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Integrated pest management of the Douglas-fir tussock moth
Forest Ecology and Management, 39 ( 1991 ) 119-130
I 19
Elsevier Science Publishers B.V., A m s t e r d a m
Integrated pest management of the Douglas-fir tussock moth R.R. Mason and B.E. Wickman ('.S. Department of Agriculture, Pacific Northwest Research Station, Forestry and Range Sciences Laboratory, 1401 Gekeler Lane, La Grande, Oregon 97850, U.S.A.
ABSTRACT Mason, R.R. and Wickman, B.E., 1991. Integrated pest management of the Douglas-fir tussock moth. l"or. Ecol, Manage., 39:119-130. The Douglas-fir tussock moth is one of the most destructive forest defoliators in western North America. Densities of most tussock-moth populations fluctuate over time with considerable regularity. Fluctuations in density on warm, dry sites where populations have a high intrinsic rate of increase are more likely to periodically reach outbreak numbers than where rates of increase are relatively small. Such cycles of abundance are inherent properties of the tussock-moth system that are not likely to change in susceptible host types. Modern pest-management programs emphasize the annual monitoring of insects in forests with outbreak histories to determine early changes in population numbers and to predict trends. When outbreaks develop, several environmentally safe chemical and microbial insecticides are effective in reducing larval numbers and preventing serious defoliation. Computer models predicting growth loss, tree mortality, and top-kill during outbreaks are available as aids to making management decisions. Silvicultural practices favoring seral nonhost species on high-risk sites may be the best prescription for reducing the impact of tussock moth outbreaks.
INTRODUCTION
Outbreaks of the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), have occurred periodically in western North America for most of the 20th Century. Because of the severe damage infestations cause, considerable effort has been made in recent years to develop an integrated pest management (IPM) plan to deal effectively with the problem. The details of this package are well documented (Brookes et al., 1978; Shepherd and Otvos, 1986 ). A series of handbooks are also available covering the practical aspects of specific subjects in tussock-moth biology, surveys, and control (e.g., Daterman et al., 1977, 1979; Beckwith, 1978; Dewey, 1978; Wickman, 1978, 1979; Mason, 1979; Paul, 1979; Campbell and Stark, 1980; Heller and Sader, 1980; Linnane and Stelzer, 1982; Mason and Torgersen, 1983a). If these IPM recommendations are carefully followed, tussock-moth outbreaks can be controlled and their effects measurably reduced.
R.R. MASON AND B.E. WICKMAN
120 POPULATION
BIOLOGY
The Douglas-fir tussock moth is a univoltine defoliator that overwinters in egg masses on its host tree. Larvae hatch in early summer and feed for about two months. The species occurs throughout the range of its primary hosts: Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco; grand fir, Abies grandis (Dougl. ex D. Don ) Lindl.; and white fir, Abies concolor (Gord. and Glend. ) Lindl. ex Hildebr. Serious outbreaks are limited, however, to a narrower range of sites that support an especially rapid rate of population growth. A variety of natural enemies attacks all life-stages, so that the number of individuals surviving to adulthood is only a small percentage of the number of eggs laid (Fig. 1 ). When generation survival is high, population density increases rapidly. Sites that are warm and dry, especially those with few natural enemies, favor a relatively high generation survival and, thus, support populations with a potentially rapid rate of increase. Most mortality factors act independently of tussock-moth density; i.e., they do not constrain rising populations by killing proportionally more individuals. Several insect parasites, however, do respond numerically to increasing hosts and eventually cause their numbers to decline (Mason and Torgersen, 1987). In outbreaks, a nucleopolyhedrosis virus disease as well as physiological and nutritional factors are frequently also major causes of population collapse (Mason, 1976, 1981 ). I00
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MANAGEMENT OF THE DOUGLAS-FIR TUSSOCK MOTH
121
EARLY DETECTION AND MONITORING
Historically, the first indication of a tussock-moth infestation was the sudden appearance of defoliated trees, often over large forested areas. AfteT two to three years of defoliation, outbreaks collapsed naturally, almost as quickly as they appeared. Although attempts were sometimes made to control infestations by spraying with insecticides, such efforts were usually too late to avoid severe tree damage (Wickman et al., 1973 ). We learned that increases in populations had to be detected early and controlled in advance of defoliation for tree damage to be prevented. Modern practices of managing the Douglas-fir tussock moth emphasize the annual monitoring of insect abundance to give an early warning of population increase. These data are the basis for making decisions on direct control and, when accumulated over many years, also give a broad picture of population behavior useful in forecasting long-term trends. Routine surveillance of populations in high-risk stands is now the backbone of all tussock-moth management programs. Annual censuses continually update the status of tussock-moth populations and give an early warning of upward trends. Censuses are vital as a first line of defence in identifying possible problem areas before trees are defoliated. Census data are collected by using practical methods of monitoring either adults or larvae.
