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Ash trees and the emerald ash borer

Updated: Jun 20, 2023

In 2002, the Emerald Ash Borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), was detected near Detroit, Michigan. This beetle is thought to have been introduced into the United States (U.S.) from China via infested wood used for shipping crates. EAB larvae develop under the bark of ash trees and if infested trees are cut into lumber, some of the beetle larvae survive in the wood and still emerge as adults. This is what happened in Michigan in 2002—adults emerged and began their life cycle on ash trees native to the U.S. But it wasn’t until 2016 when the EAB made its first appearance in northern Delaware. A single adult beetle was found on a purple sticky trap used for monitoring the spread of this destructive forest insect. It took a few more years but by 2020 all three Delaware counties were infested and the results, like in all the other 36 infested states, have been devastating.

EAB adults and exit holes

Here at Abbott’s Mill Nature Center, you can see first-hand the EAB destruction on the north end of the Boardwalk Loop Trail. Dozens of mature green ash trees (Fraxinus pennsylvanica) have died or are in severe decline. Many had to be cut because they posed a hazard to the boardwalk trail that visitors frequent.


First evidence of infestation by the EAB
Larval tunnels under the bark that interrupt the flow of nutrients to the tree.

In its native range, the EAB feeds on species of ash trees that grow naturally in China, but it doesn’t cause widespread destruction like here in the U.S. The question is, why? What causes the destructive behavior here in the U.S. by an insect that, in its native range, is considered rather innocuous? To answer this question, you need to understand the basics of population dynamics.

Clumps of dead ash trees.

All populations are subject to five principles or laws. The first two are exponential growth and cooperation. In a perfect world, all populations have the capacity to grow exponentially and if the members of that population cooperate (e.g., flocking, herding, and schooling in animals; sharing technology and information in humans), the rate of growth increases even more. We observe exponential growth with certain pest species and the human population. These first two laws create a positive feedback structure which results in unstable growth. (It should be noted here that positive feedback can go in both directions: increasing to unsustainably large population levels or decreasing to extinction of the population.) But we know that populations cannot grow infinitely and that is where the next three principles, or laws, factor in. These are competition, circular causality, and limiting factors. Competition for food, living space, mates, etc., regulates most populations. But populations also affect their own environment, sometimes negatively, which helps limit their growth. And finally, there are hurdles that all populations face such as limited habitat and resource availability. What we see in nature (scientists refer to this as empirical evidence) is an abundance of stable populations. It is true that all populations fluctuate, but in general the ups and downs are just variations about a mean or average population level.

The brittle tops of dead ash trees.

Occasionally something disrupts the system and if positive feedback dominates, we observe an outbreak or severe decline. A common way that populations enter into an outbreak phase is when negative feedback loops (regulating forces) break down or are absent. We see this occasionally in native herbivorous insects, for example, when natural enemies or other regulating factors faulter temporarily. The population increases and may cause local destruction to the host, but eventually the regulating factors recover, and stability returns to the system. However, when a herbivorous species is transported from one side of the Earth to the other, there is a greater potential for positive feedback to dominate because the species can escape from its normal regulating forces. This is the case with the EAB. The U.S. just doesn’t have the natural enemies or ash tree host resistance that normally occur in China.

What we are witnessing with the EAB is a classic example of an exotic pest species that is freed from its normal regulating forces. The population grows exponentially and spreads through space and time killing a large portion of its hosts (ash trees in this case) in its wake. In the end, some individual ash trees will survive, and new trees will sprout from stumps, but the collective ecological role of ash in the U.S. will be severely diminished. Not until the EAB’s natural regulating forces are restored here in the U.S. will there be any chance for ash trees to return to their former significant role in many forested ecosystems. We can only hope scientists will figure out a way to restore negative feedback mechanisms that will push EAB populations to lower, more stable levels.

 

IT'S MULBERRY SEASON

by Steve Childers


"All around the mulberry bush

The monkey chased the weasel.

The monkey thought 'twas all in fun.

Pop! goes the weasel."



Do little kids still know this song? Do they even know what a "mulberry bush" is? Do you?


In Delaware, the two most common species of mulberry are our native red mulberry (Morus rubra) and the imported white mulberry (Morus alba). So, what's the difference? How do you tell the invasive white mulberry from our native red mulberry? Well, it can sometimes be difficult, but here goes:

Red mulberry leaves are much longer (4-7in), are dull, dark green with a sand-papery texture, have short, fine hairs on the underside and always taper to a conspicuous, long point at their tip.

Native red mulberry leaf (underside).

White mulberry leaves are much shorter (3-4 in), shiny on the top and only sometimes taper to a long point at their tip. Some leaves may look blunt-tipped.


Invasive white mulberry leaves.

Red mulberry fruit is longer, usually about an inch long.

Red mulberry fruit
Red mulberries turn black when ripe. (Photo © kent, Public Domain.)
White mulberry fruit. (notice the shiny leaf)

White mulberry fruit is shorter, at most about 3/4" long.

(The white mulberry tree in these pictures is in fruit right now, along the side of Abbott's Pond Road, right next to the mill. Just look for the squashed berries on the walkway)


If you can't make up your mind which it is, then it's probably a hybrid of both.


Either mulberries are very good to eat and make excellent jelly or pie filling. They are also an important food source for most of our woodland animals. Keep an eye down on the ground as you walk the nature center trails and you will likely see small piles of dark poop left behind (no pun intended) by small mammals that have been feeding on the ripe mulberries. Box turtles and birds love the berries as well.


Reference: Bplant.org - https://bplant.org/compare/140-141

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