutrition, and yet previous work shows that Aspergillus flavus thrives in the presence of navel orangeworm larvae. In this study, we assessed the growth of A. flavus in or on frass from larvae fed with standard lepidopteran diet and almond potato dextrose agar (PDA) diet. We collected frass and incorporated it into an agarose medium to determine to what extent it could support fungal growth in comparison with PDA and almond PDA media. We were able to grow A. flavus on all four diets, and growth on frass media was faster than on PDA. Additionally, we collected frass from larvae that had fed on diet inoculated with A. flavus, and we examined the resulting frass at 800X and 2000X magnification for the presence of fungal spores, conidial heads, or hyphae. In frass from all diets, we observed all of these intact fungal elements. Moreover, we transferred spores from some frass samples onto PDA to assess viability. In some cases, these spores produced a colony of A. flavus, indicating that caterpillar excrement may be an alternate vehicle for phoresy and a source of compensation for substrate lost to the fungus in initial competition with navel orangeworm caterpillars.