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

We conducted a field experiment to determine the direct and indirect effect s of spraying two broad-spectrum insecticides with widely differing avian t oxicities in grassland habitat on the survival and reproductive success of nesting songbirds. Three 56-ha plots were sprayed with Decis 5F (TM), three plots were sprayed with Furadan 480F (TM), and three plots were left unspr ayed. Insecticides were applied at rates recommended for grasshopper contro l. 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 habit s of nestlings and parental foraging parameters were measured using esophag eal ligatures and observations of parental foraging flights. Applications o f both insecticides decreased grasshopper populations by more than 90%. Nev ertheless, the number of grasshoppers in nestling diets was significantly d ecreased only in nests in Decis-sprayed plots. Total arthropod biomass deli vered to the nestlings did not decrease in plots sprayed with either insect icide. Nestling weight and size were unaffected by insecticide spraying. Ra te of prey delivery also did not change; however, by two weeks after sprayi ng with Decis, parent longspurs were foraging almost twice as far from thei r 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 pl ots 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 ne sts with Furadan-exposed nestlings (n = 20) was greater than that in contro l plots during the two-week period following spray (n = 19, p = 0.03); The Baird's sparrow (Ammodramus bairdii), an uncommon prairie species of conser vation concern, was monitored using an index of productivity. The number of 3.14-ha census circles having productive Baird's sparrow territories was s ignificantly lower in Furadan-sprayed plots than in unsprayed and Decis-spr ayed plots; a larger number of sparrow territories were abandoned in Furada n plots.