Chemical Control of Depredating Blackbirds in Ontario Field Corn

INTRODUCTION Corn depredation by red·winged blackbirds (Agelaius phoeniceus) has been an agricultural problem in southwestern Ontario for many years. Large marsh areas along the shoreline of Lakes Erie and St. Clair provide ideal habitat for breeding and roosting. Concurrently, the vast monocultural field corn acreages in Kent and Essex Counties have over the years been the centers for blackbird depredation in the province. The expansion of corn production into other areas of Ontario has, however, resulted in a province·wide distribution of damage, and substantial crop losses for specific individual farmers (Tyler and Kannenberg, 1980). The Ontario Ministry of the Environment (OME) provided funding for a four·year research program on the importance, ecology and control of blackbirds in 1976 following mounting concern by Ontario corn producers. Dyer (1968) completed a major study on red-winged blackbirds in Dover Township, Kent County during the period 1964-68; however, the evaluation of 4-AP efficacy was a minor component of the project (Dyer, 1976). Thus, the OME required efficacy data on 4-AP for current Ontario conditions for registration purposes, regardless of the 1972 registration in the U.S.A. Methiocarb (3,5-dimethyl-4(methylthio) phenol methylcarbamate) (Mesurol®, Mobay Chemical Corp.) was also a candidate compound because of the known taste aversion properties (Rcigers, 1974; Crase and Dehaven, 1976). Again, the OME required data on the efficacy of methiocarb as a bird repellent in ripening field corn. Research on the potential for lethal roost control was an initial objective; however, this component of chemical control was not pursued. Conversely, lethal control at foraging sites was not an initial objective, however, ecological studies completed (Somers et aI., 1981b) suggested that the use of toxicants in a corn field may have potential. Pilot studies using Starlicide® (Ralston-Purina Co.) and a-chloralose at foraging sites were completed to evaluate this hypothesis. The ultimate objective of the four-year study was to formulate a management plan for 225 226 applications. Regardless, because non-target hazards appeared minimal (Somers et aI., 1981 a), the OME reclassified the commercial 4-AP product for controlled use in field corn in Ontario. Subsequent use of 4-AP by Ontario corn growers has, however, been limited because of the uncertainty of efficacious bird control. The results of the 1976 methiocarb pilot study in Kent County were encouraging because bird damage was marginally reduced (Table 4) in the non-treated plots. Field observations supported the illustrated reduction in the damage data in that foraging birds appeared to avoid …