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 recommended rate of Decis 5F (6.25 g deltamethrin ha(-1)). Paired control plots remained unsprayed. The density of (Acrididae) grasshoppers was monitored 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 consumption by nestlings were measured using oesophageal ligatures. Grasshoppers accounted for >85% of the biomass of the nestling diet to spraying and this proportion increased throughout the season in unsprayed plots. Applications of Decis 5F initially reduced the grasshopper density by 93%. After spraying, parent birds switched to other arthropod taxa less affected by insecticide application; the overall biomass fed to nestlings was not significantly reduced although the acridid proportion declined to <30%. The weight and skeletal size of the nestlings at fledging was unaffected. Parent birds in sprayed plots flew no further to feed their nestlings at a similar rate to that of birds in the control plots. The clutch size and nestling survival were similar between the sprayed and unsprayed plots after Decis 5F application, but egg success was lower in the sprayed plots compared to the control plots (67 versus 87%, p < 0.05).