Reduced fecundity ofPsoroptes ovis(Hering) (Acari: Psoroptidae) on calves treated with ivermectin

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Abstract
Ivermectin injected subcutaneously into calves at the rate of 200 μ/kg body weight completely eliminated females ofPsoroptes ovis(Her.) in 22 days. Injection of ivermectin also adversely affected the number and hatch of eggs and the proportion of adult males forming attachment pairs. The reduced number of eggs recovered in skin scrapings was apparently the result of a significantly lower fecundity among females that temporarily survived on treated calves. Females that were exposed to ivermectin for only three days did not recover their full reproductive potential even though they were transferred to calves not injected with ivermectin. The proportion of ovigerous females among survivors was not significantly affected five days after injection. Since the number of eggs recovered in skin scrapings was substantially reduced after only three days, the effect of ivermectin on female fecundity was not primarily gonadal. Reduction in the number ofP. oviseggs three days after treatment probably occurred because of a combination of mortality of adult females and reduced oviposition by ovigerous females surviving in the population.