Eriophyid Mites in Classical Biological Control of Weeds: Progress and Challenges

SIMPLE SUMMARY: Eriophyid mites are tiny creatures, no bigger than a speck of dust. All species feed on plants and some can cause considerable damage. These mites have an intimate relationship with the plants that they live on, and most of the known species have been collected only from a single plant species, which suggests they are very specific to their host. They reproduce extremely quickly and can build up populations of millions, if not billions, of individuals within a single season. In recent years, research to evaluate their potential for the biological control of invasive plants has increased. Working with these minuscule herbivores poses challenges and offers opportunities for researchers. We review the most updated information in the context of weed biocontrol, giving current information on the challenges already faced and possible opportunities and solutions. We cover topics on taxonomy, evaluation of safety as biological control agents, impact and efficacy on the targeted plant species, and release and post-release monitoring. By offering the lessons learned from past research in a single updated document, our goal is to equip researchers with a valuable tool to help deal with the challenges and opportunities offered by eriophyid mites for the management of invasive plants. ABSTRACT: A classical biological control agent is an exotic host-specific natural enemy, which is intentionally introduced to obtain long-term control of an alien invasive species. Among the arthropods considered for this role, eriophyid mites are likely to possess the main attributes required: host specificity, efficacy, and long-lasting effects. However, so far, only a few species have been approved for release. Due to their microscopic size and the general lack of knowledge regarding their biology and behavior, working with eriophyids is particularly challenging. Furthermore, mites disperse in wind, and little is known about biotic and abiotic constraints to their population growth. All these aspects pose challenges that, if not properly dealt with, can make it particularly difficult to evaluate eriophyids as prospective biological control agents and jeopardize the general success of control programs. We identified some of the critical aspects of working with eriophyids in classical biological control of weeds and focused on how they have been or may be addressed. In particular, we analyzed the importance of accurate mite identification, the difficulties faced in the evaluation of their host specificity, risk assessment of nontarget species, their impact on the weed, and the final steps of mite release and post-release monitoring.