Gypsy Moth in the Forest
In this article...
Management Options
Pre-salavge Timber
Timber Stand Improvement (TSI)
Convert to Less Preferred Species
Apply Bt
Do Nothing
Salvage
Wildlife
(Gypsy Moth and Oak Silviculture in the North Central Region)
Gypsy moth is a recent addition to the long list of insects that defoliate oak, aspen, and other hardwood species in the North Central United States. This exotic pest did not occur in North America until it was accidentally introduced into Massachusetts in 1869. Since then, gypsy moth populations have spread west through Pennsylvania and Michigan, and south to North Carolina. As the infestation spreads, the gypsy moth is likely to move into other North Central States besides Michigan.
Like those of other defoliators, gypsy moth outbreaks can last 1 to 5 years. Outbreak populations then decline because of the buildup of disease, natural enemies, and starvation. Low gypsy moth populations may be held in check by birds, insect predators, and small mammals for several years, before the next outbreak occurs. In oak-dominated stands, gypsy moth outbreaks usually persist 2 to 5 years, before collapsing. Populations then remain low for 4 to 12 years before increasing again. Outbreaks may build and decline faster in aspen dominated stands than in oak stands, according to recent observations in Michigan.
Effects of gypsy moth defoliation are usually most severe during the initial outbreak in a newly infested area. If more than 60 percent of the canopy is consumed by gypsy moth caterpillars, trees typically "refoliate," producing a second flush of leaves later in summer. Low rates of diameter growth are common during years of heavy defoliation and often for one or more years after defoliation ceases.
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The amount of tree mortality and topkill sustained during a gypsy moth outbreak depends on the severity and frequency of defoliation, and on tree health. Vigorous trees can often withstand severe defoliation for a few years. However, each refoliation stresses the tree, reducing its energy reserves. Subsequent stress, including additional defoliation, drought, or frost injury, may kill the tree. Defoliated oak trees often become more susceptible to attack by the two-lined chestnut borer or Armillaria root rot. Suppressed, diseased, or low vigor trees may be killed after 1 year of heavy defoliation.
Gypsy moth defoliation can reduce seed production and root-sprouting, resulting in poor regeneration. Diversity or species composition of native insects, birds and other animals may be altered during outbreaks. Severe gypsy moth defoliation, accompanied by an abundance of large hairy caterpillars and frass (fecal pellets), can be unsightly and may reduce the enjoyment of visitors at recreational areas.
Forest health specialists often evaluate hardwood stands to estimate the chance of severe defoliation and the likelihood that mortality will occur. Risk and hazard rating systems can be used to rank stands and to determine where pest problems are likely to occur.
RISK refers to the chance that a stand will be severely defoliated. Risk of gypsy moth defoliation is primarily based on tree species composition. Stands dominated by oaks, aspen, birch, basswood, willow, or other species preferred by gypsy moth will be at high risk of defoliation. On the other hand, risk will be lower in stands dominated by tree species that are less preferred by gypsy moth, such as maples, ash, and most conifers. Risk of defoliation may also be higher where many dead branches, bark flaps, and stem wounds are present, because these areas provide good hiding places for gypsy moth caterpillars and pupae.
HAZARD refers to the chance that stands will be damaged (mortality, topkill, dieback) if defoliation occurs. Hazard depends primarily on the vigor of trees growing in the stand before defoliation. Stand hazard is increased by factors that reduce stand vigor such as overmature trees, suppressed or diseased trees, compacted soil, or severe competition. Hazard also increases when drought, late spring frosts, flooding or other stressful environmental conditions occur. Trees growing off-site are especially vulnerable. In Michigan, tree mortality has been observed primarily in oak-dominated stands where gypsy moth defoliation coincided with drought or late spring frosts.
Appropriate strategies for gypsy moth management depend on:
- the specific management objectives for each stand and
- the quality of the site.
Consider the following strategies when managing for timber, wildlife, or recreation/ornamental tree objectives.
