Hormone-type herbicides such as 2,4,5-T; 2,4-D; and MCPA; were discovered during the 1940s. They do not persist in soil, are selective in their toxicity to plants, are of low mammalian toxicity, cause few direct environmental problems, but are relatively soluble and reach waterways and groundwater.

botanical derived from or relating to plants pyrethroid chemicals derived from chrysanthemums and related plants pathogenic causing illness nematocide a chemical agent which is destructive to nematodes transient present for a short time molluscicide chemical that kills mollusks herbicide a chemical pesticide designed to control or destroy plants, weeds, or grasses fungicide pesticide used to control, deter, or destroy fungi

Contact herbicides, which kill weeds through foliage applications, include dintrophenols, cyanophenols, pentachlorophenol, and paraquat. Most are nonpersistent, but triazines can persist in the soil for several years, are slightly toxic to soil organisms and moderately so to aquatic organisms. Herbicides cause few direct environmental problems other than their indirect effects, in leaving bare soil, which is free of plant cover and susceptible to erosion.

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