The toxicity of pulse-exposed insecticides and their mixtures to Melanotaenia fluviatilis and Daphnia carinata

A potential threat to the structure and functioning of aquatic communities within aquatic habitats is the presence of anthropogenic pollutants such as insecticides. The application of insecticides to crops within close proximity to freshwater habitats increases the likelihood that insecticides will be transported from their sites of application into the surrounding aquatic environment. In an attempt to determine the effects of insecticides and their mixtures to Australian freshwater organisms, three insecticides and two organisms were chosen for investigation.

The three insecticides evaluated were endosulfan (OCP), profenofos (OPP), and deltamethrin (SPP). These insecticides were chosen for investigation as they represent the major insecticide groups used to control cotton pests, are heavily applied, and are reported to be toxic to aquatic organisms. Cotton insecticides were evaluated as they are used in close proximity to many Australian freshwater environments, have been linked with a number of fish kills, and may be applied concurrently.

Crimson spotted rainbow-fish and Daphnia carinata are endemic to the freshwater environs of southeastern Australia. They represent organisms from primary consumers to top order carnivores, invertebrates and vertebrates. In addition these species are exposed to insecticides used to control cotton pests.

Pulse-exposure (PE) of insecticides was chosen for investigation as insecticides entering freshwater environments predominantly do so in short-term ‘slugs’. The effect of these short-term pulses was found to be significant on fish and daphnid test organisms. Based on 96 h PE LC50s endosulfan and deltamethrin were found to be highly toxic to crimson-spotted rainbowfish at environmentally realistic concentrations while rainbowfish were relatively insensitive to pulse-exposed profenofos. Based on 48 h PE EC50s pulse-exposed profenofos and deltamethrin were found to be highly toxic to D. carinata neonates at environmentally realistic concentrations while daphnids were relatively insensitive to pulse-exposed endosulfan.

In an attempt to replicate field conditions, fish and daphnids were exposed to binary and ternary mixtures of the three insecticides. Concentration-addition (CA) successfully predicted mixture toxicity based on single insecticide data when toxic units (TUs) of one were analysed. The potentiation of toxicity in fish and daphnids pulse-exposed to lower toxic units of endosulfan, deltamethrin and profenofos highlights the need to focus more on insecticide mixture toxicity and methods employed to prevent these toxicants from entering freshwater environments. A comparison of exposure regimes indicated that multiple pulses of deltamethrin separated by 24 h were generally more toxic than equivalent pulses separated by 1 h while the toxicity of multiple pulse-exposures separated by 1 h was generally in agreement with single pulses of equivalent duration. It was argued that concentration nd duration dependent thresholds may be responsible for the variations in toxicity noted in the various exposure regimes and that these thresholds may be linked with the induction/production of detoxifying enzymes. The significant immobility of daphnid neonates pulse-exposed to environmentally realistic concentrations and durations of
deltamethrin are a cause for concern. The findings of the current investigation perpetuate the concern for aquatic wildlife in Australian waterways following pulse exposure to technical grade insecticides.