Evolved Coexistence in Nature

What is "natural" in nature? I remember a 1990 column entitled "Bug Wars," in which NY Times columnist Anna Quindlin wrote about the gypsy moths that, back then, were defoliating vast areas of eastern forest. She used the destructive moths as fodder for reflections on nature and its ways, not knowing that the moths can hardly be thought of as natural. They were introduced from another continent, then escaped and multiplied, consuming forests that had not evolved any defenses against them. Fortunately, scientists found a very low-toxicity way to limit the gypsy moth's numbers, though sustaining that balance requires ongoing human vigilance and action.

With introduced insects and diseases having decimated first our ashes and now our beech trees, it's worth noting the more sustainable relationships between plants and animals that have co-evolved over millennia on the same continent. 

Tent Caterpillars and Black Cherry

This spring, tent caterpillars defoliated some of our black cherry trees. Were the caterpillars a threat to the trees? Well, they finally got their fill and wandered off to pupate and become moths, leaving the trees to sprout new leaves and grow unhindered for the rest of the summer. The trees may grow more slowly, but their survival (and therefore the survival of the tent caterpillars that depend upon them) is not in peril. 

 

Hibiscus Sawfly and Rose Mallow Hibiscus

Near the gazebo in the Barden at Herrontown Woods grows the local native hibiscus (H. moscheutos). Earlier this summer, its leaves were getting eaten. 

Some internet research suggests that these are larvae of the hibiscus sawfly. 

If the culprit had been an introduced insect, I might have worried. For instance, each year I see more evidence of our arrowwood Viburnums being eaten by the Viburnum leaf beetle, an insect introduced from Europe. Because the beetle's arrival is fairly recent, we don't know if it poses an existential threat to Viburnums.

But given that the native hibiscus sawfly has been interacting with the native hibiscus for thousands of years, that long track record of coexistence is reassuring.

 

Though many leaves were partially eaten, the hibiscus grew new ones that remained unaffected. I think of this relationship as akin to someone who donates a percentage of their income to good causes each year. 

An Aphid-like Insect and Hickory Trees

Earlier this year, some leaves of a young hickory tree developed these green bumps. They appear to be hickory leaf galls, caused by an aphid-like insect in the genus Phylloxera. 

The hickory leaves ultimately curled up. The insect's lifecycle sounds much like that of the introduced nematode that causes beech leaf disease. Should we be worried for the future of hickories?

Again, the aphid-like Phylloxera and the hickory tree "go way back," and through those millennia a balance seems to have been struck. Only the lower portions of the tree appear affected. The hickory continues to grow. 

Psyllids and Persimmon

Lastly, a twig on a native persimmon in Herrontown Woods was showing symptoms of contorted leaves similar to those proliferating on nearby beeches. Might we be losing our persimmons? Again, some internet research points not at apocalypse but at coexistence, between the native persimmon and a native insect called the persimmon psyllid. 

According to a NC State website,

"On native persimmon, these psyllids can be temporarily abundant; but their populations soon decline naturally, as they are attacked by their natural enemies, including parasitic wasps."

For some, even small blemishes on a plant will be annoying, but in a nature preserve, the long-sustained give and take between the plant and animal worlds is part of a complex food web to be celebrated. 

We could wish that the beech and the introduced nematode that threatens it would ultimately come into a balance that allows coexistence, but other native tree species laid low--chestnut, elm, ash--remain marginalized, even, as in the case of the American chestnut, more than a century later.

What's Eating Local Viburnums?

Each time I see a native arrowwood Viburnum growing in the woods, I take a closer look. There's a viburnum leaf beetle--a nonnative species introduced from Europe--that showed up in Princeton about a decade ago. On a visit to Pittsburgh years back, I saw native Viburnums totally skeletonized by the insect, and worried about the fate of our own Viburnums in NJ. 

I've posted about this insect pest in the past. Suffice it here to say that among the various species of Viburnum in NJ, the arrowwood Viburnum (V. dentatum) is the most vulnerable. The nightmare scenario would be for the leaf beetle to defoliate all arrowwood Viburnums, then move on to decimate the next most vulnerable species.

Preferring wet ground, blooming profusely even in the shade, the Viburnum dentatum is not as common in the woods as the blackhaw Viburnum, or the highly invasive Linden Viburnum. The last couple years, I didn't notice much insect damage. This year, observations are mixed. Some shrubs in Herrontown Woods were badly eaten, 

while an arrowwood across town in Rogers Refuge showed no damage at all. 

Hopefully, the total stripping of leaves that I witnessed in Pittsburgh will prove to have been an exception, and natural predators and diseases will keep the Viburnum leaf beetle in check. Worth keeping an eye on in coming years.