Timber
Pre-Salvage
In high hazard stands nearing economic or pathological rotation age, you may choose to log harvestable trees or stands before gypsy moth defoliation occurs. Pre-salvage harvesting can secure economic benefits while reducing stand susceptibility and vulnerability to gypsy moth defoliation. Regeneration (root- and stump-sprouting) is often high following pre-salvage harvesting.
Remove suppressed and low-vigor trees that will be highly vulnerable to damage by gypsy moth and other pests. Leave healthy trees with large crowns that are likely to survive defoliation.
If you plan to leave mature residual trees, be sure to allow trees 1 to 2 years to recover from defoliation, severe drought, or other stress before beginning pre-salvage operations. Residual trees will require that much time to recover from stress and adjust to increased exposure.
Consider conducting pre-salvage operations during winter, when frozen soils will limit soil compaction and root damage.
Carefully consider methods of stand regeneration following pre-salvage. Root and stump-sprouting can be high after harvest of relatively vigorous trees, but subsequent timber stand improvement (TSI) may be needed to adjust stocking. Residual trees from shelterwood or seed tree cuts may be repeatedly defoliated during a gypsy moth outbreak, reducing seed production.
In understocked stands, consider underplanting, especially with tree species that are less preferred by gypsy moth. However, be aware that white pine and hemlock growing under overstories dominated by oak, aspen, or birch can be severely defoliated and even killed during gypsy moth outbreaks.
Timber Stand Improvement (TSI)
Thinning to reduce stand density can increase vigor of residual trees and reduce stand risk and hazard. Thinning is most appropriate on medium- and high-quality sites, where costs are economically justified.
Increased exposure to sun and wind may cause residual trees to be stressed for a year or two after thinning. Allow trees 1 to 2 years to recover from defoliation, severe drought, or other stress before beginning TSI activities.
To reduce stand hazard, cut suppressed, wounded, diseased, or low vigor trees with poor canopies. Thin to stocking levels appropriate for the species and the quality of the site. Consult stocking guides to determine residual stocking level.
Prevent soil compaction, wounds, and other injuries to residual trees. Anything that reduces vigor of the residual trees can increase stand hazard. Consider conducting TSI during winter, when soils are frozen. Also, consider removing cattle from woodlots managed primarily for timber production. Cattle grazing will cause compacted soil and stress trees.
Thinning or partial cuts are rarely practical on poor quality sites. Consider managing for conifers or non-preferred hardwood species after harvest or salvage operations on poor sites.
Convert to Less Preferred Species
Selecting against tree species that are preferred hosts of gypsy moth will reduce long-term risk of defoliation. On poor sites, consider converting stand cover type to conifers after harvest or salvage. On fertile, mesic sites, altering the species mix can result in stands of high-quality hardwoods. Favoring ash, tulip poplar, maples, or conifers in northern or mixed hardwood stands can help increase stand diversity while reducing the risk of defoliation. Increasing species diversity within stands may also reduce the long-term risks of damage by other forest pests.
High-value oak-dominated stands approaching economic or pathological rotation age can be protected with an aerial application of Bt (Bacillus thuringiensis var. kurstaki), a short-lived microbial insecticide that causes a bacterial disease in foliage-feeding caterpillars. Bt is not toxic to other insects, or to fish, birds, humans, or other animals. It is commonly used in recreational areas to prevent gypsy moth caterpillars from consuming more than 50 to 60 percent of the canopy, the threshold for tree refoliation. Timing of Bt application is critical and is determined by caterpillar age, canopy development, weather, and other factors. Cost of Bt application varies widely; seek estimates from experienced aerial applicators.
Sometimes, taking no action is the best alternative. Young, vigorously growing stands are likely to tolerate even severe defoliation for 2 to 3 years or longer. Gypsy moth, like other defoliators, may act to "thin from below," eliminating suppressed and other low vigor trees that would have eventually died or required removal. Stands with a mixture of species are less likely to sustain severe, repeated defoliation than are stands of nearly pure oak or aspen. No action may also be the best option when stands are at or near optimal stocking levels, especially if vigorous non-host species are present. Carefully consider the cost effectiveness of Bt sprays, TSI, or other management actions in stands with low to moderate risk or hazard.
