Response of Pediculus humanus humanus (Pediculidae: Phthiraptera) to water or 70% ethanol immersion and determination of optimal times for measuring toxic effects.

Human pediculosis is caused by Pediculus humanus humanus (Linnaeus 1758) and Pediculus humanus capitis (De Geer 1767). We studied the response of body lice to immersion in water and ethanol 70% and determined the optimal times for measuring knockdown and mortality. After immersion in water, all lice remained alive from 5 min to 22 h for both times of exposure. A low proportion of lice were affected after 2 min of immersion in ethanol in the 10-min exposure test, but recovered completely after 5 min. Different proportions of lice were affected between 2 and 7 h after immersion in ethanol, depending on the immersion time. However, a high proportion of lice recovered after 22 h. The results suggest that the optimal times for measuring early knockdown effects of insecticides are the 5-min to 7-h interval for water and 5-min to 1-h interval for ethanol. On the other hand, the best time for measuring mortality is 22 h after immersion. These results should improve the interpretations of the effects of pediculicides in immersion bioassays.