Gypsy Moth

Gypsy moth is a cyclic insect that has been a problem in York county and will be again. From 2007-2009, York county experienced the outbreak phase of the gypsy moth. Following several warm, dry spring seasons, insect numbers had increased to heavy infestation levels. Insect distribution in the county was uneven, some areas were unaware of the problem, while others had caterpillars crawling up homes and tree trunks, leaving tree canopies without leaves in June and July. York County participated with the DCNR Gypsy Moth Suppression Spraying program in May of 2008 and 2009.

The combination of spraying and the insect life cycle greatly reduced egg mass numbers in late summer of 2009, when the gypsy moth population crashed and the insect entered the innocuous phase. Wet, cool spring weather favors a naturally occurring fungus and no spray program has been needed in York County since 2010.

Some time in the next 3-7 years, insect numbers will increase, egg masses will become evident and the caterpillars will plague the county again. Homeowners need to be vigilant. While surveillance is ongoing in county parks and selected areas around the county during the 'off' years, staffing limitations and legal prohibitions against trespass on private lands mean homeowners need to recognize the signs of a population increase and report it to the Gypsy Moth Program Coordinator for your county. 

To find information on gypsy moth in Pennsylvania, visit the Department of Conservation and Natural Resources Gypsy Moth website.

The gypsy moth suppression spraying program takes place ONLY when there is sufficient caterpillar activity. The insecticide used is a biological and only effective when consumed by caterpillars at the proper stage of development. Details for enrollment in the Gypsy Moth Suppression Program are made available at the appropriate time of year in the Events section of the Extension website. For the status of suppression spraying in Pennsylvania in 2013 see DCNR to Resume Spraying Woodlands to Combat Gypsy Moth Damage. The current outbreak is located in northwest and north central counties. 

The gypsy moth life cycle in York county follows this pattern: August-April: egg masses sitting silently waiting for spring. Look for them low to the ground, under rocks at the base or trees, in woodpiles, in sheltered spaces during the innocuous phase (which we are in now). Mid-late April: tiny caterpillars hatch, climb to the tops of trees and spin silken threads to be dispersed by the wind. May-June: caterpillars grow to a size of 2-3 inches and do their damage during this phase. This is the ONLY time when they feed, and the ONLY time the spraying will be effective. By mid-June or early July, the caterpillars are forming pupae. Adult moths will emerge in late July to early August. Female moths are large, white and flightless, male moths are smaller, brown and find the females by following chemical scent cues (pheromones). Eggs are laid in August. When numbers are low, gypsy moth may carry out their entire life cycle in evergreens like spruce.

There are other defoliating caterpillars in the woods. Here are some that are often confused with gypsy moth:

Forest Tent Caterpillar
Eastern Tent Caterpillar
Fall Webworm
Orange Striped Oakworm

For individuals who are exceptionally sensitive to pesticides, and should avoid application areas, please visit the Penn State Pesticide Education Hypersensitivity Registry blog for important information.

Diane Oleson
York County Gypsy Moth Program Coordinator
Phone: (717) 840-7408

Return to Pest Pages