SEX-RATIO OF HEMISARCOPTES COCCOPHAGUS, A MITE PARASITIC ON INSECTS -DENSITY-DEPENDENT PROCESSES

Hemisarcoptes coccophagus Meyer (Acari: Astigmata: Hemisarcoptidae) is an obligate parasite of armoured scale insects (Homoptera: Diaspidida e). We tested the following hypotheses: (1) the sex ratio of H. coccop hagus is affected by its population density; (2) this effect is due to an asymmetric performance and dispersal of males and females at diffe rent density levels; (3) the effect of density on sex ratio depends on the developmental stage of the mile exposed to it. Sex ratio (proport ion of females) of H. coccophagus adults in the field fluctuated betwe en male and female biases and was negatively correlated with mite dens ity. Laboratory experiments showed that during immature development mi te survival and sex ratio decreased with increasing density, indicatin g that at high density levels immature females suffered higher mortali ty than males. At high density levels females, but not males, had smal ler bodies and forelegs (which, according to the host-quality model, m ay affect the sex allocation decision of reproducing females). Longevi ty of adult males at low density was higher than that of females, whil e at high density, and while starving, females lived longer. Adult dis persal was affected by density: both sexes stayed on their hosts when density was low, while at high density levels females dispersed more t han males. We conclude that population density may cause fluctuations in the sex ratio of H. coccophagus in either male or female directions , depending on the density level and on the mite stages exposed to its influence. The operative function underlying the effect of population density on the sex ratio might be intraspecific competition.