Skvarla, Michael J.¹ ; Ochoa, Ron² ; Ulsamer, Andrew³ and Amrine, James⁴

¹✉ Department of Entomology; 501 Agricultural Science and Industries Building, University Park, PA, 16802, USA.
²USDA-ARS Systematic Entomology Laboratory, 10300 Baltimore Ave, Building 005, BARC-West, Beltsville, MD, 20705, USA.
³USDA-ARS Systematic Entomology Laboratory, 10300 Baltimore Ave, Building 005, BARC-West, Beltsville, MD, 20705, USA.
⁴West Virginia University, Division of Plant and Soil Sciences, P.O.Box 6108, Morgantown, WV 26506-6108, USA

2021 - Volume: 61 Issue: 1 pages: 121-127

Original research

Keywords

Abstract

Introduction

Tree-of-heaven, Ailanthus altissima (Mill.) Swingle (Sapindales: Simaroubaceae), which is native to China, was first introduced to North America in 1784 and was widely planted in urban industrial centers in the mid-1800’s; it is now found across most of the United States and adjacent areas of Canada and Mexico (Hu 1979, Fryer 2010, Ripka & Érsek 2014). Ailanthus altissima is commonly found in disturbed habitats (e.g., forest edges, along railway and road cuts, and even sidewalk cracks) and occasionally in undisturbed habitats. It is considered an invasive, noxious weed by many state and federal agencies due to its ability to outcompete native species and form dense, monoclonal stands, which is in part due to the production of allelopathic chemicals that inhibit the growth of native plants (Fryer 2010, Ding et al 2006, iNaturalist 2019, USDA NRCS 2019). In addition to these direct effects, Ai. altissima is a preferred host for spotted lanternfly (Lycorma delicatula (White, 1845)), an invasive fulgorid planthopper first detected in Pennsylvania in 2014 that is expected to have significant impacts on various agricultural industries (e.g., tree fruit and viticulture production), forest health and timber production, and private homeowners (PSU n.d.).

Ding et al. (2006) reported 46 species of phytophagous arthropods that feed on Ai. altissima in its native range, seven of which have been introduced to North America (Atteva aurea (Cramer, 1781), the ailanthus webworm moth, a native North American species, also feeds on Ai. altissima) . Included in the Ailanthus-feeding arthropods are four species of eriophyid mites – Aculops ailanthi Lin, Jin, & Kuang, 1997, Ac. mosoniensis Ripka & Ersek, 2014, Ac. taihangensis Hong & Xue, 2005, and Aculus altissimae Xue & Hong, 2005. Of these species, only Ac. ailanthi have been reported from the US, in a single state (Maryland) (Gardener, 2008).

De Lillo et al. (2017) suggested that Ac. taihangensis and Ac. mosoniensis are likely synonymous. Author JA agrees and additionally suggests that Ac. altissimae are junior synonyms of Ac. ailanthi as the variations between the species are slight and may be the result of various factors, including over-clearing specimens prior to slide mounting, differences in interpretation of characters of slide-mounted specimens, and poorly made illustrations . However, such taxonomic changes have not been officially proposed following ICZN rules, so we list them as distinct species herein.

Aculops ailanthi are vagrant eriophyids that live on the surface of leaves (as opposed to forming galls or seeking other forms of protection) and can build up large populations on Ailanthus that cause leaf deformation, yellowing, necrosis, premature leaf drop, and death in young, container-grown trees (Lin et al. 1997, Gardener 2008). Ripka & Érsek (2014) and de Lillo et al. (2017) also showed Ac. mosoniensis can cause similar symptoms on Ailanthus in Hungary and Italy, respectively. The majority of the mites are found on the underside of leaflets but are occasionally found on the upper side of leaflets and even on the stem when mite populations are large (Ripka & Érsek 2014, de Lillo et al. 2017).

Herein we report Ac. ailanthi from Pennsylvania and West Virginia for the first time, describe an infestation on and damage to 1.5 m, greenhouse-grown containerized trees, report successful control efforts of the mites, and provide the first scanning electron micrograph (SEM) images of specimens from North America with comments on external morphology.

Materials and methods

Wild 1–1.5 m Ai. altissima trees were collected in State College, Pennsylvania and containerized before budbreak in the spring of 2019; those wild-collected trees were comingled in a greenhouse with similarly sized two-year-old seed-grown containerized trees. On 10 July 2019, the trees appeared sickly (see Results for description of symptoms) and leaf samples were collected for analysis.

The Ailanthus leaves were examined using a Nikon SMZ18 stereomicroscope and large infestations of eriophyid mites were immediately apparent on every leaf, which the lead author suspected to be Ac. ailanthi. Specimens were sent to the USDA-ARS Systematic Entomology Laboratory for examination and species confirmation.

