Monday, March 5, 2012

Taking a bite out of the emerald ash borer

To combat the destruction of ash trees in North America, scientists suggest the release of three wasps that are native predators to the emerald ash borer (EAB). 

The EAB was first identified in the US in 2002, when people in southern Michigan realized a sudden decrease in ash tree vigor and an increase in the mortality.  Nobody knows for certain how the EAB got into North America but the best guess is that it was present in ash wood cargo that was transported into the US from Russia, China, Japan or Korea. That northeastern region of Asia is the EABs native habitat and although there are ash trees present in this region, they have co-evolved with the EAB to have increased resistance against this beetle.  Over on the other side of the world, North American ash trees (which are pretty hardy specimens against native pests) are left completely defenseless against the introduced EAB.  Since 2002 EAB has spread into 14 states in the upper mid-west and northeast and killed over 60 million ash trees.

In addition to ash trees in northeastern Asia having a greater resistance to EAB there are also several native predators of EABs including three species of parasitic non-stinging wasps: Oobius agrili, Spathius agrili and Tetrastichus planipennisi.  Each of these wasps has a different strategy for feeding on the EAB .  Oobius agrili lays its eggs inside previously laid EAB eggs, then uses the EAB eggs as food to grow to their adult form.  Spathius agrili deposits its eggs on EAB larvae and simultaneously injects a paralyzing venom.  This doesn't make for a very fair fight as it allows Spathius agrili to feed on the EAB larvae without any attempt at victim escape.  Tetrastichus planipennisi also feeds on EAB larvae, but they are the real gladiators: they eat EAB larvae that are alive and active.

O. agrili; All rights reserved to Deborah Miller

S. agrili; All rights reserved to Tracy Ayer
T. planipennisi; All rights reserved to David Cappaert 

All three of these wasp species are naturally present with the EAB in Asia, but not in North America.  Michigan State University is in the process of using these species as 'biocontrol' for the EAB.  Their hope is to introduce these native predators of the EAB to slow down the destruction of ash trees in North America.  This is a long and highly regulated process because no one knows what impact these foreign wasps could have on natural ecosystems.  Recent laboratory experiments suggest that the introduction of these wasps could have a 30-70% reduction in EAB larvae.  Although these results are from initial experiments and are highly variable, they suggest that the introduction of these wasps could help alleviate the massive devastation of North American ash trees.  (Find out more information here, here or here!)

A quick note about why we're worried about the death of our ash trees to begin with.  First from an environmental perspective: ash trees provide habitat for a variety of creatures including birds, chipmunks, squirrels and other small mammals.  They are also widely popular in the northeast, where some forests are primarily made up of ash trees.  Ash trees have a wide tolerance of environmental conditions and were able to replace the decimation of elm trees by the Dutch elm disease.  Whether a new tree can replace the decimation of ash trees is unknown.  Next from an economic standpoint: ash is an excellent hard wood for timber, furniture and tools.  The loss of millions of trees puts a huge economic strain on companies that produce these good and consumers that buy these goods.

Given the advantages of ash trees, people are nervous about their disappearance.  That being said, any time a foreign organism is introduced to a new location the utmost precautions must be taken.  Biocontrol has a rocky history (discussed in an earlier blog here) and it would be terrible to introduce a solution that ultimately causes more problems than the original problem.  Every precaution possible should be taken to ensure that the introduction of the above wasps will definitely decrease the EAB population while having minimal effects on other aspects of ecosystems.

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