Trapping adults. The most widely used method for monitoring tussock-moth abundance is the pheromone-baited sticky trap. First developed in the Pacific Northwest, the technique is now operational throughout the western United States and British Columbia. Traps are baited with a low concentration of synthetic sex pheromone formulated in plastic for slow release (Daterman et al., 1979). Each year during m o t h flight in late summer, traps are hung from low-hanging branches at permanent monitoring locations. The average number of male moths caught per trap is a relative index of population abundance at that site. Pheromone trapping is still relatively new, and long-term sets of data are yet to be generated, but the procedure has been effective in detecting critical changes in populations. For example, the average density of moths trapped in 1985 at permanent monitoring sites in northern Idaho clearly forecast a minor outbreak the next year (Stipe et al., 1987 ). Successful results are also reported from the annual monitoring of adults in British Columbia (Shepherd et al., 1985). Monitoring larvae. Although trapping of adults is the conventional method of monitoring employed in most tussock-moth IPM programs, annual sampling of small larvae is also a practical technique that has been frequently used by researchers to follow population trends (Mason, 1979; Mason and Torgersen, 1987; Mason and Wickman, 1988 ). In non-outbreak populations, larvae are monitored annually on permanent plots by beating the lower branches of
122
R.R. MASON AND B.E. WICKMAN
sample trees over a hand-held drop-cloth. Densities are easily estimated by the frequency of occurrence of larvae in the samples (Mason, 1987). The density of larvae compared with relative densities in previous years gives a picture of current trends in population behavior. Serial data from permanent plots (Fig. 2 ) illustrate two c o m m o n patterns of behavior in tussock-moth populations. Data from the Blue Mountains in northeastern Oregon are from an outbreak-prone population that has fluctuated sharply over the last 17 years (Fig. 2a). Outbreak densities in 1972 and 1973 caused tree defoliation over tens of thousands of hectares. Numbers crashed to a very low density in 1975, only to build up gradually to another peak in 1983, but this time at a density too low to cause recognizable defoliation. This example demonstrates that all rising populations do not necessarily end in outbreaks, but may be controlled naturally below the threshold where defoliation is noticed. The Cascades data are from a population on the east slope of the Cascade Range in south-central Oregon that has no outbreak history but has fluctuated at low densities for the last 14 years (Fig. 2b). This population seems to be under tight regulation by natural enemies (Mason and
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MANAGEMENTOF THE DOUGLAS-FIRTUSSOCK MOTH
12 3
Torgersen, 1983b, 1987 ). An obvious difference between the two populations is that, after reaching a suboutbreak density in 1971, the population in the Blue Mountains continued to increase rapidly for two more generations; the Cascades population could not sustain a rapid increase in density for longer than a year. The phase diagrams in Fig. 3 further illustrate this point. The clockwise spirals imply negative feedback of density-dependent mortality for both populations, but the feedback is more delayed in the Blue Mountains than in the Cascades, The result is that larval densities are more likely to fluctuate between extremes - indicated by the wide spirals in Fig. 3a - in the a. Blue Mountains 3 2
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R.R. MASON AND B.E. WICKMAN
Blue Mountains. Under these conditions, to keep abreast of current trends and to detect critical increases, annual monitoring is absolutely essential. EVALUATION AND DECISION-MAKING
Tussock-moth outbreaks evolve through a sequence of predictable phases characterized by common population attributes (Mason and Luck, 1978). For convenience, phases are numbered by years that roughly define approximate density, trend, and longevity of the typical outbreak (Table 1 ). Although larvae are common in the first year of an outbreak, densities are not yet high enough to cause noticeable defoliation of trees. Significant defoliation does not occur until the second and third years, after which the population declines sharply. The most difficult problem is the early recognition of populations in phase 1 so that direct control can be initiated before serious defoliation occurs in phase 2. Larval monitoring techniques can readily detect numbers that have reached sub-outbreak densities, but this does not necessarily portend a continuing increase. Sub-outbreaks often decline without developing into a bona-fide outbreak (e.g., Fig. 2 ). When sub-outbreak densities of larvae are detected, further evaluation is needed to determine whether TABLE 1 Description o f typical phases o f an outbreak o f the Douglas-fir tussock moth Phase
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Low density. No visible defoliation. Caterpillars u n c o m m o n , egg-masses rare. Sub-outbreak density. Usually little or no visible defoliation. May be some light feeding in tops o f crowns late in season. Caterpillars c o m m o n . New egg-masses large, o u t n u m b e r old masses.
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IV ( Postdecline )
MANAGEMENTOF THE DOUGLAS-FIRTUSSOCKMOTH
125
the population is truly in phase 1 of an outbreak or is likely to decrease by the next generation. Intergenerational trend is determined by the combined number of insects surviving in all life-stages. The survival of larvae from egg-hatch to pupation, however, is a good predictor of trend in non-outbreak populations (Mason and Overton, 1983); and Fig. 4). When less than 10% of the larvae survive, a significant increase of density in the next generation is impossible; conversely, when more than 30% of the larvae survive to pupate, rapid population growth is predicted. Larval survival in sub-outbreak populations is easily evaluated by comparing the densities of early instars with late instars from respective field samples taken 40-50 days apart (Mason and Torgersen, 1983a). Sub-outbreak populations with high larval survival have a good chance of increasing to outbreak numbers by the next generation; otherwise, densities have probably already peaked and the trend will be lateral or downward. Further evaluation of the density and survival of new eggs may also be needed for confirmation (Torgersen and Mason, 1979; Shepherd et al., 1984a). When forecasts of population trends indicate that an outbreak is imminent, computer models assist in making decisions about the need for direct control. These models project expected tree defoliation and damage for different stages of the outbreak. Two models, an outbreak model and a stand prognosis model, are currently the basis for this decision-making process. The outbreak model projects insect densities and defoliation through a typical outbreak episode, and estimates the subsequent growth loss, tree mortality, and top-kill (OverlO
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R.R.MASONANDB.E.WICKMAN
126
ton and Colbert, 1978 ). The stand prognosis model normally projects healthy stand growth (Stage, 1973 ), but when linked with the outbreak model, it likewise reflects the damage caused by defoliation (Monserud and Crookston, 1982). The ultimate effects of an outbreak on other resources can also be estimated (Fig. 5). If the above procedures are followed, incipient outbreaks can be recognized at least a year before defoliation. One year is usually enough time to project the potential effects that defoliation will have on the resources at hand and to plan intervention tactics when necessary. Although high larval survival is a good predictor of the rapid change from a sub-outbreak to an outbreak density, such a 'population release' cannot be confirmed until it has occurred. An appropriate increase in the fall density of egg-masses would be the first indication that an outbreak has begun, but the egg stage is difficult to sample with precision (Mason, 1970). Final evaluation, therefore, must wait until after egg-hatch in early summer, when outbreak densities of small larvae are easily identified (Mason, 1969).
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Fig. 5. Information inputs, flows, and outputs in the Douglas-fir tussock-moth management model (from Colbert and Campbell, 1978).
