ECONOMIC THRESHOLD FOR MANAGEMENT OF LEPIDOPTEROUS LARVAE ON BROCCOLIIN SOUTHEASTERN NEW-BRUNSWICK

Economic thresholds for the management of three lepidopterous broccoli pests, Plutella xylostella (L.), Pieris rapae (L.), and Trichoplusia ni (Hubner), were evaluated at Bouctouche, New Brunswick, during the s ummers of 1990 and 1991 on three different planting dates. Six treatme nts were tested for each planting: an untreated control, a biweekly ap plication of insecticide after insects appeared, a biweekly applicatio n of insecticide after heading, and an insecticide application when th resholds of 0.25, 0.50, or 1.0 cabbage looper equivalent per plant per week were reached. Insecticides used were Bacillus thuringiensis var. kurstaki Berliner and permethrin. Broccoli heads harvested from the 0 .25-threshold treatment of the first planting were significantly heavi er than heads from the other treatments of that planting. No significa nt differences in head weight of plants were observed from treatments of either the second or the third planting. There were no significant differences for any of the plantings in weight, diameter, and marketab ility of heads harvested from the biweekly treatment after insects app eared and those harvested from the biweekly treatment after heading. I n 1991, professional scout revenues for the biweekly after heading tre atment were $34.75 per ha greater than those of the 1.0 cabbage looper equivalent threshold treatment. With the monitoring done by the growe r, revenues were $6.07 per ha greater for the 1.0 cabbage looper equiv alent threshold treatment than for the biweekly treatment after headin g.