This story is part of CT Mirror Explains, an ongoing effort to distill our wide-ranging reporting into a "what you need to know" format and provide practical information to our readers.

Throughout this summer, a debate has raged along the Connecticut River over a little known but widely used chemical herbicide: diquat.

The controversy kicked off in June when the U.S. Army Corps of Engineers announced its plans to resume using diquat and other herbicides to combat the spread of invasive hydrilla along the river. Those plans were part of a years-long effort to study the effectiveness of using herbicides to control hydrilla. They quickly became the subject of viral social media posts.

Here’s what to know about the herbicide, the Army Corps’ plans, and risks to the public:

What is diquat?

Diquat, or diquat dibromide, is a chemical herbicide that was first developed in England in the 1950s for use in agriculture. Diquat is a fast-acting, non-selective herbicide, meaning it can be used against a wide range of weeds, invasive plants and other target species.

Dr. Melissa Perry, dean of the College of Public Health at George Mason University, said diquat can be used as a “burndown” herbicide that is applied to soils before crops are planted in order to control weeds. In agriculture, it is most commonly used in growing potatoes, oil seed crops and vineyards, though Perry said diquat remains relatively rare for use on U.S. farms.

While it is known to persist in soils for a long time, diquat dissipates quickly in water — making it much more popular as an aquatic herbicide.

Despite some claims that have circulated online, Perry said that diquat is not related to Agent Orange, another herbicide that became infamous for its use by U.S. troops during the Vietnam War.

How is it being used in Connecticut?

In Connecticut, diquat is among the most common herbicides used to rid lakes and ponds of invasive plants such as hydrilla, water chestnut and Eurasian watermilfoil.

A total of 955 permits have been issued since 2022 allowing for the use of diquat in aquatic settings.

The chemical works by destroying walls of plant cells from within and interrupting the photosynthesis process. Diquat is known to be especially effective against plants with weak root systems, like hydrilla, and is not as lethal to plants with stronger roots.

Typically, the herbicide is either sprayed at the surface level or it is mixed into a sinking solution that is injected under the water through hoses. The latter method is the one that the Army Corps of Engineers have employed as part of their ongoing studies on the Connecticut River.

That work began in 2023, when research scientists began placing dyes into the river to study its flow in order to determine what herbicides to use, in different concentrations, in areas afflicted by hydrilla. Based on those results, the Army Corps first started applying herbicides, including diquat, at five areas along the river last summer.

The Army Corps has recorded large declines in the amount of invasive hydrilla, with minimal impacts to native plants, as a result of that application. This summer, the hydrilla has returned in Chester Boat Basin and, to a larger extent, in Selden Cove, according to the Corps’ reports. Both sites were selected for additional treatments when the Corps’ resumes its studies this month.

What are the risks?

According to the U.S. Environmental Protection Agency, the potential health risks from diquat are mostly associated with people who work with the chemical or apply it to their lawns and gardens.

The agency does not report any dietary or other post-application risks associated with exposure to treated water.

“The bottom line is, if the labeled directions are being followed, there are no concerns from people swimming or coming into contact with water that’s been treated with diquat,” said Anne Overstreet, the director of the EPA’s Pesticide Re-Evaluation Division.

Overstreet said that the agency has determined that diquat can be “mildly toxic” to fish and other aquatic wildlife that are not the intended targets of its treatment. Reports released by the agency said those effects can be mitigated by applying the herbicide underneath the water, as opposed to spraying it on the surface where it can more easily drift into other areas.

Due to the potential risk to people who handle diquat in undiluted concentrations, the EPA requires certain workers to wear protective clothing, including respirators and protective eyewear.

Perry, the epidemiologist, said skin exposure can be toxic and ingesting diquat is “highly toxic,” interfering with the gut, its microbiome and other internal organs.

How is it regulated?

Diquat and other herbicides are approved for use in the U.S. by the EPA. The agency completed its most recent review of diquat in 2020, allowing its continued use in a variety of applications including lakes and waterways.

Diquat is also commercially available as an ingredient in several household landscaping products.

Critics of the Army Corps’ plans have frequently pointed out that diquat is banned in the European Union and the United Kingdom. Diquat lost its registration status in the E.U. in 2019, but it has been approved for emergency use dozens of times since, including in Ireland, Denmark and Hungary.

EPA is in charge of approving herbicides for the wider market, but Connecticut DEEP reviews and approves any requests to apply diquat or other chemicals within the state’s waterways. As part of its approval process, DEEP requires users post signs letting people know when it’s safe to swim, fish or use the water for irrigation purposes.

What are the alternatives?

Other methods for combating invasive aquatic plants include the use of mechanical harvesters or large mats to block sunlight, preventing plants from growing.

Due to the costs and inability to target specific plants without also impacting large numbers of native species, Army Corps officials determined that neither option was suitable for use along the Connecticut River.

In 2017, voters in New Fairfield adopted an ordinance to prevent officials from using diquat and other herbicides to combat a similar infestation of invasive weeds on Candlewood Lake. As an alternative to using herbicides, the Candlewood Lake Authority began introducing sterile grass carp to feed on watermilfoil and other weeds. Between 2015 and 2017, more than 8,000 carp were stocked into the lake.

Those actions effectively controlled excess vegetation, Mark Howarth, the CLA’s executive director, said in an email. But in 2022, “there was a big shift in the lake and virtually all of the vegetation was eliminated,” including native species, Howarth said. The die-off was attributed to both the voracious carp and the annual lowering of water levels by the lake’s owner, FirstLight Energy. As as result, DEEP has been working to remove the carp from the lake until the vegetation can recover.