Strength of Top-Down Forces on the Establishment of Diorhabda

Strength of Top-Down Forces on the Establishment of Diorhabda spp. on Tamarix spp. in the Southwestern U.S. 
 Kelly Tobin^1*, Anna Sher², Shannon Murphy³ 

¹University of Denver, Denver, CO, USA

²University of Denver, Denver, CO, USA

³University of Denver, Denver, CO, USA

Northern tamarisk beetles in the Diorhabda genus are specialist herbivores that were released across the western US as a biocontrol for invasive shrubs in the genus Tamarix. The biocontrol program has had varying degrees of success, with some sites supporting robust populations of beetles and others failing to establish. Previous studies have suggested that predation plays an important role, but limited data are available on how the relative strength and interactions of their predation affects establishment or how environmental factors may affect these interactions. Despite the knowledge that ecological communities include a variety of predators, how the top-down forces of vertebrate vs. invertebrate predation may cause local extinction events is not well documented in existing literature. We will evaluate how the strengths of top-down pressures from insectivorous birds and invertebrate predators can be used to predict the presence and persistence of Diorhabda by building a model with independent variables of field data from sites across the southwest, including plant cover by species, soil, climate, and geo-morphological variables. We will also quantify potential bird predation pressure using plasticine models of larvae and compare the impacts of predator-exclusion treatments. Invasive Tamarix has altered the soil chemistry, success of native species, and value of its range as wildlife habitat in riparian areas across the southwest. Our study measuring trophic cascades will help land managers make restoration decisions that consider the multi-trophic interactions in critical watersheds. Being able to predict the impacts of predation on beetle populations will provide key information on deciding which Tamarix control and removal methods are appropriate for a given site.