The effect of seeding date and plant density on infestations of root maggots, Delia spp, (Diptera: Anthomyiidae), in canola

Five seeding dates and four plant densities were evaluated in 1991 and 1992 for reducing infestations of root maggots, Delta radicum (L.) and Delta floralis (Fallen) (Diptera: Anthomyiidae), in both species of canola (Brassica rapa L. and Brassica napus L.), under field conditions in central Alberta, Canada. Infestations were assessed by numbers of eggs laid throughout the season and by degree of damage to taproots at the end of the season. Root maggot infestations were reduced by seeding in late May rather than in early or mid-May, but because seed yield also declined with late-May plantings, delaying canola seeding is not an appropriate cultural control strategy. Increasing plant density, however, resulted in reduced infestations and increased seed yield and so is considered an acceptable control practice. Increases in plant density caused corresponding and statistically significant decreases in basal stem diameter of canola. Moreover, increases in basal stem diameter resulted in corresponding and significant increases in Delia spp. egg populations and damage to taproots from larval feeding. These data suggest that ovipositing females of Delia spp. select larger canola plants, i.e., those seeded earlier or at lower densities, and these are consequently damaged to a greater degree than smaller plants.