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Management of bark and ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) with semiochemicals: letter to a prospective graduate student

Published online by Cambridge University Press:  03 April 2020

John H. Borden
1Principal Consultant, JHB Consulting, 6552 Carnegie Street, Burnaby, British Columbia, V5B 1Y3, Canada 2Professor Emeritus, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, V5A 1S6, Canada
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When I accepted the invitation to serve as the lead-off speaker for the symposium Managing Bark and Ambrosia Beetles with Semiochemicals on which this special issue is based, I noted that everything one needed to know was published in Seybold et al. (2018), a really good review of the subject. So, I took another course. About 25 years ago, I gave an oral presentation in the form of a letter to an imaginary graduate student. It seemed to go over well, so I decided to do it again. Here is a version of that letter to Sarah, the latest prospective graduate student conjured up by my imagination.

© 2020 Entomological Society of Canada

When I accepted the invitation to serve as the lead-off speaker for the symposium Managing Bark and Ambrosia Beetles with Semiochemicals on which this special issue is based, I noted that everything one needed to know was published in Seybold et al. (Reference Seybold, Bentz, Fettig, Lundquist, Progar and Gillette2018), a really good review of the subject. So, I took another course. About 25 years ago, I gave an oral presentation in the form of a letter to an imaginary graduate student. It seemed to go over well, so I decided to do it again. Here is a version of that letter to Sarah, the latest prospective graduate student conjured up by my imagination.

Dear Sarah,

I was delighted to receive your letter telling me that you have read all my available papers and asking about the prospects for a career working with semiochemicals and the management of scolytine beetles (Coleoptera: Curculionidae: Scolytinae). This letter is a little long, but since you flattered me so adroitly, I suspect that you’re worth it. Moreover, at the age of 80, it means a lot not to be forgotten.

Here’s a story that illustrates how old I am. Three summers ago, I was visiting my son and his family in Lincoln, Nebraska, United States of America. I was in the University of Nebraska bookstore wearing a United States Marine Corps T-shirt. When I brought my purchase to the cash register, the young cashier looked up and said “My best friend is in Marine Corps boot camp. Were you in the Marines?” Quite pleased with her powers of observation, I replied, “Why yes I was.” There was a moment of silence while she digested this, so I filled the dead air by saying “I served from 1957 to 1961.” There was another moment of silence, so I said, “That was before you were born.” She screwed up her brow, obviously thinking hard, and then looked up at me and sweetly said “Sir, that was before my PARENTS were born.” If you are still with me, and realising how old I really am, I do have a thing or two to tell you. It’s as much a portrayal of a way of life as it is a description of a professional discipline. It is also biased towards the western part of our continent, for that is where I did my life’s work.

Each of the many people that I have met and worked with on scolytine semiochemicals was in love with trees, forests, and the insects therein. If you are not already similarly in love, or at least falling in love, please don’t read any further and set your sights on a nice career in accounting or real estate.

Still with me Sarah? Good. You should know that you can’t build a career solely on managing bark and ambrosia beetles with semiochemicals. You will have to be much more, because these beetles are just a few of the many pestiferous agents in the forest. I suggest that you plan on being a forest health professional. That means broadening your entomological horizons by taking courses in forestry (particularly ecology and silviculture). You will also find that many lay people are in love with trees and forests (often somewhat passionately), and that they may not share your refined judgement on the principles of sustainable resource management. As part of learning how to deal effectively and constructively with these people, you may wish to take a social science course or two.

You will also realise that working in the woods is not always easy or safe. So, in addition to learning how to conduct a randomised, controlled, and replicated field experiment that exquisitely tests a well-formed hypothesis; how to analyse your data with the most appropriate statistical tests; and how to develop and implement an integrated pest management plan, you will have to learn a whole bunch of field skills. These include using a GPS, a compass (just in case you are out of reach of the satellite), and a forest cover-type map; operating a chain saw without losing a foot; and driving on radio-controlled logging roads without losing your life. You will become covered in dust, immersed in mud, and will feed thousands of biting flies in diverse taxa. Inspecting your body for ticks after a day in the woods will become routine. You will come to appreciate the invention of water-proof paper as rain streams off your hardhat onto your notebook, and you will enjoy yourself immensely when a sludge of melting snow slides off a tree branch and accurately finds the gap between your field jacket and the back of your neck.

