Host-induced assortative mating in host races of the larch budmoth

The likelihood of sympatric speciation is enhanced when assortative mating is a by-product of adaptation to different habitats. Pleiotropy of this kin d is recognized as important in parasites that use their hosts as a long-ra nge cue for finding mates, but is generally assumed to have limited applica bility for most other organisms. In the larch budmoth, Zeiraphera diniana ( Lepidoptera: Tortricidae), sympatric host races feed on larch or pine. Zeir aphera diniana females attract males (call) by releasing host-independent l ong-range pheromones. Pheromone composition differs strongly between host r aces, but we show in an experimental field study that cross-attraction call occur at a rate of 0.03-0.38. Cross-attraction to larch females increases when they call from neighborhoods (8-m radius) rich in pine or from pine tr ees. Cross-attraction to pine females similarly increases when calling from neighborhood,, rich in larch, but there is no significant effect of callin g substrate. Males, as well as females, of this species preferentially alig ht on their own host, and in neighborhoods where their own host is common. This effect of tree species and host neighborhood on assortative mating is therefore due, at least in part, to the numbers of males of each host race present within approximately 200 m(2) surrounding the female. This proximit y effect is enhanced by the clumped distributions of the hosts themselves. Host chemistry might also affect pheromone production and/or response direc tly, but we have evidence neither for nor against this. This work provides empirical evidence that host adaptation has a pleiotropic effect on assorta tive mating in a species with host-independent long-range mating signals. S ympatric speciation via pleiotropy between ecological traits and assortativ e mating may thus be more common than generally supposed: Clumped resource distributions and habitat choice by adults are widespread.