Could biological enemies slow the spread?

By P. Kerr

Popular literature often portrays evil and danger as ugly, while heroes are beautiful. The emerald ash borer is more like a character from C. S. Lewis's books or Steven Spielberg's "Men in Black" movies, because the best fiction is based on truth. Remember Will Smith's line about attacking the world's most feared alien with three worms and a mailman? The USDA is starting an attack on one of the most beautiful and dangerous invasive alien species with three insects and a handful of researchers and technicians.

Barry Lyons of the Great Lakes Forestry Centre in Sault St. Marie, Ontario, Canada, says that when EAB was discovered in North America they had half a sheet of data about the insect. It was written in Chinese and much of the material did not apply to North America. Today volumes could be written about what we know about this beetle. There are charts to predict the range when the adults will emerge according to the temperature. We also know the males have hairier chests, and that this hair acts like "Velcro" during copulation. Ken Marchant, an EAB specialist, has learned to watch for ash trees that change color early, without wilting, to monitor its spread. And, possibly most importantly, we now know EAB has enemies or parasitoids.

With 100 years of experience dealing with alien invasive insects, scientists on both sides of the U.S/Canada border collaborated on the workload and went in search of solutions. Some worked in the confined space of quarantine facilities, others in all weather, and still others crossed the globe. Following here is a sample of what a couple of them are doing to "save our plant" from EAB.

North America lacks a native beneficial insect specialized to attack and consume EAB. As a result, when the insect became established in North Amercia, its population exploded and the sheer numbers of healthy EAB feeding on trees decimated the ash population in infested areas. Dr. Leah Bauer is a research entomologist with the USDA Forest Service. Part of her work is in China searching for natural beneficial insects that control EAB in its home range. Working with colleagues, Leah discovered two enemies – Tetrastichus planipennisi and Oobius agrili.

Since no one else is nicknaming these insects, let's call them Tetra and Oobi. Tetra lays its eggs in EAB larvae (properly called a larval endoparasitoid), while Oobi lays its eggs in the EAB egg (an egg parasitoid). Thus they live and reproduce by killing EAB. Without EAB, Tetra and Oobi can't survive. In China, Tetra and Oobi produce several generations each year, consuming 74 percent of the EAB population. As a comparison, in North American, beneficial insects consume about 2 percent. (Other studies indicate the total consumption by squirrels and woodpeckers are another 2 percent.)

As specialists, Tetra and Oobi are a necessary part of the EAB's world. If quarantine, chipping and eradication had worked, we could live without the associated beneficial insects. But they failed. This was due in part to our inability to find EAB in an area before it is completely infested and the lack of public cooperation with the quarantine regulations.

After five years of field and lab studies, public consultation and regulatory investigations, Tetra started feeding in North American in the summer of 2007 and the larvae are expected to overwinter inside ash trees in the larvae they were feeding on the previous summer. Oobi, with its higher rate of parasitism than Tetra, consumes about 60 percent of EAB eggs in its home range during July and August. If its release was successful, the Oobi's larvae is spending the winter of 2007/2008 in EAB eggs awaiting a new crop of EAB eggs before emerging in June or July. Oobi successfully reproduce females without a mate, so field collections find female to male ratios of 15:1. Researchers hope to know by this spring if the specialists were successful in establishing in EAB populations in North America.

Juli Gould is with USDA Plant Health Science and Technology Laboratories. She's working with a third parasitoid, Spathius agrili. (We'll call it Spath.) Spath lays its eggs on EAB larvae, so it is a larval ectoparasitoid. It is a cousin of the tomato horn worm parasitoid and its life cycle is similar. (The tomato worm wasp lays its eggs on the green tomato worms found in gardens on tomatoes. The wasp eggs look like white hairs on the back of the green worms.) Gardeners know never to kill a tomato worm carrying eggs because, like Spath, the larvae will burrow into the host and consume it. Later in its life cycle they will emerge as a wasp and lay more eggs to consume more tomato worms. Arborists may be familiar with other Spath relatives, including the wasps that consume bronze birch borer and two lined chestnut borer.

Gould says she can't predict what percentage of the EAB the three new insects will control in North America. "The first step is to get the insects to establish. You wouldn't think it by all the exotic insects that establish in the U.S., but the probability of getting an insect to establish in a non-native country is actually quite low." Many factors influence the amount of control they will achieve, including hyperparasitoids or other predators that will attack the wasps.

A few years ago, a couple of types of beetles were released in the Ontario area to control purple loosestrife, an invasive alien plant that is harming wetlands. The purple loosestrife in the test field died. However, purple loosestrife continues in the area, but it no longer occupies whole wetlands. If the new insects released to control EAB survive in North America, in time the EAB, the wasps and the ash trees will reach levels where they can coexist as they do in East Asia. How long this transition will take is not known. Sometimes releases have to be done more then once. Monitoring after release is a huge part of the project.

