Interactive toxicity of a triazole-derivative fungicide and an organophosphate pesticide in the marine shrimp Litopenaeus vannamei (Boone, 1931)

Varied agricultural activities integrate with the shrimp farms in the state of Sinaloa on the North-Western Pacific coast of Mexico. Out of many wild species found in this area, the marine shrimp Litopenaeus vannamei was selected for this study as it is highly sensitive to a wide range of pollutants and because it is the principal marine shrimp for aquaculture in this region. L. vannamei plays an important ecological role in estuarine environments along this coast where it supports one of the most important fishery resources in Mexico. In this study, a tropical marine bioassay was developed to assess the toxicity of pesticides in the shrimp L. vannamei. A flow-through system (FTS) was designed to provide the optimal conditions for shrimp juveniles, which were individually placed in cages to evaluate the moulting rate and behaviour during the experiments, as well as mortality. The FTS was a simple apparatus, consisting of a 16-channel peristaltic pump which delivered the test solution from reservoirs into exposure chambers under controlled conditions (flow rate, temperature and photoperiod). Time-independent exposures were carried out in flow-through conditions for the organophosphate pesticide methyl-parathion, and a triazole-derivative fungicide in two preparations: Tilt (commercial formulation, 25% propiconazole) and technical grade propiconazole. The threshold median lethal concentrations (threshold LC50) were determined for methyl-parathion (4.4 μgJ\ Tilt (1137 μgJ1), and propiconazole (1716 μgJ1). During these tests, the comparative toxicity of the pesticides was examined, and in particular, the toxic contribution of propiconazole in the commercial formulation, Tilt, was established. The toxicity data of each one of the toxicants was also used to assess the interactive toxicity for each form of the fungicide (technical grade and formulation) with methyl-parathion. When methyl-parathion was combined with Tilt, a less than additive effect was found. On the other hand, when methylparathion and technical grade propiconazole were combined, results ranged from a more than additive to a simple additive effect. On the whole, exposure to methyl-parathion produced erratic movements and hypersensitivity, and no indication of moult retardation was observed. On the other hand, exposure to fungicides caused lethargy and slight moult retardation was observed. Feeding inhibition was frequently observed in all tests. When Tilt was used in a subchronic test, several morphological abnormalities occurred in the test organisms in relation to exposure concentration. Other effects observed during this exposure were changes in feeding and motor behaviour, and alterations in moulting rate