Balance and Imbalance in Nature

Here are two examples of insects eating leaves at Herrontown Woods. One is sustaining balance, while the other threatens the survival of beloved native Viburnums. Why is one insect beneficial, and the other highly destructive? The story begins with the sensitive fern, a beautiful native that graces local wetlands and gardens.

Though we tend to think of ferns as delicate, the sensitive fern is a tough, resilient plant in moist ground, sometimes even showing expansionist tendencies in a garden. The "sensitive" in the name refers primarily to its susceptibility to first frost in the fall.

Typically, the aggressiveness of native plants has been countered through the co-evolution over many millenia of other organisms that can eat them. Any plant that becomes super abundant will in turn provide abundant reward for any organism that develops a capacity to eat it, thereby bringing its population back into balance with other species. That co-evolution takes time, given that plants are brilliant chemists, with many chemical and physical defenses that must be overcome by any would-be consumer.

Since deer generally don't eat sensitive fern and our summers are getting wetter, what might keep it in check?

Recently, while weeding the new botanical garden at Herrontown Woods, I found some young sensitive ferns stripped down to their leaf veins. Sensitive ferns, it turns out, are eaten by several kinds of insects, each attacking a different part of the plant.

The culprit here was a little green caterpillar, most likely a sawfly larva.

Presumably, because I haven't heard of any new, introduced insect ravaging ferns, this insect evolved long ago a capacity to digest and detoxify the sensitive fern's chemical defenses. Any predator that consumes all of its prey will not itself survive, so relationships tend to evolve between predator and prey that are mutually sustaining, and therefore promote balance in nature.

The damage inflicted by the caterpillar is therefore reassuring.

By contrast, the insect damage on this leaf, encountered on the red trail leading up to the Veblen Cottage, was not at all reassuring. It is instead evidence of a radical change coming to Princeton's nature preserves that could largely eliminate several important shrub species from our woodlands and gardens. The leaf is of arrowwood Viburnum, one of three Viburnum species that up to now have contributed flowers, berries, and fall foliage to Herrontown Woods' ecological functionality and beauty.

Their continued presence is now threatened by an introduced species, the Viburnum leaf beetle. Past writings about this invasive beetle on this blog can be found at this link. Arrowwood Viburnum tends to be the first to succumb, followed by mapleleaf Viburnum and blackhaw Viburnum. The insect has the ability to completely skeletonize a shrub. Multiple attacks can ultimately exhaust the plant's reserve energy. I saw a skeletonized Viburnum in Pittsburgh some years ago. Its complete stripping of the plant's foliage was in contrast to a native predator that would tend to do only partial damage, leaving most of the plant alone.


http://www.hort.cornell.edu/vlb/manage.html

Below, from a Cornell University website, is one potential scenario. It suggests that there will be an initial wave of destruction as the Viburnum leaf beetle eats through all the susceptible Viburnums, after which the insect's population will crash, and become a minor pest from thereon, allowing the susceptible species to grow once again. Even if this were to prove true, the introduced pest represents one more shock to the system.

"The viburnum leaf beetle hit us hard in the Rochester area about 15 years ago. During those first few years in which the beetle population peaked most of the susceptible native species like arrowwood, that were growing in wooded areas, were killed. Some landscape plants succumbed to the defoliation then too.

"At that time I would not have recommend planting a susceptible species like the Cranberry bush viburnum. Now however the populations of the beetles are down significantly and it is safe for us to plant species again like cranberry bush and arrowwood viburnums. They’ll get a little bit of damage but nothing lethal.

"Why did the populations go down? It seems with all the very susceptible native plants that were around initially allowed the populations to reach unnaturally high levels and the beetles moved into landscapes annually. With those food sources gone the populations declined. Also, and maybe more importantly, predator insects, and nematodes that affect the larva in the soil have built up and found the Viburnum leaf beetle as a food source!

The Viburnum Leaf Beetle's Gathering Harm

This fall, before the leaves fall off, check any Viburnum shrubs you know and love, whether in your yard or in your favorite nature preserve. Do the leaves look like this, pitted with holes, or even stripped down to the veins?

Most people have heard by now about the Emerald Ash Borer's rapidly multiplying demolition of our ash trees. Lest that devastation not be enough ecological and horticultural tragedy to absorb, there is another invasion underway in Princeton, by the Viburnum leaf beetle. This one--a European insect that first showed up in Canada before spreading to the U.S., is going after our understory, one Viburnum species at a time.

This and the above photo are of Viburnum dentatum, the arrowwood Viburnum, which grows here and there in our woodlands, preferring wetter soils. The light damage could have been done by a native insect, or by the first few Viburnum leaf beetles to show up, suggesting much greater damage the next year. Listed as "highly susceptible" on Cornell's website, the arrowwood Viburnum will be the first to succumb.

The maple leafed viburnum, frequently encountered along the Princeton ridge, is less susceptible and thus isn't showing any damage this year. As the Viburnum leaf beetle increases in numbers and exhausts the most susceptible species through complete defoliation, it will presumably begin attacking those that are less susceptible.

We are not completely helpless as yet another careless introduction to the continent wreaks slow motion havoc. This site offers some ways to prevent the beetles and their larvae from consuming your favorite specimens. As with all environmental damage done, the ecological and aesthetic services these shrubs were providing for free will now start coming with a price tag for human maintenance.

The Cornell site offers another ray of hope. An extension official in Rochester, NY asserts that even highly susceptible species may in the long term become viable again, as predatory insects and soil nematodes build up to prey on the Viburnum leaf beetles. Then, perhaps, the introduced species would come into some sort of ecological balance, doing some damage but not enough to kill the Viburnums.

It's a perfectly logical and reassuring scenario, and yet observation often brings into question the speed and effectiveness of nature's capacity to restore balance and sustain diversity. More often, invasive species remain dominant and destructive, be they Phragmitis, or Japanese stiltgrass, or porcelainberry. I called up the Frick Park Environmental Center in Pittsburgh, where I had first witnessed the devastating impact of Viburnum leaf beetle a decade ago. Asked whether they had seen any signs of balance being restored, they answered, "Not yet."

Related links:

The Pennsylvania extension service also has some useful information.

Past NatureNotes posts about the Viburnum leaf beetle, dating back to 2013, can be found here.

This post explores the likely aftermath of the Emerald Ash Borer invasion currently underway.