Douglas-fir Tussock Moth
Early Warning System
Individual Plots
From 1979 to 2001, trap catches were reported for 4,332 plot-years during outbreak initiation years and for 8,958 plot-years during non-outbreak initiation years. About 15.2% of the outbreak initiation plot-years exceeded the 25-moth threshold, while 3.4% exceeded the threshold in non-outbreak initiation plot-years.
The large proportion of traps that remain below threshold even during outbreak years reflects the highly aggregated distribution of DFTM populations (Shepherd et al. 1985, Mason 1996).
Several factors should be taken into account when interpreting individual plot catch results.
First, consider the time elapsed since the last outbreak occurred in the general area. Because outbreaks tend to occur at 7 to 14-year intervals (Shepherd et al. 1988, see also a graph of subregional trends), high trap catches towards the end of this interval may be more indicative of an impending outbreak than high catches earlier in the interval.
Second, the history of catches for a particular plot or small number of individual plots may be useful. Some plots may prove to be consistent predictors of impending outbreaks, while others may consistently yield above-threshold catches that are not followed by outbreaks, and still others may always capture low numbers of moths even during outbreak years. Plots that have been monitored during the several outbreaks should be evaluated in the context of that plot's historical relation to outbreaks.
Third, both the general trend in plot catches and the distribution and density of plots in the area to be monitored should be considered when interpreting results from an individual or small number of plots. Depending on individual plot history, high catches in one or very few plots may not be significant if other plots in the general area do not exhibit similar increasing trends.
