Seasonal Shifts in the Relative Resistance to Insect Attack of Eight Commercial Cabbage Varieties1

Abstract

Throughout the 1961 growing season at Kenosha, Wis., frequent evaluations were made of the relative resistance of 8 commercial cabbage varieties to oviposition and larval attack by the imported cabbageworm, Pieris rapae (L.), and the cabbage looper, Trichoplusia ni (Hubner) ; to larvae of the diamondback moth, Plutella maculipennis (Curtis) ; and to injury by the cabbage aphid, Brevicoryne brassicae (L.); Seasonal shifts were observed in the relative resistance of the varieties to the imported cabbageworm, the diamondback moth, and the cabbage aphid, but no significant shifts were found in relative resistance to the cabbage looper, which does not occur in appreciable numbers until the latter part of the season. The red cabbage varieties proved least susceptible to early-season oviposition by the imported cabbageworm and to host selection by the alate cabbage aphid, but their susceptibility to both these species increased as the season progressed; the red varieties apparently are more favorable hosts for both these insects than are the green varieties. The varieties least injured by first-generation populations of the imported cabbageworm tended to be relatively more susceptible to second-generation oviposition. Diamondback moth populations always were low, but varietal preferences became more pronounced as the season advanced. Two green varieties showed significant shifts in resistance to this species.