The Emerald ash borer OR Release the wasps! Part II
This post is a continuation from my previous post, “Release the wasps (no, really, release the wasps) Part I,” which I put up on Wednesday. If you haven’t yet read it, please do, as it will provide a bit more context as to what I’m writing about today.
In my earlier post, I wrote about two parasitic, non-native wasps, which I released as a part of the USDA APHIS EAB Biocontrol program. This post will be used to provide background on the Emerald ash borer and why the decision was made to release non-native wasps in order to control it.
The Emerald ash borer (Agrilus planipennis) is a member of the sun-loving family,Buprestidae (~15,000 known species), commonly known as the Jewel beetles or the Flat-headed borers. These beetles can be found feeding from the crown downwards, on hardwoods, tending to be specialists, meaning that each beetle will often only feed on a particular species or genera of tree (This specialization is often emphasized in the common name of the beetle, e.g. the Bronze birch borer, Twolined chestnut borer, Bronze poplar borer, Emerald ash borer.) Behind this specialization are physiological and morphological implications for the beetles. Organisms that have spent a great deal of time feeding on one, or a few, closely related species will evolve mechanisms that improve their ability to feed upon that species, or genera.
I think it’s worthwhile to break this down and think about it. Evolution always favors adaptations that improve fitness.
In order to understand how the fitness of the beetle is being changed, I’m going to define a three words I have mentioned, to provide a more clear picture.
What is a mechanism? A mechanism is a process that achieves a result.
For instance, you may have seen numerous comics or jokes that involve this sort of setup (forgive me for using an academic setup here…),
Step 1. Read papers on topic of interest, decide to study it.
Step 2. ???
Step 3. Publish results!!!
Step 2., is what could be referred to as a “black box,” because one understands the input (the knowledge required to study a topic), as well as the output (publishing a paper on it), and does not necessarily need to know (unless YOU are the one performing the study), the mechanism (or work) that allowed Step 1. to proceed to Step 3.
To be more specific about my usage of mechanism now, I refer to mechanisms that improve a beetle’s ability to feed upon a tree. A general explanation could be, “evolution,” whereas more specific explanations could be, “larger or smaller mandibles” to chew with, or “chemicals specific to dissolve a particular wood tissue”, etc.
Mechanisms are what generally answer questions such as, “How does X species do Y?”
What is an adaptation? An adaptation is a particular change in an organisms biology specific to a certain situation. Examples of adaptations are: the ability to regulate body temperature (These organisms, of which we are one, are known as endotherms), coloration that assists in hiding from, or escaping predators, (also known as camouflage), and the evolution of cognition.
It is important to recognize that in the case of the beetles being able to feed more efficiently, these adaptations ARE the mechanisms which produce the resultant efficiency of particular beetle species performing well on particular trees.
What is fitness? Fitness can be defined in a few different ways, but they all fundamentally refer to the health of the organism. For the sake of synthesis, I am going to use the Emerald ash borer as an example. When the Emerald ash borer was first discovered in the United States, researchers performed a study to determine what the “host preferences" of the beetle were. These preferences refer to its propensity to eat the foliage of, as well as lay eggs upon, and chew into the phloem of particular tree species. (also see: 1,2) It was found that the Emerald ash borer would occasionally lay its eggs on species aside from ash (Fraxinus spp.), under both laboratory and field conditions, but would only eat ash foliage. In spite of laying eggs on these non-ash species, Emerald ash borer larave that attempted to bore into the phloem and construct galleries always failed, ultimately ending in death.
To be utterly facetious, death is the complete opposite of fit. I hope you’re beginning to see where I’m going- there is something about the nature of the tissues within the wood, specific to particular tree species, that makes it difficult, or rather, impossible, for an Emerald ash borer to develop on a non-ash species.
This evokes the concept of a trade-off: if you want one thing, you have to give something else up. A generalist is able to eat a little of a lot of things, whereas a specialist is able to eat a lot of a few, or one particular thing.
