Spread of Aspergillus flavus by navel orangeworm (Amyelois transitella) on almonds

Authors: Palumbo, Jeffrey - Jeff; Mahoney, Noreen; Light, Douglas; Siegel, Joel; RYAN, PUCKET - University Of California; THEMIS, MICHAILIDES - University Of California

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2013
Publication Date: 9/30/2014
Citation: Palumbo, J.D., Mahoney, N.E., Light, D.M., Siegel, J.P., Ryan, P., Themis, M. 2014. Spread of Aspergillus flavus by navel orangeworm (Amyelois transitella) on almonds. Plant Disease. 98(9):1194-1199.

Interpretive Summary: The navel orangeworm (NOW, Amyelois transitella) is the primary lepidopteran moth pest of almonds in California. This insect causes damage by both directly feeding on almond kernels and by increasing the likelihood that damaged kernels will become contaminated with aflatoxin-producing fungi (Aspergillus flavus and Aspergillus parasiticus). Once these fungi become established in the tree canopy, they can invade undamaged almonds, contaminating them with fungal toxins. Aflatoxin contamination is a major quality concern, and, if levels exceed regulatory thresholds, the almonds are rejected by the buyer or destroyed at the point of entry into foreign markets. Navel orangeworm damage has long been associated with aflatoxin contamination, but there have been no studies conducted examining how this insect transmits Aspergillus flavus or parasiticus to almonds. In this study,which focused on Aspergillus flavus, we demonstrated that NOW hatching from eggs contaminated with fungal spores can leave a trail of A. flavus and contaminate nuts while feeding. An increase in feeding on a nut is associated with higher levels of aflatoxin contamination. In the field the prevalence of Aspergillus species recovered from almonds varied seasonally. It was lowest in the spring and early summer and increased to higher levels in late summer and early fall, following the same pattern as increases in the NOW population. Moths were trapped throughout the season and plated onto special growth plates that only allowed Aspergillus species to grow. The level of aflatoxin-producing fungi recovered from these moths also showed a seasonal pattern, and recovery peaked in late summer/early fall, confirming the association between NOW and Aspergillus flavus. This information in turn helps justify the amount of money spent to control this insect in order to prevent direct damage and decrease aflatoxin contamination of almonds. This increase in quality benefits nut growers because fewer kernels are damaged and by protecting and expanding the overseas market for these nuts.

Technical Abstract: Navel orangeworm (NOW) damage to almonds is correlated with increased incidence of aflatoxin contamination caused by Aspergillus flavus. However, no reports demonstrate a causative relationship between NOW feeding and A. flavus infection. To demonstrate the potential of NOW to act as a vector of A. flavus on almond, NOW eggs were dusted with A. flavus and incubated in microchambers adjacent to, but not touching, agar plates or almond kernels. Following egg hatch, A. flavus colonies developed on agar along trails left by NOW larvae. Almond kernels damaged with A. flavus-carrying NOW showed higher incidence of A. flavus colonization and aflatoxin contamination than control treatments. Interestingly, levels of aflatoxin in NOW-damaged, A. flavus-infected almonds were significantly higher than control treatments, even in the absence of visible fungal growth. Commercial almond orchards had a relatively low level of contamination with Aspergillus section Flavi in spring and early summer and a high level during summer, corresponding with the higher level of NOW infestation of the crop. Our study demonstrates that NOW is capable of vectoring A. flavus to almonds, and that monitoring and sorting of almonds for insect damage is warranted to limit aflatoxin contamination potential both before and after harvesting.