Print PageThe Douglas-fir tussock moth (Orgyia pseudotsugata) is an important pest of northwestern forests, causing extensive damage to Douglas-fir (Pseudotsuga menziesii) and other host trees. Insect outbreaks are usually terminated by epizootics of a nucleopolyhedrovirus and the virus has been incorporated into tussock moth pest-management systems in a pesticide spray (TM Biocontrol-1). Questions remain regarding the efficacy of TM Biocontrol-1 because historically defoliation has not been significantly reduced in treated areas relative to controls. We examined the dynamics of an epizootic in (N = 3, each) sprayed and control plots during an extensive pest management program in N. Central Washington State (USA) with field observations of disease spread, followed by fitting of stochastic models of disease transmission. In a greenhouse experiment, we tested an additional hypothesis that the efficacy of TM Biocontrol-1 varies with temperature. Baculoviruses such as NPV have been shown to decay with temperature and exposure to UV radiation in other study systems; thus temperature has the potential to affect the efficacy of pesticide spray programs. Populations of tussock moth larvae were maintained on Douglas-fir seedlings at two temperatures (mean temperature difference ~2.5 ºC) with no virus or treated with virus allowed to decay for 0, 1, or 3 days.
Results/Conclusions
Models of disease transmission indicated that, although an epizootic was started earlier in sprayed populations, initial infection rates in control populations may have been high enough to prevent defoliation through high transmission late in the season. Peak infection rates were ~0.65 in the sprayed plots and ~0.35 in the control plots after the first 20-30 days post-spray. Continued transmission caused the late-season epizootic in the control plots resulting in a second increase in infection rates after 30-40 days to ~ 0.85. In the greenhouse experiment warmer temperatures increased the speed of kill by TM Biocontrol-1 by an average of ~1.5 days relative to cooler temperatures. There was, however, no significant effect of virus decay time on speed of kill although a trend toward decreasing efficacy with increasing decay time was evident, indicating the need for field experiments given that greenhouses attenuate UV radiation. Therefore, the geographic extent of the spray program may have led to differences in efficacy of TM Biocontrol-1 through temperature variation with elevation heterogeneity among plots. Furthermore, long-term climate change may affect the efficacy of spray programs, at least at the level of timing their initiation.