MANAGEMENT OF THE DOUGLAS-FIR TUSSOCK MOTH
127
CONTROL
Direct suppression. Outbreaks can be stopped before significant defoliation has occurred by treating larvae with insecticides, sometimes by the aerial spraying of large forested areas. Effective chemical and microbial insecticides are available. A nucleopolyhedrosis virus that frequently causes natural disease in tussock-moth outbreaks has been produced in large quantities from laboratory-reared larvae, and is stockpiled. Aerial or ground applications of virus to small larvae starts an epizootic that eventually reduces population density (Stelzer et al., 1975; Shepherd et al., 1984b; Otvos et al., 1987). Because of the long incubation period of the virus, considerable defoliation may still occur the first year before the epizootic takes effect (Shepherd and Otvos, 1986 ). Formulations of the bacterium Bacillus thuringiensis (B. t. ) are being tested against the tussock moth; they show promise for termination of feeding sooner than virus disease (R.C. Beckwith, personal communication, 1988 ). Other innovative techniques for directly reducing populations may be developed. Tests have been conducted on the effectiveness of releasing large amounts of synthetic pheromone to confuse adult males in finding mates (Sower and Daterman, 1977 ). If practical, this mating disruption technique might be used to curtail the increase of populations previously identified as having a high probability of becoming outbreaks. Silvicultural control. The ideal approach to solving tussock-moth problems would be to manage stands such that populations rarely or never reach outbreak numbers. In theory, this could be done by creating conditions in which insects are naturally constrained below the density where they are damaging. Such conditions undoubtedly already exist in much of the host type where outbreaks have never been a problem. Here, populations remain at low densities and fluctuate with minimal magnitude because intrinsic rates of population increase are low and the response of negative feedback forces is fast (Berryman et al., 1987; Mason and Torgersen, 1987 ). As a rule, outbreaks are most frequent in warm, dry environments where inherent rates of increase are relatively high and negative-feedback processes are slow. Although these processes are ultimately stabilizing, the delay of feedback causes population densities to fluctuate widely and to frequently reach outbreak numbers (Mason and Wickman, 1988 ). The physical conditions favoring outbreaks are caused by climatic and physiographic factors not amenable to alteration. In the Pacific Northwest, for example, the most severe outbreaks occur in fir stands growing on relatively shallow soils on upper slopes and ridgetops (Stoszek et al., 1981 ). Many of these sites were once dominated by ponderosa pine, Pinus ponderosa Dougl. ex Laws., but since the inception of selective logging, they have been occupied by susceptible stands of fir that are perpetuated by fire-exclusion practices (Williams et al..
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R.R. MASON AND B.E. WICKMAN
1980). As long as susceptible host types prevail on these high-risk sites, periodic outbreaks will likely continue. The really effective practice to reduce susceptibility is to return the sites to non-host seral species. Other silvicultural approaches, not yet tried for the tussock moth, may have application for reducing the vulnerability of stands to damage. Fertilizing susceptible stands before an outbreak of the western spruce budworm, Choristoneura occidentalis Freeman, has shown promise in limiting the effects of defoliation (Wickman et al., 1988). Managing soil fertility, although not necessarily affecting insect numbers or outbreak frequencies, may have longterm benefits by enabling stands to withstand short periods of defoliation (Stoszek, 1986). Undoubtedly, much still remains to be learned about improving management of the Douglas-fir tussock moth.
REFERENCES Beckwith, R.C., 1978. Larval instars of the Douglas-fir tussock moth. USDA, Agric. Handb., 536, 15 pp. Berryman, A.A., Stenseth, N.C. and Isaev, A.S., 1987. Natural regulation of herbivorous forest insect populations. Oecologia, 71 : 174-184. Brookes, M.H., Stark, R.W. and Campbell, R.W. (Editors), 1978. The Douglas-Fir Tussock Moth: a Synthesis. USDA Tech. Bull. 1585:338 pp. Campbell, R.W. and Stark, R.W., 1980. The Douglas-fir tussock moth management system. USDA, Agric. Handb., 568:19 pp. Colbert, J.J. and Campbell, R.W., 1978. The integrated model. In: M.H. Brookes, R.W. Stark and R.W. Campbell (Editors), The Douglas-Fir Tussock Moth: A Synthesis. USDA, Tech. Bull., 1585: 216-224. Daterman, G.E., Livingston, R.L. and Robbins, R.G., 1977. How to identify tussock moths caught in pheromone traps. USDA, Agric. Handb., 517:15 pp. Daterman, G.E., Livingston, R.L., Wenz, J.M. and Sower, L.L., 1979. How to use pheromone traps to determine outbreak potential. USDA, Agric. Handb., 546:11 pp. Dewey, J.E., 1978. You could spot the next tussock moth outbreak! Here's how. USDA, Agric. Handb., 543:11 pp. Heller, R.C. and Sader, S.A., 1980. Rating the risk of tussock moth defoliation using aerial photographs. USDA, Agric. handb., 569:23 pp. Linnane, J.P. and Stelzer, M.J., 1982. Protecting ornamental and shade trees. USDA, Agric. Handb., 604:11 pp. Mason, R.R., 1969. Sequential sampling of Douglas-fir tussock moth populations. USDA, Res. Note, PNW-102:11 pp. Mason, R.R., 1970. Development of sampling methods for the Douglas-fir tussock moth, Hemerocampa pseudotsugata (Lepidoptera: Lymantriidae). Can. Entomol., 102: 836-845. Mason, R.R., 1976. Life tables for a declining population of the Douglas-fir tussock moth in northeastern Oregon. Ann. Entomol. Soc. Am., 69: 948-958. Mason, R.R., 1977. Advances in understanding population dynamics of the Douglas-fir tussock moth. Bull. Entomol. Soc. Am., 23: 168-171. Mason, R.R., 1979. How to sample larvae of the Douglas-fir tussock moth. USDA, Agric. Handb., 547:15 pp.
