Bacillus thuringiensis kurstaki
Bt (kurstaki)- Rate
- Label rate — ½–4 tsp per gallon on home concentrates; varies by product
- Best for
- Young caterpillars with all-black heads, in May
Pest
Spongy moth
Lymantria dispar
The short answer
The spongy moth (Lymantria dispar) is the insect most Americans still know as the gypsy moth. In July 2021 the Entomological Society of America dropped "gypsy moth" from its list of common names because it uses a derogatory term for the Romani people. On 2 March 2022 it adopted "spongy moth", after the moth's sponge-like egg masses and the French name spongieuse. It is the same European insect under either name. The hairy caterpillars feed on more than 300 kinds of trees and shrubs, with oaks among the favourites, and in outbreak years they strip whole stands of trees in May and June.
hero macro · older caterpillar showing blue and red dot pairs
Late-instar Lymantria dispar caterpillar on an oak leaf. Photo: pending.
Identification
The caterpillar is the stage people notice. Down its back run five pairs of blue dots followed by six pairs of rusty red dots, which is the tell to look for. It is hairy, and the hairs can irritate skin. It hatches at about 1/16 inch and grows to 2 inches or more; UMass Extension gives over 3 inches just before pupation. Young caterpillars have all-black heads, and older ones have obvious yellow markings on the head, which matters for timing Btk.
The egg masses are what gave the moth its new name. They are tan to brown, fuzzy and sponge-like, about 1½ inches long and ¾ inch wide, and each holds 500 to 1,000 eggs. Females lay them on trunks, branches, stones, walls and outdoor household articles such as patio furniture, grills and trailers, which is how the moth hitches rides into new areas. Adult males are brown with black markings and feathered antennae. Females are larger and white with dark zigzag markings. In the European strain established in North America, the females do not fly.
Easily confused with
- Eastern tent caterpillarAlso a pestBuilds a white silk tent in a branch fork and has a white stripe down its back with blue spots along the sides. Spongy moth caterpillars make no tent and carry paired blue and red dots.
Damage symptoms
Newly hatched caterpillars chew small holes in the leaves. Older ones eat the whole leaf, often leaving only the larger veins. The University of Wisconsin puts about 90% of the season's feeding in the last two instars, which is why damage seems to appear all at once in June. In an outbreak the defoliation is regional: UMass records 923,000 acres of it in Massachusetts in 2017.
- Small holes in young leaves in May, then whole leaves eaten down to the main veins
- Oaks and other hardwoods stripped bare by late June
- Brown, rounded droppings and shredded leaf pieces piling up under the trees
- Hairy caterpillars with blue and red dots crawling on trunks, walls and fences
- Tan, fuzzy egg masses on bark, stones and outdoor furniture from late summer through winter
- Dead caterpillars hanging head-down on trunks after a wet spring: the fungus Entomophaga maimaiga at work
Lifecycle & degree days
One generation a year, and about three-quarters of it is spent as eggs. The eggs hatch in spring, the caterpillars feed for five to six weeks, and the moths fly and lay the next egg masses in midsummer. Timings below are for Massachusetts and Wisconsin.
- Egg hatch
- First week of May (MA); 80–100 degree days, base 50 °F
- Feeding
- 5–6 weeks, to about the third week of June (MA)
- Moths
- Late June to July; neither sex feeds
- Generations / yr
- 1
- Overwinters
- Egg masses, 500–1,000 eggs each
J
F
M
A
M
J
J
A
S
O
N
D
Host crops
OaksBirches, especially black birchPoplar, willow and alderBasswood, hawthorn and larchApple, cherry, maple, hemlock, pine and spruce (other hosts)
USDA APHIS counts more than 300 species of trees and shrubs as hosts. UMass lists its preferred hosts as alder, basswood, birch, hawthorn, larch, oak, poplar, willow and witch hazel. UMass also lists apple, cherry, elm, hemlock, hickory, maple, pine and spruce among the other hosts, and says it mostly avoids American holly, arborvitae, ash, balsam fir, black walnut, catalpa, flowering dogwood, locust, mountain laurel, red cedar, rhododendron, sycamore and yellow poplar. The moth is established across the Northeast and into the upper Midwest, and APHIS lists it in 20 states and the District of Columbia.
Monitoring & action thresholds
Action threshold
No numeric threshold for a garden tree
None of the extension sources we checked gives an egg-mass count for a single home tree. What they agree on is tolerance. An otherwise healthy tree can usually withstand one year of complete defoliation, and most trees put out new leaves in July. Repeated years of defoliation cause branch dieback and can kill trees, so a tree that was stripped last year is the one most worth protecting this year.
From autumn to early spring, look for egg masses on trunks, branches, stones, walls and outdoor furniture; each one you find and destroy is up to 1,000 caterpillars fewer. In early May, watch young leaves for pinholes and tiny black-headed caterpillars. Check their heads before spraying Btk: all-black heads mean it will work, and yellow markings mean it is late. After a wet spring, look on trunks for dead caterpillars hanging head-down. That is the fungus, and in Wisconsin it and the virus helped crash the last outbreak.
Beneficial
Biological control
Two naturally occurring diseases do most of the work. The fungus Entomophaga maimaiga, probably introduced from Japan, is now common across the moth's range. It needs a wet spring, can kill caterpillars even when their numbers are low, and leaves dry, dead caterpillars hanging head-down on the trunks. A nucleopolyhedrosis virus (NPV) behaves the other way: it builds up at high densities, overwinters in the soil and is what collapses outbreaks. The US Forest Service produces and registers an NPV product, Gypchek, for suppression programmes run by government agencies. The University of Wisconsin credits both the fungus and the virus with ending that state's recent outbreak. Neither has a profile here. The one control a homeowner can apply is Btk, and only while the caterpillars are small.
Cultural controls
The moth spends about three-quarters of its life as an egg mass, and that is where hand work pays off.
- Scrape egg masses you can reach into a can of soapy water from autumn to early spring
- Wrap burlap or sticky bands around trunks in June and check them daily; living caterpillars can crawl over a band full of dead ones
- Check patio furniture, grills, trailers and toys for egg masses before moving them out of a quarantined area
- Do not move firewood from infested areas
Chemical, last resort
Sprays only work on young caterpillars, and by July the year's damage is doneCheck any product before it goes on. Open the compatibility matrix →
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
Entomological Society of America, 'Spongy Moth' adopted as new common name for Lymantria dispar (2022)
USDA APHIS, Spongy Moth (plant pests and diseases page, accessed 2026)
UMass Extension, Spongy Moth fact sheet (Simisky & Brown, updated 2025)
University of Wisconsin–Madison Extension, Spongy Moth (Liesch & Williamson, 2026)
University of Minnesota Extension, Spongy moth (reviewed 2022)
Bonide Thuricide concentrate label, EPA Reg. 4-226 (2016)
Monterey B.t. label, EPA Reg. 70051-106-54705 (2014)
USDA Forest Service, Gypchek: Bioinsecticide for Gypsy Moth Control (FHTET-2012-01, 2nd ed.); EPA Reg. 27586-2
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Spongy moth by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.