Most of the concern about pesticides on non-target species focuses on direct mortality. Scientists answer the question, “What dose is required to kill non target species?” Where, when and how much of a pesticide can be used are all factors that can be regulated to minimize the amount of pesticides reaching a non-target insect and keep the pesticides below the toxic dose. Common non-target insects of concern are honey bees, and aquatic insects such as dragonflies.
Pesticides that act on the nervous system can have indirect effects on mortality. The nervous system controls insect behavior. Behaviors that are critical to the survival of an insect can be altered by exposure to insecticides. I first encountered this phenomenon when working with insect pheromones. The company I worked for had evidence that a sub-lethal dose of insecticide placed on a pheromone dispenser could affect the ability of male moths to detect female pheromone and follow the pheromone trail of the female to the source. This was of interest because a small amount of insecticide that would not affect non-target insects could increase the efficacy of the pheromone in mating disruption strategies.
Recent evidence suggests that one of the effects of neonicotinoid insecticides on honey bees is a behavioral effect on the ability of honey bees to return to their hive. Honey bees are social insects and do not survive long outside the hive. A lost honey bee will soon die.
Another example is an effect on damselfly behavior caused by sublethal doses of the insecticide endosulfan. In the journal, Aquatic Toxicology, researchers from Belgium have reported that endosulfan can alter the response of damselfly larvae to predator threat. Damselfly larvae are commonly eaten by fish and other aquatic predators. To avoid the predators, damselflies must be able to swim faster than the predators. Endosulfan can slow the escape speed of the damselflies making them more susceptible to mortality by predation.
How important are sublethal effects of pesticides? In most instances, the sublethal effects do not cause large changes in populations. However, when large changes in insect populations are noted and they are associated with insecticide use, sublethal effects of pesticides are one factor that should be considered.