FORESTENTOMOLOGY Methods for Monitoring the Spread of Gypsy Moth (Lepidoptera: Lymantriidae) Populations in the Appalachian Mountains ALEXEI A. SHAROV, ANDREW M. LIEBHOLD,l AND E. ANDERSON ROBERTS Department of Entomology, VirginiaPolytechnic Institute and State University, Blacksburg,VA24061 Downloaded from https://academic.oup.com/jee/article/90/5/1259/2216731 by guest on 24 September 2021 J. Econ. Entomol. 90(5): 1259-1266 (1997) ABSTRACT Gypsy moth, Lymantria dispar (L.), is gradually spreading in North America from New England to the west and south. Monitoring this expansionis important for evaluating effects of population management on the rate of gypsy moth spread, for planning areas regulated by domestic quarantine, and for accurate timing of preventive si\viculturalmeasures. Spread rate was measured as the distance between population boundaries in consecutive years. Gypsy moth population boundaries from 1988to 1995were estimated in northwestern Virginia and southeastern West Virginia using counts of male moths in pheromone-baited traps. Popu- lation boundaries estimated usingthe 10moths per trap threshold were moststable in space and time compared with the boundaries estimated for other thresholds ranging from 1to 300moths per trap. Thus, the 10 moths per trap threshold is reliable for the monitoring of gypsy moth spread. Local spread rates were significantlyautocorrelated in space (range, 80 km) but not in time. The rate of gypsy moth spread decreased from 16.9km/yr in 1984-1990to 8.8 km/yr in 1991-1996.An 8-km intertrap distance was adequate for detecting this decline in the rate of gypsy moth spread. KEY WORDS Lymantria dispar, biological invasion, spread rate, monitoring, pheromone traps, autocorrelation THE GYPSYMOTH,Lymantria dispar (L.), was acci- rate of spread is relatively slow, the full range of the dentally introduced from France to Medford, MA, potential area will probably not become infested in either 1868 or 1869 (Liebhold et al. 1989). Erad- for many years. Prediction of when various areas ication was attempted several times but failed, and will become infested would be useful for: proper the range of the gypsy moth has since spread timing of silvicu\tural measures which can reduce through most of northeastern North America the adverse impact of gypsy moth defoliation (Liebhold et at. 1992). The current distribution of (Gottschalk 1993), planning sampling programs in the gypsy moth includes most of the northeastern areas at risk of defoliation, planning areas for quar- United States and parts of bordering Canadian antine regulation, and planning and evaluating provinces. Another, discrete population that origi- strategies to slow the spread of the gypsy moth nated from a secondary introduction exists in Mich- (McFadden and McManus 1991). igan (Dreistadt and Weber 1989). The primary- and In 1993, the U. S. Forest Service initiated the secondary-infested regions continue to expand. Slow-the-Spread (STS) Program, a pilot project de- The relatively slow rate of spread of the gypsy signed to test the feasibility of slowing the spread moth may be related to its limited dispersal ability. of the gypsy moth over large regions (Leonard and Females in North American populations are unable Sharov 1995). The following 3 project areas were to fly, thus the primary natural mechanism of gypsy established along the advancing front of gypsy moth dispersal is wind-borne movement of 1st in- moth populations: (1) the Appalachian Mountains stars (Mason and McManus 1981). The expansion in Virginia and West Virginia, (2) northeastern of an infested area and the founding of isolated North Carolina, and (3) the upper peninsula of populations also may occur when egg masses or Michigan. The strategy used in this project was to other life stages are accidentally transported on detect and eradicate (or suppress) isolated gypsy human-made objects (McFadden and McManus moth colonies that occurred just beyond the ex- 1991, Liebhold et al. 1992). panding front of gypsy moth populations. Suppres- The gypsy moth feeds on a wide variety of tree sion is considered here as an intermediate step to species (Liebhold et al. 1995). Thus, it is likely that eradication. However, if the colony is located too populations ultimately will invade most of the close to the population front, there may not be United States and Canada. However, because the sufficient time to eradicate the colony. Eradication- suppression of newly established colonies should 1 u.s. Forest Service, 180 Canfield Street, Morgantown, WV reduce their growth and coalescence and thereby 26505. reduce the rate of gypsy moth spread. The Appa- 1260 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 90, no. 5 lachian Integrated Pest Management (AIPM) Project was conducted from 1988 to 1992 in Vir- ginia and West Virginia that was designed to sup- press both isolated populations and high-density populations near the expanding front (Reardon 1991). Reduction of gypsy moth spread rate was one of the AIPM objectives which was