Modeling interactions of hemlock woolly adelgid and two biological control predatory beetle species in the Great Smoky Mountains National Park

Abstract

Hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), is an invasive insect pest of Tsuga canadensis (L.) Carrière, eastern hemlock, in the eastern US. An A. tsugae infestation often results in the death of T. canadensis within years, and has caused significant changes to hemlock forests. We construct a model to represent the population dynamics of A. tsugae and two introduced biological control organisms, Laricobius nigrinus Fender (Coleoptera: Derodontidae) and Sasajiscymnus tsugae (Sasaji and McClure) (Coleoptera: Coccinellidae), released to control A. tsugae populations and allow T. canadensis survival. Using the model, we study the dynamics of these three species and the joint impact of the two predator species on A. tsugae densities, as not much is known about the coexistence or joint impact of these predators in natural settings. We use field data of A. tsugae, L. nigrinus and S. tsugae densities observed over 3 years in Great Smoky Mountains National Park to develop the model and to perform parameter estimation. The model is life stage structured, with two classes per species, and represents seasonality with time dependent parameters and by using 15 systems of ordinary differential equations throughout the year. We show that the presence of the predators reduce A. tsugae densities in the model. We also demonstrate that the structure of the model allows for the coexistence of the predators as seen in the field data but that this coexistence can be affected by reducing the value of the parameter representing T. canadensis health.