Yield Responses in Lettuce Plants at Various Density Treatment Levels of Lepidopterous Larvae

Abstract

The impact of various densities of lepidopterous larval populations on lettuce between thinning and head formation stages was investigated. Populations of lepidopterous larvae were maintained below preestablished density treatment levels of 0, 0.5, 1, 2, and 5 larvae per plant on four lelluce plantings in California from 1977 to 1979. The predominant lepidopterous larval pests in these lettuce plantings were Trichoplusia ni (Hübner), Spodoptera exigua (Hübner), Hefiothis virescens (F.), and H. zea (Boddie). Total larval populations were highest in the 1977 Imperial County plantings, where they reached a level of ca. 10 larvae per plant, and were the lowest in the 1978 Imperial County planting, where they reached a level of ca. 1.3 larvae per plant. It was found that maximum yields were obtained when insecticide applications were initiated at 0.5 and 1 larva per plant in the period between thinning and head formation. Also, after the lelluce head is formed, it must be kept insect free to insure high-quality, marketable produce at harvest. Thus, pesticide applications may be reduced after thinning and before heading without any reduction in the marketable product.