Evaluation of reflective mulch and insect exclusion coverings for allium leafminer (Diptera: Agromyzidae) management in allium crops.

Allium leafminer (Phytomyza gymnostoma Loew) is a recent invasive pest in the United States causing serious economic loss in organic allium crops. Organic management of P. gymnostoma is currently limited to foliar applications of spinosad, but this strategy is not always sufficient under high infestations. Nonchemical management tools used either alone or in combination with spinosad are needed to improve P. gymnostoma management. Reflective mulch alone or combined with spinosad as well as insect exclusion coverings were evaluated for managing P. gymnostoma in allium crops in New York from 2018 to 2021. Reflective mulch alone reduced the numbers of oviposition marks by 16% and densities of larvae plus pupae by 40% compared with those in standard plastic mulch. Reflective mulch combined with 1 spinosad application reduced P. gymnostoma densities to levels lower than those in reflective mulch alone, but 2 spinosad applications were required to provide an acceptable control level. Combining with reflective mulch, row covers, and insect netting reduced P. gymnostoma densities by 76% compared with those without physical barriers, and the level of control was comparable to that provided by 2 spinosad applications. Phytomyza gymnostoma densities in allium crops grown under row covers deployed throughout the entire period when flies were active tended to be lower than those protected during shorter periods (80% reduction). Future management of P. gymnostoma in allium crops should consider either combining reflective mulch with 2 foliar spinosad applications or deploying insect exclusion coverings. The advantages and challenges of using these management strategies are discussed.

Optimizing Spinosyn Insecticide Applications for Allium Leafminer (Diptera: Agromyzidae) Management in Allium Crops.

Allium leafminer, Phytomyza gymnostoma Loew (Diptera: Agromyzidae), is an invasive pest of allium crops in North America. Spinosyn insecticides, spinetoram and spinosad, have been effective choices for managing P. gymnostoma infestations in allium crops, but their use should be optimized for economical and resistance management purposes. In New York from 2018 through 2020, performance of each spinosyn insecticide was evaluated by making two applications spaced either 1 or 2 wk apart beginning at various intervals after P. gymnostoma was first detected in the field; a weekly spray program also was included. Results indicated that weekly applications of each spinosyn insecticide provided ≥98% reduction of P. gymnostoma densities in scallions and leeks relative to the untreated control. Spinetoram applied twice, regardless of initial timing and duration between sprays, provided an acceptable level of P. gynostoma control (71 to 98% reduction in densities relative to the untreated control). Spinosad also was effective when applied twice (85 to 95% reduction in densities relative to the untreated control), but not when sprays were made consecutively beginning when P. gymnostoma was first detected and not when the P. gymnostoma infestation was extremely high (i.e., 38 insects per plant in the untreated control). Management of P. gymnostoma with spinosyns can be successful with only two applications, but control tended to be best when first applied 2 to 3 wk after initial detection. Optimizing applications of spinetoram and spinosad will save growers time, reduce insecticide costs, and mitigate resistance development without significantly increasing the risk of yield reduction.

Managing Allium Leafminer (Diptera: Agromyzidae): An Emerging Pest of Allium Crops in North America.

Allium leafminer, Phytomyza gymnostoma Loew, is the newest invasive pest of allium crops in North America. Larvae initially feed in the upper canopy before mining toward the base of the plant to pupate. Crop loss occurs when larvae destroy vascular tissue, facilitating infection by bacterial and fungal pathogens that cause rot. Contamination also occurs when larvae and pupae are present at harvest. In response to this invasion, efficacy of 14 insecticide active ingredients applied via foliar sprays, transplant treatments, and drip chemigation was evaluated for managing P. gymnostoma. Multiple field studies were conducted in onions, leeks, and scallions in Pennsylvania and New York, United States in 2018 and 2019. The highest and most consistent levels of P. gymnostoma control occurred using foliar applications of dinotefuran, cyantraniliprole and spinetoram (84-89% reduction in damage; 95% reduction in P. gymnostoma densities). Despite the success of dinotefuran and cyantraniliprole applied as foliar sprays, neither was effective in controlling P. gymnostoma when administered via drip chemigation. Other foliar-applied insecticides that significantly reduced densities of P. gymnostoma in one or two experiments included abamectin, acetamiprid, cyromazine, imidacloprid, lambda-cyhalothrin, methomyl, and spinosad. Active ingredients that never controlled P. gymnostoma included azadirachtin, kaolin clay, pyrethrin, and spirotetramat. Spinosad applied to bare-root and plug-tray transplants immediately before transplanting reduced P. gymnostoma damage in the field by >90%. Implications of using these insecticides and application strategies are discussed within the context of developing a sustainable IPM program.