Direct toxicity and emigration: evaluation of herbicide interactions with a biological control agent for Brazilian peppertree (Schinus terebinthifolia)

Abstract

Invasive plant management is costly. Integrating multiple management strategies such as herbicides and biocontrol may improve long-term control of invasive plants, while lowering costs. A South American thrips, Pseudophilothrips ichini (Hood) (Thysanoptera: Phlaeothripidae), was released in Florida, USA in 2019 as a biocontrol agent for Brazilian peppertree, Schinus terebinthifolia Raddi (Anacardiaceae), a highly invasive subtropical plant. However, biocontrol alone may not be enough to effectively manage this plant and integration with other methods will likely be necessary. We examined the impacts of the herbicides glyphosate, imazapyr, two formulations of triclopyr, and a methylated seed oil adjuvant on adult P. ichini, through both direct application and feeding studies of treated plants. Thrips survival was significantly reduced by direct application of glyphosate, imazapyr, and triclopyr amine. However, overall, > 75% of thrips survived all treatments to 120 h (five days) post-application. Survival of P. ichini was reduced by 50% after feeding on triclopyr-treated plant material. However, P. ichini demonstrated aversive behaviors toward triclopyr-treated plant material. Emigration from treated plants occurred for multiple herbicides applied via foliar and basal bark applications. Furthermore, P. ichini that emigrated from herbicide-treated plants to untreated plants were able to produce offspring, suggesting that movement to refugia may facilitate population survival. These data provide insight into one of the key interactions between biocontrol and herbicides, both key components of integrated pest management for S. terebinthifolia. They further demonstrate the need for additional research on how to effectively utilize refugia in integrated management programs for invasive plants.