Management of Navel Orangeworm (Lepidoptera: Pyralidae) Using Four Commercial Mating Disruption Systems in California Almonds

Pest risk assessment of Amyelois transitella for the European Union

Following a request from the European Commission, the EFSA Panel on Plant Health performed a pest risk assessment of Amyelois transitella (Lepidoptera: Pyralidae), the navel orangeworm, for the EU. The quantitative assessment considered two scenarios: (i) current practices and (ii) a requirement for chilled transport. The assessment focused on pathways of introduction, climatic conditions and cultivation of hosts allowing establishment, spread and impact. A. transitella is a common pest of almonds, pistachios and walnuts in California, which is the main source for these nuts imported into the EU. Based on size of the trade and infestation at origin, importation of walnuts and almonds from the USA was identified as the most important pathways for entry of A. transitella. Using expert knowledge elicitation (EKE) and pathway modelling, a median estimate of 2,630 infested nuts is expected to enter the EU each year over the next 5 years (90% certainty range (CR) from 338 to 26,000 infested nuts per year). However, due to estimated small likelihoods of transfer to a host, mating upon transfer and survival of founder populations, the number of populations that establish was estimated to be 0.000698 year-1 (median, 90% CR: 0.0000126-0.0364 year-1). Accordingly, the expected period between founding events is 1,430 years (median, 90% CR: 27.5-79,400 year). The likelihood of entry resulting in establishment is therefore considered very small. However, this estimate has high uncertainty, mainly concerning the processes of transfer of the insect to hosts and the establishment of founder populations by those that successfully transfer. Climate matching and CLIMEX modelling indicate that conditions are most suitable for establishment in the southern EU, especially around the Mediterranean basin. The median rate of natural spread was estimated to be 5.6 km/year (median, 90% CR 0.8-19.3 km/year), after an initial lag period of 3.1 year (mean, 90% CR 1.7-6.2 year) following the establishment of a founder population. If A. transitella did establish, estimated median yield losses in nuts were estimated to be in the order of 1-2% depending on the nut species and production system. A scenario requiring imports of nuts to be transported under chilled conditions was shown to provide potential to further reduce the likelihood of entry.

Pest mites and their interaction with Phytoseiidae and other arthropod predators in an almond orchard in South-West Spain

The almond crop in Spain has increased in importance in recent years and consequently there is a need to improve knowledge about pests, diseases, and weeds. The present study was conducted from 2017 to 2020, with the objective of determining the fauna of spider mites and their natural enemies, with a special emphasis on phytoseiids. The main spider mite species was Tetranychus urticae Koch, and secondary species were Bryobia rubrioculus (Scheuten) and Eutetranychus banksii (McGregor). Phytoseiidae were the most abundant group of natural enemies, with 59% of the individuals observed; Euseius stipulatus (Athias-Henriot) was the predominant species, accounting for 96% of adult females identified, Typhlodromus (Anthoseius) athenas Swirski & Ragusa accounted for the remaining 4%. Other (potential) natural enemies were Chrysopidae, Scolothrips longicornis Priesner, and Stethorus sp. with 36, 2, and 3%, respectively, of the natural enemy individuals. The seasonal pattern of T. urticae indicated population peaks from July to September, and its control was based on miticides in most seasons. Euseius stipulatus and T. athenas appeared mainly in May-June and did not show interaction with the spider mite population. Chrysopidae were present throughout the season, from May to October in the 4 years, but no direct relationship with the spider mite population was observed. In contrast, the seasonal pattern of both S. longicornis and Stethorus sp. coincided with the most important peaks of spider mites and these predators were seen in the spider mite colonies, although in very low numbers. The importance of these latter specialized spider mite predators and ways to strengthen them are discussed.

Diel Periodicity in Males of the Navel Orangeworm (Lepidoptera: Pyralidae) as Revealed by Automated Camera Traps

Navel orangeworm, Amyelois transitella (Walker), is a key pest of walnuts, pistachio, and almonds in California. Pheromone mating disruption using timed aerosol dispensers is an increasingly common management technique. Dispenser efficiency may be increased by timing releases with the active mating period of navel orangeworm. Past work found that the peak time of sexual activity for navel orangeworm females is 2 h before sunrise when temperatures are above 18°C. Inference of male responsiveness from data collected in that study was limited by the necessity of using laboratory-reared females as a source of sex pheromone emission to attract males and the inherent limitations of human observers for nocturnal events. Here we used camera traps baited with artificial pheromone to observe male navel orangeworm mating response in the field over two field seasons. Male response to synthetic pheromone exhibited diel patterns broadly similar to females, i.e., they were active for a brief period of 2-3 h before dawn under summer conditions and began responding to pheromone earlier and over a longer period of time during spring and fall. But contrary to the previous findings with females, some males were captured at all hours of the day and night, and there was no evidence of short-term change of pheromone responsiveness in response to temperature. Environmental effects on the response of navel orangeworm males to an artificial pheromone source differ in important ways from the environmental effects on female release of sex pheromone.

