Mountain Plover responses to deltamethrin treatments on prairie dog colonies in Montana.

Pyrethroid insecticides containing deltamethrin provide broad spectrum insect control that can adversely affect food supplies of insectivorous birds. I hypothesized that this could result in lowered nest survival for a ground-nesting insectivorous bird, the Mountain Plover (Charadrius montanus), which preferentially nests on prairie dog colonies. I studied Mountain Plover nest survival in 2003-2010 at a small cluster of black-tailed prairie dog (Cynomys ludovicianus) colonies in north-central Montana. Three colonies were treated with deltamethrin to control fleas and limit the spread of plague; four untreated colonies served as controls. I monitored 412 plover nests during the 8 year study (264 on treatment colonies and 148 on control colonies) and found a strong negative effect of deltamethrin treatments on nest survival (β(Dust) = -1.24, 95 % CI was -2.00 to -0.48) in the years following the actual treatment (2004-2006). I conclude that the observed treatment effect most likely occurred because of changes in insect (food) availability for the plover, and this in turn lowered nest survival because adults spent more time off nests or switched to less desirable insect prey. These results lend support to the need to consider the indirect effects of insecticide treatments on non-target species and suggest a potential conflict in current plague management strategies for prairie dogs.