As a forest health professional, you will probably want to specialise in chemical ecology. In doing so, you will learn how pheromones of conspecific and heterospecific scolytine beetles and host and nonhost volatiles interact in surprisingly complex communication systems. All of them need to be much better understood.

You will enter the profession of forest health management in a disturbing period of profound climate change. Let me share some thoughts based on my personal experience.

In 1983 and 1984, I did some early work testing attractive semiochemicals to contain and concentrate spot infestations of the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae). Our research in the west Chilcotin Plateau of British Columbia, Canada, then ceased abruptly for lack of beetles. In 1984 and again in 1985, the temperature dropped to −35 °C in the fall, killing the beetles before they completely hardened off. Temperatures in the interior of British Columbia used to drop to −40 °C for a couple of weeks every winter. That was enough to kill the larvae, which can supercool to −38 °C. We haven’t seen winters like that since 1985. That’s 33 years and counting.

You can see another effect of climate change in the uncertain fate of whitebark pine, Pinus albicaulis Engelmann (Pinaceae). Palynologists tell us that this species used to occur at low elevations throughout the West. We also know that whitebark pine is a preferred host for the mountain pine beetle, and that the tree produces monoterpenes that are particularly attractive to the beetle. We hypothesise that after the last ice age, the mountain pine beetle killed off whitebark pine in the low-lying stands and relegated the survivors to high elevations where fall and winter temperatures were usually too low for the beetle to survive. Now it rarely gets that cold, and the mountain pine beetle has killed many stands of previously invulnerable whitebark pine. Forest health professionals from the Forest Service, United States Department of Agriculture have valiantly used the antiaggregation pheromone verbenone to protect some of these trees and stands, but in the end, I think that almost all the mature trees will be doomed.

Most of the trees that you encounter in western forests will have grown from natural seeds or planted seedlings adapted to a local climate that no longer exists. You may be able to use semiochemicals as part of the tool kit used to save these trees from bark beetles, but you may also find yourself part of a team that selects trees (and species) better adapted to new and ever-changing climatic conditions.

Let me switch your attention to the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Curculionidae). This minute creature existed as an obscure noncuriosity in New Mexico, Arizona, and California, United States of America, until some time in the 1990s. Before that, two remarkable events had occurred. Beetles in two distinct genetic lineages hybridised, and the hybrid hooked up with a previously unknown species of symbiotic fungus, that proved to be highly lethal to walnut, Juglans Linnaeus (Juglandaceae), trees. A tiny monster emerged and spread across the continent, attacking living walnut trees, inoculating them with the mysterious fungus, and leaving damaged and dead trees in its path. Who knows when the next monster will emerge? You might be there to witness that event.

I know where I’m going here Sarah, so bear with me. There are 1430 species of scolytine beetles in North and Central America. We understand something about the chemical ecology for a handful of them. For the rest, we know nothing. That was true for the walnut twig beetle until 2011 when a group led by the Forest Service, United States Department of Agriculture along with university collaborators filed a Provisional Patent Application describing a new pheromone for the beetle. Part of the research used coupled gas-chromatographic electroantennographic detection analysis (what we call GC-EAD) to aid in identification of the pheromone. The antennae of this beetle are virtually invisible to the naked eye. Yet a clever researcher was able to insert the tip of a microelectrode into the antennal club and record an electrical impulse in response to the compound that turned out to be the pheromone. The synthetic pheromone is now used to detect and monitor for the beetle, and it has the potential for other uses, for example, in insecticide-treated, pheromone-baited trap trees.

