Gypsy Moths

By Linda Peacock, Master Gardener, SCMG

We’ve all heard about Gypsy Moths lately – what are they?  Why are they a problem?  What can we do?

Gypsy Moths (Lymantria dispar dispar) are Eurasian in origin.  Their common name comes from their ability to attach themselves to objects by a silken thread and travel.  When they attach themselves to foliage near the top of a tree, they can be carried away in the wind current for up to one kilometre!

Are They Causing The Webs In The Trees We See?

No. Gypsy Moths do not make nest or webs in trees like other caterpillars.

What Do They Look Like?

Larvae hatch at the same time tree buds are opening usually late April to early June.  They feed on the new growth of their host tree for 7 weeks as they travel up toward the crown.  They grow from 3mm to 7mm and start out either brown or black some with flecks of yellow with long hairs (dark or golden) until they produce 5 pairs blue dots 6 pairs red dots along their back. Some variations have all blue dots.

Pupae are 5 cm long, have a dark brown shell and are covered in hairs.  They appear June to July.

Female adults (moths) are white with dark markings, cannot fly and have a 5 cm wingspan.  Male adults are brown, can fly, have a 2.5 cm wingspan, and have more feathery looking antennae than females.  Adults appear late July to early August for two weeks they do not eat, their sole purpose is to mate and lay eggs.

They overwinter in egg masses that are 4 cm long, tan colour covered with hairs from the female moth’s abdomen.  The colour will bleach out in the sunlight. You can tell how bad an infestation year it is by the size of the egg masses.  When the population is in decline, they are the size of a dime.  A larger mass indicates a stable or growing population.  In an outbreak year they can be the size of a loonie and will hold 100-1000 eggs each.  They can be found on the trunk and underside of branches of host trees, sides and eaves of buildings, outdoor furniture, swing sets, boats, trailers, and firewood.

Are We Having An Outbreak Year?

Outbreak year means nearly all broad-leafed trees can be defoliated in woodland or urban areas.  Outbreaks occur every 7 – 10 years in a region, usually localized areas have 2-3 years of defoliation.  An outbreak is followed by a crash in population caused by competition for the depleted food supply, natural predators, or disease.  We have seen an increased population in many areas of our region.  That will continue for a year or two before the population declines on its own.  The best way to know how it is impacting your area is to monitor your trees each season.

Why Are They a Problem?

Gypsy Moth larvae can defoliate host trees.  They feed on over 300 species of plants – mostly hardwoods: oak, birch, aspen, beech, maple and softwoods eastern white pine and Colorado Blue Spruce.   When a broad-leafed tree is defoliated, they do have the ability to send out a new flush of growth, thankfully this occurs after the larvae (eating) stage is over.  This foliage is tougher and less nutritious most likely to seem less attractive to future attackers.  This second growth takes a lot of energy from the tree and although it can survive one or maybe two years of defoliation if this happens year after year it will weaken and kill the tree.  An evergreen could die after one defoliation.  When the larvae have defoliated their tree hosts, they will feed on understory shrubs and plants, however, up until now not poison ivy.  If food is scarce, they will also feed on unripe tissues of annual flowers and buds.  One larva will eat the equivalent of 1 square metre of foliage.  Forests can lose biodiversity and ultimately some wildlife. When oak stands are defoliated in a forest setting it can severely affect the wildlife in the area.  In a harsh winter, many animals depend on acorns to get their nutrition, especially the deer. Orchard defoliation causes the weakened trees to attract agricultural pests and diseases. And worse yet – large amounts of caterpillars in host trees can cause large amounts of frass to fall.

What You Can Do

Start with the least harmful to your environment, each treatment has its limits – using a variety of methods is most effective.

Healthy trees ward off attacks easier, defoliation is harder on a tree during a drought or when it is already unhealthy.  Urban tree owners need to water trees during dry spells and protect their root zones.  In a natural area, good forestry practices must be maintained.

Hand pick caterpillars while within reach April through June.  Wear gloves when handling larvae – their long hairs can irritate the skin or cause allergic reactions.

May through August place a band of burlap 45 cm wide placed around the trunk of affected tree at chest height.  Secure with string around the middle and fold the upper half down over the string.  This gives them a place to hide from the heat.  Simply lift the burlap and pick them off and destroy them.  This method can also catch adult females climbing up the tree to lay egg masses.  Larvae will also use the protected area to transform into the pupae.  All can be crushed or dropped into a bucket of soapy water to destroy.

Sticky tape around the trunk can catch smaller caterpillars.

Btk (Bacillus thuringiensis “kurstaki”) is a natural bacterium found in soil, it can be used as a foliar spray to poison the caterpillars at all stages.  The caterpillar stops feeding and dies within 5 days.  This bacterium is only effective on insects and will not harm other wildlife, however, it is not selective to Gypsy Moth larvae and will destroy any caterpillars feeding on the foliage including native species.   It is most effective on Gypsy Moth larvae May to early June while they are in the early feeding stages before they defoliate too much.

Small wasps (Encyrtidae family) were introduced to North America in 1909 as a parasite for the Gypsy Moth egg masses.  It is now commonly found here and is an important biological control.  They are capable of parasitizing 30% of the egg mass near surface only, unable to reach eggs in centre of mass.

Native predators include White-Footed Mice, American & Fowlers Toads. Carpenter Ants eat pupae and egg masses.  Robins and Chickadees eat all stages even egg masses.  Blue Jays, Eastern Towhees, Red-Eyed Vireos, Grey Catbirds, Baltimore Orioles, and Indigo Buntings will eat the hairy caterpillars.

Disease caused by the Entomophaga maimaiga fungus and the Nucleopolyhedrosis virus (NPV) are most effective during cool, wet Spring weather.  Both result in carcass disintegration within a few days.  They are believed to be the reason populations crash for many years.

Dormant Oil combined with Lime Sulphur applied to tree late winter will smother eggs before they hatch.

Pheromone Traps attract males July through August before mating can occur.

Scrape away egg masses with a dull knife November through April.  Either burn or drop into soapy water to destroy.

TreeAzin injections – a natural extract from Neem seeds.  When leaves are eaten, it prevents caterpillar from growing any larger, limiting damage.  Will protect tree for one season only.  Timing is important and should be administered by a tree expert.  Only trees that will become defoliated are chosen.

Green traps are seen in trees in many municipal areas.  These are used to monitor populations.

The Asian Gypsy Moth is also now found in Canada.  This moth prefers conifers, is better adapted to our cold climate and the females can fly. That is a topic for another day!