BIOMONITORING WITH SENTINEL PLANTS TO ASSESS EXPOSURE OF NONTARGET CROPS TO ATMOSPHERIC DEPOSITION OF HERBICIDE RESIDUES

In regions of highly diversified agriculture, herbicides used in one f ield may not be compatible with crops growing in nearby fields. Injury to nontarget crops may occur from drift or localized atmospheric tran sport of herbicide residues. Pea, bean, and corn seedlings have been u sed as sentinels to detect atmospheric deposition of trace residues. S eedlings were exposed at various locations throughout south central Wa shington and exchanged on a weekly basis with a new set of plants. Pla nts were returned to a lath house and observed over 3 weeks for develo pment of symptoms indicative of probable exposure to four classes of h erbicides-sulfonylureas, phenoxyacetates, aminophosphonic acids, and b ipyridiliniums. The most frequently observed symptoms on sentinel plan ts were chlorotic spots on the upper leaf surfaces; such symptoms were created under controlled conditions by exposing plants to aerosols of the sulfonylurea (SU) herbicide chlorsulfuron. For 3 years, the great est incidence of probable SU symptoms was detected during April; howev er, occurrence of symptoms was not correlated with periods of maximum use of SU herbicides on wheat in the region. The results suggested a g eneral atmospheric loading of herbicides with occasional deposition, r ather than point source drift. Further tests are needed to quantitate the exposures of sentinel plants and determine if chlorotic spots are associated with adverse physiological effects.