Larval preference and performance of Amyelois transitella (Navel orangeworm, Lepidoptera: Pyralidae) in relation to the fungus Aspergillus flavus

Authors: AMPT, ELINE - University Of Illinois; BUSH, DANIEL - University Of Illinois; Siegel, Joel; BERENBAUM, MARY - University Of Illinois

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2015
Publication Date: 10/21/2015
Citation: Ampt, E.A., Bush, D.S., Siegel, J.P., Berenbaum, M.R. 2015. Larval preference and performance of Amyelois transitella (Navel orangeworm, Lepidoptera: Pyralidae) in relation to the fungus Aspergillus flavus. Environmental Entomology. 45(1):155-162.

Interpretive Summary: The navel orangeworm, Amyelois transitella (Walker), is an important lepidopteran pest of California nut crops, and its damage is increasing as more acres are planted with almonds, pistachios, and walnuts. It damages crops directly by feeding and contaminating nuts with frass excrement and webbing, and it causes indirect damage by transferring aflatoxin producing fungi belonging to the genus Aspergillus into damaged nuts. Once these fungi are established in a cluster they can infect other nuts. In order to reduce this damage, we must first understand the relationship between species of Aspergillus and navel orangeworm. If both insect and fungus obtain benefits from their association, known as mutualism, it will be much harder to reduce fungal damage associated with navel orangeworm, because the fungus species and insect are tightly linked. In laboratory studies we demonstrated that navel orangeworm larvae seek out agar where this fungus is growing, and this fungus in turn grows better in the presence of orangeworm frass. Navel orangeworm larvae also grow larger and faster when they eat Aspergillus fungi. Since this relationship benefits both species, it will be important to develop new management strategies that take this association into effect in order to reduce both navel orangeworm damage and aflatoxin contamination of tree nuts, in order to maintain or increase the quality of these nuts for the domestic and foreign market.

Technical Abstract: The navel orangeworm, Amyelois transitella (Walker), is a polyphagous pest of California nut crops and is responsible for extensive losses in the United States. It directly damages crops by feeding and contaminating nuts with frass and webbing and vectors saprophytic fungi that infect crops. The navel orangeworm is specifically associated with Aspergillus species, including the toxigenic Aspergillus flavus, which causes crop loss by producing carcinogens including aflatoxin B1. This lepidopteran-fungal complex is the most economically serious pest complex in Central Valley orchards, and evidence indicates that this relationship is mutualistic. We assessed preference and performance of navel orangeworm larvae associated with A. flavus in behavioral bioassays in which neonates were allowed to orient within arenas to media with or without fungal tissue, and performance bioassays in which larvae were reared with and without A. flavus on potato dextrose agar (PDA) and a semi-defined almond PDA diet to evaluate effects on development and pupal weight. Navel orangeworm larvae were attracted to A. flavus and developed faster in its presence, indicating a nutritional benefit to the caterpillars. Larvae reached pupation ~33% faster on diet containing A. flavus, and pupal weights were ~18% higher for males and ~13% higher for females on this diet. Our findings indicate that A. flavus plays an important role in larval orientation and development on infected hosts. The preference-performance relationship between navel orangeworms and Aspergillus flavus is consistent with a facultative mutualism that has broad implications for pest management efforts and basic understanding of Lepidoptera-plant interactions.