Winter Cover Crops Reduce Spring Emergence and Egg Deposition of Overwintering Navel Orangeworm (Lepidoptera: Pyralidae) in Almonds

Habitat diversification has been shown to positively influence a variety of ecosystem services to agriculture, including biological control of arthropod pests. The impact of increased biodiversity tends to be species specific though, and practices therefore need to be developed on a case-by-case basis for each cropping system. In perennial systems, numerous studies have demonstrated that cover crops can have positive impacts on soil quality and other ecosystem services, such as pollination and pest management. However, few studies have focused on the use of cover crops to enhance pest control in almond orchards, especially winter cover crops. The primary pest of almonds in North America is navel orangeworm, Amyelois transitella Walker, which overwinter as larva or pupa on remnant nuts, many of which remain on the orchard soil surface. In the spring, first flight adults subsequently use these remnant nuts as reproductive substrate. An experiment was conducted to evaluate the influence of two distinct winter cover crop mixtures on overwintering mortality and spring egg deposition of A. transitella. Remnant nuts placed into cover crop plots produced fewer adult A. transitella in the spring, suggesting increased overwintering mortality. Additionally, spring egg deposition was reduced on remnant nuts in the cover crops, possibly due to the ground covers interfering with host location and access. In this way, winter cover crops appear to contribute to the reduction of A. transitella populations in the orchard by altering abiotic and physical conditions, although studies to document specific mechanisms are still needed.

Individual and Additive Effects of Insecticide and Mating Disruption in Integrated Management of Navel Orangeworm in Almonds

Simple Summary

Mating disruption is an increasingly important part of pest management for the navel orangeworm Amyelois transitella. Industry groups have long supported mating disruption research and development with the divergent objectives of both minimizing damage from this key pest and reducing insecticide used on these crops. It is therefore important to know whether the benefits of mating disruption and insecticide are additive or, alternatively, if using both together provides no additional benefit over either alone. Ten years of data from research trials in a large commercial almond orchard found that the benefits of mating disruption are generally additive with lower damage if both are used together than either alone. Substantial year-to-year variability in navel orangeworm damage was also evident, even with stringent management. These findings indicate that the combination of mating disruption and insecticide can reduce the impact of navel orangeworm damage on the almond industry. Further improvements in monitoring and predictions of navel orangeworm abundance and damage are necessary for mating disruption to effectively contribute to the industry goal of reduction of insecticide use by 25%.

Abstract

Damage from Amyelois transitella, a key pest of almonds in California, is managed by destruction of overwintering hosts, timely harvest, and insecticides. Mating disruption has been an increasingly frequent addition to these management tools. Efficacy of mating disruption for control of navel orangeworm damage has been demonstrated in experiments that included control plots not treated with either mating disruption or insecticide. However, the navel orangeworm flies much farther than many orchard pests, so large plots of an expensive crop are required for such research. A large almond orchard was subdivided into replicate blocks of 96 to 224 ha and used to compare harvest damage from navel orangeworm in almonds treated with both mating disruption and insecticide, or with either alone. Regression of navel orangeworm damage in researcher-collected harvest samples from the interior and center of management blocks on damage in huller samples found good correlation for both and supported previous assumptions that huller samples underreport navel orangeworm damage. Blocks treated with both mating disruption and insecticide had lower damage than those treated with either alone in 9 of the 10 years examined. Use of insecticide had a stronger impact than doubling the dispenser rate from 2.5 to 5 per ha, and long-term comparisons of relative navel orangeworm damage to earlier- and later-harvested varieties revealed greater variation than previously demonstrated. These findings are an economically important confirmation of trade-offs in economic management of this critical pest. Additional monitoring tools and research tactics will be necessary to fulfill the potential of mating disruption to reduce insecticide use for navel orangeworm.