Biology and Management of Navel Orangeworm (Lepidoptera: Pyralidae) in California

Abstract

Navel orangeworm, Amyelois transitella (Walker), is a primary pest of almonds, pistachios, and walnuts in California. These specialty tree nut crops are widely planted across the state and account for a significant share of total agricultural revenue, with 1.7 million combined acres generating a total farm-gate value of $8.9 billion. Larvae of A. transitella cause direct damage to the nut, burrowing into the kernel and contaminating it with frass and webbing, while adults are able to introduce fungi during oviposition that produce aflatoxin, a known human carcinogen that is heavily regulated both domestically and in key foreign markets. As such, there is little tolerance for A. transitella infestation, and most operations aim for <2% crop damage from this pest. Currently, integrated management of A. transitella involves a combination of orchard sanitation, well-timed insecticide sprays, timely harvest, and, most recently, mating disruption. Additional novel tools, such as sterile insect technique, are currently being explored. This species has a strong dispersal capacity, and given the extensive, and many times contiguous, acreage of tree nuts in California, long-term management will require the development of an effective area-wide management strategy. Tools, tactics, and conditions are in an ongoing state of change, and therefore pest management for this economically important species is a work in progress. Here, we discuss the biology, seasonal phenology, monitoring, and management of A. transitella across almonds, pistachios, and walnuts.

Pest categorisation of Amyelois transitella

“…Although there are lighter and darker forms of this species, and adult size can vary considerably (Wilson et al., 2020), this intraspecific phenotypic diversity has not been related to any feature affecting the conclusions of this categorisation.…”

“…removal and destruction of remnant 'mummy' nuts in and around the orchard), the importance of early harvest to reduce A. transitella infestation has long been a key to the management of this pest, and has been demonstrated in walnuts and almonds. Early or timely harvest can reduce crop damage by minimising the amount of time new crop nuts are exposed to A. transitella populations late in the season (Wilson et al, 2020).…”

“…In California, A. transitella overwinter as larvae inside either unharvested tree nuts that have been left in the orchard (i.e. ‘mummies’ or ‘mummy nuts’) or other vulnerable agricultural commodities such as apples, figs and oranges (Wilson et al., 2020).…”

“…On nuts, it is able to introduce spores of Aspergillus flavus which produces toxic aflatoxins; infestation of the nut by A. transitella larvae provides a suitable environment for A. flavus colonisation (Palumbo et al., 2014; Ampt et al., 2015). Due to this association between A. transitella infestation and aflatoxin, growers, processors and exporters have allow tolerance for A. transitella damage in all nut crops, typically aiming for < 2% infestation (Wilson et al., 2020). Navel orangeworm is a moth that infests both almonds and pistachios.…”

“…The hindwings of the species are a uniform white to dusky grey with some darkening one the wing edge and veins. There are lighter and darker forms, and adult size can vary considerably (Wilson et al., 2020).…”