Risk assessment and risk management options for neonicotinoids

Organisms in the environment are exposed to a cocktail of chemicals that may be detrimental to their health. The risk assessments of pesticides is an integral part of the decision-making process which usually evaluate the effects per individual pesticide, rather than considering their effects when combined. It is only recently that the interactions between agrochemicals have been considered, predominantly at ecologically relevant levels. This is an important step towards understanding the impact of these chemical combinations on the environment. A more streamlined investigation is still required to provide scientifically factual data on improved use of pesticides that can help control pests proficiently as well as contribute towards effective decision-making. The aim of this study was, therefore, to address the risk assessment process of pesticides in mixtures on target and non-target organisms to understand how these risks could be managed in the future. Lethal and sublethal endpoints were assessed for acetamiprid and thiacloprid neonicotinoid insecticides, and tebuconazole fungicide. Chrysoperla carnea, was taken as a model non-target insect and Plutella xylostella, as a model target insect. In Plutella xylostella, the lethal toxicity of pesticides was age dependent with younger instars being more susceptible and their mortality rate increased with increasing the dose of pesticides both individually and in mixtures. Furthermore, the toxicity of mixtures of acetamiprid and tebuconazole was greater for Plutella xylostella in comparison with Chrysoperla carnea. Low LC50 values of acetamiprid for Plutella xylostella indicated its high toxicity to target relative to Chrysoperla carnea. Tebuconazole however, was found to be highly toxic to both insects’ larvae. Moreover, for Chrysoperla carnea, lethal toxicity of thiacloprid and thiacloprid-tebuconazole mixtures was not significantly different. Sublethal endpoints such as the avoidance rate of Plutella xylostella larvae was affected by higher concentrations of pesticides because it declined due to a higher mortality rateat increased concentrations and remained higher at lower concentrations. In Chrysoperla carnea, it increased with the increasing concentrations of the pesticides treatments. Speed of Plutella xylostella declined and number of stationary periods increased with rise in concentrations of acetamiprid, tebuconazole and acetamiprid – tebuconazole mixtures whereas, only the feeding rate declined with increasing concentrations of tebuconazoleand acetamiprid – tebuconazole mixtures. In Chrysoperla carnea, only its feeding rate declined with exposure to tebuconazole and thiacloprid – tebuconazole mixtures. Exposure time to these pesticides and mixtures mostly affected the lethal and sublethal parameters of Plutella xylostella. The maximum difference (53.3%) in mortality rate existed between Chrysoperla carnea and second instar larvae of Plutella xylostella in 24 h of exposure to mix3 (0.5 mlL-1 tebuconazole + 0.25 gL-1 acetamiprid). However, this difference declined to 45% over 48 h due to a relative increase in the mortality rate of Chrysoperla carnea larvae. Based on these findings, recommendations on optimizing the use of pesticides mixtures have been outlined in this study. This study highlights theimportance of timing the spray mixtures accurately to optimise their impact on target insects while maximizing the conservation of non-target insects