The Relative Resistance to Insect Attack of Three Cabbage Varieties at Different Stages of Plant Maturity1

Abstract

In field observations at Kenosha, Wis., in 1962, the cabbage varieties Racine Market, Red Acre, and Flat Dutch were planted at 5 different dates 20 days apart. One row of each plot in the first planting, given regular applications of insecticide, served as a control protected from insects. Three times during the season, resistance ratings of the varieties were made, to egg deposition and larval attack by the imported cabbageworm, Pieris rapae (L.), and the cabbage looper, Trichoplusia ni (Hubner), and to larvae of the diamondback moth, Plutella maculipennis (Curtis). These evaluations were synchronized with peak occurrence of first- and second-generation oviposition and peak first-generation larval populations of the imported cabbage worm. In a given count, the relative resistance of the 3 cabbage varieties to each of the insect species was virtually identical in all the plantings. The actual insect populations supported by the plants of any 1 variety varied greatly with plant maturity. The species composition of the lepidopterous complex supported by the different varieties similarly varied in the different plantings. The insecticide-protected plants of all varieties were twice as susceptible to late-season oviposition by the imported cabbageworm and the cabbage looper as those unprotected from prior insect infestation. These observations indicate that seasonal shifts in the relative resistance of the cabbage varieties result from prior insect injury rather than from physiological changes in the plants.