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Mason, R.R., 1981. A numerical analysis of the causes of population collapse in a severe outbreak of the Douglas-fir tussock moth. Ann. Entomol. Soc. Am., 74:51-57. Mason, R.R., 1987. Frequency sampling to predict densities in sparse populations of the Douglas-fir tussock moth. For. Sci., 33: 145-156. Mason, R.R. and Luck, R.F., 1978. Quantitative expression and distribution of populations. In: M.H. Brookes, R.W. Stark and R.W. Campbell (Editors), The Douglas-Fir Tussock Moth: A Synthesis. USDA, Tech. Bull., 1585:39-41. Mason, R.R. and Overton, W.S., 1983. Predicting size and change in nonoutbreak populations of the Douglas-fir tussock moth (Lepidoptera: Lymantriidae). Environ. Entomol., 12: 799803. Mason, R.R. and Torgersen, T.R., 1983a. How to predict population trends. USDA, Agric. Handb., 610:7 pp. Mason, R.R. and Torgersen, T.R., 1983b. Mortality of larvae in stocked cohorts of the Douglasfir tussock moth. Orgyia pseudotsugata (Lepidoptera: Lymantriidae). Can. Entomol., 115: 1119-1127. Mason, R.R. and Torgersen, T.R., 1987. Dynamics ofa nonoutbreak population of the Douglasfir tussock moth, Orgyia pseudotsugata (McD.) (Lepidoptera: Lymantriidae) in southern Oregon. Environ. Entomol., 16:1617-1627. Mason, R.R. and Wickman, B.E., 1988. The Douglas-fir tussock moth in the interior pacific Northwest. In: A.A. Berryman (Editor), Dynamics of Forest Insect Populations: Patterns, Causes, Implications. Plenum Press, New York, pp. 179-209. Monserud, R.A. and Crookston, N.J., 1982. A user's guide to the combined stand prognosis and Douglas-fir tussock moth outbreak model. USDA, Gen. Tech. Rep., INT-127:49 pp. Otvos, I.S., Cunningham, J.C. and Friskie, L.M., 1987. Aerial application of nuclear polyhedrosis virus against Douglas-fir tussock moth, Orgyia psuedotsugata (McDunnough) (Lepidoptera: Lymantriidae). I: Impact in the year of application. Can. Entomol., 119: 697-706. Overton, W.S. and Colbert, J.J., 1978. The outbreak model. In: M.H. Brookes, R.W. Stark and R.W. Campbell (Editors), The Douglas-Fir Tussock Moth: A Synthesis. USDA, Tech. Bull., 1585:209-210. Paul, H.G., 1979. How to construct larval sampling equipment. USDA. Agric. Handb., 545:11 PP. Shepherd, R.F. and Otvos, I.S., 1986. Pest management of Douglas-fir tussock moth: procedures for insect monitoring, problem evaluation and control actions. Can. For. Serv. Pac. For. Res. Cent. Inf. Rep., BC-X-270:14 pp. Shepherd, R.F., Otvos, I.S. and Chorney, R.J., 1984a. Pest management of Douglas-fir tussock moth (Lepidoptera: Lymantriidae): a sequential sampling method to determine egg mass density. Can. Entomol., 116: 1041-1049. Shepherd, R.F., Otvos, I.S., Chorney, R.J. and Cunningham, J.C., 1984b. Pest management of Douglas-fir tussock moth (Lepidoptera: Lymantriidae): prevention of an outbreak through early treatment with a nuclear polyhedrosis virus by ground and aerial applications. Can. Entomol., 116: 1533-1542. Shepherd, R.F., Gray, T.G., Chorney, R.J. and Daterman, G.E., 1985. Pest management of Douglas-fir tussock moth, Orgyia pseudotsugata (Lepidoptera: Lymantriidae): monitoring endemic populations with pheromone traps to detect incipient outbreaks. Can. Entomol., 117: 839-848. Sower, L.L. and Daterman, G.E., 1977. Evaluation of synthetic sex pheromone as a control agent for Douglas-fir tussock moths. Environ. Entomol., 6: 889-892. Stage, A.R., 1973. Prognosis model for stand development. USDA, Res. Pap., INT-137:332 pp. Stelzer, M.J., Neisess, J. and Thompson, C.G., 1975. Aerial applications of a nucleopolyhedrosis virus and Bacillus thuringiensis against the Douglas-fir tussock moth. J. Econ. Entotool., 68: 269-272.
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Stipe, L.E., James, R.L., Livingston, R.L., Schwandt, J.W., Beckman, D.P., Knapp, K.A., Hoffman, J. and Weatherby, J., 1987. Idaho forest pest conditions and program summary 1986. Idaho Dep. Lands/USDA For. Serv., North. Intermount. Regions, Rep. No. 87-l" 32 pp. Stoszek, K.J., 1986. Nutrient stress, insect-caused disturbances and forest ecosystem stability. In: Proc. 18th IUFRO World Congress, Div. 1, Vol. 1. 7-21 September 1986, Ljubljana, Yugoslavia, pp. 97-109. Stoszek, K.J., Mika, P.G., More, J.A. and Osborne, H.L., 1981. Relationships of Douglas-fir tussock moth defoliation to site and stand characteristics in northern Idaho. For. Sci., 27: 431-442. Torgersen, T.R. and Mason, R.R., 1979. Predation and parasitization of Douglas-fir tussock moth egg masses. USDA, Agric. Handb., 549: I 1 pp. Wickman, B.E., 1978. How to time the sampling of tussock moth larvae. USDA, Agric. Handb., 532:6 pp. Wickman, B.E., 1979. How to estimate defoliation and predict tree damage. USDA, Agric. Handb., 550:15 pp. Wickman, B.E., Mason, R.R. and Thompson, C.G., 1973. major outbreaks of the Douglas-fir tussock moth in Oregon and California. USDA, Gen. Tech. Rep., PNW-5" 18 pp. Wickman, B.E., Mason, R.R. and Savage, T.J., 1988. The effects of thinning and fertilization on western spruce budworm, Choristoneura occidentalis Freeman (Lepidoptera: Tortricidae), and grand for growth. In" Proc. 18th Int. Congress of Entomology, Vancouver, B.C., p. 419. Williams, J.T., Martin, R.E. and Pickford, S.G., 1980. Silvicultural and fire management implications from a timber type evaluation of tussock moth outbreak areas. In: Proc. 6th Conf., Fire and Forest Meteorology. Society of American Foresters, Washington, DC, pp. 191 - 196.
