When reflecting on the H1N1 crisis last year, one can’t help but get the feeling the whole thing was way overblown. Still, it was impressive to see the massive mobilization of vaccines, hand sanitizer and ad campaigns the government was able to muster so quickly. This admirable political resolve will be needed once more as the city of Winnipeg is about to face another epidemic, except this time it is our ash trees which are under threat from a scourge known as the emerald ash borer (EAB).
The EAB, Agrilus planipennis, is a small insect, which in its adult stage resembles a tiny grasshopper at first glance. The adults are shiny green and are about two centimetres long and 0.4 cm wide.
In the adult stage these insects will nibble on ash tree foliage, but this grazing poses little threat to the tree; it is the larval stage that is most dangerous. EAB larvae are creamy white and worm-like in appearance. When the adults are feeding on the tree foliage they will lay their eggs on the branches or trunk of the tree. When the eggs hatch, the first instar (newly hatched) larvae will bore through the bark and begin to feed on the phloem of the tree. This girdling of the tree causes a disruption in the flow of nutrients and will cause death within two years. EAB will attack all species of ash trees except the mountain ash which is not a true ash. This pesky insect is native to eastern Asia and was only discovered in the United States and Canada in 2002.
While invasive species usually spell trouble, this one in particular has cut a massive swathe of destruction in a short time. Even with its recent arrival, EAB has killed tens of millions of ash trees in southeastern Michigan alone, with tens of millions more killed in the neighbouring states and provinces. The EAB is found as far south as Virginia and Kentucky and recently has moved into Minnesota, much to the alarm of Manitobans. It is believed EAB originally arrived on solid wood packing material transported from Asia. As a precaution against further spread, regulatory agencies have enacted quarantines to stop the transport of ash logs or firewood from infected areas. Even with these precautions it is only a matter of time before the EAB makes the trip from Minneapolis to Winnipeg, so what can be done to stop it?
The only way to save the ashes is to prevent the borer from taking up residence; once the damage is done by the larvae there is no going back. To that end, a company called Arborjet has released a product known as “Tree-age” with which to inoculate trees against EAB. Tree-age is a chemical pesticide whose main ingredient is emamectin benzoate. It is injected directly into the trunk of the tree and is subsequently carried throughout the tree as it naturally draws up water and nutrients from its roots. Within 6-8 weeks after the injection, none of the wood of the tree is suitable for the borer and thus the tree is saved from its destructive colonization. This product is not yet approved for use in Canada but is approved in 11 states. The drawback to this seeming wonder treatment is the cost. It must be applied to the tree every two years and costs about $10 per inch of tree diameter. On the scale of inoculating all the city’s ashes, this cost becomes almost prohibitive.
One other option for protecting the ash trees is to kill off the emerald ash borers before they can do any damage. To this end, scientists in the U.S. began looking for potential biological control agents shortly after the EAB was first discovered in North America. Studying the native habitat of the EAB in China showed a spotty distribution of the EAB and low population densities in native ash stands. This is likely due to a combination of host plant resistance and natural predators. Investigating its natural predators yielded three promising parasitoid wasp species. Spathius agrili and Tetrastichus planipennisi are larval parasitoids, meaning they lay their eggs on or in the EAB larvae. The third species, Oobius agrili, is an egg parasitoid meaning it lays its eggs on the eggs of EAB. These species are believed to be the primary natural control agents of the EAB in Asia.
The U.S. government has set up a breeding facility for these species in Michigan and in 2007 began several small scale releases, which appear to show that the S. agrili and O. agrili can survive a Michigan winter. While more research is needed and the long-term effect on EAB populations is yet to be seen, these species may be key to fighting EAB.
These methods of biological and chemical control may help solve the EAB epidemic but neither is a perfect solution. It will take time, money, and political will to combat this menace which is quickly approaching Manitoba. As much as we might wish to live in a biological autarky, these types of invasive species problems are likely to become more common as the world becomes a more interconnected place. Let’s hope our society can muster the will to combat these threats.