Crowding and a Plant's Ability to Defend Itself Against Herbivores and Diseases

Populations of spider mites were reduced on cotton seedlings that had been damaged as cotyledons by a brief bout of feeding by mites. However, this induced resistance was found only for seedlings grown in relatively uncrowded conditions. The strength of induced resistance against both spider mites and Verticillium wilt diminished as plant density increased. Results were qualitatively similar when differences in foliage quantity were factored out. This suggests that induced resistance in cotton does not result from a passive removal of plant tissue but is caused by an active response. Crowded plants may be unable to express the response to damage. If this result is general, then preventing induced resistance against herbivores and diseases may represent an important and overlooked consequence and mechanism of plant competition.