I 19
Elsevier Science Publishers B.V., A m s t e r d a m
Integrated pest management of the Douglas-fir tussock moth R.R. Mason and B.E. Wickman ('.S. Department of Agriculture, Pacific Northwest Research Station, Forestry and Range Sciences Laboratory, 1401 Gekeler Lane, La Grande, Oregon 97850, U.S.A.
ABSTRACT Mason, R.R. and Wickman, B.E., 1991. Integrated pest management of the Douglas-fir tussock moth. l"or. Ecol, Manage., 39:119-130. The Douglas-fir tussock moth is one of the most destructive forest defoliators in western North America. Densities of most tussock-moth populations fluctuate over time with considerable regularity. Fluctuations in density on warm, dry sites where populations have a high intrinsic rate of increase are more likely to periodically reach outbreak numbers than where rates of increase are relatively small. Such cycles of abundance are inherent properties of the tussock-moth system that are not likely to change in susceptible host types. Modern pest-management programs emphasize the annual monitoring of insects in forests with outbreak histories to determine early changes in population numbers and to predict trends. When outbreaks develop, several environmentally safe chemical and microbial insecticides are effective in reducing larval numbers and preventing serious defoliation. Computer models predicting growth loss, tree mortality, and top-kill during outbreaks are available as aids to making management decisions. Silvicultural practices favoring seral nonhost species on high-risk sites may be the best prescription for reducing the impact of tussock moth outbreaks.
INTRODUCTION
Outbreaks of the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), have occurred periodically in western North America for most of the 20th Century. Because of the severe damage infestations cause, considerable effort has been made in recent years to develop an integrated pest management (IPM) plan to deal effectively with the problem. The details of this package are well documented (Brookes et al., 1978; Shepherd and Otvos, 1986 ). A series of handbooks are also available covering the practical aspects of specific subjects in tussock-moth biology, surveys, and control (e.g., Daterman et al., 1977, 1979; Beckwith, 1978; Dewey, 1978; Wickman, 1978, 1979; Mason, 1979; Paul, 1979; Campbell and Stark, 1980; Heller and Sader, 1980; Linnane and Stelzer, 1982; Mason and Torgersen, 1983a). If these IPM recommendations are carefully followed, tussock-moth outbreaks can be controlled and their effects measurably reduced.
R.R. MASON AND B.E. WICKMAN
120 POPULATION
BIOLOGY
The Douglas-fir tussock moth is a univoltine defoliator that overwinters in egg masses on its host tree. Larvae hatch in early summer and feed for about two months. The species occurs throughout the range of its primary hosts: Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco; grand fir, Abies grandis (Dougl. ex D. Don ) Lindl.; and white fir, Abies concolor (Gord. and Glend. ) Lindl. ex Hildebr. Serious outbreaks are limited, however, to a narrower range of sites that support an especially rapid rate of population growth. A variety of natural enemies attacks all life-stages, so that the number of individuals surviving to adulthood is only a small percentage of the number of eggs laid (Fig. 1 ). When generation survival is high, population density increases rapidly. Sites that are warm and dry, especially those with few natural enemies, favor a relatively high generation survival and, thus, support populations with a potentially rapid rate of increase. Most mortality factors act independently of tussock-moth density; i.e., they do not constrain rising populations by killing proportionally more individuals. Several insect parasites, however, do respond numerically to increasing hosts and eventually cause their numbers to decline (Mason and Torgersen, 1987). In outbreaks, a nucleopolyhedrosis virus disease as well as physiological and nutritional factors are frequently also major causes of population collapse (Mason, 1976, 1981 ). I00
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MANAGEMENT OF THE DOUGLAS-FIR TUSSOCK MOTH
121
EARLY DETECTION AND MONITORING
Historically, the first indication of a tussock-moth infestation was the sudden appearance of defoliated trees, often over large forested areas. AfteT two to three years of defoliation, outbreaks collapsed naturally, almost as quickly as they appeared. Although attempts were sometimes made to control infestations by spraying with insecticides, such efforts were usually too late to avoid severe tree damage (Wickman et al., 1973 ). We learned that increases in populations had to be detected early and controlled in advance of defoliation for tree damage to be prevented. Modern practices of managing the Douglas-fir tussock moth emphasize the annual monitoring of insect abundance to give an early warning of population increase. These data are the basis for making decisions on direct control and, when accumulated over many years, also give a broad picture of population behavior useful in forecasting long-term trends. Routine surveillance of populations in high-risk stands is now the backbone of all tussock-moth management programs. Annual censuses continually update the status of tussock-moth populations and give an early warning of upward trends. Censuses are vital as a first line of defence in identifying possible problem areas before trees are defoliated. Census data are collected by using practical methods of monitoring either adults or larvae.
Trapping adults. The most widely used method for monitoring tussock-moth abundance is the pheromone-baited sticky trap. First developed in the Pacific Northwest, the technique is now operational throughout the western United States and British Columbia. Traps are baited with a low concentration of synthetic sex pheromone formulated in plastic for slow release (Daterman et al., 1979). Each year during m o t h flight in late summer, traps are hung from low-hanging branches at permanent monitoring locations. The average number of male moths caught per trap is a relative index of population abundance at that site. Pheromone trapping is still relatively new, and long-term sets of data are yet to be generated, but the procedure has been effective in detecting critical changes in populations. For example, the average density of moths trapped in 1985 at permanent monitoring sites in northern Idaho clearly forecast a minor outbreak the next year (Stipe et al., 1987 ). Successful results are also reported from the annual monitoring of adults in British Columbia (Shepherd et al., 1985). Monitoring larvae. Although trapping of adults is the conventional method of monitoring employed in most tussock-moth IPM programs, annual sampling of small larvae is also a practical technique that has been frequently used by researchers to follow population trends (Mason, 1979; Mason and Torgersen, 1987; Mason and Wickman, 1988 ). In non-outbreak populations, larvae are monitored annually on permanent plots by beating the lower branches of
122
R.R. MASON AND B.E. WICKMAN
sample trees over a hand-held drop-cloth. Densities are easily estimated by the frequency of occurrence of larvae in the samples (Mason, 1987). The density of larvae compared with relative densities in previous years gives a picture of current trends in population behavior. Serial data from permanent plots (Fig. 2 ) illustrate two c o m m o n patterns of behavior in tussock-moth populations. Data from the Blue Mountains in northeastern Oregon are from an outbreak-prone population that has fluctuated sharply over the last 17 years (Fig. 2a). Outbreak densities in 1972 and 1973 caused tree defoliation over tens of thousands of hectares. Numbers crashed to a very low density in 1975, only to build up gradually to another peak in 1983, but this time at a density too low to cause recognizable defoliation. This example demonstrates that all rising populations do not necessarily end in outbreaks, but may be controlled naturally below the threshold where defoliation is noticed. The Cascades data are from a population on the east slope of the Cascade Range in south-central Oregon that has no outbreak history but has fluctuated at low densities for the last 14 years (Fig. 2b). This population seems to be under tight regulation by natural enemies (Mason and
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Fig. 2. Population trends of small larvae of the Douglas-fir tussock moth at two independent locations: a, Blue Mountains (Umatilla National Forest, northeastern Oregon); b, Cascade Range (Winema National Forest, south central Oregon (modified from Mason and Wickman, 1988) ).