You may choose to salvage stands damaged during gypsy moth outbreaks. Salvage logging should occur within 6 to 12 months of tree death, before economic value is substantially reduced by stain or decay. It can be used to modify species composition, reducing long-term stand risk and hazard. Salvaging oak after a gypsy moth outbreak may reduce the cost of species conversion on poor quality sites. Avoid damage to residual trees and advance regeneration.
Managing stands to provide wildlife habitat may result in greater numbers of natural enemies that feed on gypsy moth. Understory plants and shrubs, trees with cavities, and fallen trees are important for maintaining populations of predatory and parasitic insects, insectivorous birds, rodents, and other small mammals.
In low vigor stands, severe defoliation may reduce acorn production. Consider planting or favoring other food plants for wildlife, such as serviceberry (Amelanchier sp.), hazel (Corylus sp.), American beech (Fagus grandifolia), and hickories (Carya sp.). When leaving trees for wildlife, consider selecting species that are not preferred by gypsy moth.
Avoid using Bt to protect foliage in stands managed primarily for wildlife. Bt can kill caterpillars of "non-target" butterfly and moth species that are present during spraying. However, high gypsy moth populations may also temporarily reduce diversity and abundance of some native insect species.
Recent research in some Eastem States indicates that closed-canopy bird species may leave severely defoliated areas, while open-canopy bird species typically increase in abundance.
Recreational areas experiencing a gypsy moth outbreak can be unpleasant places for camping or picnics. The swarms of large caterpillars and frass are distasteful, and the hair on the caterpillars may trigger allergic reactions in some people. Defoliation of ornamental and shade trees in late June and July contributes to the annoyance that this pest can cause.
Consider applying Bt in high-use recreational areas during outbreaks to reduce the defoliation and nuisance caused by an abundance of gypsy moth caterpillars.
Consider using burlap, sticky or Teflon-coated bands on the lower trunk of gypsy moth host trees in recreation areas. Use burlap and sticky bands to trap or destroy caterpillars as they move up and down tree stems. Use Teflon-coated bands to prevent caterpillars from crawling up into tree canopies to feed. Banding is economically viable only for high-value ornamental or shade trees. More information on banding for gypsy moth management is available from the Michigan State University Extension Service.
Where practical, water defoliated trees during warm dry periods. Prevent soil compaction and wounds to gypsy moth host trees.
Anonymous. 1993.. Gypsy moth in Michigan: homeowner's guide. Ext. Bull. E-2302. East Lansing, Ml: Michigan State University. 8 p.
Bauer, L.S.; McCullough, D.G.; DuBois, N.R. 1993. Using Bt to control gypsy moth. Ext. Bull. E-2421. East Lansing, Ml: Michigan State University. 2 p.
Gottschalk, K.W. 1993. Silvicultural guidelines for forest stands threatened by the gypsy moth. Gen. Tech. Rep. NE-171. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 50 p.
Hall, D.J.1993. Silvicultural guidelines for oak stands threatened by gypsy moth In tine Lake States. Madison, Wl: Wisconsin Department of Natural Resources. 4 p.
Haack, R.A.; Acciavatti, R.E.1992. Twolined chestnut boron For. Insect 8 Dis. Leafl.168. Washington, DC: U.S. Department of Agriculture, Forest Service. 12 p.
Katovich, S.; Haack, R. 1991. Gypsy moth in the northern hardwood forest. Northern Hardwood Notes 7.10. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 6 p.
McManus, M.; Schneeberger, N.; Reardon, R.; Mason, G. 1989. Gypsy moth. For. Insect & Dis. Leafl. 162. Washington, DC: U.S. Department of Agriculture, Forest Service. 13 p.
Document Author(s): Program Staff
Revised: September 11, 1997
URL: http://www.ent.msu.edu/gypsyed/docs/forest.html