Michel Cusson and the Laurentian Forestry Center in Quebec, Canada, have started testing a United Kingdom invention. It is at the proof of concept stage, but a hand-held portable version is under development. The goal is to acoustically identify what species of bark beetle is chewing a specific tree. The current testing is geared toward EAB.

In Sault St. Marie, Ontario, Dr. Blair Helson and his colleques did extensive testing with Neem on various insects and different modes of inoculation, including aerial spraying. Neem is a trade name for an oil extracted from neem trees in East India. You can buy neem products in health food stores. It has insecticial, antifungal and antibacterial properties. It can be used as a mosquito repellent. In East India, it is used in soap and toothpaste. If it works on EAB, customers will love this organic, tree-based product.

Dr. Barry Lyons, also of Sault St. Marie, Ontario, took the active ingredient from Neem, azadirachtim, and injected it into ash trees in one of the quarantine areas. The public response was so positive homeowners begged for "their" tree to be included in the experiment. Follow-up studies are ongoing to determine if residual material was in the leaves when they fell in the fall (to determine if there are environmental issues), the latency period and the health of the trees. Tape was also applied to the tree's bark to establish EAB landing. Results are expected in the fall of 2008.

The blue ash is an endangered species in Canada. Observers note EAB seems to prefer green ash foliage before blue ash foliage. In order to determine if any chemical differences exist between the two tree species, extracts from the leaves and bark of both trees were performed; the extracts were tested against the EAB. An experiment known as choice feeding was conducted. In this experiment, known amounts of both extracts (green ash vs. blue ash) as solutions are applied to separate green ash leaves. Then EAB is allowed to choose between the two food sources. This helps the scientist determine if there is potentially some compound or compounds in the blue ash extract that will deter EAB feeding. An electronic scanner measured the amount of leaves eaten by the EAB on each leaf. Additional choice feeding experiments were put on hold until spring 2008, when additional fresh green ash foliage is available. Once the numerous tests are conducted, the results will be analyzed and recommendations for further work implemented. The process is mind numbing and tedious, but when complete it should provide another tool for arborists in the battle with the EAB.

Mamdouh Abou-Zaid, a natural products research scientist from the Great Lakes Forestry Centre, is also working on the fact that EAB attack vigorous trees in North America. As with the other researchers, he has a number of different projects on the go. One is to establish the biochemical profiles of host trees. Assuming Bauer's and Gould's wasps do their jobs, this will be necessary information for arborists. He is also establishing the characteristics of the host tree crowns under stress that could predict an EAB attack.

Stephen Marshal of Guelph University in Ontario is monitoring the crabronid wasp, Cerceris fumipennis. This native wasp collects beetles from the EAB's family as feed for its young. He is using the wasp to monitor the EAB's spread. This program has one problem – the crabronid wasp is not common. But it does prove that one native wasp species is adapting to the EAB. Juli Gould also says Spathius floridanus, another native wasp, is also consuming EAB, but again not in high enough numbers.

The best guess is that is has been 17 years since EAB arrived in North America, though only five years since we found out about it. Millions of ash trees have died. The slash and chip program was not as successful at slowing the spread as was originally hoped, but it appears researchers are now so very close to at least some partial solutions for ash trees. A little more patience is needed.
Pat Kerr is a freelance writer living in Fraserville, Ontario, Canada.

About the Emerald Ash Borer

Emerald Ash Borer is a deadly threat to ash trees, killing host trees within three to five years of infestation. The pest belongs to a group of insects known as the metallic wood-boring beetles. Adults are dark metallic green in color, 1/2 inch in length and 1/8 inch wide, and fly only from early May until September. Larvae spend the rest of the year developing beneath the bark of ash trees, and when they emerge as adults, leave D-shaped exit holes in the bark about 1/8 inch wide.

Note the characteristic D-shaped exit hole. Once they become adult beetles, they crawl through the bark, leaving these exit holes.	EAB larvae choke the Ash tree by eating through the cambium layer just under the bark.

The pest is difficult to detect in its first year of infestation, as it first infests the tops of ash trees and works its way down. Infested ash trees are essentially choked to death by larvae feeding on the cambium layer -- the live tissue just beneath the bark responsible for transporting nutrients throughout the tree. Signs of EAB include dieback at the tops of ash trees, D-shaped exit holes, S-shaped larval galleries, epicormic sprouts, woodpecker damage, and bark splits.

Research has shown that the natural spread of the pest is less than one-half mile per year. However, EAB can be easily, yet unknowingly spread through the movement of infested ash material such as logs, nursery stock, firewood, etc. The majority of infestations were introduced through the artificial spread of this insect.

Many states and provinces are now aggressively spraying ash trees to stop the spread of this deadly pest.