MANAGEMENTOF THE DOUGLAS-FIRTUSSOCK MOTH
12 3
Torgersen, 1983b, 1987 ). An obvious difference between the two populations is that, after reaching a suboutbreak density in 1971, the population in the Blue Mountains continued to increase rapidly for two more generations; the Cascades population could not sustain a rapid increase in density for longer than a year. The phase diagrams in Fig. 3 further illustrate this point. The clockwise spirals imply negative feedback of density-dependent mortality for both populations, but the feedback is more delayed in the Blue Mountains than in the Cascades, The result is that larval densities are more likely to fluctuate between extremes - indicated by the wide spirals in Fig. 3a - in the a. Blue Mountains 3 2
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R.R. MASON AND B.E. WICKMAN
Blue Mountains. Under these conditions, to keep abreast of current trends and to detect critical increases, annual monitoring is absolutely essential. EVALUATION AND DECISION-MAKING
Tussock-moth outbreaks evolve through a sequence of predictable phases characterized by common population attributes (Mason and Luck, 1978). For convenience, phases are numbered by years that roughly define approximate density, trend, and longevity of the typical outbreak (Table 1 ). Although larvae are common in the first year of an outbreak, densities are not yet high enough to cause noticeable defoliation of trees. Significant defoliation does not occur until the second and third years, after which the population declines sharply. The most difficult problem is the early recognition of populations in phase 1 so that direct control can be initiated before serious defoliation occurs in phase 2. Larval monitoring techniques can readily detect numbers that have reached sub-outbreak densities, but this does not necessarily portend a continuing increase. Sub-outbreaks often decline without developing into a bona-fide outbreak (e.g., Fig. 2 ). When sub-outbreak densities of larvae are detected, further evaluation is needed to determine whether TABLE 1 Description o f typical phases o f an outbreak o f the Douglas-fir tussock moth Phase
0
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Low density. No visible defoliation. Caterpillars u n c o m m o n , egg-masses rare. Sub-outbreak density. Usually little or no visible defoliation. May be some light feeding in tops o f crowns late in season. Caterpillars c o m m o n . New egg-masses large, o u t n u m b e r old masses.
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Outbreak density. Defoliation repeated on all trees. Caterpillars abundant early in season, but numbers decline sharply before pupation. New egg-masses scarce, and smaller than usual.
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IV ( Postdecline )
MANAGEMENTOF THE DOUGLAS-FIRTUSSOCKMOTH
125
the population is truly in phase 1 of an outbreak or is likely to decrease by the next generation. Intergenerational trend is determined by the combined number of insects surviving in all life-stages. The survival of larvae from egg-hatch to pupation, however, is a good predictor of trend in non-outbreak populations (Mason and Overton, 1983); and Fig. 4). When less than 10% of the larvae survive, a significant increase of density in the next generation is impossible; conversely, when more than 30% of the larvae survive to pupate, rapid population growth is predicted. Larval survival in sub-outbreak populations is easily evaluated by comparing the densities of early instars with late instars from respective field samples taken 40-50 days apart (Mason and Torgersen, 1983a). Sub-outbreak populations with high larval survival have a good chance of increasing to outbreak numbers by the next generation; otherwise, densities have probably already peaked and the trend will be lateral or downward. Further evaluation of the density and survival of new eggs may also be needed for confirmation (Torgersen and Mason, 1979; Shepherd et al., 1984a). When forecasts of population trends indicate that an outbreak is imminent, computer models assist in making decisions about the need for direct control. These models project expected tree defoliation and damage for different stages of the outbreak. Two models, an outbreak model and a stand prognosis model, are currently the basis for this decision-making process. The outbreak model projects insect densities and defoliation through a typical outbreak episode, and estimates the subsequent growth loss, tree mortality, and top-kill (OverlO
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R.R.MASONANDB.E.WICKMAN
126
ton and Colbert, 1978 ). The stand prognosis model normally projects healthy stand growth (Stage, 1973 ), but when linked with the outbreak model, it likewise reflects the damage caused by defoliation (Monserud and Crookston, 1982). The ultimate effects of an outbreak on other resources can also be estimated (Fig. 5). If the above procedures are followed, incipient outbreaks can be recognized at least a year before defoliation. One year is usually enough time to project the potential effects that defoliation will have on the resources at hand and to plan intervention tactics when necessary. Although high larval survival is a good predictor of the rapid change from a sub-outbreak to an outbreak density, such a 'population release' cannot be confirmed until it has occurred. An appropriate increase in the fall density of egg-masses would be the first indication that an outbreak has begun, but the egg stage is difficult to sample with precision (Mason, 1970). Final evaluation, therefore, must wait until after egg-hatch in early summer, when outbreak densities of small larvae are easily identified (Mason, 1969).
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Fig. 5. Information inputs, flows, and outputs in the Douglas-fir tussock-moth management model (from Colbert and Campbell, 1978).
