EFFECTS OF CARBARYL GRASSHOPPER CONTROL ON NESTING KILLDEER IN NORTH-DAKOTA

There is currently special concern for the effects of range grasshoppe r control on insectivorous grassland birds, such as upland shorebirds. The direct and indirect effects on killdeer (Charadrius vociferus) of carbaryl applications for grasshopper control were investigated in No rth Dakota in 1992. Specimens were collected from two rangeland sites aerially sprayed with 1.44 and 1.12 kg/ha of Sevin-4-Oil (0.56 and 0.4 5 kg/ha carbaryl active ingredient, respectively) and two untreated ar eas. Potential exposure to carbaryl, estimated from pesticide residue concentrations of whole-body and gizzard contents, was highest 2 d aft er treatment and then decreased over time. Whole-body carbaryl residue s averaged significantly higher (p < 0.05) in specimens from sprayed a reas, particularly from sites where the higher carbaryl application wa s used. There was no difference in the mean brain AChE activity and wh ole-body lipid content of killdeer from treated and untreated areas. T he relative abundance of arthropods (principal prey of killdeer) was a lso estimated on the treatment area. As expected, grasshopper densitie s declined significantly posttreatment but the spray caused a temporar y increase in the availability of affected grasshoppers. The two appli cation rates caused similar reductions in grasshopper numbers. Killdee r did not exhibit a functional response of a prey switch as a result o f the spray, but they did demonstrate an increased arthropod capture r ate for a short period (8 d) after treatment. Although exposure was do cumented on the higher application rate area, this exposure was appare ntly not high enough to cause an impact on the parameters we measured in this study. These results support the conclusions of other studies that carbaryl, applied at the rate of 1.12 kg/ha, has little impact on birds and is equally as effective in reducing grasshopper numbers as the higher application rate.