Response of Black Bears to Gypsy Moth Infestation in Shenandoah National Park, Virginia  

 John W. Kasbohm, March, 1994



This study was conducted from 1985 through 1991 to assess the effects of gypsy moth infestation and subsequent defoliation on the resident population of black bears in the Shenandoah National Park.  Significant data exists documenting pre-infestation conditions (see Garner and Carney studies) enabling a comparison based on radio telemetry, population, and behavioral data.

Gypsy moth defoliation (>60% canopy loss) increased from 546 ha in 1986, to 2,304 ha in 1987, 6,227 ha in 1988, and 17,736 ha in 1989 (34% of the study area).  Chestnut oak and red oak habitat types received the greatest defoliation, with 60% and 45% of these types suffering greater than 60% loss in the Northern and Central Districts respectively.  Infestation resulted in a 99% reduction in acorn production in defoliated stands.  Long-Term Ecological Monitoring System (LTMS) sites experienced a 17.6% oak mortality in areas of 2 - 3 years defoliation.  Areas with multiple years of defoliation were projected to have 50% oak mortality over 20% of the oak habitats in SNP from the initial gypsy moth infestation.

Short-term gypsy moth-induced habitat alterations included dramatic increases in soft mast production (blackberries, raspberries, cherry, grape, and pokeweed).  Thus, short-term impacts have not yet jeopardized the SNP bear population.  The reoccurrence of extensive defoliation events will dictate oak mortality and long-term habitat quality, and ultimately will determine bear habitat suitability in the future.

Bears were able to switch from hard mast (acorns) to soft mast (previously mentioned) with little detectable effect on bear nutrition, physical condition, reproduction, or survival, despite acorn failure.  Litter size did not differ, nor did females skip opportunities (years) to reproduce.  Survival rates were not different from pre-defoliation levels.

Bears used cove hardwood and black locust habitats more often than expected, likely due to increased cherry abundance.  Bears did not avoid defoliated habitat, nor did they significantly increase use of gypsy moth non-host stands (tuliptree forests).  Fall range areas were significantly greater during infestation than before, implying that acorn loss stimulated fall bear movements.  In defoliated areas of Virginia, 220 bear were “harvested” compared to 138 harvested prior to defoliation; a reflection of greater bear movement.  Den entry dates were equivalent, but den emergence and length of denning were significantly later and longer during infestation.  Of the 70 dens used from 1896 to 1990, cavities in live (mostly oak) trees were the predominant den types.  Defoliation resulted in 54% mortality of oak den trees during this period.



Since the accidental release of gypsy moths in 1869 in Medford, Massachusetts, the gypsy moth has slowly spread south and west, and has defoliated up to 25 million ha of forest since 1924.

Gypsy moths defoliate forest cover from early June to late July and can result in permanent forest species composition changes through large scale mortality of host species.  Preferred hosts starts with chestnut oak, followed by northern red oak, white oak, scarlet oak, and black oak.  Heavy defoliation aborts acorn production and can prevent full acorn production for ten years.

Effects of this defoliation on wildlife has been virtually never studied (except a few bird studies).  Black bears in the southern Appalachians are adapted to habitats that are based on acorn hard mast production.  Thus, the impacts of the gypsy moth defoliation has the potential to significantly modify bear habitat through forest cover and yearly food supply changes that can be expected to alter many aspects of bear behavior and population dynamics.

The gypsy moth was first discovered in SNP in 1984 and began extensively defoliating the Park by 1987.  With the intensive black bear radio telemetry study conducted by Carney (1985) and Garner (1986) documenting the basic biology of the population, a unique opportunity exists to investigate the effects of gypsy moth infestation, and the resultant habitat defoliation, on the SNP black bear population.

 The specific objectives for this study were:

1. To determine the short-term effects of gypsy moth infestation on aspects of the SNP black bear habitat.

2. To quantify short-term demographic changes in the SNP bear population during years of extensive gypsy moth infestation.

3. To identify bear behavioral alterations attributable to gypsy moth defoliation.



The 522km2 study area was the North and Central Districts of SNP.



Extensive defoliation of canopy trees results in an immediate decrease in overstory foliage and can cause large scale tree mortality, resulting in short-term and long-term changes in vegetation structure, wildlife cover, and hard and soft mast food abundance. Potentially significant habitat composition shifts away from oak toward non-gypsy moth host species such as red maple, black birch, and tulip tree.




