An Assessment of a Regional Crash of American Toad Populations. A. G. Westerman, Natural Resources and Environmental Protection Cabinet
Population blooms of Eastern tent caterpillar (Malacasoma americana) were seen in Kentucky in Spring 2000, 2001. They were particularly heavy in 2001. Caterpillars defoliated their preferred food of wild cherry (Prunus serotina) and moved in-mass from the trees to find a pupation site. The leaves of the cherry tree contain 3000 mg/kg or greater cyanide. Within days of the migration, a number of American toads (Bufo americanus) were observed to be lethargic or dead. Upon inspection of toad stomach contents, frogs were found to be feeding exclusively on caterpillars. The obvious initial conclusion was that the caterpillars were poisoning the toads. However, toads and caterpillars have coexisted for a long time, both are native. To test the hypothesis that food-mediated cyanide was the causative agent and to determine the aerial extent and persistence of effects, eight 2.0 ha plots were evaluated across Kentucky. Each plot had a pond (0.1-0.2 ha) and a variety of habitats. Toads were surveyed in five plots in the Inner Bluegrass Region (BG) and three plots some 200 km west in the Pennyrile Region (PR) of Kentucky. Contamination levels, stomach contents, age, size, sex, mortality and reproductive effort were determined. Mortality in BG adult toad plots ranged from 52-72% and in PR frogs from 0-1%. Caterpillars in the stomachs of BG dead animals ranged from 28.4-39.0 per frog and 7.0-19.2 per individual in live BG toads. PR stomach contents ranged from 0.0-5.0 caterpillars per individual. Stomach contents cyanide (HCN) levels averaged 51 mg/kg in dead BG frogs, 17.5 mg/kg in live BG frogs and <0.05 mg/kg in PR toads. One spawn was found in five BG ponds in 2001 and five spawns in 2002. All spawns were small, 190-405 eggs. By comparison at the PR sites, over 100 spawns containing 4000-8000 eggs/spawn in the three ponds were noted in 2001 and 2002. Egg survival in the BG ponds averaged 37.2% as compared to 93.8% in the PR ponds.
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