Using permethrin to reduce mosquito numbers and improve health of nestling Barn Owls

Mosquitoes can negatively impact nestling birds by causing blood loss, irritation, and transmission of diseases. Reducing mosquito numbers could increase the health and fledging success of nestling birds. The objectives of this study were 1) test if an insecticide, permethrin, could reduce numbers of mosquitoes entering these boxes to feed on nestlings and 2) assess the effects of hematophagy on the health status of nestling Barn Owls by analyzing blood samples. We used mosquito nest traps fixed to the outside of nest boxes to collect mosquitoes over a 24 hour period once weekly, from incubation until all nestlings fledged.  Blood samples were taken from each nestling at 25 days of age to evaluate their health by quantifying total white blood count, heterophil:lymphocyte ratio, plasma proteins and corticosterone levels. Culex nigripalpus was the predominant species collected and up to 2,100 mosquitoes were collected overnight from a single nest trap. Survivorship between nestlings from treated and untreated nests was only significantly different for nestlings that hatched in September and October, months when the highest numbers of mosquitoes were collected. Additionally, the plasma protein, gamma, was higher in nestlings from untreated nests during this period. Changes in white cell counts and plasma proteins were also noted. In addition to addressing questions regarding mosquito attack rates on nestling Barn Owls, this study also provides valuable information regarding barn owl clinical pathology associated with mosquito feeding pressure.  Our results may indicate that the use of a permethrin insecticide could reduce the number of mosquitoes attempting to feed on nestling birds.