MP4 Toxic Effects of Pesticide Mixtures
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() The interaction of a predator stress and two organophosphorous pesticides on Ceriodaphnia dubia survival.

Maul, J1, Farris, J1, Lydy, M2, 1 Arkansas State University, State University, AR, 724672 Southern Illinois University, Carbondale, IL, 62901

ABSTRACT- Communities of aquatic organisms are typically exposed simultaneously to multiple environmental conditions and anthropogenic compounds that can reduce the viability of populations or disrupt community interactions. Some aquatic invertebrate species (e.g., cladocerans) are commonly used to identify the toxicity of agricultural chemicals, but often in the absence of population-mediating ecological processes such as predation, competition, and resource limitation. Despite recent attention given to the topic of chemical mixtures, the combined effects of ecological and anthropogenic stressors are relatively unknown. In this study, we examined the interactive effects of an ecological stressor (presence of a predator), and two organophosphorous pesticides (malathion and dicrotophos) on survival of Ceriodaphnia dubia using factorial experiments. Although no main effect of predator was detected, a significant interaction effect of predator stimulus (aliquot of a whole body natural predator [Pimephales promelas] homogenate) and malathion on percent survival was observed: presence of predator stress enhanced the toxicity of malathion on C. dubia survival. Additional experiments supported this interaction, and suggest that the effect of both compounds on C. dubia survival is enhanced with the addition of the predator stressor. An interaction effect between malathion and dicrotophos was also observed for survival, suggesting a greater than additive (synergistic) response. Percent mortality in binary mixtures of the two compounds ranged from 11.5 to 26.5 times greater than predicted for an additive response. Compounds with similar modes of action (i.e., both cholinesterase inhibitors) are predicted to be strictly additive. Similar patterns for the malathion × dicrotophos interaction were observed over two different experiments. Based on these results, we suggest that researchers consider local ecological processes when assessing contaminant impacts to an ecosystem. In addition, there is a need to further examine the dicrotophos × malathion interaction, since both compounds are used simultaneously and extensively on crops within the Mississippi Alluvial Plain ecoregion.

Key words: synergism, predator stress, multiple stressors, pesticide interactions

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