PA Bureau of Forestry briefs Lycoming County on spongy moth biology, thresholds and treatment options

Rose Yu, a forest health specialist with the Pennsylvania Bureau of Forestry, told Lycoming County commissioners on April 16 that the invasive spongy moth (formerly called the gypsy moth) can cause large-scale defoliation and tree mortality if high populations persist for multiple years. She outlined the insect's life cycle, how the bureau determines whether suppression is required and the tools and timing most effective for control.

Yu said the spongy moth caterpillar prefers oaks but will feed on many plants once preferred hosts are exhausted and that repeated heavy defoliation over two to three years can kill trees, especially if other stressors such as drought are present. "The females don't fly," Yu said, describing a limit on adult moth dispersal; she added that people unintentionally move egg masses and that young caterpillars can balloon on silken threads and be carried by wind.

The bureau uses two main methods to decide where to intervene: aerial surveys followed by ground egg-mass surveys. Yu explained the egg-mass threshold the agency uses for suppression decisions: a single tree with about 12–13 egg masses translates to roughly 500 egg masses per acre, a level at which the bureau typically recommends suppression operations.

She described treatment options and timing. The bureau's primary biological pesticide is Bacillus thuringiensis kurstaki (Btk), which is most effective on the first and second instars when caterpillars are small; an insect growth regulator sold under the trade name Mimic (novaluron) is used in other spray blocks. "Timing matters," Yu said. "This product is most effective on the first and second instar of the caterpillar." She and staff emphasized that spraying later, when many caterpillars are in later instars and trees are already defoliated, is unlikely to protect that year's foliage and is generally not recommended.

Yu described delivery options and cost differences. For large areas the bureau uses fixed-wing aircraft or helicopters; state contract costs she provided were about $20 per acre for fixed-wing applications using Mimic and about $50 per acre for helicopter applications using Btk or similar products. She said aerial applications are typically economical starting around 50 acres; smaller treatments can be handled by helicopters, arborists (for systemic injections), or newer drone applicators on a company-by-company basis.

Biological controls present additional, weather-dependent checks on populations. Yu said a native fungus (Entomophaga) and a spongy-moth virus help reduce outbreaks in cool, wet springs and when populations are high; both agents were observed in some areas in recent years. She urged communities to conduct egg-mass surveys in late summer or fall to help determine whether a spray program is warranted in the following year.

Yu said the bureau conducts its planning many months in advance: aerial surveys and mapping begin in June–July the year before operations, then proposed spray blocks are drawn and submitted for environmental review. For 2025 the bureau's proposed blocks were already posted on an interactive map and will be updated as work progresses. "We have all of our proposed blocks on the interactive map. As we go through our program, that is updated daily," Yu said.

After the presentation, commissioners and members of the public asked questions about winter mortality, impacts on predators such as birds and small mammals, and whether homeowner actions such as burlap bands are useful. Yu said burlap banding and hand removal of egg masses can reduce local numbers but are unlikely to meaningfully reduce populations in heavy-outbreak areas. A member of the public, Thomas Adams, urged consideration of predator effects and said several bird species and small mammals also consume the caterpillars and egg masses.

The bureau provided contact information and said additional guidance, the interactive map and a list of licensed aerial applicators are available on the DCNR website.