Since the Emerald ash borer is more of a specialist than a generalist, it would be adaptive for it to develop means to eat more ash, and less of other species. An example of a strategy it MAY (I put this in caps because there is no evidence to this claim, it is my speculation-but, if research were done, I believe results would fall along this line) employ is secreting chemicals via its saliva, that help detoxify tree defense compounds that are specific to ash species. If this speculation is true, then it would add some context as to why the Emerald ash borer performs so poorly and cannot develop on non-ash species: it doesn’t have compounds to detoxify tree defenses, the defenses overcome the beetle, and therefore, the Emerald ash borer dies.
To finally define fitness in terms of the Emerald ash borer, a fit beetle would lay more eggs and produce more of, and larger larvae, than an unfit, or less fit beetle. What I just explained above, in terms of fitness, are components that could leads to more fit beetles, with respect to the evolution of the beetle (I say this as I am not taking into account any abiotic or external biotic effects).
Now that I have defined those terms, and given context to the situation, you hopefully understand how beetles which tend to feed on and develop inside of, a few or particular species of tree, may begin to form a close evolutionary relationship with its host, over time.
This close evolutionary relationship is termed, coevolution, also known as the Red Queen Hypothesis. The Red Queen posits that if one organism is high specialized to another, so that, it could potentially cause the other to go extinct, the second organism must evolve (like a counter-defense), in order to persist. This leads to the famous Lewis Carroll quote (from which the Red Queen is named),
"…it takes all the running you can do to keep in the same place."
This implies that the two organisms, continue to counter-evolve to each other’s evolutionary adaptations to improve their respective fitness’, in an endless struggle. Can you imagine ash trees and the Emerald ash borer, over generations upon generations, struggling against one another for dominance?
Most likely you can’t imagine this, because you probably live somewhere in the United States. One thing I haven’t mentioned yet (that if you are familiar with the Emerald ash borer, you already know), is that the Emerald ash borer is not a species native to North America, it is endemic (native to), East Asia. Because it isn’t native consider the situation above, regarding coevolution. There are two possible outcomes: (1) The Emerald ash borer is too specialized upon Asian ash, that it will fail to survive on North American ash or (2) The Emerald ash borer will be able to use its adaptations and feed very heavily upon ash that have NOT evolved defenses to it like its East Asian hosts.
Before I respond to the obvious answer above, I want to note that the Emerald ash borer in East Asia responds to East Asian ash just like our North American Agrilus respond to their respective hosts (you can find some context a few posts back, here), they become noticeable only during stressful events. What I mean, is that, Agrilus only become a problem when there is a drought, or a flood, or an outbreak of another insect, that weakens the host trees of Agrilus. Once the trees has less vigor, the beetles go to work attacking the trees. After the area has had time to recover, the beetles are maintained at low levels and are not an issue until the next big stressful event.
In North America, the Emerald ash borer does not wait until trees are weakened to attack, because it does not need to. That is why the Emerald ash borer has been deemed invasive and dangerous to the North American ash.
Look at this map of where the Emerald ash borer is as of May 2nd, 2011. It was only found in Detroit, MI in 2002 (was likely around since 1992, just unnoticed), and has spread rapidly through its own feeding, and most likely, transport of firewood.
Because the Emerald ash borer is so voracious and moves quickly due to various factors, is why biological control (the wasps I released), in conjunction with systemic insecticides, cutting of ash, and environmental education, is being used to slow the spread of the beetle; so that it may be able to be more effectively managed in the future.
Some Emerald ash borer resources:
Emerald ash borer: Invasion of the urban forest and the threat to North America’s ash resource
The Biology and Ecology of the Emerald ash borer (Agrilus planipennis)
The Chemical Ecology of the Emerald ash borer (Agrilus planipennis)
Cost of potential emerald ash borer damage in U.S communities, 2009-2019