Effects of fall and spring seeding date and other agronomic factors on infestations of root maggots, Delia spp. (Diptera: Anthomyiidae), in canola.

Several agronomic benefits can result from fall seeding of canola (Brassica spp.), but extensive research data are lacking on the potential impact of this practice on infestations of root maggots (Delia spp.) (Diptera: Anthomyiidae), which are major pests of the crop in western Canada. Field experiments making up 13 location by year combinations were conducted in central Alberta, Canada, from 1998 to 2001 to determine the effect of fall versus spring seeding of canola on root maggot damage. Depending on the experiment, interactions with seeding rate, seed treatment, timing of weed removal, and canola species (cultivar) also were investigated. Root maggot damage declined with an increase in seeding rate for plots seeded in May but not in fall or April. Susceptibility to infestation was greater for plants of Brassica rapa L. than Brassica napus L., but seed treatment had no effect on damage by these pests. Combined analysis using data from all experiment by location by year combinations indicated that seeding date had no significant effect on root maggot damage. The extended emergence of Delia spp. adults, which spans the appearance of crop stages vulnerable to oviposition regardless of seeding date, prevented reduced root maggot attack. Covariance analysis demonstrated the importance of increasing seeding rate for reducing root maggot infestations, a practice that can be especially beneficial for May-seeded canola when growing conditions limit the ability of plants to compensate for root maggot damage. Results determined with the small plot studies described here should be validated in larger plots or on a commercial field scale, but both the combined and covariance analyses indicate that seeding canola in fall does not predispose plants to greater damage by larval root maggots than seeding in spring.