MONITORING EXPOSURE OF NESTLING SONGBIRDS TO AGRICULTURAL APPLICATIONOF AN ORGANOPHOSPHORUS INSECTICIDE USING CHOLINESTERASE ACTIVITY

In June 1992 we collected 53 blood plasma samples from nestling red-wi nged blackbirds (Agelaius phoeniceus), house sparrows (Passer domestic us), and brown-headed cowbirds (Molothrus ater) at five study sites in northwestern Minnesota to evaluate the feasibility of using plasma an d brain cholinesterase (ChE) activity and reactivation as a means of a ssessing exposure of nontarget organisms to the operational use of ChE - inhibiting insecticides in an agricultural setting. Three sites were adjacent to sugar beet fields that were likely to be treated with chl orpyrifos as Lorsban(R) (an organophosphorus [OP] insecticide) for con trol of sugar beet root maggot (Tetanops myopaeformis), and two sites were distant from fields likely to be treated (reference sites). Appli cation of chlorpyrifos in fields surrounding study plots was monitored through contact with landowners and direct observations. Cholinestera se activity levels (total cholinesterase [ChE], acetylcholinesterase [ AChE], and butyrylcholinesterase [BChE]) in nestling plasma were measu red and tested for reactivation (ChE and AChE) in the presence of 2-PA M, an indication of exposure to an organophosphorus insecticide. In ad dition, 11 nestlings were euthanatized and in these samples we measure d brain ChE activity and reactivation, and we analyzed gastrointestina l tracts and carcass washes for chlorpyrifos residues. Total ChE and B ChE activity were lowest in similar-aged nestlings at sites adjacent t o treated beet fields (ChE, t = -2.51, d.f. = 21, p = 0.033; BChE, t = -2.56, d.f. = 21, p = 0.043). Nestlings from sites that were near fie lds where chlorpyrifos was applied were more likely to exhibit plasma AChE reactivation than nestlings from reference sites where OP or carb amate insecticide application was improbable (chi(2) = 3.805, d.f. = 1 , p similar to 0.05). The magnitude of plasma ChE reactivation was hig hest within 1 to 3 d of insecticide application, although significant reactivation was measured up to II d after application of chlorpyrifos . Plasma AChE reactivation in the presence of pyridine-2-aldoxime meth ochloride (2-PAM) appeared to be a more sensitive indicator of exposur e to chlorpyrifos than plasma ChE or BChE activity levels.