Prior to 1989, defoliation was restricted primarily to the North District.  Most areas were defoliated only one year, but significant acreage of the North District was defoliated 2 - 3 years form 1986 - 1989.  Defoliation occurred in SNP as a patchwork of heavily defoliated (>60%) mixed with areas with lesser levels of defoliation (<60%), and stands relatively untouched by the moth.  Non-defoliated areas tended to occur in drainages, primarily along streambeds, where non-gypsy moth host trees (tulip trees) were in greater abundance.


Effects of Defoliation on Acorn Production

Overall, non-defoliated plots produced 119,500 acorns/ha compared to a yield of only 1,600 acorns in areas of first time defoliated stands; a 99% reduction in hard mast production.  The one plot defoliated for two consecutive years produced no acorns the second year.


Effects of Defoliation on Forest Temperature

Maximum daily temperatures in defoliated stands averaged 4.7 degrees C higher than in non-defoliated stands during peak defoliation.  After refoliation was complete, maximum daily temperatures continued to be 2.5 degrees C warmer in defoliated areas.


Effects of Defoliation on SNP Vegetation

Sites with 2 - 3 years defoliation experienced the largest oak and maple density declines and the greatest mortality per year.  For oak species, yearly mortality was greater than 20% in three of the five sites with 2 -3 years defoliation.  The most severely affected site experienced 85% oak mortality over a three year period (from 304 trees/ha to 43 trees/ha).  At sites of 0 to 1 year defoliation, oak tree losses were less than 4% /year with one exception, which experienced a 17% oak mortality/year.  Hickory mortality was related directly to the number of years of defoliation, with some significant plot losses, although overall loss of hickory and maple averaged only 3 hickories and 5 maples/ha died during gypsy moth infestation.  Depressed growth rates were found among hickories and oaks with 2 to 3 years of defoliation.  Soft mast trees (dogwood, hackberry, blackgum, sweet and black cherry, sassafras, and elderberry) exhibited significantly greater growth in increased years of defoliation.  Similarly, soft mast shrubs and vines (spicebush, Virginia creeper, gooseberry, black raspberries, greenbriar, poison ivy, viburnums and grape) increased dramatically in three of the five 2 - 3 year defoliated sites.



Despite extensive gypsy moth defoliation and immediate habitat and microclimate changes, bears in general did not avoid defoliated areas.  Elevated temperatures did not affect bear ranges.  Hard mast loss was compensated for by soft mast production.  Hard mast is known to be an important fall bear food for maintaining bear nutrition and reproduction.  Areas of 60% defoliation were subjected to a complete acorn failure, and even moderate defoliation (>20% and <60%) resulted in a 50% reduced acorn production.  Limited studies show acorn reduction lasts for 1 to 5 years following foliage defoliation.  With oak mortality, acorn production will decrease, in all cases, except where less than 25% mortality occurs, in which case, increased growth of surviving trees may offer partial compensation and actually increase production as much as 20%.

Multiple years of defoliation resulted in substantial, short-term oak mortality (17% on average), up to 85% in 3 years at one site, and decreased oak density.  Another study found one year defoliation resulting in 18% oak mortality, with 2 and 3 year defoliation resulting in 89% and 98% mortality respectively.  Such statistics indicate that a conservative estimate suggests that a greater than 50% oak mortality is likely over at least 20% of the oak habitats and 15% of all habitat types in the North District.  (This is higher than PA studies that reported only 4% of stands received greater than 50% mortality, but about 30% suffered 20 - 50% mortality.  Although not born out by this study, other studies often reveal overall oak regeneration failure due to competition to less preferred species (such as black birch, beech, red and striped maple), loss of acorns as a seed reserve, the inability of oaks killed following defoliation to produce vigorous stump sprouts, and the death of existing seedlings.  Studies have also described reduced woody seedling regeneration due to increases in pioneer species such as blackberries and understory trees and shrubs.

The immediate and short-term effects of gypsy moth infestation reveal trade-offs with respect to long-term bear habitat quality.  While substantial and widespread oak decline will unquestionably reduce hard mast production, especially with replacement by birch and maple forests, moderate oak decline will result in increased acorn production from surviving oak trees, diversify Park habitat, increase production of soft mast, and thereby increase habitat quality.  Gypsy moth infestation in SNP has not yet resulted in sufficient habitat alterations to jeopardize the Park bear population.  However, repeated defoliation events will dictate oak mortality and long-term habitat changes and ultimately will determine SNP bear habitat suitability in the future.