Biological control and how its being used to combat the Emerald Ash Borer

In the popular 1979 movie Alien, the world was introduced to quite possibly the most terrifying creature ever conceived. This fictional creature is called a ‘Xenomorph’ and has a bazaar and terrifying breeding process that is not too dissimilar to an insect currently being studied today to combat the spread of the Emerald Ash Borer beetle.

Folks familiar with the movie know that the process starts with the egg stage. These eggs are biomechanical in nature, laid in clusters and when approached by a viable host they open launching an arachnid-like “Facehugger” that attach themselves to the face of the host implanting an embryo down their throat. After the embryo has been implanted the “Facehugger” dies. Meanwhile, the embryo gestates inside the host, eventually violently bursting from the chest of the host. This is referred to as the “chestburster,” stage resulting in the birth of a juvenile Xenomorph (awe, how cute). From there on chaos ensues for our viewing pleasure.

This process is eerily similar to the breeding process of Tetrastichus planipennisi, a parasitic wasp native to China, the original home of the Emerald Ash Borer beetle (EAB). This parasitic wasp is a natural predator of EAB and is currently being studied by scientists at the USDA Forest Service and Michigan State University. The method of control being studied is referred to as biological control. Here is how it works:

  • Tetrastichus planipennisi locates Emerald Ash Borer larvae beneath the bark of an Ash tree.
  • Using its stinger, it plunges beneath the trees bark and injects itself inside the EAB larvae, depositing its eggs inside the hosts body.
  • The wasps eggs hatch inside the larvae and eat it from the inside out all while the host larvae is still alive.
  • The wasp larvae time the death of its host about the same time the wasps are mature enough to live outside of its host. At that time, the newly formed wasps eat their way out.

The potential here would be another tool to help protect our native Ash trees from annihilation. Studies out of Michigan State University suggest that Tetrastichus planipennisi will likely play a critical role in suppressing the Emerald Ash Borer populations. Tetrastichus planipennisi is highly specialized in its host preference, primarily targeting Emerald Ash Borers. It is not known to be a threat to other insects or organisms, as its life cycle and reproductive strategy are closely tied to the presence of Emerald Ash Borers. This makes Tetrastichus planipennisi a valuable tool in the effort to manage and reduce the impact of Emerald Ash Borer infestations on ash trees.

Key findings from the study include:

  • Approximately 3 to 4 years after the release of Tetrastichus planipennisi in Emerald Ash Borer -infested areas in southern Michigan, this parasitoid became well-established and common on ash trees in both release and control plots at all six study sites.
  • Over the course of several years, the number of Tetrastichus planipennisi broods on ash trees increased from about one brood per tree to three broods per tree.
  • While knowledge of the spread and dispersal of Tetrastichus planipennisi adults is limited, observations indicated that the parasitoid spread at least 1 km per year in some areas. In other cases, it was found in control plots relatively far from the initial release sites, suggesting a potentially higher spread rate. Additional studies are needed to understand the spread and dispersal patterns to optimize release strategies.
  • Tetrastichus planipennisi appears to prefer smaller diameter ash trees. Larger ash trees with thick bark provide a refuge for Emerald Ash Borer larvae from Tetrastichus planipennisi This may affect the effectiveness of the parasitoid on larger ash trees.
  • In addition to Tetrastichus planipennisi parasitism, Emerald Ash Borer larvae also face other biotic factors that contribute to their mortality, including predation by woodpeckers, potential tree resistance, diseases, and parasitism by other native parasitoids.
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