SEASONAL OCCURRENCE AND INFESTATION POTENTIAL OF CABBAGE MAGGOT, DELIA-RADICUM (L) (DIPTERA, ANTHOMYIIDAE), ATTACKING RUTABAGA IN MANITOBA AS DETERMINED BY CAPTURES OF FEMALES IN WATER TRAPS

The seasonal activity and infestation potential of the cabbage maggot, Delia radicum (L.) (Diptera:Anthomyiidae), attacking rutabaga was studied at two Manitoba locations over 4 years based on captures of females in yellow water traps baited with allyl isothiocyanate. The activity peak for the overwintering generation occurred on 8 June .+-. 4.1 days (mean .+-. SD), with degree-day accumulations, base 5.degree.C, of 159 .+-. 6.2 for soil at 5 cm and 387 .+-. 69.4 for air for both locations combined. The activity peak for the first generation occurred on 12 August .+-. 11.7 days with degree-day accumulations of 981 .+-. 229.3 and 1295 .+-. 175.7 for soil and air temperatures, respectively. Correlations between numbers of females trapped per week and of eggs in soil samples from around rutabaga plants for individual seasons at the two locations generally were not significant. A regression of numbers of eggs on females trapped based on all data was significant but had a low coefficient of determination (r2 = 0.289). The water traps are adequate for monitoring seasonal activity of adult flies but the captures are unreliable as quantitative indicators of potential infestation.