Economic threshold for management of lepidopterous larvae on broccoli in 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 summers of 1990 and 1991 on three different planting dates. Six treatments were tested for each planting; an untreated control, a biweekly application of insecticide after insects appeared, a biweekly application of insecticide after heading, and an insecticide application when thresholds 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 heavier than heads from the other treatments of that planting. No significant 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 marketability of heads harvested from the biweekly treatment after insects appeared and those harvested from the biweekly treatment after heading. In 1991, professional scout revenues for the biweekly after heading treatment were $34.75 per ha greater than those of the 1.0 cabbage looper equivalent threshold treatment. With the monitoring done by the grower, revenues were $6.07 per ha greater for the 1.0 cabbage looper equivalent threshold treatment than for the biweekly treatment after heading