Change in behavioral response to herbivore-induced plant volatiles in a predatory mite population

Damage by herbivorous spider mites induces plants to produce volatiles that attract predatory mites that consume the spider mites. A clear attraction to volatiles from Lima bean plants infested with the spider mite Tetranychu s urticae has been consistently reported during more than 15 years for the predatory mite Phytoseiulus persimilis. We have monitored the response to v olatiles from spider-mite infested Lima bran plants for a laboratory popula tion of the predatory mite from 1991 to 1995 on a regular basis. A reductio n in the level of attraction in the laboratory population of P. persimilis was recorded in mid-1992. The attraction of the laboratory population was w eaker than that of a commercial population in the latter part of 1992, but the responses of these two populations were similarly weak in 1994 and 1995 . Therefore, a behavioral change has also occurred in this commercial popul ation. Experiments were carried out to address the potential causes of this change in attraction. The attraction of predators from a commercial popula tion with a strong response decreased after being reared in our laboratory. Within a predator population with a low degree of attraction, strongly res ponding predators were present and they could be isolated on the basis of t heir behavior: predators that stayed on spider-mite infested plants in the rearing set-up had a strong attraction, while predators that had dispersed from the rearing set-up were not attracted to prey-infested bean plants. Fr om our laboratory population with a low degree of attraction, isofemale lin es were initiated and maintained for more than 20 generations. All isofemal e lines exhibited a consistently strong attraction to spider mite-induced p lant volatiles, similar to the attraction recorded for several populations in the past 15 years. Neither in a population with a strong attraction nor in two with a weak attraction was the response of the predators affected by a starvation period of 1-3 hr. Based on these results, possible causes for the observed reduction in predator attraction to spider mite-induced bran volatiles are discussed. The predatory mite P. persimilis is a cornerstone of biological control in many crops worldwide. Therefore. the change in for aging behavior recorded in this predator may have serious consequences for biological control of spider mites.