EFFECTS OF ECTOPARASITISM ON HOST CONDITION IN THEDROSOPHILA–MACROCHELESSYSTEM

Abstract

The effect of ectoparasitism by Macrocheles subbadius, a cosmopolitan mesostigmatid mite, on testis and somatic condition (expressed as tissue dry mass per unit body length) of its natural host Drosophila nigrospiracula was evaluated experimentally. Male flies were infested 5 h posteclosion, and the condition of their testes and soma was determined following 4 and 6 d, i.e., at the age males of this species reach sexual maturity at 25°C. Mite burdens were within the range encountered in the field. Both testis and body condition were significantly reduced among infested males compared with control, uninfested males. Moreover, both measures of host condition decreased progressively with mite burden, demonstrating a dose-dependent effect of ectoparasitism. The slope of the function between ectoparasitism and testis condition was significantly steeper in males that had carried mites for 6 d as opposed to 4 d, demonstrating that the strength of the effect of ectoparasitism on testis condition depended also on the duration of infestation. A field study revealed negative relationships between mite burden and both body and testis condition, but only the relationship with body condition was statistically significant. Although thorax length (estimate of overall body length) was strongly positively correlated with dry testes and body mass, the interaction between thorax length and mite burden was not significant in regression analyses, in which either testis condition or body condition served as dependent variables. This lack of significant interaction effects indicates that larger males were not better able to buffer the physiological cost of ectoparasitism than smaller individuals. Testis volume and testis width of infested males were also found to be significantly reduced relative to uninfested individuals, suggesting that infested males produce and store fewer sperm than uninfested males, and that infestation may reduce a male’s effectiveness in sperm competition.