Orientation of navel orangeworm larvae and adults (Amyelois transitella: Lepidoptera: Pyralidae) toward Aspergillus flavus

Authors: BUSH, DANIEL - University Of Illinois; LAWRANCE, ALLEN - University Of Illinois; Siegel, Joel; BERENBAUM, MAY - University Of Illinois

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2017
Publication Date: 3/31/2017
Citation: Bush, D.S., Lawrance, A., Siegel, J.P., Berenbaum, M.R. 2017. Orientation of navel orangeworm larvae and adults (Amyelois transitella: Lepidoptera: Pyralidae) toward Aspergillus flavus. Environmental Entomology. 46(3):602–608. https://doi.org/10.1093/ee/nvx068.
DOI: https://doi.org/10.1093/ee/nvx068

Interpretive Summary: The navel orangeworm is an imporant pest of tree nuts in California, and this insect is often associated with the fungus Aspergillus flavus. Navel orangeworm larvae grow faster and survive better on diets containing this fungus, and we investigated whether larvae respond to chemical cues indicating fungal presence and plant damage. In our assays we found a strong response to three compounds, two associated with damaged plants (1-octen-3-ol and 2-phenylethanol) and one which is an inhibitor of fungal growth (ethyl benzoate). More eggs were laid on diet containing the fungus than on plain diet. Interestingly, there were twice as many fertilized eggs laid on the fungus diet than on the plain diet. We investigated whether navel orangeworm females had a strategy to lay more unfertilized eggs on poor diet to help feed the larvae that hatched, and found that larvae lived 2.5X as long when provided with eggs compared to larvae that did not have eggs as a supplement to their diet. Although this could be an artifact of our experimental protocol, there are numerous insects that lay unfertilized eggs as a dietary supplement. Further research is needed to determine if this truly is a strategy used by navel orangeworm. We clearly demonstrated that larvae reacted strongly to three compounds that were either associated with damaged plants or had antifungal properties, and this information could be used to develop attractants or repellents to help manage this pest.

Technical Abstract: The navel orangeworm (Amyelois transitella), a pest of California tree nuts, is associated with the fungus Aspergillus flavus, and mounting evidence suggests that these two species are facultative mutualists. Navel orangeworm larvae exhibit improved growth and survival on diets containing this fungus, therefore orientation toward host plants infected with A. flavus could be beneficial. We conducted a series of behavioral assays to determine if larvae respond to the same chemical cues that elicit behavioral or electrophysiological responses in adults. In Petri dish arenas, larvae showed a strong preference for 1-octen-3-ol and 2-phenylethanol, volatiles characteristic of damaged plants, as well as methanolic extracts of almond meal with 1-octen-3-ol and the fungal volatile conophthorin. Larvae displayed aversion to ethyl benzoate, a known inhibitor of fungal growth. When we assessed oviposition behavior relative to substrates with and without A. flavus present, females laid almost twice as many eggs on fungus-inoculated substrates. Moreover, 62% of eggs on the inoculated substrates were fertilized, compared to only 26% of eggs on uninoculated sites. An assay comparing larval survivorship in the presence and absence of unfertilized eggs was conducted to test the hypothesis that unfertilized eggs are laid on nutrient-poor substrates to provide supplemental nutrition for larvae. Neonates provisioned with eggs survived 2.5 times longer than unprovisioned neonates (208.8 h versus 85.2 h), indicating that this species may use cannibalism to compensate for oviposition on lower-quality food sources. We conclude that larvae orient to host plant and fungal volatiles and document a possible strategy that could enable neonate larvae to establish on nutritionally poor hosts.