More on Western Tent Caterpillars (Malocosoma californicum pluviale)


The Western Tent Caterpillar is probably one of the most studied and also one of the most loathed insects in the Pacific Northwest. I’m hoping to change attitudes by shining a light on some of the ecological facets of the species and how it connects to the larger food web. We often deem something a pest before really considering the whole picture. Is there anything good about a caterpillar eating leaves off a tree? It depends on a lot of factors. Why not take time to examine the web…and I’m not referring to the tent here either.

It was just last week in my community (San Juan Island), that I heard a story about a woman who fell and hit her head after getting on a ladder to BURN the tent caterpillars out of her fruit trees. Hmmm. Please don’t try this at home. It isn’t safe. Burning the tents out of trees can actually do more damage to the tree than the caterpillars do by eating the leaves.

The photos below show something that happens to the tent caterpillars we may not notice in our panic to eradicate them from our trees. The egg on the caterpillar was laid by a parasitic Tachinid fly. It chose the head, so the caterpillar can’t chew it off its body. The egg is shed when the caterpillar molts, but the fly is already developing inside the caterpillar. It will literally eat the caterpillar from the inside out. So, when you clip off those tents and throw them into the fire, you are also killing the natural and best pest predators along with them. Naturus interruptus! We do more harm than good by intervening.

Western Tent Caterpillar with Tachinid Fly egg on head
Western Tent Caterpillars (Malocosoma californicum pluviale) with deceased individuals in background likely affected by nucleopolyhedrovirus 

The Western Tent Moth caterpillars are affected by a few other parasitoids. Braconid wasps also attack them. Some lay eggs on the cocoons. There is also a nucleopolyhedrovirus that infects them when populations are high. In my rush to get this out, I may come back and edit, but I’ve referenced lots of great information below so you can read more about this on your own.

To add to all of this, over the weekend, my daughter and I found some tents in the orchard trees on our property. I might just be the ONLY resident in the San Juans excited to see them. Hmmm. Well, what I found was even more interesting. The tents had dead caterpillars inside and living family groups of earwigs. We also found a super cute jumping spider!

I was curious about this because earwigs are known to be garden pests, I did find some studies about earwigs that are PREDATORY on species of Lepidoptera. While these studies addressed other species of moths, the gist was that the plant species sends out a chemical signal that calls pest predators when it is being attacked by caterpillars. Every plant and pest predator sends and responds (respectively) to various signals, some very specific to each relationship. The plant is calling in the army! It may not always be earwigs, but there are wasps, flies, and others that come to aid the plant when it is under attack. Yes, it is very cool!!!

Earwigs and Jumping spider eat tent caterpillars

Oh, and those Western Tent Caterpillars turn into adult moths in mid summer. They are attracted to light. Turn off your outdoor lights. Nature will thank you and you will be less attractive to the mating moths. Many moth species also tend to fly off en-masse when they are mate seeking. These periodic, seasonal pulses of terrestrial invertebrates in our region end up in nearshore marine habitats when they fly out over the ocean.

Various studies have surveyed the stomach contents of Chinook and Coho Salmon, and other fishes in nearshore marine habitats during their first year at sea. Two studies I found reported finding Western Tent Moths and Spruce Budworm Moths (species considered as pests in northern boreal forests) in sampled gut contents. Brodeur et al., (1987) reported the following from one survey, “The incidence of several juvenile coho collected after the storm which had stomachs that were distended with over 100 of these insects exemplifies the ability of these juvenile coho to readily exploit these allochthonous inputs into the marine environment.” They were referring to the “pest” species, (Choristoneura occidentalis) or Spruce Budworm Moth in this instance. In Brennan et al. (2002), sampling of salmon in Central Puget Sound found insect prey included Western Tent Moths (Malocosoma sp.), and that “Lepidoptera in 2002 diets were gravimetrically dominated by tent caterpillar moths (Malocosoma sp.) 51% of Lepidoptera category by weight.” They also reported that Lepidoptera in their samples “were only abundant in 2002.” Coincidentally perhaps, this was a year of a recorded outbreak of tent caterpillars in WA state.

Other studies acknowledge terrestrial invertebrates as a better quality food than marine crustaceans for developing salmon. Periodic, cyclic, or seasonal events resulting in abundant insect flotsam in marine habitats may be missed, or difficult to record, but undoubtedly play a role in feeding fish in nearshore marine habitats.

