[Focus On Stinging Insects] Attack Of The Sap Addicts

The strange case of bark-stripping hornets.

October 7, 2003

This is the story of the European hornet, a giant wasp with a powerful thirst. Their drink of choice is the fresh sap of certain woody plants, oozing from wounds that the hornets make by gnawing off bark with their powerful jaws. And when these bad boys — er, girls — decide to stay and party in a particular tree or shrub, it can turn into an all-night event.

Compulsive bark stripping is by far the source of most homeowner complaints about hornets, but it’s just one of the many noteworthy habits of this amazing insect. Let’s begin with some general biology.

SIZE MATTERS. Originally confined to the Old World and found mainly in Southeast Asia, the so-called "true hornets" (Vespa spp.) are among the world’s largest wasps. The European hornet (Vespa crabro) is the only member of the group to range into north temperate regions. It’s quite an eye catcher. Flaunting a deep, almost glowing yellow against a rich, reddish brown, the queens are almost an inch and a half in length (see figure 1 below and figure 3 on page 70), with the workers about two-thirds this size. Possibly they rely on sheer scariness for much of their defense, since although their sting is exceptionally painful — believe me, I know — European hornets are noticeably more mild-mannered than their smaller cousins, the yellowjackets (Vespula and Dolichovespula spp.). Nests are typically located in hollow trees (see figure 2 on page 66), but any man-made cavity will also do, such as sheds, attics and wall voids. (See PCT, August 2003.) A distinctive and memorable aspect to mature nests is their extremely foul smell, due to copious amounts of excreted droppings that steadily accumulate beneath the combs.

In addition to its protective value, great size also confers great predatory privilege. European hornet workers are capable of subduing prey as large and vigorous as cicadas and dragonflies, although the majority of their victims are smaller fare. They frequently capture other stinging insects, including honey bees, which has earned them the ill will of many beekeepers. (In truth, however, they are an insignificant source of honey bee mortality.) Workers also dine on ripe fruit, either on the tree or off, excavating deep scoops that resemble rodent or rabbit feeding. They occasionally do substantial damage in late-bearing apple orchards (see figure 4 on page 72).

Sometime back in the mid-1800s, European hornets began showing up in the New York area, most likely the offspring of fertilized queens that had stowed aboard transatlantic ships. The species has since spread across most of the eastern United States and — I think I can say this without fear of contradiction — is presently the most spectacular social wasp in North America. But right from the get-go, starting with a report in 1878, it began attracting attention mainly for its peculiar habit of stripping bark off the twigs and young branches of ornamental plants. The wounds often girdle a branch, thereby killing it.

Over the years, observers of this behavior persisted in thinking that the hornets were collecting the bark for nest construction and you’ll still see this erroneous assumption in some modern references. In fact, the bark is nibbled away in tiny bits that drop to the ground. What the hornets are really after is a liquid sugar fix.

SAP HAPPY. Sap is a complex, highly nutritious source of carbohydrates and many animals will go to great lengths to get at it. Squirrels and other rodents are well known bark peelers, but this method of sap seeking is also practiced by deer, bears, primates (from marmosets to orangutans) and various human cultures worldwide. Most sap-feeding insects have piercing-sucking mouthparts and extract the substance by drilling through leaves or stems. Hornets are unusual in that they are large enough to strip mine for it. Although they’re attracted to more than a dozen different types of trees and shrubs, lilac is indisputably their favorite, with ash and birch as runners-up. Other sources include horsechestnut, alder, elm, dogwood, rhododendron and boxwood.

Now, it’s clear that if this feeding was limited to a few well-mannered foragers, most homeowners would never even realize the insects were there. But it doesn’t work that way. Especially in late summer and autumn, when colonies are at their peak, dozens of the brutes may congregate at one site. They tend to be quite protective of their food source. The exposed cambium attracts an abundance of freeloaders, such as flies, ants, yellowjackets and paper wasps (Polistes spp.) and when not jostling each other, feeding hornets will repeatedly lunge at these intruders to drive them off. Occasionally they’ll take one as prey. They’re also not shy around people, launching into the air when disturbed and closely circling in what seems to be an effort at territorial intimidation. This is undeniably disconcerting, but bear in mind that like any other social wasp, hornets reserve their sting for nest defense or to save their life if seized. Stinging is not used to defend food sources or to subdue prey.

The most spectacular displays of this group behavior are when hundreds of hornets from several nearby colonies are drawn to a single area. Bad enough for an entomophobic suburbanite, but what do you think are the consequences if this area happens to be a commercial nursery? At last year’s national conference of the Entomological Society of America, Dr. Jason Oliver of Tennessee State University reported that two growers in his state had estimated $4,000 and $15,000 in losses due to dieback on hornet-girdled stock — not to mention the obvious nuisance that the creatures caused for their field hands.