MANAGEMENT OF THE DOUGLAS-FIR TUSSOCK MOTH
127
CONTROL
Direct suppression. Outbreaks can be stopped before significant defoliation has occurred by treating larvae with insecticides, sometimes by the aerial spraying of large forested areas. Effective chemical and microbial insecticides are available. A nucleopolyhedrosis virus that frequently causes natural disease in tussock-moth outbreaks has been produced in large quantities from laboratory-reared larvae, and is stockpiled. Aerial or ground applications of virus to small larvae starts an epizootic that eventually reduces population density (Stelzer et al., 1975; Shepherd et al., 1984b; Otvos et al., 1987). Because of the long incubation period of the virus, considerable defoliation may still occur the first year before the epizootic takes effect (Shepherd and Otvos, 1986 ). Formulations of the bacterium Bacillus thuringiensis (B. t. ) are being tested against the tussock moth; they show promise for termination of feeding sooner than virus disease (R.C. Beckwith, personal communication, 1988 ). Other innovative techniques for directly reducing populations may be developed. Tests have been conducted on the effectiveness of releasing large amounts of synthetic pheromone to confuse adult males in finding mates (Sower and Daterman, 1977 ). If practical, this mating disruption technique might be used to curtail the increase of populations previously identified as having a high probability of becoming outbreaks. Silvicultural control. The ideal approach to solving tussock-moth problems would be to manage stands such that populations rarely or never reach outbreak numbers. In theory, this could be done by creating conditions in which insects are naturally constrained below the density where they are damaging. Such conditions undoubtedly already exist in much of the host type where outbreaks have never been a problem. Here, populations remain at low densities and fluctuate with minimal magnitude because intrinsic rates of population increase are low and the response of negative feedback forces is fast (Berryman et al., 1987; Mason and Torgersen, 1987 ). As a rule, outbreaks are most frequent in warm, dry environments where inherent rates of increase are relatively high and negative-feedback processes are slow. Although these processes are ultimately stabilizing, the delay of feedback causes population densities to fluctuate widely and to frequently reach outbreak numbers (Mason and Wickman, 1988 ). The physical conditions favoring outbreaks are caused by climatic and physiographic factors not amenable to alteration. In the Pacific Northwest, for example, the most severe outbreaks occur in fir stands growing on relatively shallow soils on upper slopes and ridgetops (Stoszek et al., 1981 ). Many of these sites were once dominated by ponderosa pine, Pinus ponderosa Dougl. ex Laws., but since the inception of selective logging, they have been occupied by susceptible stands of fir that are perpetuated by fire-exclusion practices (Williams et al..
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R.R. MASON AND B.E. WICKMAN
1980). As long as susceptible host types prevail on these high-risk sites, periodic outbreaks will likely continue. The really effective practice to reduce susceptibility is to return the sites to non-host seral species. Other silvicultural approaches, not yet tried for the tussock moth, may have application for reducing the vulnerability of stands to damage. Fertilizing susceptible stands before an outbreak of the western spruce budworm, Choristoneura occidentalis Freeman, has shown promise in limiting the effects of defoliation (Wickman et al., 1988). Managing soil fertility, although not necessarily affecting insect numbers or outbreak frequencies, may have longterm benefits by enabling stands to withstand short periods of defoliation (Stoszek, 1986). Undoubtedly, much still remains to be learned about improving management of the Douglas-fir tussock moth.
REFERENCES Beckwith, R.C., 1978. Larval instars of the Douglas-fir tussock moth. USDA, Agric. Handb., 536, 15 pp. Berryman, A.A., Stenseth, N.C. and Isaev, A.S., 1987. Natural regulation of herbivorous forest insect populations. Oecologia, 71 : 174-184. Brookes, M.H., Stark, R.W. and Campbell, R.W. (Editors), 1978. The Douglas-Fir Tussock Moth: a Synthesis. USDA Tech. Bull. 1585:338 pp. Campbell, R.W. and Stark, R.W., 1980. The Douglas-fir tussock moth management system. USDA, Agric. Handb., 568:19 pp. Colbert, J.J. and Campbell, R.W., 1978. The integrated model. In: M.H. Brookes, R.W. Stark and R.W. Campbell (Editors), The Douglas-Fir Tussock Moth: A Synthesis. USDA, Tech. Bull., 1585: 216-224. Daterman, G.E., Livingston, R.L. and Robbins, R.G., 1977. How to identify tussock moths caught in pheromone traps. USDA, Agric. Handb., 517:15 pp. Daterman, G.E., Livingston, R.L., Wenz, J.M. and Sower, L.L., 1979. How to use pheromone traps to determine outbreak potential. USDA, Agric. Handb., 546:11 pp. Dewey, J.E., 1978. You could spot the next tussock moth outbreak! Here's how. USDA, Agric. Handb., 543:11 pp. Heller, R.C. and Sader, S.A., 1980. Rating the risk of tussock moth defoliation using aerial photographs. USDA, Agric. handb., 569:23 pp. Linnane, J.P. and Stelzer, M.J., 1982. Protecting ornamental and shade trees. USDA, Agric. Handb., 604:11 pp. Mason, R.R., 1969. Sequential sampling of Douglas-fir tussock moth populations. USDA, Res. Note, PNW-102:11 pp. Mason, R.R., 1970. Development of sampling methods for the Douglas-fir tussock moth, Hemerocampa pseudotsugata (Lepidoptera: Lymantriidae). Can. Entomol., 102: 836-845. Mason, R.R., 1976. Life tables for a declining population of the Douglas-fir tussock moth in northeastern Oregon. Ann. Entomol. Soc. Am., 69: 948-958. Mason, R.R., 1977. Advances in understanding population dynamics of the Douglas-fir tussock moth. Bull. Entomol. Soc. Am., 23: 168-171. Mason, R.R., 1979. How to sample larvae of the Douglas-fir tussock moth. USDA, Agric. Handb., 547:15 pp.
