The effects of the insecticides Guthion (technical grade) and Guthion 2S (commercial formulation) on survival and growth of tadpoles of the Pacific treefrog Pseudacris regilla, and larvae of the Northwestern salamander Ambystoma gracile and the spotted salamander Ambystoma maculatum were determined in continuous-flow exposures in the laboratory. Ninety-six-hour LC50 values were >3.6 mg/L for P. regilla with technical grade Guthion and 1.47 mg/L with the formulation Guthion 2S (measured as active ingredient Guthion). Ten-day LOAEL and NOAEL values based on length and weight for P. regilla with Guthion were 3.60 and 0.98 mg/L. LOAEL and NOAEL values based on length and weight for P. regilla with Guthion 2S were 0.17 and 0.07 mg/L. The 96-h LC50 for Guthion 2S was 1.67 and 1.90 mg/L for A. gracile and A. maculatum, respectively. LOAEL and NOAEL values based on length and weight for A. gracile with Guthion 2S were 0.22 and 0.10 mg/L; they were 0.11 and 0.03 mg/L for A. maculatum based on weight. These species are as similar in sensitivity as some fish species but are more tolerant to Guthion than most invertebrates.
The American bullfrog Lithobates catesbeianus has been introduced around the world, with invasive populations reported from almost all South American countries. A population of this species was introduced in the Calingasta department of San Juan province, which is an arid environment in western Argentina. This work provides information on the dietary composition of an invasive population of L. catesbeianus, and compares the degree of dietary overlap between adults and juveniles. Stomach contents of 169 bullfrogs (82 adults and 87 juveniles) were analysed. Adults consumed 40 prey taxa and Hymenoptera (Insecta) was the most numerous prey item (41.8%), followed by Araneae (13.6%) and Aeglidae (13.4%). Juveniles consumed 29 prey taxa and Hymenoptera constituted the highest percentage in prey number (77.2%). The trophic overlap niche index at the same level shows a value of 0.64 overlap in dietary community between adults and juveniles of this bullfrog. Aeglidae was volumetrically the most important trophic item (25.4%), followed by Anura (25.02%). Our results showed that cannibalism in bullfrogs is more common than the consumption of native anurans, coinciding with that reported in other populations of introduced bullfrogs. The high similarity in the diets of both size classes and the association between the size of the predator and prey suggest that the impact caused by bullfrogs throughout their ontogeny is high and probably has an impact on their prey. Freshwater crabs are the main items in the diet of Lithobates catesbeianus in other introduced populations and are usually the most conspicuous at our study site. The crabs in freshwater ecosystems are part of the lowest trophic level in the food chain. The major threats to the southern region's freshwater crabs include deforestation, farming and exotic species. Lithobates catesbeianus has a generalist diet and high overlap between adults and juveniles.
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