Economic Injury Levels for Flea Beetles (Phyllotreta spp.; Coleoptera: Chrysomelidae) in Spring Oilseed Rape (Brassica napus; Brassicales: Brassicaceae)

Abstract

Abstract Flea beetles (Phyllotreta spp.) are major insect pests in spring oilseed rape (SOSR; Brassica napus L.). Prohibited use of three neonicotinoid insecticides in the European Union means that there are currently no insecticide seed treatment options available. Insecticide spraying remains as a control option, but there is a need to estimate the economic threshold for crop injury. As a first step to this end, economic injury levels were determined for flea beetles in SOSR. Data from 16 field experiments were used to quantify the relationship between flea beetle crop injury and SOSR yield, and additional data from paired sprayed and unsprayed plots in 12 commercial SOSR fields were used to determine the reduction in crop injury from a pyrethroid spray. There was a strong linear negative effect of flea beetle injury with 19 kg/ha yield loss per percent crop injury to seedlings and a pyrethroid spray reduced crop injury by 39%. These results gave an economic injury level of 11% defoliation of SOSR seedlings under average oilseed rape prices and insecticide use costs in 2017. This is considerably lower than previously used nominal thresholds of 25–30% injury to cotyledons. Increased yields and increasingly cheaper pyrethroids might be the reason for the lower levels of crop injury that warrant chemical control. The economic injury levels presented here can be used to construct economic thresholds that preferably should also take into account crop growth stage, crop growth rate, and anticipated flea beetle activity.