Take away point here. Even bugs we see as pests have a role in ecosystems. Salmon and other species of wildlife don’t have grocery stores to visit when they need a meal. They rely on seasonal and periodic availability of food. It’s all they have, and it’s important for us to appreciate that.

Please take a moment to scroll through some of the photos below. Definitely check out the fantastic animation by April Randall about the adult moths flying out over the shoreline and being eaten by salmon! Don’t miss checking out those references and reading material too. If you are curious to know more, shoot me an email and I’m happy to send you literature for further reading.

Thank you!

Malocosoma californicum pluviale with Tachinid fly egg on head
Malocosoma californicum pluviale caterpillars affected by nucleopolyhedrovirus 

Jumping spider eating Malocosoma californicum pluviale caterpillar (photo credit to Alex Maas and artistic rendering by Cynthia Brast-Bormann)
Parasitized Malocosoma californicum pluviale cocoon (Tachinid fly parasite)
Animation by April Randall, Orcas Island

References and Further Reading

Bell, K., Naranjo-Guevara, N., Santos, R., Meadow, R., & Bento, J. (2020). Predatory Earwigs are Attracted by Herbivore-Induced Plant Volatiles Linked with Plant Growth-Promoting Rhizobacteria. Insects11(5), 271. https://doi.org/10.3390/insects11050271

Clark, E. C. (1958). Ecology of the Polyhedroses of Tent Caterpillars. Ecology39(1), 132–139. https://doi.org/10.2307/1929975

Ciesla, W. , Ragenovich, I.R. 2008. Western Tent Caterpillar. USDA Forest Insect and Disease Leaflet 119. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_042847.pdf

Cooper, Dawn & Cory, Jenny & Theilmann, David & Myers, Judith. (2003). Nucleopolyhedroviruses of forest and western tent caterpillars: Cross-infectivity and evidence for activation of latent virus in high-density field populations. Ecological Entomology. 28. 41 – 50. 10.1046/j.1365-2311.2003.00474.x.

Dahlhoff, V. Woods, A. and B. Larkin. 2019. The Western Tent Caterpillar, Malocosoma californicum pluviale. MPG North Field Notes. https://www.mpgnorth.com/field-notes/2019/08/western-tent-caterpillar-malacosoma-californicum-pluviale

Furniss RL, Carolin VM. 1977. Western forest insects. U.S. Department of Agriculture, Forest Service, Washington, D.C. Miscellaneous Publication 1339. 654 p.

Knight, G. A.; Lavigne, R. J.; and Pogue, M. G. 1991. “The Parasitoid Complex of Forest Tent Caterpillar,
Malacosoma Disstria (Lepidoptera: Lasiocampidae), in Eastern Wyoming Shelterbelts,” The Great Lakes Entomologist, vol 24 (4) Available at: https://scholar.valpo.edu/tgle/vol24/iss4/7

Rodstrom, R & Resources, Greenwood & Portland, Oregon & John, J & Brown, John. (2017). FOREST AND WESTERN TENT CATERPILLARS Insect Pest Management in Hybrid Poplars Series. 10.13140/RG.2.2.24262.37442.

Stehr, F.W. & E.F. Cook 1968. A revision of the genus Malacosoma Hubner in North America (Lepidoptera: Lasiocampidae): systematics, biology, immatures, and parasites. Bulletin of the United States National Museum, (276): 1-321. https://archive.org/details/bulletinunitedst2761968unit/page/n6/mode/1up?view=theater

Witter JA, Kuhlman HM. 1972. A review of the parasites and predators of tent caterpillars (Malacosoma spp.) in North America. Minnesota Agricultural Experiment Station. Technical Bulletin 289. 48 p.

Additional References***Updated 06.23.2022

Brennan, J.S., K.F. Higgins, J.R. Cordell, and V.A. Stamatiou. 2004. Juvenile Salmon Composition, Timing Distribution, and Diet in Marine Nearshore Waters of Central Puget Sound in 2001-2002. King County Department of Natural Resources and Parks, Seattle Wa. 164pp.

Brodeur, R. D., Mundy, B. C., Pearcy, W. G., & Wisseman, R. W. 1987. The neustonic fauna in coastal waters of the northeast Pacific: abundance, distribution, and utilization by juvenile salmonids. Oregon State University Publication ORESU-T-87-001.

Brodeur, R. D. (1989). Neustonic feeding by juvenile salmonids in coastal waters of the Northeast Pacific. Canadian Journal of Zoology67(8), 1995-2007.

