Comparative toxicities of two common agricultural chemicals to toads

Abstract

Amphibians are limited in their movements compared to other terrestrial vertebrates and are therefore at risk of having entire local populations destroyed by agricultural pesticides. This study provides basic toxicological information about two toad species, Bufo valliceps Wiegmann, 1833, and B. velatus Bragg & Sanders, 1951, in relation to two major agricultural pesticides, carbofuran and trifluralin, that are widely used and applied directly to the soil. Toads of both species were collected at a site in East Texas in 1993, 1994, and 1995, and maintained in captivity for several months to observe feeding behavior and general health. Animals were exposed dermally by ventral absorption for 6 hr to carbofuran or trifluralin. Toads were checked prior to the exposure and at 2, 4, and 6 hr for loss of righting reflex and hind-leg retraction, hyperactivity, iris contraction, and mass increase. Animals were also tested pre-and post-dosing for feeding response. Toads were frozen at 1 hr and 1 d post-dosing for brain cholinesterase (ChE) analysis. Trifluralin showed differences from controls only within the post-feeding test; dosed animals did not eat. There were significant differences between controls and toads exposed to carbofuran in all behavioral categories. A carbofuran concentration of approximately 5.5% of the highest recommended field application rate (2.5 gal/acre) caused loss of righting reflex in 30% of B. valliceps and 60% B velatus at 6 hr. Behavioral tests between species, with the exception of hyperactivity, revealed differences in sensitivities to carbofuran with B. velatus being more sensitive. This coincides with the recent reduction in its geographic range, however, the cause is unknown. Tests between sexes showed no significant differences. juveniles showed differences from the adults in iris contraction and mean brain ChE levels at 1 hr. There was a difference between baseline levels of brain ChE activity between the two species, with B. valliceps having higher normal levels. B. valliceps showed 41.2% and B. velatus 46.5% inhibition of brain ChE after carbofuran exposure. There were no differences in brain ChE responses between animals exposed to trifluralin and their controls.