Immunological and physiological responses to predation risk and sublethal concentrations of chlorothalonil and β-endosulfan in Lithobates taylori (Anura: Ranidae) tadpoles

Pesticide exposure and its interaction with other natural stressors can play a role in amphibian population declines because disruptions in stress hormone regulatory mechanisms may inhibit immune responses during metamorphosis. Here, we determined the interactive effects of predation risk and sublethal concentration of two pesticides on immunological and physiological responses in tadpoles of the tropical frog Lithobates taylori. Using mesocosms, we used chronic exposure to three levels of chlorothalonil and β-endosulfan in the presence or absence of Odonate larvae. Our results show that β-endosulfan in high concentrations reduced the weight of the tadpoles and increased the neutrophil count and corticosterone (CORT) levels. Larval development was accelerated by high concentrations of chlorothalonil. Also, this pesticide in low and high concentrations increases the absolute values of lymphocytes. Tadpoles exposed to chlorothalonil increased the numbers of monocytes (in low concentrations), and lymphocytes (in high and low concentrations). The interactions of the low concentrations of both pesticides with and without the predator's presence also increased the number of lymphocytes. A combination of pesticides increases the number of lymphocytes in the blood due to synergistic cytotoxicity. This study proves that β- endosulfan elevates circulating CORT and thus generates physiological stress in tadpoles. Given that both pesticides are widely used within the distribution of L. taylori in Costa Rica, it is likely that tadpoles’ development and immune function are altered by pesticide use. In combination with stressors such as emerging diseases and altered precipitation regimes, widespread agrochemical uses likely caused this species enigmatic decline in recent decades.