Effect of a Parasitic Nematode,Chondronema passaliLeidy (Incertae sedis), on the Size and Strength of the Horned Passalus,Odontotaenius disjunctusIlliger (Coleoptera: Passalidae)

Abstract

The horned passalus, Odontotaenius disjunctus (Illiger), is host to a variety of parasites, including a little-studied nematode, Chondronema passali (Leidy), that can number in the thousands in a single beetle. We attempted to determine the effects of this parasite on two measures of host fitness, physical strength and body size of adult beetles collected from hardwood logs at two sites in Georgia, USA. For a subset of the beetles, we measured their individual pulling strength using a dynamometer and data-logging apparatus. We also recorded the live weight of all beetles and from another subset a composite measure of body size based on digital images. Beetles were dissected and nematode infection status was assessed for all beetles. Of all beetles examined in this study (49 females, 44 males), we detected C. passali in 63 (67.7%) individuals. Prevalence in males did not differ from females. Infections ranged from 10 to over 1,000 individual nematode larvae per beetle. There was no significant effect of infection on maximum pulling force after accounting for body size and gender. Beetles with nematode infections weighed significantly more than those without nematodes and were significantly larger in body size. These results suggest the effects of this parasite are minimal to the host, and infections may even confer an advantage via the increase in size. Based on a review of the relevant literature, this appears to be a rare phenomenon among the many insect-nematode parasite relationships.