Emerald Ash Borer Biocontrol via Interactions with Native and Introduced Parasitoid Wasps

Emerald Ash Borer Biocontrol via Interactions with Native 
and Introduced Parasitoid Wasps

Devin Jameison, University of Maryland

Introduction

The emerald ash borer (EAB; Agrilus planipennis) is an invasive beetle species that has wrought immense havoc upon ash trees in deciduous forests across the eastern US and Canada. First detected in Michigan in 2002, EAB was detected in Maryland just one year later, although the pest likely escaped detection for at least a decade. It is estimated that EAB has caused billions of dollars in environmental and industry damage (Kovacs et al. 2010). EAB adults are easily identifiable due to their iridescent, metallic green hue and body size measuring about 1 cm in length. In contrast, the larvae stay out of sight, boring into the inner cambium of ash trees. Here they feast upon the nutrient-rich wood tissue in serpentine, S-shaped patterns known as galleries. With high density galleries carved throughout their inner bark, ash trees’ phloem nutrient supply is cut off, ultimately leading to their demise. Once larvae grow in size through four instars, larvae bore deeper chambers within the wood to pupate and then later emerge as adults.

Ongoing Research: Biological Control

The Gruner Lab at the University of Maryland is currently partnered with the USDA in EAB biocontrol management. Initially, the use of pesticides as a large-scale biocontrol method was investigated. More specifically, emamectin benzoate tree injections were administered in infested ash trees and found to confer a protective neighboring effect between trees. However, high tree proximity is necessary for this to occur. Not only is such spacing seldom seen in nature, but large-scale tree injection treatments are too costly an endeavor to sustain for such a widespread invasive species. Given this, the focus shifted towards classical biological control, in which natural predators of the invasive species are introduced into non-native environments. In this case, the introduced predators are four parasitoid wasp species from Eurasia, namely Tetrastichus planipennisi, Oobius agrili, Spathius agrili, and Spathius galinae. Of the four, Spathius galinae currently shows the greatest potential for biocontrol. To collect relevant data, the lab identifies Maryland sites with heavy EAB damage, extracts infested trees to incubate and rear larvae in barrels, and then assesses parasitoid presence through analysis of parasitism rates. Larvae retrieval also involves debarking trees, in which smaller diameter trees have their bark stripped off. Parasitism rates are measured primarily as a function of parasitoid wasp presence per year for a given site rather than EAB counts. In the last two years there has a been a sharp uptick in parasitism rates across all our field sites. However, our data is still under further analysis and more time is necessary to conclusively state that the introduced species are established.

     

 

Left: Left: Spathius (S.galinae), one of the the four parasitoids wasps introduced for EAB biocontrol.
Photo credit: Gruner Lab

Right: One of the 20-gallon barrels in the UMD greenhouse used to incubate EAB from collected logs. The urine sample cups are used to capture emerging EAB adults and associate arthropods on from logs.
Photo credit: Gruner Lab

EAB and Ash Tree Identification
Ash Tree Identification
Ash trees are hydrophytes, which means that they are naturally abundant in swamps, wetlands, or deciduous forests with high water tables. The most common species of ash in Maryland are green ash, white ash, and black ash. Ash trees possess the following distinguishing characteristic from other trees:
1.)Bark: Mature ash trees possess corky, ridged bark that often forms diamond shaped patterns (Fig. 1). How pronounced the corky bark is varies between species. For immature ash, the bark is not as corky, but still displays the diamond pattern and often is covered with splotches of moss in swampy environments.

2.)Opposite Branches: Branches split from nodes opposite each other rather than in an alternating fashion (Fig 2).

3.)Compound Leaves and Bud Shape: Ash tree leaves are pinnately compound; there are at least 5 leaflets, but usually 7-9, alongside 1 terminal leaflet on separate stalks attached to the main stalk (Fig 3). Pairs of leaflets are positioned opposite each other along the petiole. Moreover, the axillary buds are shaped like Hershey kisses (Fig 4).

4.)Leaf Color and Smoothness: Newly formed ash leaves for white ash may be reddish green in color (Fig 5). Moreover, some species of ash (such as blue ash) have leaves that are slightly toothed or serrated (Fig 6).

Ash trees are commonly confused with hickory, box elder, elm, walnut, and mountainash. Careful observation and looking for the traits mentioned above will always help to distinguish between them

 

 

 

  

Signs of EAB Infestation
The presence of current or previous EAB infestation is often made obvious by numerous dead ash trees accompanied by trees that are half dead. Half dead ash trees exhibit at least one of the following “signs” of ongoing EAB infestation:

1.)D-shaped Exit Holes: When adult EAB bore their way out of host trees they leave behind D-shaped exit holes that are similar in size to this “D” right here. Exit holes are markedly obvious when located and are the most reliable indicator of EAB presence (Fig 7).

2.)Bark Splits and Galleries: The bark will have long, irregular splits due to the tree attempting to grow over and engulf the larvae (Fig 8). Trees also do this to repair phloem damage caused by EAB galleries. Bark splits are usually where old galleries are visible, and are also common where the bark is falling off since the galleries result in dead cambium tissue.

3.)Woodpecks: Heavily infested ash trees typically are covered in woodpecks across the entire length of the tree (Fig 9). Since woodpeckers will predate upon all larval stages, woodpecks are almost always found near galleries.

4.)Thinning Crown: The crown of an infested ash tree is usually in poor health, lacking fully flushed leaves on some branches and exhibiting evidence of dieback (Fig 10).

5.)Epicormics: Offshoots from the main trunk that trees produce under immense stress. Most occur around the base of trees, but some can form higher up as well (Fig 11).

Lastly, infested ash trees may be covered with vines, such as wild grape or ivy, especially in swampy habitats. This is due to thinned crowns allowing light to penetrate further below and the weakened state of stressed trees.

Outreach
Given the speed at which EAB decimate ash populations, field sites provide trees and larvae for only a few years at most. As such, research labs at University of Maryland and USDA-ARS are always in search of new leads for EAB-infested ash trees or stands to supply us with further infestation data, ash health recovery, and EAB larvae, which we require to rear parasitoid wasps for biocontrol release. The best way to contribute to our research and EAB management as a whole begins with learning how to identify ash trees and EAB infestation signs, as described in the section above. If you have or know of ash trees that we could potentially use in the future, we strongly encourage you to reach out to us at the following contacts:

Daniel Gruner, Gruner Lab PI- 301-405-3957 dsgruner@umd.edu
Renee Dollard, Gruner Lab Manager rdollard@umd.edu
Gruner Lab website: https://www.google.com/url?q=https://www.grunerlab.org/emerald-ash-borer.html&sa=D&source=docs&ust=1686961145051291&usg=AOvVaw3Ko04dYoAtLH28HIM9wUF_

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