REPRESSION OF ARTOGEIA-RAPAE (L) (LEPIDOPTERA, PIERIDAE) AND PLUTELLA-XYLOSTELLA (L) (LEPIDOPTERA, YPONOMEUTIDAE) ON FRESH-MARKET AND PROCESSING CABBAGE, USING COMPOSITE ACTION THRESHOLDS FOR CHEMICAL AND BIOLOGICAL-CONTROL

Seasonal population fluctuations of the imported cabbage worm Artogeia rapae (L.) and the diamondback moth Plutella xylostella (L.) were stu died in southern Quebec (Canada) from 1984-1988, in nontreated check, permethrin and granulosis virus treated cabbage-plots. Timing of perme thrin treatment based on an action threshold, equal to 57% of plants i nfested with larvae of either two species of the caterpillars, produce d quality heads corresponding to the standards for storage or fresh ma rket. This threshold approximate one larva of A. rapae or P. xylostell a per plant. A threshold of 87% of infestation (or about 3 larvae of A . rapae and 3.5 larvae of P. xylostella per plant) yielded cabbages ma rketable for processing. From one to three insecticide treatments a ye ar were required with the 57% threshold, and one to two with the highe r threshold. The granulosis virus of A. rapae (ArGV) was very effectiv e to control this pest. The proportion of cabbages marketable for proc essing was not statistically different between ArGV and weekly permeth rin-treated plots. As this virus is ineffective against P. xylostella, for fresh market in which cosmetic standards are more important, a mi xture of ArGV and a half dose of permethrin yielded a high marketabili ty crop.