The Effects of the Commercially Formulated Neonicotinoids Imidacloprid and Thiamethoxam on the Survival of Infectious Stages of Two Trematode Parasites

Neonicotinoids are a popular class of systemic insecticides registered for use in over 120 countries. Recent global water surveys have found neonicotinoid insecticides in waterbodies adjacent to agricultural fields. The presence of neonicotinoids in surface waters raises the concern of the potential risk of exposure to non-target aquatic species, with relevance for both free-living and parasitic taxa. The susceptibility of aquatic free-living parasitic stages to the lethal effects of neonicotinoids may influence the prevalence and intensity of host infection, and their availability as prey for a variety of organisms (e.g., fish, invertebrates). We examined the effects of two commonly used neonicotinoids on the survival of free-living infectious stages (cercariae) from two trematode (flatworm) species (Diplostomum sp. and Haematoloechus sp.). We found that exposure to commercially formulated imidacloprid and thiamethoxam, individually and in a binary mixture, had no significant effect on cercarial survival for either species at environmentally relevant concentrations. Further study is required to understand if the ability of the surviving cercariae to infect the next host in their life cycle is compromised in the presence of neonicotinoids, or if host defenses are compromised, as changes in parasite infectivity or host susceptibility could influence infection dynamics and have community- and ecosystem-level implications.