Periodic outbreaks of the Douglas-fir tussock moth (Orgyia pseudotsugata) can result in severe defoliation, posing a major problem for forest conservation, national parks, and commercial logging interests. A naturally occurring baculovirus causes natural epizootics, sometimes leading to collapse of moth populations. Managers may consider spraying additional virus to increase the likelihood or severity of an epizootic in order to minimize defoliation of affected stands. Due to the cost and effort of applying such microbial control, we are interested in identifying the situations in which natural epizootics will result in rapid population crashes, obviating the need for expensive management interventions.
We develop a mathematical model of baculovirus spread in Douglas-fir tussock moth in order to project the likely course of naturally occurring epizootics, with the model parameters informed by a combination of laboratory experiments and field observations of prior outbreaks, and fit the model using a line-search MCMC procedure. Additionally, we compare transmission parameters across sites and years in our observational data set in order to evaluate whether a uniform approach to modeling tussock moth population dynamics yield appropriate management recommendations.
Results/Conclusions
Our model suggests that naturally occurring epizootics are likely to lead to crashes in Douglas-fir tussock moth populations for substantially lower initial infection rates than those detectable in previously published sampling protocols (as low as 1/1000 infected insects in high density populations), indicating that current management practice may intervene unnecessarily in settings where defoliator populations were already likely to collapse. However, additional sampling effort and more exhaustive laboratory assays are required to accurately identify the scenarios where managers can reasonably refrain from acting, and at such low densities measurement uncertainty in the initial conditions can be more influential in predicting the ultimate course of the outbreak. Additionally, preliminary work suggests that site-specific transmission parameters linked to stand composition may impact whether natural epizootics will lead to outbreak-terminating epizootics.