CORVALLIS, Ore. - A common household and agricultural chemical touted for its low toxicity to human and mammal populations may disrupt the life cycles of caddisflies and increase rates of predation on the larvae by other aquatic insects and fish even at doses as minute as 2 parts per billion, according to a study by Oregon State University researchers.
Pyrethroids, a class of synthetic pesticides derived from naturally occurring chemicals in chrysanthemums, can enter rivers and streams through a variety of pathways, including run-off from agriculture and home use. Once in a waterway, pyrethroids settle quickly out of the water column, attaching to organic matter and algae in the water and adhering to stones and sediment in the channel's bottom.
Caddisfly larvae use this detritus to build hard tubular casings that protect them from their environment and predators. Under normal circumstances the larvae will not leave their casings until they are ready to pupate, however exposure to pyrethroids through case material or ingestion causes a disruption in the nervous system that sends a scrambled signal causing the larvae to perform an early evacuation and leaving them vulnerable.
"The larvae are being exposed to a sublethal dose of pyrethroids that results in a behavior ending with death," said Jeffrey Jenkins, a professor in OSU's College of Agricultural Sciences and an Extension Service faculty member. "It's no different than if they would have been killed outright by the chemical. It may be harder to quantitate, but it's still a death sentence."
Caddisfly larvae that are not immediately eaten upon their forced evacuation may try to rebuild their casings, but these structures are not the well-ordered tidy domiciles the larvae usually build, said Katherine Johnson, the primary researcher on the project and a graduate student in OSU’s Department of Environmental and Molecular Toxicology. Tests conducted by Johnson, Jenkins and Paul Jepson, director of OSU's Integrated Plant Protection Center and a faculty member in the department, found that the post-contamination casings tended to be chaotically constructed, lacking in adequate ventilation and not structurally sound.
"Overall, rebuilt casings were 70-90 percent weaker than stream-built cases in our test study," said Johnson. "The insects in the rebuilt cases were extremely vulnerable to predators and more susceptible to predation."
In the course of rebuilding, the larvae also use incredible amounts of energy, affecting their overall fitness. These larvae tend to become smaller winged adults and have lower reproductive rates, said Jenkins.
"Caddisfly that have been exposed to pyrethroids as larvae are unable to reproduce at the same level as an unaffected population," said Jenkins. "Exposure to this chemical comes down to a decrease in the growth of the population and a decline in overall ecological health."
The use of pyrethroids has increased in recent years as businesses and farmers moved away from broad-spectrum organophosphate pesticides because of the environmental and health impacts associated with their use. Today, pyrethroids are one of the most common pesticides used in the home, and are found in products ranging from head lice treatments to lawn fertilizers.
"With the synthetic pyrethroids there is much less concern for human health, but the risk to aquatic life is potentially a huge issue," said Jenkins. "These chemicals are considered relatively benign to humans and other mammals, but they have the potential to negatively impact our waterways in a significant manner."
According to the U.S. Environmental Protection Agency, pyrethroids are hazardous to both fish and aquatic insects, but concentrations commonly found in rivers and streams are usually too low to cause immediate fatalities in populations. The OSU study was one of the first studies to look at the sublethal affect of pyrethroids on aquatic insects that live in the water.
Caddisflies are a food source for trout, salmon and steelhead populations. They are also an indicator of stream health, and an integral part of nutrient cycling, said Jenkins. The larvae have been know to leave their casings when exposed to extreme environmental conditions, including drought, low levels of dissolved oxygen, freezing temperatures and other situations. But the inability to successfully rebuild a new casing is so far unique to pyrethroid exposure, said the researchers.
Johnson and other OSU researchers will present their findings at an upcoming American Chemical Society symposium on synthetic pyrethroids Sept. 10-14.