The research on the walnut twig beetle pheromone was a far cry from what we used to do, beginning in the 1960s. In my laboratory, for example, we needed 25 000 female striped ambrosia beetles, Trypodendron lineatum (Olivier) (Coleoptera: Curculionidae), laboriously collected from the forest floor where they were overwintering, to produce 450 g of pheromone-laden frass, barely enough for chemists to isolate and identify the pheromone lineatin. With modern technology, chemicals can be extracted or captured from pheromone-saturated air and identified by GC-EAD followed by coupled GC-mass spectrometry, usually using fewer than 100 insects. So, here’s a challenge for you. Why not collaborate with some cool chemists and get started on identifying the pheromones and host tree synergists for the over 1200 scolytine species about which we know nothing? In addition to contributing to our knowledge base, you might unexpectedly encounter the next walnut twig beetle. And then you would be ahead of the game.

Even for the beetles for which active semiochemicals are known, there are mysteries yet to be solved. Here are two that I would have liked to pursue myself.

The western balsam bark beetle, Dryocoetes confusus Swaine (Coleoptera: Curculionidae), kills subalpine fir, Abies lasiocarpa (Hooker) Nuttall (Pinaceae), and the fir engraver, Scolytus ventralis LeConte (Coleoptera: Curculionidae), does the same for white fir, Abies concolor (Gordon) Lindley ex Hildebrand, and grand fir, Abies grandis (Douglas ex Don) Lindley (Pinaceae). These species are often sympatric, yet the beetles almost never attack the “wrong” tree. I would like to know why, because the knowledge gained might provide new semiochemical tools for managing these tree-killing bark beetles. In addition, it could contribute to our capacity to genetically engineer sterile trees that smell like nonhosts.

Here’s my other pet project. The aggregation pheromone of the striped ambrosia beetle has been used for over 30 years in operational mass-trapping programmes. Yet for longer than that evidence for a potentially useful antiaggregation pheromone has been ignored. A researcher from the Canadian Forest Service used greenhouse cages that allowed odours from logs in separate chambers to be combined as they exited through a screen at the top. When logs infested by females producing aggregation pheromone were in one chamber, hundreds of beetles were caught in traps placed above the cages. When a log infested by both sexes was placed in the other chamber, the number of beetles captured in the traps declined strikingly. Because virgin and mated females were equally attractive, the researcher postulated that the male beetles produced a potent antiaggregation pheromone. Some time in the future, if I’m still alive, I hope that you can tell me what it is and how it is being used.

Based on my experience with the sort of career you espouse Sarah, you will undoubtedly have numerous unexpected adventures and encounters. Here are some of mine. After a hard day in the woods, I and an assistant got back to our truck to find two flat tires. It was a long walk back into town in the dark. Once I had a class of students with me doing field work. After I got them started on an exercise, I retreated some distance away to answer a call of nature. Suddenly, I heard a grunt, and I looked up to see a large black bear, Ursus americanus Pallas (Mammalia: Ursidae), 10 m in front of me intently observing the proceedings. I froze in fright, but surprisingly, it uttered another grunt and ambled off, obviously recognising that I was doing the right thing by marking my territory. One day, we had a cow moose, Alces alces (Linnaeus) (Mammalia: Cervidae), who wouldn’t let us onto one of our field plots. On another, it was a very large Hereford bull who was guarding a salt lick. Every time we approached, it lowered its massive head and menacingly pawed the dirt. We decided to go find our own salt lick.

Spring comes late in northern British Columbia and logging roads remain impassable due to snow. So, we were checking out potential research sites by helicopter. Up one valley, we left the helicopter at a landing and walked in the snow about 400 m up the road to a mountain pine beetle infestation that we had spotted from the air. Returning after about an hour, we found a THIRD set of footprints in the snow made by a gray wolf, Canis lupus Linnaeus (Mammalia: Canidae). He had followed us the entire way, and had probably been watching us, and maybe thinking about dinner, as we checked out the infested trees.

