The uptake of phorate, a systemic insecticide, applied as a slurry to wheat and mustard seeds

SUMMARY The absorption by plants of wheat and mustard of a systemic organophosphorus insecticide (phorate), applied as a slurry seed dressing, was studied by caging the aphid Rhopalosiphum padi (L.) on the foliage of wheat, and the aphid Brevicoryne brassicae (L.) and the Chrysomelid beetle Phaedon cochleariae (F.) on white mustard, grown from phorate-treated seed. Wheat and mustard plants quickly lost their toxicity to insects when they were transplanted, suggesting that most of the insecticide from a slurry seed treatment passes into the soil and is picked up by the roots. That phorate or its derivatives occur in the soil was shown by tests of anticholinesterase activity. Insecticide can also pass into the seed of wheat and move to the growing embryo. Phorate becomes closely bound to the testa of mustard, but does not penetrate it to reach the cotyledons or other parts of the embryo. Mustard cotyledons can become contaminated by insecticide as they emerge through the soil. Young and old leaves of both wheat and mustard depend on continued absorption of insecticide from the soil to maintain their toxicity. No insecticide moves from old to young leaves. Old leaves lose their toxicity to insects more slowly than young ones. When treated seeds are sown close together, the overlapping zones of insecticide round each seed can increase strength and persistence of insecticidal effect. This happens more with dimethoate, which readily dissolves in water, than with phorate, which is almost insoluble. At normal sowing rates the zones of insecticide round each seed would rarely overlap. Roots of wheat and mustard from treated seed did not excrete insecticide, and the roots did not carry insecticide through the soil.