An assessment of the potential for pesticide exposure and effects on the Rio Grande leopard frog (Rana berlandieri)

Worldwide population declines and widespread discoveries of deformities in ranid frogs illustrate the need for research on effects of environmental contaminants on this group of amphibians. I studied the impact of exposure to insecticides (organochlorine, pyrethroid, organophosphate, and carbamate) and triazine herbicides on populations of Rio Grande leopard frogs ( Rana berlandieri) in South and West Texas. Frogs were collected from five locations: the (1) Santa Ana/(2) Lower Rio Grande Valley National Wildlife Refuge complex; (3) Falfurrias, TX; (4) a ranch in South Texas; and (5) Big Bend National Park. Sites were considered to represent varying levels of pesticide contamination from agricultural chemical applications, with the least contaminated site located in Big Bend. During 2000, adult frogs (8/site; four males and four females) were collected twice from each site (Feb./March and Oct./Nov.) and assessed for gross abnormalities. Body weight and snout-vent length (SVL) were recorded, tongue was collected for cholinesterase (ChE) activity analysis, plasma testosterone was measured, and flow cytometry was run on red blood cells. Liver was collected for pyrethroid and organochlorine residue analyses. Gonads were collected for histological examination, and a gonadosomatic index (GSI) was calculated for each frog. Biomarker and residue analyses, gonad histology, and morphological measurements were compared between collection sites to assess the impacts of agricultural-use pesticides on populations sampled. No gross abnormalities were found in any frogs captured. Morphological measurements revealed that Big Bend frogs were smaller than those from other sites, but had higher GSIs. There were no differences in ChE activity among sites, indicating no recent exposure to organophosphate or carbamate insecticides at collection times. Overall liver pesticide residues were highest in Santa Ana frogs and lowest in Big Bend frogs, with site-specific differences in compounds. Gonad histology indicated that R. berlandieri exhibits continuous spermatogenesis, and is likely an opportunistic breeder at the sites evaluated. Liver pesticide loads did not appear to interfere with amplexus and egg-laying. Corresponding laboratory toxicity tests with late-stage R. berlandieri tadpoles revealed that two pesticides commonly applied to crops in South Texas, atrazine and esfenvalerate, administered singly at sublethal doses did not interfere with metamorphosis.