Simply Science, Part IV: Ghosts under the oyster bed: why the Washington Department of Ecology rejected a new pesticide for oyster farms

By Danielle Edelman

Photo Credit Danielle Edelman

On April 9, 2018, the Washington State Department of Ecology (DoE) responded to a request from the Willapa-Grays Harbor Oyster Growers Association (WGHOGA) to approve the use of the pesticide imidacloprid to control burrowing ghost shrimp in oyster beds. Specifically, the WGHOGA had applied for a National Pollutant Discharge Elimination System (NPDES) permit, a key component of the national Clean Water Act which allows for the discharge of chemicals and wastewater into the environment. The DoE’s tentative decision to reject the permit application raises some questions about water quality standards, agricultural practices, and the future of shellfish farming in Washington, especially for members of the public who may not have been following the three-year process from permit application to the DoE’s rejection letter.

What are ghost shrimp? Why do oyster farmers care about them?

Bay ghost shrimp (Neotrypaea californiensis) are pale, crayfish-sized shrimp which burrow in muddy sediments. Despite their scientific name, they are found from Alaska to Baja California. Observant beach-goers can spot their burrows by the small, conical mounds of sand surrounding the main surface entrance. Ghost shrimp are considered a pest by oyster farmers because their main crop, the Pacific oyster, uses the same habitat as the shrimp. Burrowing shrimp disturb the mud and sand around oysters on the tidal flats. As the sediment particles soften oysters, the oysters sink and suffocate. Shellfish farmers have spent many decades trying to find ways of eliminating ghost shrimp from their oyster beds, including setting out electrified spikes, injecting shrimp burrows with cayenne pepper, and crushing shrimp burrows. Chemical pesticides had been used in the past, specifically carbaryl. When carbaryl was banned for use on oyster beds due to concerns about using a pesticide on a native estuarine species, oyster farmers and regulators sought a new solution. Frustrated by years of suffocated oysters, Washington shellfish farmers asked the state for permission to use the chemical insecticide imidacloprid in 2015.

What is imidacloprid? Is it dangerous?

Imidacloprid is a chemical insecticide commonly used in agriculture in the U.S. It belongs to a class of pesticides called neonicotinoids, which are synthetic chemicals that resemble naturally-occurring nicotine, a known insect repellent. Imidacloprid, like other neonicotinoids, is a neurotoxin which attacks the nervous system of insects, paralyzing and killing them. Since neonicotinoids have little to no effect on mammals and other animals, they are commonly used as pesticides by farmers worldwide.

Imidacloprid applied in agriculture has been targeted by concerned scientists and activists as a possible culprit behind the global decline in bee populations. The claim about neonicotinoids’ impact on bees is still a matter of debate, however, as large-scale field trials showed mixed results. Imidacloprid can have far-ranging impacts on insect populations outside of the area in which it is applied, as it is water soluble and can persist in the environment for approximately 1,000 days. Even small concentrations of imidacloprid in the environment can have negative impacts on insect populations.

Even though ghost shrimp aren’t insects — they are crustaceans—and imidacloprid also has an effect on the nervous systems of crustaceans. In fact, the pesticide’s ability to affect a wide range of marine organisms such as worms and crustaceans is one of the primary reasons the DoE rejected the application to use it on oyster beds. A secondary reason behind the ban is the lack of aquatic studies regarding the impacts of this pesticide.

Why did the DoE reject the permit to use imidacloprid? What will oyster farmers do now?

In 2015, when shellfish farmers in Washington state first put forth their plan to use imidacloprid on oyster beds threatened by ghost shrimp, a massive public outcry caused most shellfish farms to back away and look for other options to control ghost shrimp. Consumers and restaurants were concerned about the possibility of ingesting the pesticide when eating oysters. The WGHOGA initially backed down from the plan, but later submitted a scaled-down NPDES permit application which the DoE recently rejected. The DoE’s rationale for not allowing this pesticide to be used on oyster farms has less to do with consumer protection and more to do with lack of certainty about imidacloprid’s effects in aquatic environments. The official response from the DoE lists several reasons for denying the permit, including concerns about imidacloprid building up in the benthos? (due to its slow breakdown process), negative impacts on marine organisms besides ghost shrimp, and the potential for imidacloprid to spread far beyond the confines of the oyster farms (because of the tide and water flow). The DoE has estimated that a one-acre application of imidacloprid could affect up to five acres of marine habitat.

The issue of controlling ghost shrimp in commercial oyster farms remains a problem, and the rejection of the imidacloprid solution is yet another setback for oyster farmers. Shellfish aquaculture is an important part of Washington’s economy, but the DoE’s decision makes it clear that the state is uncertain about the environmental effects of imidacloprid and is therefore prioritizing environmental health and safety over industry preferences. The response to the NPDES permit request is tentative, as opposed to a final decision, and this is likely to remain an ongoing issue for the foreseeable future.