Impact of decreasing ratios of insecticide-treated seed on flea beetle (Coleoptera: Chrysomelidae, Phyllotreta spp.) feeding levels and canola seed yields

Abstract

Field studies were conducted at two locations on the Canadian prairies to investigate use of reduced ratios of insecticide-treated seed in controlling flea beetle (Coleoptera: Chrysomelidae, Phyllotreta spp.) damage to canola (Brassica napus L. and Brassica rapa L.). Five treatments were evaluated: bare seed control, fungicide-only (OX), and three ratios of insecticide plus fungicide in proportions of all (IX), two thirds (0.67X), or one third (0.33X) of the seeds coated with insecticide. Decreasing treated seed ratios by one third had no consistent deleterious effects on flea beetle damage, seedling growth, plant density, seed yield, or net cash return. Flea beetle injury to seedlings in the IX treatment was similar to that of seedlings in the 0.67X treatment, with only two exceptions, and it was almost always lower than that of seedlings without insecticide. The 0.33X treatment generally had flea beetle feeding levels between those of the two high and the two noninsecticide treatments. Plant stand and seedling growth rates with IX and 0.67X treatments were similar and higher than with bare seed or fungicide-alone treatments. Seed yields were inversely proportional to flea beetle feeding levels. Under very heavy flea beetle feeding, seed yields and net cash returns were highest in IX plots, but when flea beetle feeding pressure was less extreme and canola growing conditions were favorable, 0.67X seed yields and profits from them were comparable to those in IX treatments. On an economic basis, currently there is no advantage to decreasing the level of insecticide treated canola seed, but other considerations may affect this assessment.