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 sawfly larva, described as Hemitaxonus dubitatus.

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.

Check Your Viburnums for Imported Leaf Beetle

The list of destructive, imported insects affecting Princeton is getting longer. Most people are aware of the gypsy moth and the Emerald ash borer. Add the Viburnum leaf beetle, which skeletonizes Viburnums in people's yards and in the wild. A post in May last year documented the larva chewing on a Viburnum in Princeton's Harrison Street Park. I killed every larvae I could find, and haven't seen a recurrence this year. But badly eaten leaves on a Viburnum in a friend's yard in nearby Lawrence this summer mark a second sighting, this time with damage coming from the adult beetle.  

Here's what appears to be an adult on a Viburnum commonly called highbush cranberry.

They do a thorough job of it. Completely skeletonized shrubs seen in Pittsburgh years ago were a chilling omen for what could happen throughout the eastern U.S. It should be noted, in this age when so much focus in the news is on intentionally destructive behavior, that the greatest damage to our world is being wrought completely without malice, as insects and diseases hitchhike on transported nursery stock, and the products of combustion slip silently, invisibly from our exhaust pipes and chimneys.

An arrowwood Viburnum (V. dentatum) in my backyard is showing early signs of an infestation. These are two of the most susceptible species of Viburnum. The most striking incidence of arrowwood Viburnum in Princeton is in low woodlands along the DR canal. Despite growing in deep shade, it can have surprisingly showy clusters of white flowers in the spring. Sad to think those spring vistas could become a thing of the past.

If you check any backyard Viburnums you may have and find early signs, you may be able to limit next year's damage with some of the techniques described at this link.

The blackhaw Viburnum, our most common Viburnum in the wild, is less susceptible.

Why the Native/Non-native Distinction Matters

Here are two examples of leaves being stripped by insects in the backyard. One example is likely harmless, and actually shows that there's a functional foodchain in this backyard habitat. The other signals the arrival of yet another severe blow to our local ecology.

Most people recognize jewelweed, the annual with tubular orange flower that feeds hummingbirds, and has a fleshy stem that some use to rub on poison ivy skin rash. The flowers hang like earrings, and turn into spring-loaded seedpods that are fun to put in the palm of the hand and explode. Used to seeing it grow robustly in low wet areas, I was surprised this morning to find one defoliated.

A few new leaves had sprouted from stems made bare by a caterpillar.

Closer inspection showed a busy caterpillar on the underside of new leaves. Some internet research (here and here) suggests there are several species of moth that feed upon it.

Though this one plant was defoliated, others in the garden are prospering. If the caterpillar is native, one can assume it's been consuming jewelweed for thousands of years, and so poses no threat to its favored plant species' survival.

Here is a much different situation--an arrowwood Viburnum showing signs of insect damage. Chances are good that this native Viburnum, which has flourished for a decade in our backyard, is not being eaten by a native insect with which it has coexisted for millenia. More likely the hungry caterpillar is the Viburnum Leaf Beetle. Imported from Europe, it has been spreading across the eastern U.S. Since it did not evolve here, the native Viburnums have had no chance to develop defenses to limit its consumption. As with the Emerald Ash Borer that is beginning to devastate our ash trees, the introduced Viburnum Leaf Beetle will likely cause radical changes, not only in our forests but in backyards as well.

Local nature, in effect, is sustaining one body blow after another, due to the introduction of non-native species, some of which unleash radical change. It's important to note how our world is being transformed by unintended acts while the news media focus on intentional acts of destruction. If a rogue arborist began randomly cutting down people's trees and shrubs, there'd be a great outcry and the destruction stopped. But if an accidentally introduced insect does the same, causing millions of dollars in damage, its accepted with a shrug and a sigh. Add the collateral damage known as climate change into the mix, and you see how profoundly vulnerable is our world, no matter how big the military or well trained the police force.

A useful approach to defining "native": 
There's a lot of confusion about what the word "native" means, and why it's an important distinction in ecology. Some contend that all plants and animals are native to the planet, and so all species should be welcomed everywhere with open arms. That supposedly openminded point of view requires denying the reality of co-evolution. It's really quite simple. When plants and animals evolve together for thousands of years, they adapt to each other's presence. A balance develops, in which plants evolve chemical or physical defenses that discourage animals from eating them into oblivion. Animals, in turn, develop the capacity to crack those defenses sufficiently to get the food they need to survive. There's a balance of consumption.

Here's a well-written column explaining how differently a plant like garlic mustard behaves where it evolved, vs. after being introduced to a new continent.

