Plant Resistance to Insects in Cotton

Abstract

Entomologists and plant breeders have noted that glandless lines of cotton generally are susceptible to phytophagous insects. Gossypol, the yellow pigment present in the glands, has been shown to be the most important allelochemical that provides resistance. In field tests, high gossypol lines have repeatedly been correlated with lessened insect damage, probably due to toxicity of gossypol to Heliothis sp. and other cotton insects. Recent behavioral-histochemical studies of newly hatched larvae of tobacco budworm, Heliothis virescens (F), feeding on cotton showed that first-stage larvae avoided consuming the glands. However, when these molt between 48 and 72 h of age, they nonselectively consume the glands, suggesting a metabolic adaptation. Young tobacco budworm larvae prefer to feed along the margin area of the calyx crown of the square, an area devoid of gossypol glands. On resistant lines having glands in the calyx crown, the larvae feed sporadically on the tissue thus avoiding the glands. The numbers of gossypol glands on bracts of small squares, calyx crown, bract mid-rib, and the entire calyx differed significantly between the susceptible and resistant lines. First instar larvae placed on squares of cotton genotypes with high gland density in the small square bract and in the calyx crown, were significantly smaller than larvae placed on line with fewer glands. These and related studies suggest that breeding strategies should concentrate on placing gossypol glands in the calyx crown, the primary site of insect attack.