Outbreaking gypsy moth populations can be extremely detrimental to the health of forest ecosystems.  The most effective treatments available to managers are spray formulations of the NPV virus (Lymantria dispar nucleopolyhedrovirus, marketed as Gypchek) or the bacterial insecticide Bt (Bacillus thuringiensis).  Bt is broadly toxic to all species of moths and butterflies and does not replicate after application, whereas the NPV is species-specific to the gypsy moth and does create active infections. However, the NPV is much more costly to produce.  We analyzed the effectiveness of these two control strategies using a system of coupled delay-differential equations that effectively reproduces observed patterns of host-pathogen dynamics.  In the model, we varied the timing and amount of pathogen introduced within a single season, in addition to exploring long-term strategies of spray application across multi-year host population cycles.
Results/Conclusions

Our results suggest that increases in the frequency of control often have a larger impact than increases in amount of control agent used, and highlight the need for population monitoring to inform effective control strategies.  Our model also shows the importance of within-season processes in systems with multiple control strategies and examines the relationship between system behavior and the level of stochasticity. Our analysis should be useful to managers seeking affordable control strategies, as well as providing new insights into the behavior of host-pathogen systems in response to within-and-between season perturbations.