—–By Amanda Lindeman, PhD Candidate, Carleton University—–
In April 2015, I coauthored a paper on what bark beetles are trying to say to each other when they interact with potential mates (1). No one knew for sure – since bark beetles, as their name implies, live under the bark, males could simply be announcing their presence as they wander the surface of the bark trying to join a mate in her gallery below, they could be advertising their species identity to make appropriate mate pairings or to say “Hey! I’m not a predator, let me in!” But the thing that always struck me is that many of the 5000+ species of bark beetles produce sounds, and their sounds are complex — they produce more than one kind of sound, and they can be multi-component. For a group of animals that already produces intricate attraction pheromones, why produce sound at all? Is it just for the sake of redundancy?
Before I go too far, perhaps I should go waayyyy back, and explain why I find insect sounds to be so interesting in the first place. I think that animal communication has always captured human attention and imagination as we consider both the beauty in animal sounds and what they mean. The dawn choir of birds; the roar of a lion; the squeak of a mouse. But, as Frank E. Lutz (1924) said: “probably the first definite sounds made by land-animals on this earth were made by insects. Before ever birds sang or even frogs croaked”. Insects led the way. Indeed, many insects have beautiful songs appreciated by people since antiquity when crickets were kept as domestic pets in ancient China (3) and cicadas were kept in cages in Greece and Rome (4), not unlike how we would keep a pet bird today. Apart from those musically talented insects, however, we need to remember that even in “the lowest insect tribes, many a rough, rasping note, though awakening no particular delight in us, serves as great a purpose as the more pleasant sounds” – F. C. Clark (1875). The trouble in research often comes down to finding out what that purpose is.
No group embodies this sentiment more than the beetles, an order with more ways of producing sound than any other, and yet with a very poor and widely neglected understanding of the purpose of those sounds (5). Bark beetles are an incredibly interesting group of beetles, who likely first caught our interest because of their destructiveness. The members of the genus Dendroctonus in particular have been hailed by forest entomologists as being “the most destructive enemies of the coniferous forests of North America” (6) and “the greatest tree killers known”(7).
One species in the genus that is no stranger to Canadians is the mountain pine beetle, and to put things in perspective, this beetle has impacted over 18 million hectares of forest in BC, and killed about 50% of the total volume of commercial lodgepole pine in only two decades. And, as I mentioned above, I personally find them particularly interesting because of their complex sounds which many of them invariably make as they approach the gallery of a potential mate and try to enter.
So, getting back on point, what do these sounds mean? In one species of the destructive Dendroctonus genus, the red turpentine beetle, I found that many aspects of a male’s courtship song correlated to his size. Since male size is linked to his ability to produce more offspring, this means that the male might be using his chirping as a way to honestly tell the female how fit he is. One important chirp variable related to size was the number of components per chirp. Chirps with just one component are termed “simple” while chirps with more than one component are termed “interrupted” and sound like a stutter in the chirp to the human ear. It turns out larger males have more components in their chirps. Also, and importantly, females always admitted a male into her gallery if he made interrupted chirps, while if he only made simple chirps, or was experimentally muted to produce no sound at all, he would only be successfully accepted approximately 60% of the time.
Even though I find the question of why an animal produces sound to be inherently interesting, someone who has unfortunately been a bark beetle victim and has seen local communities and businesses devastated by these insects might not care so much about the why and instead wonder what next? Now that we know that sounds may be important to the life history of bark beetles and that their chances of successful mating might depend to some extent on these signals, can this help us manage them? Probably! Acoustic technologies have helped control pest insects by using the sounds the pests rely on against them (8). This can mean anything ranging from detecting their presence to manipulating their behaviour. These kinds of technologies have not yet been applied in bark beetle management because we haven’t known enough about their sounds to develop strategies. Hopefully, as we begin to understand more about the purpose of their sounds, we can use acoustic technology to develop new targeted solutions to this serious problem.
(1) Lindeman, A.L. & Yack, J.E. (2015) What is the password? Female bark beetles (Scolytinae) grant males access to their galleries based on courtship song. Behav. Proc. 115:123-131
(2) Lutz, F.E. (1924) Insect sounds. Am. Mus. Nat. Hist. 50:333-372.
(3) Laufer, B. (1927) Insect Musicians and Cricket Champions of China. Field Museum of Natural History Leaflet
(4) Clark, F.C. (1875) The song of the cicada. Nat. 90(2):70-74.
(5) Wessel, A. (2005) Stridulation in the Coleoptera – An overview. Insect Sounds and Communication. 397-430
(6) Hopkins, A.D. (1909) Practical information on the Scolytid beetles of North American forests. I. Bark beetles of the genus USDA Bur. Entomol. Bull. 83. 169 pp.
(7) Wood, S.L. (1963) A revision of the bark beetle genus Dendroctonus Erichson (Coleoptera: Scolytidae). Great Basin Nat. 23:1-116.
(8) Mankin, R.W. et al. (2011) Perspective and promise: a century of insect acoustic detection and monitoring. Ent. 57(1):30-44.