Using sentinel plants as biomonitors of herbicide drift and deposition

Abstract

Chemical analyses of air and precipitation have documented a widespread occurrence of pesticide residues over both intensively farmed regions and areas remote from agriculture. The health or ecological significance of the low levels detected is obscure, but drift of herbicide residues during spraying has sometimes damaged nearby nontarget crops. Monitoring of herbicide residue deposition, whether resulting from direct drift or following long‐range transport, is highly desirable to determine possible adverse effects on yield. Although chemical assays can confirm the identity of residues, their use in intensive regional sampling 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 specific as chemical analyses, biological assays with sentinel plants can be used to detect classes of herbicides with unique modes of action and characteristic injury patterns.

We used bean plants to study the movement of chlorsulfuron aerosols and to assess the relationship among plant response, spray droplet spread diameter, and concentration of the herbicide solution. The droplets produced by nebulizing an aqueous solution of chlorsulfuron simulated the reported diameter size range of aerosols found in spray drift clouds; the nebulized spray was enriched in aerosols under 8 μm in diameter. The characteristic symptom used in the plant bioassays to detect chlorsulfuron deposition was the appearance of discrete chlorotic spots on primary or trifoliate bean leaves. Trifoliate leaves of 3‐week old bean plants were more sensitive to chlorsulfuron aerosols than 2‐week old primary or trifoliate leaves. Chlorotic spots could be induced by exposure to chlorsulfuron‐containing aerosols sprayed from a 50 ppb solution. Effective movement of drifting aerosols likely to produce significant injury to bean plants was 10 m, but drift could be detected up to 500 m from the point of spray release. The suite of experiments showed that sentinel plants can be used to biomonitor herbicide drift and deposition while providing information about potential biological effects on nontarget crops.

Key words:

• Herbicide drift
• chlorsulfuron
• biomonitoring