ABSTRACT
Although agricultural chemicals are designed to act selectively, they sometimes act directly or indirectly on non-target organisms. Herbicides may be toxic to aquatic or terrestrial insects, including some that are as toxic as widely used insecticides. The physiological effects of herbicides vary among insects, and the negative effects of some herbicides result in reduced growth rates or prolonged development time rather than mortality. Also, in some cases it is the herbicide adjuvant rather than the toxicant that affects insects, and some herbicides synergize the toxicity of insecticides. Some herbicides also stimulate or benefit insects. Indirect effects of herbicides generally result from modification of the plant community. Some weeds serve as important alternate hosts of insects, so in this case herbicide application may be beneficial by reducing the number of herbivorous insects. On the other hand, the selectivity of herbicides may reduce the availability of weeds to predatory and parasitic insects, which often benefit from plant diversity due to the availability of floral and extrafloral nectar sources, or pollen. Herbicide use affects not only insects, but vertebrate organisms that depend on weed seed or insects as a source of food. Avifauna, in particular, can be indirectly affected by herbicide use. Microbial plant pathogens also are harbored in weeds, with insects serving to transmit the pathogens to crops plants, resulting in plant disease. Herbicides thus can disrupt the occurrence of plant disease in crops if they eliminate weeds that either harbor insect vectors or the microbial pathogens they transmit. Despite our extensive knowledge of herbicide-weed-insect-plant pathogen relationships, our ability to predict outcomes of herbicide application, especially as related to insect-vectored plant disease, is rudimentary.
