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.