Geographic parthenogenesis has been explained as resulting from parasite pressure (Red Queen hypothesis): several studies have found high degrees of sexuals where the prevalence of parasites is high. However, it is important to address whether prevalence of parasites mirrors risk of infection. We explored geographic parthenogenesis of Ips acuminatus bark beetles and their nematodes. Local climate is crucial for nematode stages outside the host, in spring and summer, and prevalence should thus be associated with those temperatures if prevalence reliably reflects exposure risk across populations. This was the case; however, high prevalence of a virulent nematode species was not associated with many sexuals, whereas highly sexual populations were characterized by high infection risk of benign nematodes. Low virulence of the latter makes Red Queen dynamics unlikely. Geographical patterns of parthenogenesis were instead associated with winter temperature and variance in temperature.

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Previous research has left confusion about central features of the mating system of the common ash bark beetle Hylesinus varius; in particular, reports conflict as to whether this species is monogynous or bigynous. It was found that females initiated egg tunnel construction; copulation takes place either on the bark in the initial phase of egg tunnel construction, or inside the tunnel system. Observations of one male copulating with several females and of solitary females in galleries with eggs suggested that the males sometimes leave after copulation. In contrast to findings for populations in Slovakia, Norwegian male H. varius were most commonly paired with only one female. Bigynous galleries contained significantly more eggs than monogynous galleries, but bigynous males did not double their reproductive success as found in some other bigynous bark beetles. This suggests that bigynous galleries at first are the work of one female but some males are later joined by a second female.