Douglas-fir Tussock Moth
Early Warning System
Recommendations
The early warning system has been used throughout the range of DFTM in the western United States for over 20 years. The system provided a one- to three-year early warning for seven of the nine outbreaks in which pheromone plot data could be evaluated, allowing managers to focus their attention on those limited areas where direct treatments may be warranted. A figure summarizes key steps in the early warning system and follow-up ground sampling. Following are recommendations for applying the early warning system:
In most areas, permanent plots should be evenly distributed throughout host-type forests at a density of at least 1 plot for every 3,000 acres. Host-type includes forests with significant amounts of Douglas-fir or true firs, excluding coastal forests. Susceptible forests may include areas with no recorded history of DFTM defoliation. The recommended plot density can also be described as 8 plots per township (36 square miles).
Additional permanent plots may be warranted in high-value areas. As shown in the northern Idaho case studies, higher plot densities generally provide earlier alerts to potential defoliation. Establishing artificial shelters may be a cost-effective method for ground sampling pupae, cocoons, and/or egg masses in high-value areas.
Supplementary plots should not be used to augment permanent plots after the latter have indicated population increases. Past applications of such temporary, supplementary traps have resulted in less time to conduct ground sampling and plan management options. A better approach is to have a higher density of appropriately distributed permanent plots in areas of interest.
Rising trap catches should be evaluated in the context of several factors. First, consider the time elapsed since the last outbreak occurred, and be especially watchful if seven or more years have elapsed. Second, the subregional average catch level that signals an outbreak varies among subregions (figure 2), so consider the historic subregional trends in numbers of moths caught in relation to subsequent outbreaks. Third, consider the distribution of those plots with trap catches above the 25-moth threshold, paying particular attention to those areas with many plots above the threshold; in addition, bear in mind that due to within- and among-plot variation, average trap catches of 17 moths or more should spark concern. Fourth, the value of the resources at risk if heavy defoliation occurs should be balanced with the costs of ground sampling to measure DFTM populations in specific locations. For example, if the cost of ground sampling in a critical wildlife habitat area is minor compared to the change in habitat suitability that would follow if extensive tree mortality occurred, then managers may be more willing to conduct ground sampling -- even though ground sampling may reveal low DFTM populations that are not likely to cause serious damage.
When trap catches in a geographic subregion rise above the 25-moth threshold, conduct ground sampling in areas of concern. Within a subregion with rising trap catches, the general location of a potential outbreak is signaled by individual plots with catches near or above the 25 moth threshold. If there are areas of concern to managers within that general area, ground sampling for larvae and egg masses (Mason 1979, Shepherd et al. 1985, Mason and Paul 1994, Mason et al. 1998) becomes necessary to locate patches of high DFTM populations. Ground sampling can be focused on specific locations such as campgrounds, parks, and administrative sites where high DFTM populations could be particularly damaging.
