6.3 Using Forest Survey Data to Map the Defoliation Potential of Gypsy Moth
David A. Gansner
Summary
A model that uses forest stand characteristics to estimate the likelihood of gypsy moth (Lymantria dispar) defoliation has been developed. It was applied to recent forest survey plot data to calculate susceptibility ratings for counties in a seven-state area of the U.S.
where gypsy moth is an immediate threat. Locations of individual ground plots have been digitized, so the defoliation potential of plots could be used to develop a map showing the spatial distribution of critical locations in the region. Declines in susceptibility have been recorded for forest areas that have been subjected to intensive defoliation. The susceptibility ratings and maps give resource and pest managers a better understanding of what lo expect from the gypsy moth and thus a basis for improved decisions for coping with the pest.
Keywords: Forest survey, forest health, Lymantria dispar L., defoliation, susceptibility, hazard classification, pest management
6.3.1 Introduction
The gypsy moth may be the most notorious tree pest of our time. Accidentally released in 1869 by a French scientist living in Massachusetts, the gypsy moth has generally spread south and west leaving its mark in the form of millions of defoliated acres of forested land and dead trees.
Resource and pest managers are constantly being challenged to show that the benefits of gypsy moth control outweigh the costs. Unfortunately, levels of defoliation vary greatly within areas infested by the pest. So, practical methods for identifying highly susceptible locations (that is, those most likely to suffer heavy defoliation during an infestation) would be a great aid to cost effective control efforts.
6.3.2 A Guide for Estimating Defoliation Potential
separating stands at risk of heavy defoliation from those where defoliation is likely to be light - and not for predicting actual defoliation in any given stand.
6.3.3 Forest Survey Data Base
The USDA Forest Service updates statewide timber resource information approximately every 10 years. The last inventory of Pennsylvania was completed for 1989. Latest reinventories for the six other states included in this analysis were: Delaware-1986, Kentucky-1988, Maryland-1986, Ohio-1990, Virginia-1986, and West Virginia-1989.
Thousands of ground plots were measured in these seven states. Measurements from those plots provided the data base for mapping defoliation potential.
6.3.4 Procedure
The defoliation potential model was used to calculate susceptibility ratings for all of the forest inventory ground plots in Pennsylvania and six surrounding states. By design, each inventory plot represents a given proportional share of the forest area in a county. So, appropriate weights could be applied to plot data to derive average susceptibility ratings for each county. Exceptions are counties designated nonforest (such as Philadelphia county in Pennsylvania) where no ground plots have been sampled.
Only three of the four variables included in the defoliation potential model could be used in the analysis. Crown condition was not measured by forest inventory crews and no appropriate surrogates for the crown condition variable are available. This is not a serious problem because crown condition does not account for a large amount of the variation in defoliation.
The process of assigning susceptibility ratings to counties began with Pennsylvania.
Values were calculated for each of Pennsylvania's forested counties using data from the 1978 inventory. Ratings ranged from a high of 26.8 for York to a low of 9 for Erie (Fig. 1).
Rating class boundaries of <12, 12 to 17.9, and 18+ were used to sort Pennsylvania's counties into three groups representing low, medium, and high levels of defoliation potential (Fig. 2). This grouping correlates well with the history of annual gypsy moth defoliation recorded on rough sketch maps since 1978. Satisfied with these results for Pennsylvania, we applied the methodology to the most recent forest inventory plot data available to get defoliation potential for counties in all seven states. (Fig. 3).
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warren 11.7
Mckean 9.5 102
Mercer 16.0
Lawrenc 12.6 Beaver
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Buller 12.9
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AlleghenyI 13.8 ./
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Jwashington c:..} 13.9
Jefferson 14.3
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Greene 15.2 � 14.9
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;Fig. 1 . Gypsy moth susceptibility ratings for Pennsylvania counties, 1978 .
Bradford 13.3
susquehanna 102
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Susceptibility Rating (%)
c::::=l < 12 r:::2:3 1 2 to 1 7.9
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Fig. 3. Gypsy moth defoliation potential in seven threatened states, by county.
Using county ratings to estimate the distribution of susceptible forest has its limitations.
Counties with low susceptibility ratings can contain areas with high defoliation potential and vice versa. Average ratings for counties tell us nothing about the amount or location of susceptible forest within a county. For example, Pike and Lebanon Counties in Pennsylvania both have high ratings, but Pike has five times more forest land. Also, ratings for counties with very little forest are based on data from very few ground plots and may be subject to high sampling errors.
Ratings for individual ground plots provide a more specific view of defoliation potential. The locations of individual plots have been digitized, so susceptibility ratings for each plot can be mapped. This map provides a better look at the spatial distribution of critical spots in the seven-state region (Fig. 4).
Susceptibility Rating
t\) less than 20 %
• 20 % plus
Fig. 4. Gypsy moth defoliation potential: each dot represents approximately 1 0,000 acres of forest land.
6.3.5 Results and Implications
The final products of this effort are susceptibility ratings and maps showing current defoliation potential in a seven state area where gypsy moth is an immediate concern.
There are no major surprises here. Areas with the highest defoliation potential are those where oaks, especially chestnut and black are major components of the forest. Low ratings reflect the prevalence of species such as yellow-poplar, ash, red maple, black cherry, hemlock and pine that rank lower on the list of preferred hosts.
A comparison of ratings based on data from current forest inventories with those based on previous inventories reveals some shifts in defoliation potential. For example, Bedford County, Pennsylvania had a susceptibility rating of 20.2 in 1978. But heavy defoliation and drought during the 1980's led to high mortality. salvage cutting, and growth reduction in the oaks. At the same time, less preferred host species such as hemlock, yellow-poplar,
6.3.6 References
FOSBROKE, D.E.; HICKS, R.R. Jr., 1993: Predictability of gypsy moth defoliation in central hardwoods: a validation study. 156-170. In: Proceedings of the 9th Central Hardwoods Conference, Gillespie, A.R.; PARKER, G.R.; P OPE, P.E.; RINK, G. eds.: March 8-10, 1993, West Lafayette, IN, U.S. Dep. Agric. For. serv. North Central Forest Experiment Station, St. Paul MN, General Technical Report NC-161. 515 pp.
GANSNER, D.A.; QUIMBY, J.W.; KING, S.L.; ARNER, S.L.; DRAKE, D.A.: Tracking changes in the susceptibility of forest land infested with gypsy moth. U.S. Dep. Agric. For. serv. North
eastern Forest Experiment Station, Radnor, PA res. pap. (in press).
HERRICK, O.W.; G ANSNER, D.A., 1986: Rating forest stands for gypsy moth defoliation. U.S. Dep.
Agric. For. serv. Northeastern Forest Experiment Station, Broomall PA, res. pap. NE-583. 4 pp.
