USING SENTINEL PLANTS AS BIOMONITORS OF HERBICIDE DRIFT AND DEPOSITION

Chemical analyses of air and precipitation have documented a widesprea d occurrence of pesticide residues over both intensively farmed region s and areas remote from agriculture. The health or ecological signific ance of the low levels detected is obscure, but drift of herbicide res idues during spraying has sometimes damaged nearby nontarget crops. Mo nitoring of herbicide residue deposition, whether resulting from direc t drift or following long-range transport, is highly desirable to dete rmine possible adverse effects on yield. Although chemical assays can confirm the identity of residues, their use in intensive regional samp ling studies can be expensive. Certain herbicide groups are difficult to detect. Furthermore, the mere presence of a residue does not easily translate into an assessment of biological effect. Although not as sp ecific as chemical analyses, biological assays with sentinel plants ca n be used to detect classes of herbicides with unique modes of action and characteristic injury patterns. We used bean plants to study the m ovement of chlorsulfuron aerosols and to assess the relationship among plant response, spray droplet spread diameter, and concentration of t he herbicide solution. The droplets produced by nebulizing an aqueous solution of chlorsulfuron simulated the reported diameter size range o f aerosols found in spray drift clouds; the nebulized spray was enrich ed in aerosols under 8 mu m in diameter. The characteristic symptom us ed in the plant bioassays to detect chlorsulfuron deposition was the a ppearance of discrete chlorotic spots on primary or trifoliate bean le aves. Trifoliate leaves of 3-week old bean plants were more sensitive to chlorsulfuron aerosols than e-week old primary or trifoliate leaves . Chlorotic spots could be induced by exposure to chlorsulfuron-contai ning aerosols sprayed from a 50 ppb solution. Effective movement of dr ifting aerosois likely to produce significant injury to bean plants wa s 10 m, but drift could be detected up to 500 m from the paint of spra y release. The suite of experiments showed that sentinel plants can be used to biomonitor herbicide drift and deposition while providing inf ormation about potential biological effects on nontarget crops.