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Mason, R.R., 1981. A numerical analysis of the causes of population collapse in a severe outbreak of the Douglas-fir tussock moth. Ann. Entomol. Soc. Am., 74:51-57. Mason, R.R., 1987. Frequency sampling to predict densities in sparse populations of the Douglas-fir tussock moth. For. Sci., 33: 145-156. Mason, R.R. and Luck, R.F., 1978. Quantitative expression and distribution of populations. In: M.H. Brookes, R.W. Stark and R.W. Campbell (Editors), The Douglas-Fir Tussock Moth: A Synthesis. USDA, Tech. Bull., 1585:39-41. Mason, R.R. and Overton, W.S., 1983. Predicting size and change in nonoutbreak populations of the Douglas-fir tussock moth (Lepidoptera: Lymantriidae). Environ. Entomol., 12: 799803. Mason, R.R. and Torgersen, T.R., 1983a. How to predict population trends. USDA, Agric. Handb., 610:7 pp. Mason, R.R. and Torgersen, T.R., 1983b. Mortality of larvae in stocked cohorts of the Douglasfir tussock moth. Orgyia pseudotsugata (Lepidoptera: Lymantriidae). Can. Entomol., 115: 1119-1127. Mason, R.R. and Torgersen, T.R., 1987. Dynamics ofa nonoutbreak population of the Douglasfir tussock moth, Orgyia pseudotsugata (McD.) (Lepidoptera: Lymantriidae) in southern Oregon. Environ. Entomol., 16:1617-1627. Mason, R.R. and Wickman, B.E., 1988. The Douglas-fir tussock moth in the interior pacific Northwest. In: A.A. Berryman (Editor), Dynamics of Forest Insect Populations: Patterns, Causes, Implications. Plenum Press, New York, pp. 179-209. Monserud, R.A. and Crookston, N.J., 1982. A user's guide to the combined stand prognosis and Douglas-fir tussock moth outbreak model. USDA, Gen. Tech. Rep., INT-127:49 pp. Otvos, I.S., Cunningham, J.C. and Friskie, L.M., 1987. Aerial application of nuclear polyhedrosis virus against Douglas-fir tussock moth, Orgyia psuedotsugata (McDunnough) (Lepidoptera: Lymantriidae). I: Impact in the year of application. Can. Entomol., 119: 697-706. Overton, W.S. and Colbert, J.J., 1978. The outbreak model. In: M.H. Brookes, R.W. Stark and R.W. Campbell (Editors), The Douglas-Fir Tussock Moth: A Synthesis. USDA, Tech. Bull., 1585:209-210. Paul, H.G., 1979. How to construct larval sampling equipment. USDA. Agric. Handb., 545:11 PP. Shepherd, R.F. and Otvos, I.S., 1986. Pest management of Douglas-fir tussock moth: procedures for insect monitoring, problem evaluation and control actions. Can. For. Serv. Pac. For. Res. Cent. Inf. Rep., BC-X-270:14 pp. Shepherd, R.F., Otvos, I.S. and Chorney, R.J., 1984a. Pest management of Douglas-fir tussock moth (Lepidoptera: Lymantriidae): a sequential sampling method to determine egg mass density. Can. Entomol., 116: 1041-1049. Shepherd, R.F., Otvos, I.S., Chorney, R.J. and Cunningham, J.C., 1984b. Pest management of Douglas-fir tussock moth (Lepidoptera: Lymantriidae): prevention of an outbreak through early treatment with a nuclear polyhedrosis virus by ground and aerial applications. Can. Entomol., 116: 1533-1542. Shepherd, R.F., Gray, T.G., Chorney, R.J. and Daterman, G.E., 1985. Pest management of Douglas-fir tussock moth, Orgyia pseudotsugata (Lepidoptera: Lymantriidae): monitoring endemic populations with pheromone traps to detect incipient outbreaks. Can. Entomol., 117: 839-848. Sower, L.L. and Daterman, G.E., 1977. Evaluation of synthetic sex pheromone as a control agent for Douglas-fir tussock moths. Environ. Entomol., 6: 889-892. Stage, A.R., 1973. Prognosis model for stand development. USDA, Res. Pap., INT-137:332 pp. Stelzer, M.J., Neisess, J. and Thompson, C.G., 1975. Aerial applications of a nucleopolyhedrosis virus and Bacillus thuringiensis against the Douglas-fir tussock moth. J. Econ. Entotool., 68: 269-272.
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Stipe, L.E., James, R.L., Livingston, R.L., Schwandt, J.W., Beckman, D.P., Knapp, K.A., Hoffman, J. and Weatherby, J., 1987. Idaho forest pest conditions and program summary 1986. Idaho Dep. Lands/USDA For. Serv., North. Intermount. Regions, Rep. No. 87-l" 32 pp. Stoszek, K.J., 1986. Nutrient stress, insect-caused disturbances and forest ecosystem stability. In: Proc. 18th IUFRO World Congress, Div. 1, Vol. 1. 7-21 September 1986, Ljubljana, Yugoslavia, pp. 97-109. Stoszek, K.J., Mika, P.G., More, J.A. and Osborne, H.L., 1981. Relationships of Douglas-fir tussock moth defoliation to site and stand characteristics in northern Idaho. For. Sci., 27: 431-442. Torgersen, T.R. and Mason, R.R., 1979. Predation and parasitization of Douglas-fir tussock moth egg masses. USDA, Agric. Handb., 549: I 1 pp. Wickman, B.E., 1978. How to time the sampling of tussock moth larvae. USDA, Agric. Handb., 532:6 pp. Wickman, B.E., 1979. How to estimate defoliation and predict tree damage. USDA, Agric. Handb., 550:15 pp. Wickman, B.E., Mason, R.R. and Thompson, C.G., 1973. major outbreaks of the Douglas-fir tussock moth in Oregon and California. USDA, Gen. Tech. Rep., PNW-5" 18 pp. Wickman, B.E., Mason, R.R. and Savage, T.J., 1988. The effects of thinning and fertilization on western spruce budworm, Choristoneura occidentalis Freeman (Lepidoptera: Tortricidae), and grand for growth. In" Proc. 18th Int. Congress of Entomology, Vancouver, B.C., p. 419. Williams, J.T., Martin, R.E. and Pickford, S.G., 1980. Silvicultural and fire management implications from a timber type evaluation of tussock moth outbreak areas. In: Proc. 6th Conf., Fire and Forest Meteorology. Society of American Foresters, Washington, DC, pp. 191 - 196.