Effects of a Carbaryl Bait Treatment on Nontarget Wildlife

Abstract

We studied the effect on nontarget birds, mammals, and insects of 2% carbaryl bran bait applied for grasshopper control in western North Dakota. Total bird density did not differ 2, 10, or 21 d or 1 yr after application. Though carbaryl is an acetylcholinesterase (AChE) inhibiting chemical, we found no evidence of depressed brain AChE in birds or small mammals collected from the treated area. We sampled arthropod biomass on the treated and untreated areas with pitfall traps and stickyboards. Grasshopper densities were estimated with ring counts. Biomass of Coleoptera declined on the treated relative to the untreated area 2 and 10 d and 1 yr after treatment. Biomass of ants (Formicidae) and arthropods consumed by birds declined 2 d after treatment but recovered during later sampling periods. Grasshopper numbers declined on the treated area between the before and after 2-d count and remained unchanged during later counts. We found no evidence of a decline in Halictid bees, which are the primary pollinators of some native plant species, but our inferences regarding changes in arthropod numbers are limited because we did not identify insects to species. These results indicate that low rate (1.68 kg/ha) carbaryl bait applications have minimal potential for direct toxic effects on birds and mammals but may have limited indirect effects on species that depend on certain arthropod groups for food or seed dispersal. It has been suggested that bait applications are less hazardous to wildlife than liquid sprays because there is less exposure to nontarget organisms. Our results generally support this hypothesis, but there is potential for indirect effects on some species.