adopted by STS. However, STS was designed to slow gypsy moth spread while using fewer pesticide applica- tions than in the AIPM project (Leonard and Sha- Gypsy moth rov 1995). boundaries for thresholds Downloaded from https://academic.oup.com/jee/article/90/5/1259/2216731 by guest on 24 September 2021 To evaluate the effect of these projects on the (mothsllrap) -1 rate of population spread, it is important to have -3 -10 reliable methods for measuring population spread •••• 30 Virginia -100 rates. Gypsy moth populations are traditionally 100km -300 monitored using any of the following 3 methods: (1) aerial maps of forest defoliation, (2) numbers Fig. 1. Study area, its relation to the AIPM project of overwintering egg masses (Kolodny-Hirsch area, and gypsy moth boundaries in 1989 estimated for 6 1986), and (3) numbers of male moths in phero- thresholds of moth counts in pheromone traps. Bound- aries are mathematical functions in a rotated Cartesian mone-baited traps (Talerico 1981, Ravlin et al. coordinate system (x-y). 1987). Egg mass counts are the most reliable method for assessing densities of medium- and high-density populations; thus they are widely used ginia and southern West Virginia were used for for decision-making concerning suppression of out- analysis. Basic intertrap distance was 3 km in West break populations (Ravlin et al. 1987). Counts of Virginia and 2 km in Virginia, although intensive adult males are widely used to detect new isolated trapping grids (lor 0.5 km) were applied in several gypsy moth infestations because pheromone traps places of particular interest. Trap competition may are effective in detecting low-density populations cause a bias in moth counts if traps were located and are less labor-intensive than egg mass sampling too close (e.g., at 0.5 km). Elkinton and Carde (Schwalbe 1981). (1988) found competition between traps separated Population spread can be quantified using popu- by :580 m, but there were no studies on the com- lation boundaries which are the lines that separate petition between traps separated by a large dis- areas where population densities are generally tance (e.g., ~0.5 km). The area covered by dense above or below a specific threshold (Sharov et al. grids of pheromone traps was always <5% of the 1995). Sharov et al. (1996) detected a reduction in total study area. Thus, we believe that the effect of the rate of gypsy moth spread in the central Appa- trap competition on population boundaries was lachians from 1988 to 1994, which likely resulted small. from pest management activity in the area (AIPM Egg masses were sampled using O.OI-ha fixed- and STS projects). radius plots. The density of samples varied from 4 2 Our objectives in this study were (1) to compare to 1011 km . Defoliation was recorded using high- spatio-temporal variability of population bound- altitude optical bar photography (Ciesla and Accia- aries estimated from different population thresh- vatti 1982). The threshold for detecting defoliation olds (including male moth counts, egg mass counts, was ""'30%.Most data were collected as part of the and defoliation) and to select the threshold that is U.S. Forest Service AIPM and STS projects (Rear- most stable and hence most reliable for monitoring don 1991, Leonard and Sharov 1995). Complete population spread; (2) to analyze the autocorrela- details of sampling methods were described by Sha- tion of spread rates in space, time, and among rates rov et al. (1995, 1996). The area of analysis was derived from different population thresholds; this restricted to the mountain region where most his- information is important for planning the spatio- torical data were collected (Fig. 1). temporal scope of monitoring programs and for Boundary Estimation. A best-cell classification measuring change in spread rates; and (3) to assess method (Sharov et al. 1995, 1996) was used to es- the accuracy of population spread rates estimated timate "regular" population boundaries. A bound- from male moth counts in pheromone traps and to ary is considered regular if it has no islands, gaps, or examine the relationship between the accuracy and folds. If a grid of cells is applied to the area, then a the density of pheromone traps. boundary line classifies some cells as occupied by the population and other cells as unoccupied. The best-cell classification method minimizes the num- Materials and Methods ber of grid cells that are misclassified. Population Area and Data. Historical pheromone trap data thresholds of 1, 3, 10, 30, 100, and 300 moths per (1984,1988-1995), egg mass count data (1988-1991), trap; and 1, 3, 10, and 30 egg masses per 0.01 ha and aerial sketch maps of defoliation (1988-1994) were used to estimate boundaries. Defoliation data from the Appalachian Mountains in northern Vir- did not require thresholds. Boundary points were October 1997 SHAROV ET AL.: MONITORING GyPSY MOTH SPREAD 1261 estimated in l-km intervals, then averaged in non- ray with coordinates: (1) space (distance along the overlapping 5-km blocks.