Brodeur, R. D., Lorz, H. V., & Pearcy, W. G. (1987). Food habits and dietary variability of pelagic nekton off Oregon and Washington, 1979-1984. NOAA Technical Report NMFS 57.  U.S. Department of Commerce National Oceanic and Atmospheric Administration National Marine Fisheries Service.

Cheng L, Birch M. 2008. Insect flotsam: an unstudied marine resource. Ecol Entomol 3:87–97.

Cheng L. 1975. Marine pleuston: animals at the sea-air interface. Oceanogr Mar Biol Annu Rev. 13:181–212.

Cheng, L., M. C. Birch. 2009. Terrestrial insects at sea.  Journal of the Marine Biological Association of the United Kingdom57, 4, (995-997).

DNR TreeLink. Tenting in the Trees. 2012. WSU Extension Puget Sound Stewardship E-Newletter 5:4

Drake, V.A., D. R. Reynolds, Radar Entomology: Observing Insect Flight and Migration (CABI, Wallingford, UK, 2012).

Duffy, E.J., D.A. Beauchamp, R. Sweeting, R. Beamish, and J. Brennan. 2010. Ontogenetic diet shifts of juvenile Chinook salmon in nearshore and offshore habitats of Puget Sound. Transactions of the American Fisheries Society. 139:803-823. 

Glick P. 1939. The distribution of insects, spiders, and mites in the air. Washington D.C.: US Department of Agriculture. 

Green K., 2011. The transport of nutrients and energy into the Australian Snowy Mountains by migrating bogong moths Agrotis infusaAustral. Ecol. 36, 25–34.

Gutierrez, L. 2011. Terrestrial invertebrate prey for juvenile Chinook salmon: Abundance and environmental controls on an interior Alaskan river. MS Thesis, University of Alaska Fairbanks, Fairbanks, AK. 

Hardy AC, Cheng L. 1986. Studies in the distribution of insects by aerial currents. III. Insect drift over the sea. Ecol Entomol. 113:283–90.

Helm RR. 2021. The mysterious ecosystem at the ocean’s surface. Plos Biology. Apr;19(4):e3001046.

Holland RA, Wikelski M, Wilcove DS. How and why do insects migrate? Science. 2006 Aug 11;313(5788):794-6. doi: 10.1126/science.1127272. PMID: 16902129.

Hu G, Lim KS, Horvitz N, Clark SJ, Reynolds DR, Sapir N, Chapman JW. Mass seasonal bioflows of high-flying insect migrants. Science. 2016 Dec 23;354(6319):1584-1587. doi: 10.1126/science.aah4379. PMID: 28008067.

Landry J. S., Parrott L., Could the lateral transfer of nutrients by outbreaking insects lead to consequential landscape-scale effects? Ecosphere 7, e01265 (2016).

Locke, A., S. Corey. 1986. Terrestrial and freshwater invertebrates in the neuston of the Bay of Fundy, Canada. Canadian Journal of Zoology64(7): 1535-1541. https://doi.org/10.1139/z86-228

Myers, J. 2000. Population fluctuations of the western tent caterpillar in southwestern British Columbia. Popul Ecol 42, 231–241. https://doi.org/10.1007/PL00012002

Peterson, C. 2013. Where Are the Yellow-billed Cuckoos? https://www.birdnote.org/listen/shows/where-are-yellow-billed-cuckoos

Satterfield, Dara & Sillett, T & Chapman, Jason & Altizer, Sonia & Marra, Peter. 2020. Seasonal insect migrations: massive, influential, and overlooked. Frontiers in Ecology and the Environment. 18. 10.1002/fee.2217.

Zaitsev, Y. P. (1971). Marine neustonology.

3 comments

  • What a gorgeous website! Is this a new form of your “bugs in San Juan island” venture, or just my first time seeing it?
    Also, a longtime friend of my husbands was talking about her PNW childhood in which her father would announce the tent caterpillars arrival as a sign it was august….are they now on a different time frame?

    Liked by 1 person

  • Hi Jamie!!! Adult emergence can range from early July through August and likely correlates with weather cycles and timing of development. I did read today that our cycles in the San Juans and Gulf Islands can differ from mainland cycles. Myers, J. Population fluctuations of the western tent caterpillar in southwestern British Columbia. Popul Ecol 42, 231–241 (2000). https://doi.org/10.1007/PL00012002

    Like

  • Really nicely done, Cynthia! I learned some things! The relationship to chinook was particularly fascinating. Thank you!!!

    Liked by 1 person

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