I have had research plots wiped out by forest fires, floods, destroyed by loggers, and made inaccessible by a huge landslide. Several species of squirrels (Mammalia: Sciuridae) have chewed up my tree baits, cows and equipment operators have destroyed my multiple-funnel traps, and porcupines, Erethizon dorsatum (Linnaeus) (Mammalia: Erethizontidae), have eaten my radiator hoses. On the cool side, I have twice found myself sitting in my truck immobile for an hour or so, surrounded by cattle, cowboys, and border collies as ranchers moved their herds down from the high country in the fall. Events and encounters like these may become part of your job Sarah. Be prepared and enjoy the ride.

When you become a forest health professional, you will be using strategies and tactics. Many people confuse these terms, so let me aid your clarity of comprehension. Both are military terms; this is appropriate, for we have been at war with our scolytine friends since we met them. Strategies are broad plans, and tactics are specific actions taken to meet strategic objectives. I have boiled my list of possible strategies for insect pest management down to four:

  1. 1. Prevent a problem from occurring (not the pest, but the problem),

  2. 2. Maintain the problem at a tolerable level,

  3. 3. Suppress the problem to a tolerable level, and

  4. 4. Do nothing; let nature run its course.

There are many tactics that can come into play, usually as a selected group that is specific to the problem at hand. A recent review identified numerous semiochemical-based tactics for managing scolytine beetles (Seybold et al. Reference Seybold, Bentz, Fettig, Lundquist, Progar and Gillette2018). These include:

  • employing semiochemical-baited traps to detect and monitor scolytines in forests, cities, and ports of entry,

  • using attractant-baited trees to detect new or expanding infestations when population levels are very low,

  • mass-trapping to suppress populations in small and concentrated outbreaks,

  • using trap trees to absorb populations that can later be removed by loggers,

  • baiting stands of trees with attractive semiochemicals to contain and concentrate small infestations prior to sanitation harvesting,

  • combining semiochemical baiting with insecticide treatment of trees or logs in a bait-and-kill tactic,

  • using antiaggregation pheromones alone or with other repellents to protect individual trees or stands from infestation, and

  • combining antiaggregants with attractive semiochemicals in a push-pull tactic, to push beetles away from susceptible trees or stands and to draw them into another area where they can be disposed of.

In your future career, you may want to apply one or more of these tactics in an integrated pest management programme that meets a strategic objective. You may also want to improve certain tactics. One possibility might be to use blends of antiaggregants to create a complex bogus message for host-seeking beetles, for example, this stand is composed of nonhost trees already occupied by heterospecific beetles, so keep moving. Let me give you an example. The antiaggregation pheromone verbenone is marginally effective when used alone against the western pine beetle, Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae). I was part of a study led by a researcher from the Forest Service, United States Department of Agriculture testing the effect of adding one new pheromone component and two nonhost volatiles to verbenone. We called the composition Verbenone Plus. It was much more effective than verbenone in preventing attack on ponderosa pine trees and stands. We proposed that it be adopted as an operational treatment.

Therein lies a problem. Because Verbenone Plus is used to control a pest it must by law be registered as a pesticide in both Canada and the United States of America. Registering a blend of compounds, even if they are all natural, is difficult and costly, because the safety for human health and the environment of each compound, as well as the blend, must be tested. Meanwhile, well-meaning legislation is directed mainly at large companies that market a small number of broad-spectrum high-revenue pesticides. Their cut-off annual revenue for a potential product almost always exceeds $25 million USD. They can afford the cost of registration. For semiochemical-based companies, the cost of registration must be weighed against the potential revenue from a species-specific product that may not even be used every year. It is rarely more than $50 000 USD. Thus, despite the effectiveness, apparent safety, and a critical need for Verbenone Plus, it was never registered. It is unavailable for use, even by well-educated and responsible professionals like your future self. Perhaps some time in your career Sarah, your help will be needed to register multi-component semiochemical-based products. Unfortunately, this may demand that you get a tad involved in politics.

