Eriophyid mites in classical biological control of weeds: Progress and challenges

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

Field assessment of the host range of aculus mosoniensis (Acari: Eriophyidae), a biological control agent of the tree of heaven (ailanthus altissima)

Tree of heaven (Ailanthus altissima) is a fast-growing deciduous tree native to China, considered a serious invasive species worldwide, with several socio-economic and ecological impacts attributed to it. Chemical and mechanical methods have limited efficacy in its management, and biological controls may offer a suitable and sustainable option. Aculus mosoniensis (Ripka) is an eriophyid mite that has been recorded to attack tree of heaven in 13 European countries. This study aims to explore the host range of this mite by exposing 13 plant species, selected either for their phylogenetic and ecological similarity to the target weed or their economic importance. Shortly after inoculation with the mite, we recorded a quick decrease in mite number on all nontarget species and no sign of mite reproduction. Whereas, after just one month, the population of mites on tree of heaven numbered in the thousands, irrespective of the starting population, and included both adults and juveniles. Significantly, we observed evidence of damage due to the mite only on target plants. Due to the specificity, strong impact on the target, and the ability to increase its population to high levels in a relatively short amount of time, we find A. mosoniensis to be a very promising candidate for the biological control of tree of heaven

Open field evaluation of Aculodes altamurgensis, a recently described eriophyid species associated with medusahead (Taeniatherum caput-medusae)

Medusahead (Taeniatherum caput-medusae, Poales: Poaceae) is an annual grass native to central Asia and the Mediterranean region. It is a noxious, invasive weed in much of western North America. As such, it is the target of a classical biological control programme established by USDA-ARS. During explorations carried out in 2014 a new species of eriophyid mite, Aculodes altamurgiensis de Lillo & Vidovic, 2018 (Acari: Eriophyidae), was discovered on medusahead in southern Italy; it was also collected from medusahead in Bulgaria, Serbia, Turkey, and Iran in subsequent years. In the field A. altamurgensis has consistently been associated with the target weed while never having been detected from sympatric grass (Poales: Poaceae) species (e.g. Stipa austroitalica, Avena sativa, Triticum durum, T. aestivum), suggesting that A. altamurgensis is highly specific to medusahead. An open-field host-specificity test was carried out in Rome, Italy in 2016 in which an Italian population of A. altamurgensis was infested onto 11 different grass genotypes, including five crop species and five different populations of medusahead (two from Italy and three populations that are invasive in the USA). The results supported the previous observations that A. altamurgiensis is highly specific to medusahead and merits further evaluation as a candidate for biological control of this invasive grass. However, the Italian population of A. altamurgiensis showed variable colonisation rates on different medusahead populations, indicating that populations of A. altamurgiensis from other regions should also be tested to determine if they can colonise medusahead at more uniformly high rates, particularly on the targeted invasive populations in the USA

Drought stress promotes the colonization success of a herbivorous mite that manipulates plant defenses

Climate change is expected to bring longer periods of drought and this may affect the plant’s ability to resist pests. We assessed if water deficit affects the tomato russet mite (TRM; Aculops lycopersici), a key tomato-pest. TRM thrives on tomato by suppressing the plant’s jamonate defenses while these defenses typically are modulated by drought stress. We observed that the TRM population grows faster and causes more damage on drought-stressed plants. To explain this observation we measured several nutrients, phytohormones, defense-gene expression and the activity of defensive proteins in plants with or without drought stress or TRM. TRM increased the levels of total protein and several free amino acids. It also promoted the SA-response and upregulated the accumulation of jasmonates but down-regulated the downstream marker genes while promoting the activity of cysteine—but not serine—protease inhibitors, polyphenol oxidase and of peroxidase (POD). Drought stress, in turn, retained the down regulation of JA-marker genes and reduced the activity of serine protease inhibitors and POD, and altered the levels of some free-amino acids. When combined, drought stress antagonized the accumulation of POD and JA by TRM and synergized accumulation of free sugars and SA. Our data show that drought stress interacts with pest-induced primary and secondary metabolic changes and promotes pest performance