At the USDA-ARS-SEL, the leaves were sectioned into small (approximately 1 × 0.5 cm) pieces and placed onto a sticky carbon pad, which was mounted on to a steel viewing stub at room temperature. The steel stub was placed into the mounting ring of a Hitachi TM3030Plus tabletop scanning electron microscope (SEM), which was cooled to -25 °C with a Deben Standard SEM Coolstage (Suffolk, UK). The specimens were then imaged without further manipulation. Species identification was confirmed by J. Amrine based on the resulting scanning electron micrographs.

Photographs of the Ailanthus leaves and mite infestations were taken using a Samsung Galaxy S6 cell phone and stereomicrographs taken using the cell phone in combination with the SMZ18. After the eriophyid infestation was discovered, the trees were treated with a 2% solution of M-Pede insecticidal soap (active ingredient: Potassium salts of fatty acids) (Gowan USA, Yuma, Arizona, USA).

On 18 August 2018, eriophyid mites were collected from new shoots of wild Ai. altissima along Morgan’s Run Trail, Morgantown, West Virginia. Specimens were cleared in KOH, slide mounted in Hoyer’s and Berlese medium, and identified as Ac. ailanthi. Stereomicrographs of slide mounted specimens were taken using a Zeiss Axioskop microscope.

Results and discussion

The Ac. ailanthi reported here represent the first records of the species in Pennsylvania and West Virginia and the second and third records of them in the United States – Gardener (2008) had previously reported it from Maryland. While it is likely that Ac. ailanthi is present in North America wherever tree-of-heaven is found given the tree has been established on the continent for more than 200 years, all three reports of it are from a limited region in the Mid-Atlantic. Additional surveys should be taken to establish its range in North America given its potential as a biological control agent of Ailanthus (see Smith et al. (2010) for a review of eriophyids that have been proposed or released as biocontrol agents of weeds).

The greenhouse-grown Ai. altissima exhibited similar symptoms to heavily infested wild tree-of-heaven, namely leaf yellowing, leaf deformation and edge curling, stippling, and small necrotic lesions (Figure 1). Because the trees were grown for experiments with spotted lanternfly, they were treated to control the eriophyid infestation before more severe symptoms could develop. The application of 2% insecticidal soap effectively eliminated the mites after one application: two weeks post treatment, MS could not locate a single living mite despite examining more than a dozen leaves from trees that had previously hosted heavy infestations. While tree-of-heaven are not often grown in greenhouses as ornamentals, there may be an increasing need for Ailanthus IPM as the North American spotted lanternfly infestation grows and research to combat it increases.

The SEM images presented here are the first such images of Ac. ailanthi in North America. We were able to capture the eriophyids feeding on the ventral side of the leaf and congregating around the trichomes (Figure 2c). Several individual mites were imaged while feeding directly from the lateral surface of the trichome (Figure 2d). Eggs were located in small groups of 3-6 and larvae were encountered near the eggs (Figure 2e). Spermatophores were randomly distributed across the surface of the leaf.

The SEM images provide an undistorted view of how morphological structures appear on uncleared specimens that are not slide-mounted. The anterior lobe over the gnathosoma is acuminate (which is characteristic of Aculops) (Figures 2a, 2f) but may appear rounded in some specimens (which is characteristic of Aculus) due to specimen orientation on the slides or because the lobes are slightly declivous (Figure 2b). The prodorsal shield is subtriangular with a network pattern with broad cells medially, laterally with granules. The ridges that create the cells are obvious in the SEMs (Figure 2f) but may be obscured or destroyed during clearing and slide mounting (Figure 2a); some of the ridges are absent in nymphs (Figure 2e). Dorsal tubercles are present on rear margin. Dorsal annuli 37–42, ventral annuli 77–87. The female genital flap has 10–12 longitudinal ridges (Figures 3b–d), which may be difficult to observe or destroyed in slide mounted specimens (Figure 3a). The number of rays on the empodia has been used to differentiate species of Ailanthus-feeding Aculops (4 in Ac. altissimae, 5 in the other 3 species); unfortunately, we were unable to crisply image the empodia and cannot at this time determine how many rays the specimens we collected possess.

These images confirm the species present in North America is Ac. ailanthi and may inform the discussion of whether Ac. ailanthi, Ac. taihangensis, Aculus altissimae, and Ac. mosoniensis are synonymous.

Acknowledgements

We thank Dana Roberts, David Long, Scott DiLoretto, and Julie Urban for bringing the mite infestation to our attention, allowing us to photograph their trees, and providing information about the trees and control measures implemented against the eriophyids; and Dr. Cheryle O$’$Donnell and Mr. Armando Rosario-Lebron (APHIS-USDA) for the use of the Tabletop SEM. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA; USDA is an equal opportunity provider and employer.

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