Understanding Gypsy Moth Outbreaks

gypsy moths on tree
Photo: Tom Worthley

Gypsy moths (also known as the North American Gypsy Moth or the European Gypsy Moth) were imported to North America from Eurasia in 1869 for a silk production experiment. They have caused periodic defoliations in New England since then and particularly severe defoliations in the early 1980s and again in Connecticut and Massachusetts in 2016 and 2017. (Gypsy moths do not build webs – the webs you see in cherry trees are tent caterpillars.)

Female moths lay between 500 and 1,000 eggs that overwinter until spring when they hatch. Eggs are usually found underneath the bark scales of trees, on trunks, branches or other protected sites. Eggs last for 8-9 months before they hatch. Adults only live for about one week while they mate and lay eggs. Gypsy moth populations can persist with very low numbers for years but under the right conditions can have outbreak years where populations explode.

The caterpillars feed on leaves of most deciduous trees and many conifers as well. After feeding for some time they descend to the ground by means of silk threads to change size (molt). Silk threads and numerous hairs on the bodies of small, early-instar (stage) caterpillars allow them to be spread by the wind. These caterpillars change size three times before entering the pupal stage and maturity.

Gypsy moths only damage trees during the larval (caterpillar) stage when they are feeding on the leaves, and leaf-feeding and defoliation is the only type of damage they do. In high numbers they can completely defoliate the trees. One gypsy moth caterpillar can eat as much as eleven square feet of leaf area.

Most deciduous trees have the ability to re-set buds and produce a second set of leaves following defoliation.

dead caterpillars on tree
Photo: Tom Worthley

Coniferous trees do not have this ability. Multiple defoliations can be problematic for the trees. Gypsy moth caterpillars will feed on most tree and shrub species (500 total species!) but prefer oak and beech. Tulip trees (yellow poplar) are not affected. Pines and hemlocks are likely to die after one defoliation.

Multiple defoliations combined with drought are causing individual tree and stand-level mortality in some areas. Trees that have not “leafed-out” in 2018 can be seen in numerous locations around the state. In some places, large individual roadside trees and trees near structures that have died present potential future safety hazards. In forest stands on state and private forest lands, sufficient oak mortality can be observed to warrant consideration of forest harvesting activities to salvage timber value. Private woodland owners are well advised to consult with a CT-Certified Forester to evaluate their woodland conditions. (Links provided below.)

Since the 1980s, a fungus from Japan, Entomophaga maimaiga, has been keeping gypsy moth populations under control but during dry conditions the fungus is less active. Gypsy moth populations seem to explode when there are dry conditions during the spring and summer months.

Natural controls include:

  • Birds (limited effectiveness, small instars only)
  • Vertebrates (deer mice and shrews)
  • Invertebrates (ants and ground beetles, parasitic flies and wasps)
  • Viral Disease Wilt
  • Pathogens like Entomophaga maimaiga fungus

There are a few management tools available:

  • Bacteria-based treatments exists.
  • Soapy water sprays (horticulture soap/oils mixed with water).
  • Traps to catch adults.
  • Finding and destroying egg masses (too late for 2018).

Pesticides are not commonly used because of chemical toxicity and are impractical for entire forests. If used for an individual tree be sure to read the label.

On the UConn campus in Storrs the arborist crew will spray some campus trees using bio-based spray. Trees that don’t leaf out will be removed. Some salvage of dead trees in the UConn Forest will take place as appropriate for fuel wood and saw logs. Inspections will be done in early summer.

A Certified Forester should be consulted for stand-level management and a licensed arborist for individual trees near homes and buildings. More information is available from the Department of Energy and Environmental Protection: http://www.ct.gov/deep/cwp/view.asp?a=2697&q=589362&deepNav_GID=163 and the Connecticut Agricultural Experiment Station; http://www.ct.gov/caes/site/default.asp .

To find an arborist link to the CT Tree Protective Association, www.ctpa.org.

Article by Tom Worthley

[Read More …]