Induced responses in clover to an herbaceous mite.

Halotydeus destructor feeding on subterranean clover cotyledons can cause severe damage. The mites live on the soil surface and move up onto plants to feed. Foraging behaviour consists of palpating, probing, and feeding with frequent transitions between them. Sustained feeding is made up of a series of short (1-2 min) feeds separated by periods of palpating. The mites tend to feed in aggregations, and are attracted to cotyledons damaged by other mites feeding or by mechanical damage. Mites can distinguish between resistant and susceptible cotyledons within 30 min and resistance is antixenotic due to deterrence. Study of the mechanisms shows this to be induced plant resistance. Several green leaf volatiles are involved in the plant/mite interaction. After feeding commences, 2-E-hexenal is released that at low concentrations is attractive to mites, perhaps causing the feeding aggregations. The wound-induced C(8) compound, 1-octen-3-one, plays a significant role in the deterrence of cotyledons of resistant subterranean clover varieties to H. destructor. Damaged cotyledons of resistant varieties produce more 1-octen-3-one that those of susceptible varieties. Screening for resistance has identified varieties from Italy showing resistance. H. destructor does not occur in Europe. Production of damage-induced volatiles by the resistant plants may have resulted from invasion by herbivores or pathogens, but not from coevolution with these mites. The responses of H. destructor are probably an adaptation to these general plant defensive compounds.