DEC reports on herbicide treatment in battle against invasive Ludwigia in Peconic River

The first year of a five-year project aimed at eliminating the infestation of Ludwigia in the the Peconic River with the targeted application of herbicides was very successful, state officials reported in a public meeting last night.

The state for the first time this summer used the widespread application of herbicides in its ongoing — and previously “consistently ineffective” — effort to eliminate the invasive plant, which grows and spreads aggressively to form a dense mat on the surface of the water, upending the aquatic ecosystem and interfering with recreational activities.

“Despite thousands of hours of hand-pulling efforts by volunteers since 2006 — Ludwigia was first discovered the Peconic in 2003 — the 2022 infestation spanned from Connecticut Avenue and Calverton to Peconic Avenue in Riverhead,” Cathy McGlynn, aquatic invasive species coordinator for the State DEC said during an online presentation last night. Ludwigia has not been found east of the Grangebel Park dam, she said.

The herbicide application was conducted on July 27. Additional treatments are planned once each year for the next two years.

“We are anticipating having to do spot treatments and additional work in these areas,” McGlynn said. “We’ll observe every portion of the river to see if there’s any regrowth next year. We plan to do that as part of the overall management effort next year and into the future,” she said.

The treatment this summer was conducted following a successful 2021 pilot study in a small (less than one acre) portion of the river known as Brown’s Bog.

The DEC in July approved five-year permits for the application of a combination of two systemic herbicides that were determined, through lab and field testing, to be effective against the target species with minimal impacts on non-target species, McGlynn said.

The DEC, for purposes of surveying and documenting growth of the plant, divided the affected freshwater portion of the river into nine sections. McGlynn reviewed the results of the 2022 treatment for each section in detail.

The data show “very good control six weeks after treatment,” McGlynn said. Known Ludwigia sites were reduced by 50% to 86% overall among the nine sections.

Treated plants degrade slowly over a period of weeks, which prevents rapid reduction in the river’s dissolved oxygen resulting from the decomposition.

Ludwigia, a federally listed noxious weed, is “notoriously difficult to manage because it reproduces in several ways,” so control even through the application of effective herbicides can take several years of treatment before it is considered successful, according to the DEC’s Peconic River Ludwigia Control Project website.

Also known as floating primrose-willow, Ludwigia peploides, because of its aggressive manner of growth, crowds out other aquatic plants, blocks sunlight and reduces dissolved oxygen in the water body, which negatively impacts fish and other marine life.

Sampling and surveying done pre-treatment and at intervals of up to six weeks after treatment showed that the herbicides did not harm other plants or significantly affect aquatic macroinvertebrates, which are insects in their nymph and larval stages, as well as snails, worms, crayfish and clams that spend at least part of their lives in water.

Randy Wade, who owns a home on the river, said before the treatment there was an abundance of dragonflies hovering over the water and they were gone after the treatment. She asked if the DEC staff could explain that.

The surveys did not sample flying insects, McGlynn said. The method of application, with a wand directly to the leaves of the plants, done by staff members standing in a boat, minimized any dispersement of the chemical through the air.

A significant amount of larval dragonflies were found in the DEC’s post-treatment survey.

“So we know, based on the post treatment macroinvertebrate survey that dragonflies were still able to live in the water column and survive in the water column after the treatment,” said  Nicole White, who is with the DEC’s Invasive Species Coordination Section staff member who did not identify herself before speaking during the presentation. “So the herbicides themselves were not killing the dragonflies that lived there… and would have been exposed to herbicide” at the time of treatment,” she said.

An explanation for the dragonflies disappearing could be that the dragonflies were attracted to the Ludwigia’s bright yellow flowers and once the Ludwigia died off, the flowers were gone and the dragonflies moved to a different spot to look for flowers and habitat, said James Gorman, an aquatic specialist at the Lake and Pond Connection, a DEC contractor on the project.

The DEC is repeating the online presentation and meeting today at 12 noon.

Interested persons can register for today’s meeting at

https://meetny.webex.com/meetny/j.php?RGID=rdefc3f2d191817cabe81005026c38aef