PHYTOTOXICITY OF PESTICIDE DEGRADATION PRODUCTS

Pesticide metabolites and degradation residues can accumulate in soils and plants. Relatively little is known of the potential phytotoxicity of these compounds. High levels of certain pesticide metabolites, particularly herbicides, are phytotoxic. For instance, N-methyl-N'-[3-(trifluoromethyl)phenyl]urea, a metabolite of fluometuron, is a weak photosynthesis inhibitor. Crops are not likely to encounter sufficient concentrations of a single weakly phytotoxic pesticide degradation product to cause symptoms in most field situations. Some scientists have speculated that accumulated herbicide degradation products in soils have contributed to a less than anticipated growth of agricultural productivity in cotton and some other crops. However, there is little scientific evidence to validate this and some data indicate that this is not the case. For instance, levels of trifluralin metabolites in soil equivalent to many years of accumulation from high trifluralin use rates have been shown to have no effect on yield of cotton or soybeans. Similar results were obtained with the metabolites of diuron and fluometuron on cotton. In crops genetically engineered to be highly resistant to certain herbicides, metabolite accumulation at high herbicide application rates could be sufficient to cause phytotoxicity in some cases. In the field, there are generally a large number of different degradation products of many different pesticides in the soil. Little is known of possible interactions of these combinations of metabolites on plant health.