PS 11-159 - Can tamarisk beetle pheromone lures lead to reduced re-growth of tamarisk following tree cutting?

Abstract: Tamarisk (Tamarix spp.), an invasive phreatophyte prevalent in riparian areas across the southwestern United States, threatens native cottonwood (Populus spp.) galleries, which are critical ecological and cultural resources. Tamarisk is notorious for re-sprouting after being cut, even when treated with herbicide. We used green leaf volatile (GLV) compounds and beetle aggregation pheromone (PHE) to attract tamarisk beetles (Diorhabda spp.). We hypothesized that the presence of pheromone lures would attract tamarisk beetles and result in reduced tamarisk re-sprout vigor through increased herbivory. Tamarisk trees in both pheromone treated and untreated plots (N = 4 each) were cut or masticated, but not treated with herbicide, 1 or 3 years before this study, respectively. We hung 10 GLV lures in each treatment plot, approximately equidistant from three target trees, during spring tamarisk leaf-out at the beginning of June 2022. One PHE lure was hung in each of the three target trees on three dates, approximately 3 weeks apart, mid-June and through the end of July, 2022. We counted beetles and measured tree canopy volumes in pairs of pheromone treated and untreated (control) plots. Repeated measures, mixed effect analysis of variance detected no difference in beetle numbers between trees with and without pheromone lures (p = 0.83), but trees with pheromone lures had smaller canopy volumes than those without lures (p < 0.0001). Thus, tree defoliation, but not beetle numbers, indicated reduced growth due to pheromone lures. Beetles were not prevented from frequenting control trees without lures, and we suspect they moved between pheromone treated and untreated trees because the study areas were too small to separate them by distances greater than beetle flight ranges. Our statistical power to detect treatment effects was limited by the small size of the study. Despite mixed results, there is enough evidence supporting our hypothesis to repeat the study on a larger scale.