Pesticides and the Wyoming Toad

By Dickerson, Kim | Endangered Species Bulletin, May 1999 | Go to article overview

Pesticides and the Wyoming Toad

Dickerson, Kim, Endangered Species Bulletin

The preceding article on efforts to recover the Wyoming toad highlights the captive breeding and release program. However, the reintroduction of this species is not enough to secure its future. Until we understand and address the reasons for its decline, the Wyoming toad's survival in the wild is far from guaranteed. Possible causes include climate change, increased predation, changes in agricultural practices, disease, and pesticide use.

For more than a decade, biologists have looked into possible environmental causes of the toad's decline. Studies conducted at Mortenson National Wildlife Refuge (NWR) by our Cheyenne, Wyoming, Field Office's Environmental Contaminants Division during 1989-1991 showed that concentrations of trace elements in water, sediment, and vegetation were below levels harmful to the Wyoming toad. Studies conducted by the Wyoming Game and Fish Department revealed that predation, habitat modification, and soil and hydrologic conditions were not identified as serious threats to the toad. The Wyoming State Veterinarian Laboratory confirmed that the bacterial disease "redleg" was present in the Wyoming toad population, but the lab was unable to determine if this disease was a cause for the drastic population decline.

In 1998, our Cheyenne office conducted a study to determine if pesticides were entering Mortenson NWR through aerial drift and affecting the Wyoming toad. During the 1970's and early 1980's, fenthion (Baytex) was sprayed for mosquito control on lands adjacent to Mortenson NWR. This pesticide was subsequently not reregistered with the Environmental Protection Agency after 1992 for use as a mosquitocide and therefore was taken off the market. It was replaced with malathion, which is very toxic to fish, amphibians, and aquatic invertebrates, although less so than fenthion. Coincidentally, the last toad population was found in 1987 on lands of the future Mortenson NWR adjacent to a ranch that did not spray for mosquitoes.

To determine if aerial drift was occurring and what effects malathion would have on Wyoming toads in their natural environment, we used a non-endangered species, Woodhouse's toad (Bufo woodhousii), as a surrogate for research purposes. Because side effects of a non-lethal dose of malathion include lethargy, intoxication, and paralysis, it is important to know if the pesticide is affecting the toad's ability to escape predation. To have a basis for camparison before and after the nonlethal dose, we conducted behavioral tests called "righting trials," in which the toads are flipped onto their backs to see how long it takes to right themselves. We also took blood samples from some of the toads to compare the level of cholinesterase activity in toads before and after spraying. Cholinesterase is an enzyme essential for normal nerve function. Certain pesticides, such as malathion, inhibit cholinesterase activity. A significant decrease in the enzyme's activity usually leads to uncontrolled tremors, convulsions, and ultimately death. Pesticides that affect cholinesterase activity attack the nervous systems of all animals.

It was also important to determine if the adult Wyoming toad's food source was being reduced by aerial drift of pesticides. Adult Wyoming toads primarily eat ants and beetles, but they will also consume aquatic insects. To collect terrestrial insects, we placed insect pitfall traps at sampling sites overnight for two consecutive nights prior to spraying. Pitfall traps are containers set into the soil so that their tops are flush with the surrounding ground. Insects fall into the trap but are unable to get out. We also collected aquatic invertebrates from each site. We recorded the species and quantity of terrestrial and aquatic insects captured to estimate the total abundance of insects and the number of individual species. Insects were submitted for chemical analysis prior to the spraying so that results can be compared to chemical levels found in insects collected after the pesticide spraying. …

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