Sex-biased parasitism of adult spring beetles, Heteronyx dimidiata and Heteronyx crinitus, by Tachinidae in Eucalyptus nitens plantations in Australia

Parasitism of adult <i>Heteronyx dimidiata</i> (Erichson) and <i>H. crinitus</i> Blackburn (Scarabaeidae: Melolonthinae), that attack <i>Eucalyptus nitens</i> plantation seedlings in southern Australia, was recorded over 3 years. An undescribed genus of Blondeliini (Tachinidae: Exoristinae) was reared from both scarab species while a <i>Palpostoma</i> sp. (Tachiinae) was reared from <i>H. crinitus</i>. Flies were observed parasitising flying beetles by intercepting them mid-air. Eggs were laid externally on the exoskeleton, mainly on the dorsum of abdominal tergites. Male <i>H. dimidiata</i> were parasitised significantly more (40%) than female conspecifics (13%) and male or female <i>H. crinitus</i> (both 13%), which was attributed to their greater flight activity searching for mates. Superparasitism was common in both species, particularly in male <i>H. dimidiata</i> (56% of parasitised beetles, up to 22 eggs/beetle), despite evidence that only a single tachinid could complete development in a beetle. Early stages of parasitism in <i>H. dimidiata</i> females did not appear to affect ovarian development nor the ability of beetles to feed on young trees. However, we infer that the tachinids had a significant impact on <i>Heteronyx</i> spp. populations through a reduction in adult beetle longevity and fitness.