Yield Responses in Broccoli Plantings Sprayed with Bacillus thuringiensis at Various Lepidopterous Larval Density Treatment Levels

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Abstract
The impact of various levels of lepidopterous larval populations on broccoli prior to heading was investigated. Dipel® (Bacillus thuringiensis Berliner var. kerstaki) was applied to 4 consecutive broccoli plantings in southern California to maintain populations of lepidopterous larvae below preestablished levels of 0–1, 2, and 5 larvae/plant. Trichoplusia ni (Hübner) was the predominant lepidopterous larval pest in spring, summer, and fall plantings. Plutella xylostella (L.) and Pieris rapae (L.) were present at low levels in all plantings and Spodoptera exigua (Hübner) occurred only in spring and fall crops. Total larval populations were highest in summer and fall plantings, reaching levels of 4.6 and 9.1 larvae/plant, respectively, in untreated plots. Yields in any planting did not differ between untreated controls and plots treated to maintain larval populations at preestablished levels. Levels of 1st-instar T. ni on plants in summer and fall plantings were independent of Dipel application schedules. The necessity for treating broccoli for lepidopterous larval infestation prior to head formation is discussed as an adjunct to pest management on this crop.

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