ALERT: Viburnum Leaf Beetle Spotted in Princeton

During an otherwise very positive weeding session in Harrison Street Park, I spotted a small Viburnum shrub that appeared to be a ghost of its former self. Every last leaf had been skeletonized. As I took a closer look, I was afraid of what I might find. Native to Europe, the Viburnum Leaf Beetle was transported to our continent and has been spreading across the eastern U.S., decimating native and exotic Viburnums as it goes.

This closeup corresponds with what can be found on the web. As far as I know, and I'll check with the extension service, this is the first documented occurrence in Mercer County. (Update, May 20: I called today, and they are in the process of confirming a sighting in Pennington.) The map on the NJ Invasive Species Strike Team website shows documented occurrences in other areas of NJ. My first encounter with it was some five years ago in a park in Pittsburgh, where a grove of arrowwood Viburnums (V. dentatum) had been transformed by the insect from robust green to brown ooze.

The only good news is that only two Viburnum shrubs in Harrison Street Park had been visually affected. This photo shows the early stages of attack. I tried to kill every last larva I could find.

Chances are, this is not the only infestation in Princeton. Homeowners should check any Viburnums in their yards for signs of damage. Some suggestions for control can be found at this link. Not all Viburnums are equally vulnerable, as this Cornell University list shows. Among our three most common native species, arrowwood Viburnum (V. dentatum) is the most vulnerable, followed by mapleleaf Viburnum (V. acerifolium), with blackhaw Viburnum (V. prunifolium) being the most resistant. Arrowwood Viburnum is generally found in low-lying woodlands. Mapleleaf Viburnum is a small shrub, rarely encountered except along the ridge. The blackhaw Viburnum is the largest and most common native Viburnum in Princeton. A Penn State website suggests that the insect prefers Viburnum species with less pubescence on the leaves.

Most people have heard of Emerald Ash Borer by now, which was spotted for the first time in Mercer County last year. One ecological consequence of losing most of our ash trees over the next ten years is that more sunlight will then reach the understory, which is dominated by non-native shrubs like honeysuckle, privet, asian Photinia and winged Euonymus. Viburnums are a main component of the native understory. If they are severely weakened by Viburnum leaf beetle, then they'll be even less able to compete with exotics for that additional sunlight. A recent presentation by entomologist Douglas Tallamy in Princeton offered evidence that not only is the foliage of native plant species necessary for the survival of countless species of insects, but their berries are much more nutritious for birds. How we are to keep foodchains intact in our forests is something of a question.

In addition to being vigilant and acting quickly if an infestation is found, one thing to do is take a moment to appreciate the Viburnums and ashes we have--the play of light on their leaves, the colors they turn in fall. In a largely unregulated world where powerful forces of change continue to be unleashed, there's a heightened poignancy in what we all too often take for granted.

A Pre-requiem for Viburnums

Ignorance, I suppose, is bliss. If one has never learned of the existence of Arrowwood Viburnums (V. dentatum), which flower along streambanks and in wet woods near the canal,

or the Mapleleaf Viburnums (V. acerifolium) common among the boulders along the Princeton Ridge, then their future absence will go unnoticed. So, is it better to maintain ignorance, or cultivate awareness so that these and other native Viburnums can at least be more consciously treasured during their remaining years in our woodlands?

The Viburnums look perfectly healthy now, but there's a certain leaf beetle coming our way, specifically the Viburnum leaf beetle (Pyrrhalta viburni). First noticed in the U.S. in 1994, it's a species from Europe and Asia that skeletonizes the leaves, exhausting the shrub's energy reserves after several years of defoliation. According to an information sheet prepared by Cornell (http://www.hort.cornell.edu/vlb/), the Arrowwood Viburnums are the most vulnerable. Mapleleaf Viburnums will be slower to succumb, and the other common native, blackhaw Viburnum, may show greater resistance. According to a comment on a Cornell University blog post, the beetle had already shown up at Duke Farms in 2011, just 15 miles north of Princeton.

This arrowwood Viburnum (lighter green foliage below is buttonbush and Virginia sweetspire) has reached ten feet high, along an ephemeral stream in the backyard. The mapleleaf Viburnums don't grow much past four feet in Princeton's woodlands.

As with the flowering dogwoods that have become more rare along the east coast due to an anthracnose disease introduced in the 1970s, the impact may be most felt by migrating birds that depend on the berries to fuel their flight south in the fall.

Knowing of the leaf beetle's imminent arrival at least gives us a chance to appreciate these shrubs while they can still spread a full array of leaves. It also deepens awareness of the importance of those who inspect plant material entering the country.