Sexually transmitted parasites and sexual selection in the milkweed leaf beetle, Labidomera clivicollis

Milkweed leaf beetles (MLBs; Labidomera clivicollis Chrysomelidae) are para sitized by a subelytral mite, Chrysomelobia labidomerae (Tarsonemina: Podap olipidae). We show that C. labidomerae is transmitted between MLBs when the y copulate, and can reduce the survival of nutritionally stressed beetles. We investigated the effect of this sexually transmitted parasite on mate ch oice and male-male competition in MLBs, and the consequences of variation i n these behaviours for mite transmission. We found no evidence of parasite avoidance by MLBs, and evidence for high rates of parasitism in the MLB pop ulations we surveyed. In the absence of females, parasitized males contacte d unparasitized males more often and for longer than controls, and they ten ded to displace rival males from females more often than did unparasitized males, a result consistent with the interpretation that parasitized beetles compensate for loss of fitness by increasing reproductive effort. These ch anges can also benefit mites, because longer and possibly more contacts bet ween beetles provide more opportunities for transmission, but there is no e vidence that these changes in male behaviour result from parasite manipulat ion.