INDIRECT EFFECTS OF THE PYRETHROID INSECTICIDE DELTAMETHRIN ON REPRODUCTIVE SUCCESS OF CHESTNUT-COLLARED LONGSPURS

An experiment was conducted to determine whether spraying with a broad -spectrum pyrethroid insecticide in grassland habitat for the control of grasshoppers could affect nesting songbirds through the removel of insect food resources. Three 81 ha plots were sprayed at the recommend ed rate of Decis 5F (6.25 g deltamethrin ha(-1)). Paired control plots remained unsprayed. The density of (Acrididae) grasshoppers was monit ored throughout the spring and summer. The nests of chestnut-collared longspurs (Calcarius ornatus) were monitored to determine the nest and nestling survival rates, size at fledging and food habits. Attributes of parental foraging were quantified. Food selection by parents and c onsumption by nestlings were measured using oesophageal ligatures. Gra sshoppers accounted for >85% of the biomass of the nestling diet to sp raying and this proportion increased throughout the season in unspraye d plots. Applications of Decis 5F initially reduced the grasshopper de nsity by 93%. After spraying, parent birds switched to other arthropod taxa less affected by insecticide application; the overall biomass fe d to nestlings was not significantly reduced although the acridid prop ortion declined to <30%. The weight and skeletal size of the nestlings at fledging was unaffected. Parent birds in sprayed plots flew no fur ther to feed their nestlings at a similar rate to that of birds in the control plots. The clutch size and nestling survival were similar bet ween the sprayed and unsprayed plots after Decis 5F application, but e gg success was lower in the sprayed plots compared to the control plot s (67 versus 87%, p < 0.05).