Effects of two grasshopper control insecticides on food resources and reproductive success of two species of grassland songbirds

Abstract

AbstractWconducteda field experimenttodetermine the direct and indirect effects of spraying two broad‐spectrum insecticides with widely differing avian toxicities in grassland habitat on the survival and reproductive success of nesting songbirds. Three 56‐ha plots were sprayed with Decis 5F™, three plots were sprayed with Furadan 480F™, and three plots were left unsprayed. Insecticides were applied at rates recommended for grasshopper control. Grasshopper (Orthoptera: Acrididae) density was measured throughout the spring and summer. Nests of chestnut‐collared longspurs (Calcarius ornatus) were monitored to determine survival rates and nestling growth. Food habits of nestlings and parental foraging parameters were measured using esophageal ligatures and observations of parental foraging flights. Applications of both insecticides decreased grasshopper populations by more than 90%. Nevertheless, the number of grasshoppers in nestling diets was significantly decreased only in nests in Decis‐sprayed plots. Total arthropod biomass delivered to the nestlings did not decrease in plots sprayed with either insecticide. Nestling weight and size were unaffected by insecticide spraying. Rate of prey delivery also did not change; however, by two weeks after spraying with Decis, parent longspurs were foraging almost twice as far from their nests as were birds in control plots (p < 0.05) to maintain prey delivery rates. Clutch size and egg and nestling success were similar among treated and control plots within specific two‐week periods during the season. Age‐corrected brain acetylcholinesterase activities of longspur nestlings in plots sprayed with Furadan were significantly depressed compared to controls; a single case of insecticide‐induced mortality was detected in a nestling that was severely infested with blowfly larvae. Nevertheless, success of nests with Furadan‐exposed nestlings (n = 20) was greater than that in control plots during the two‐week period following spray (n = 19, p = 0.03). The Baird's sparrow (Ammodramus bairdii), an uncommon prairie species of conservation concern, was monitored using an index of productivity. The number of 3.14‐ha census circles having productive Baird's sparrow territories was significantly lower in Furadan‐sprayed plots than in unsprayed and Decis‐sprayed plots; a larger number of sparrow territories were abandoned in Furadan plots.