NOW FOR OUR BAD NEWS. At this point, there is really no practical means, chemical or otherwise, to make these pests go away. Since the source of the hornets may be concealed anywhere within a half-mile radius from where they’re feeding, locating and destroying the nest(s) is simply not a feasible option. Trapping so far remains in the realm of wishful thinking, since nobody’s yet discovered any bait that out-competes the lure of the sap. I’m sure that PCOs could suggest that some lightweight netting could be draped over smaller shrubs, but this is not a route most homeowners are going to take. And finally, a liquid spray of Sevin (carbaryl) is almost universally recommended in extension bulletins, but I think this is mainly because (a) it’s legal to use in this fashion and (b) it’s readily available to the public. Over the years, I’ve talked to numerous pest management professionals who are certified to treat ornamentals, but have never found anybody who has discovered any pesticide labeled for the site, including Sevin, that does anything besides contact-kill the foragers that happen to be present at the time of spraying. Since there are usually plenty more where they came from (typical mature colonies contain several hundred workers), the hornets always tend to come back before long. I would be most interested in hearing if anybody out there has come up with a method that gives consistently good results in dealing with this problem.

Until then, it seems to me that the most powerful tool you have at your disposal is to convey accurate information to your client. The bottom line is that the insects are not a stinging threat unless you’re thrashing around in the bushes with them and that, for most homeowners, the damage done by the wasps is really quite minimal.

ADAPTATION OR ADDICTION? Now I have to tell you about what I think is the most fascinating aspect of all this. My own interest in sap feeding by Vespa crabro began a few years back when I was approached by the late Dr. Frank Santamour of the U. S. National Arboretum. At the time, I was studying hornet life within the nest by means of free-foraging colonies maintained inside glass-bottomed nest boxes, similar in principle to the familiar observation honey bee hive. Dr. Santamour had noticed hornets "attacking" some of his research plots of birch and ash and wondered if there was any pattern to their selection of specific plants. Together with Dr. Nancy Breisch, we began making detailed observations in the field.

It soon became apparent that the craving of European hornets for sap and the effort they expend to secure it goes far beyond any other known type of social wasp foraging behavior. Considering the total amount of damage recorded in the plots, coupled with how gradually the insects whittle away the bark, we concluded that an enormous number of hornet-hours must be devoted to this task. At least 140 trees at the site displayed damage and up to about 80 hornets were counted per tree. The single-mindedness of their quest was remarkable. When gently offered a piece of freshly stripped and crushed ash bark, the insects would pounce on it, lapping furiously. We began to wonder how in the world the colonies were getting anything else done.

In fact, when I would peer into my observation boxes during the late summer and autumn, the nests often seemed strangely empty, even though I knew large numbers of adults had recently emerged. It was just as perplexing after dark. Unlike most other social wasps, European hornets readily fly at night. However, none of the returning foragers at that time ever seemed to bring anything noticeable back with them. What were they doing out there? At one point, I sacrificed a colony in early September, sealing off the entrance hole at 1 a.m. Four workers returned within 15 minutes, then no more for the rest of the night, for a total of 139 individuals in the nest. As soon as the air warmed up the following morning, 49 additional workers showed up. More than a quarter of the colony hadn’t come home that night.

What could those rascals have been up to? A midnight visit to Dr. Santa-mour’s plots later that month suggested the answer. Just as many hornets were present on the branches as had been during the daytime! With temperatures too cold for flight, the insects slowly and feebly chewed and lapped or simply remained motionless with their mouthparts on the exposed cambium.

Essentially, they had passed out on their plate.

One has to wonder — is this curious obsession actually adaptive for the hornets, representing a laborious mining for essential nutrients that cannot be obtained elsewhere? (A parallel might be drawn with Australian sugar gliders, sap-feeding marsupials that spend more than 80 percent of their time feeding, more than any other type of mammal.) Or is it actually more of an addiction, a detrimental preoccupation with a highly attractive but non-essential substance and thus represents a serious drain on colony resources? In his fascinating book "Intoxication: Life in Pursuit of Artificial Paradise" (E. P. Dutton, 1989), Dr. Ronald Siegel presents a wealth of evidence that the quest for intoxication should be considered as a universal drive throughout the animal kingdom. Could the products of fermentation be behind the bark-stripping phenomenon? Does the sap mimic the sweet oral secretion of larval hornets that is compulsively solicited by the adults in the process known as trophallaxis? All I know for sure as I write this late one autumn night is that out there in the dark, all over the eastern United States, hosts of giant wasps are silently clinging to branches as if in a trance.

It’s 10 p.m. Do you know where your nestmates are?

The author gratefully acknowledges the late Dr. Frank Santamour and Dr. Nancy Breisch, who collaborated on the research reported here. The opinions expressed herein are the views of the author and do not necessarily reflect the official policy or position of the U.S. General Services Administration.

All photos are ©Al Greene/Nancy L. Breisch.

The author is regional entomologist for the U.S. General Services Administration in Washington, D.C. He can be reached at agreene@pctonline.com.