From that dismal state-of-affairs, let me turn to the aesthetic and emotional delights that you will certainly experience during a career working in the forest. Here are some of my favourites:

  • inhaling the rich intoxicating odours of conifer resin in the hot sun,

  • stopping for a moment to wipe my brow and looking down to discover a motionless fawn lying right next to my feet,

  • hearing the distant drumming of a woodpecker on an otherwise silent summer day,

  • looking up in wonder at the golden snow of aspen leaves falling all around me on a crisp fall day, and

  • feeling exhilarated, but slightly unsettled, when I found myself alone, away from all human contact, in a beautiful but potentially deadly forest, and understanding, as do few of my friends, how small and insignificant mankind really is in the face of nature.

My best aesthetic experience occurred as the sun was dipping low over the interior British Columbia mountains on a late August day. That morning I had worked my way uphill through the forest. As I finished evaluating our tree-baiting experiment, I did not relish the thought of returning downhill, dragging my tired legs over the fallen trees that I had nimbly stepped over earlier in the day. I checked my map and found that if I walked about 100 m west, I would emerge into a cutblock that had been planted with lodgepole pine, Pinus contorta Douglas ex Loudon, seedlings about three years before. Walking through this cutblock would take me directly to my truck. As I emerged into the block, I found abundant little pines about thigh high. Amongst them wild roses had seeded in and grown in profusion. Being late in the summer, millions of rose hips had appeared; each one bright red and highlighted by the setting sun. I walked downhill in amazed delight through a field of pine trees and glowing rubies. It was incredibly beautiful.

Finally, let me end by paraphrasing, a noted British Columbia biologist, and advising you that managing scolytine beetles with semiochemicals is not rocket science. No, it’s not. It is infinitely more complicated and considerably more intellectually challenging. It demands knowledge of a multitude of subjects: entomology, forestry, plant pathology, law, climatology, ecology, meteorology, economics, hydrology, sociology, statistics, and of course chemical ecology. There are a daunting number of possibilities for combining strategies and tactics in forest health management plans that differ in concept and application with each forest site and infestation.

Unlike rocket science, forest health management inevitably involves people, who both foster and perturb the management of forest pests. People elect politicians to make laws, educate lawyers to interpret them, and employ police of various sorts to enforce them. Although there are noted exceptions, none of these people can be expected to know much about nature or science. Some people love trees and forests to death, some use and manage them judiciously, and others see them as removable objects that are in the way of some other enterprise. Each of these viewpoints conflicts with the other two. You must gain the knowledge, the ability, and the wisdom to deal with all these factors in ever-changing natural and social environments. Sarah, I trust that you are up to the challenge, and I wish you every success in what will certainly be a very interesting and rewarding career.


John H. Borden

Principal Consultant, JHB Consulting

Professor Emeritus, Simon Fraser University


Dr. Steven J. Seybold, to whom this special issue is dedicated, and I emerged 26 years apart from Dr. David L. Wood’s laboratory at the University of California, Berkeley, California. Despite the disparity in our ages, we shared a similar rewarding experience as graduate students and were both deeply appreciative of the role of that experience in shaping our future useful and fulfilling professional lives. After my presentation, Steve told me how meaningful it was to him as he reflected on his formative years. I could not know at the time that his days with us were so limited. Knowing that now, I respectfully dedicate this paper in honour of Dr. Seybold, one of the finest scientists and persons that one could ever have as a colleague and friend.


Subject editor: Chris Fettig


Seybold, S.J., Bentz, B.J., Fettig, C.J., Lundquist, J.E., Progar, R.A., and Gillette, N.E. 2018. Management of western North American bark beetles with semiochemicals. Annual Review of Entomology, 63: 407432.CrossRefGoogle ScholarPubMed

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