Portland’s parks department will be treating Deering Oaks for browntail moths this spring, and the park will be closed during and immediately after treatment.

Treatment is scheduled for the week of May 16-20, when the city’s Parks, Recreation & Facilities Department will be applying an organic insecticide to infested trees through ground spraying and misting into the tree canopy.

Deering Oaks will be closed to all guests during treatment and for four hours following each application, the city said in a news release. Applications are expected to take place prior to 8 a.m., and police, park rangers and signs will be placed around entrances during the closures.

“It is imperative to treat the trees now because of the public health risks associated with browntail moths’ poisonous hairs, and because the moths can harm and even kill the trees over a couple of seasons,” the city said. “The organic insecticide treatment aligns with leaf-out, which is necessary to minimize defoliation and prevent the caterpillars from molting and releasing their hairs into the air.”

The Portland Farmers’ Market will relocate to Payson Park during the treatment and for three weeks afterward. From May 16 through June 12, residents can visit the market Wednesdays and Saturdays in Payson Park.

Last year, the city relocated the farmers’ market for six weeks following treatment because it was unclear how long it would take to see a decline in the browntail moth population, parks Director Alex Marshall told the City Council this week.

“After evaluation from 2021, we’ve decided to decrease that time away from Deering Oaks,” he said.

Browntail moths are an invasive species first seen more than 100 years ago in Massachusetts that pose health risks to both humans and trees. The moths have tiny, poisonous hairs that can cause a dermatitis reaction in humans similar to poison ivy.

The hairs become prevalent once the caterpillars emerge from their webs in late April or early May and begin feeding on foliage. They typically cocoon and emerge as moths in mid-July, at which point the poisonous hairs become most rampant, according to a memo presented to the council.

The moths can also harm native tree species such as white and red oaks, birches and fruit trees. In Deering Oaks, where there are many heritage white oaks, the moth population has accelerated to a level where there could be extensive defoliation if treatment is not done quickly, the memo said.

Over the last few years, city staff have attempted to manage the browntail moth population by clipping nests and injecting infested trees with a pesticide. In 2021, staff sprayed insecticide throughout the park to mitigate the high numbers seen in a January 2021 survey.

“This treatment, along with perimeter tree injections, has proven to be extremely successful and has set us up for a much lighter year of caterpillar activity and treatment coverage,” the memo said.

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