Field Assessment of the Host Range of Aculus mosoniensis (Acari: Eriophyidae), a Biological Control Agent of the Tree of Heaven (Ailanthus altissima)

The Invasive Ailanthus altissima: A Biology, Ecology, and Control Review

Tree of Heaven (Ailanthus altissima (Mill.) Swingle) is a tree native to China which has invaded disturbed areas in many regions worldwide. Its presence endangers natural ecosystems by displacing native species, modifying habitats, changing community structures, and affecting ecosystem processes. Its invasive nature is enhanced by its high ability to reproduce both vegetatively through root regrowth and sexually through seeds. Seeds, which are wind dispersed, are the main mechanism by which this species reaches new habitats. When they germinate and develop the root system, roots emit new shoots that contribute to a rapid increase in the tree density and the subsequent expansion of the population nearby. The contradictory results about the ecological requirements for seeds to germinate and their degree of dormancy and longevity indicate the complexity and difficulty of understanding the mechanisms that govern the biology and adaptability of this plant. The management of this weed aims at its eradication, with programs based on herbicide applications carried out by injecting the active ingredient directly to the trunk. But, not many active ingredients have shown total control, so new ones should be tested in order to increase the range of available herbicides. During the last few decades, some biological agents have been identified, but their efficacy in controlling the tree and their safety for the local flora have not yet been determined. A correct management strategy should take into account all these aspects in order to contain the expansion of this species and, ultimately, allow its eradication.

Integrative Taxonomy and Synonymization of Aculus mosoniensis (Acari: Eriophyidae), a Potential Biological Control Agent for Tree of Heaven (Ailanthus altissima)

Simple Summary

Tree of heaven, Ailanthus altissima, is a deciduous tree indigenous to China and introduced for ornamental purposes into North America and Europe. It shows a highly invasive profile in field, urban, and suburban areas, posing a serious threat to ecosystems in the introduced ranges. The current management of this noxious infesting plant by mechanical and chemical measures remains ephemeral and incomplete. A multi-tactic approach emphasizing classical biological control appears highly relevant. Eriophyid mites are well known for their high specificity and for the impact caused to the associated hosts, resulting in relevant potential biological control agents of infesting plants. The partially unresolved taxonomy of the eriophyid mite species reported on the tree of heaven is currently seen as an impediment to their further concern as biocontrol agents. This paper investigated morphological and molecular characters of Aculus mosoniensis in order to better clarify its taxonomic status. A paratype specimen of Aculops taihangensis was also studied, and this mite species was reassigned to the genus Aculus. The strong congruence between morphological and molecular analyses for all mites collected on tree of heaven in Europe led to the conclusion that A. mosoniensis is a junior synonym of Ac. taihangensis.

Abstract

The taxonomy of Aculus mosoniensis appears to be an unresolved question and its clarification is required, owing to the potential relevance of this mite species as a biological control agent of the tree of heaven. This paper is aimed at giving accurate details on a previously and shortly announced synonymization with Aculops taihangensis, using a morphological and molecular approach. A fusiform morph of A. mosoniensis was distinguished from a vermiform morph and this latter was recognized as deutogyne, which was herein documented. Phylogenetic relationships between Chinese Ac. taihangensis and all A. mosoniensis mites collected in twenty localities in Europe were examined through the analysis of the mitochondrial cytochrome c subunit I (CO1) protein and the nuclear ribosomal internal transcribed spacer 1 region (ITS1). CO1 sequences of Ac. taihangensis from the Shandong province in China and those from mites collected in Austria and Slovenia were 100% identical; the ITS1 sequence of an Ac. taihangensis paratype matched for 99.8% with those obtained from protogynes and deutogynes of A. mosoniensis collected in Italy. All these data supported the announced synonymization of A. mosoniensis with Ac. taihangensis. Aculusmosoniensis was found genetically variable, with five CO1 haplotypes in Europe (becoming eight along with those of Ac. taihangensis) clustering in two highly supported maternal lineages and eight ITS1 haplotypes (becoming nine along with those of Ac. taihangensis) distributed in four supported clades. No overlap between intra- and interspecies distances was observed for both markers and all studied A. mosoniensis populations clustered in one monophyletic mitochondrial clade, suggesting that only one single species might occur in Europe. However, more mite clades may be related to more tree of heaven biotypes with potential ecological differences, which might have potential effects on the biological control and should be investigated.