Last week, while checking on our prairie, I noticed a couple big congregations of ‘woolly bear’ caterpillars. I found several clusters of hundreds, which was impressive. They didn’t seem to be tied to any particular plant species or habitat structure – just apparently random groups. The numbers were impressive.
Yesterday, I was back out at the prairie to catch the sunrise and I saw the caterpillar clusters again, but this time, there was a stark difference. Many of the caterpillars were way up high in the canopy – at the tops of grasses, forbs, and even fence posts. And many of those high elevation caterpillars were dead. Some were still intact, but others were withered or empty-looking shells of their former selves. The calm winds, dewy conditions, and warm-colored sunlight made for some great photography, but I spent much of the time distracted by the mystery – why were all these caterpillars climbing up high and dying?
I believe the caterpillars I saw were larvae of the Virginian tiger moth (Spilosoma virginica). The species can be really variable in color, so while I found both pale creamy-colored caterpillars and almost bright orange caterpillars, they could still all be the same species. (It’s also very possible that there were multiple species – I’m not an expert on moth caterpillar identification!)
The climbing behavior reminded me of stories I’ve read and heard about insects that are attacked by parasitoids, fungi, or viruses that alter their behavior before eventually killing them. In the case of some fungi and viruses, insects are compelled to climb up high before dying, which helps the spores of the infecting agent spread widely after the larva dies. It’s certainly possible something like that is at work here, though the appearance/condition of the dead caterpillars I saw didn’t match the kinds of descriptions I found during an online search last night.
Parasitoids seem like a likely possibility – either flies or wasps. Stephen Spomer of the University of Nebraska-Lincoln says he has seen similar behavior in the past with tiger moth larvae and has reared parasitoids from them (entomologists are the coolest…). However, neither of us could explain why parasitoids would stimulate climbing behavior. Stephen suggested there could also be multiple factors at work (parasitoids and fungi, for example). If so, that’s pretty rough for those cute little caterpillars.
I did see some caterpillars that were still acting normally – crawling around on plants and feeding. They were also down at a more reasonable elevation in the vegetation, rather than at the pinnacle of the highest plant they could find. I hope they escaped whatever afflicted their colleagues and weren’t just in an early stage of infection that hadn’t fully kicked in yet.
I’d appreciate hearing from anyone who knows more about what I was seeing and can provide more explanation. I love a good mystery, but I love fascinating natural history stories even more. There’s got to be at least one good story here and I’d really like to hear it. It’s easy to feel badly for the caterpillars, and that’s certainly a reasonable emotion. At the same time, parasitoids and other possible infection agents also have an important role to play in controlling the populations of insects like this. I think it’s possible to feel bad for individuals while appreciating the broader picture that led to their affliction (even if we don’t yet understand that picture fully).
Tiger moths have been around for a long time and are likely to continue existing long into the future. I’m sure they’ve been dealing with whatever parasitoid or other organism is attacking them for a very long time. I’m fascinated by what I saw yesterday and would like to learn more about it. I also wish both organisms luck. The caterpillars that survive have a long winter ahead of them and the mystery organism will also have to get through the winter so it can continue its own life cycle next year. Complex interactions keep prairies healthy and resilient. Carry on, little friends!
Many thanks to everyone who submitted questions in response to my last post celebrating 10 years of this blog. I got enough questions that I’ve decided to split my responses into two batches. I’ll do one here and save the others for later this week or early next.
Before I get to the questions, here’s a quick notification. Tomorrow night (Thursday, September 24) I’ll be a featured speaker for a conservation webinar hosted by Pollinator Friendly Alliance. The session begins at 6pm Central Time and you can find out more at this link. I’ll be talking about how plants, insects, and other small organisms are really what make ecosystems resilient and functional, regardless of how much attention gets paid to birds, big mammals, and other large charismatic animals. If this sounds interesting to you, please consider joining in! (You can pepper me with hard questions afterward if you want.)
Ok, here is the first batch of questions and answers. I hope you find them interesting.
Angie Miller asks: Aside from reading The Ecology and Management of Prairies in the Central United States (which is a given), what advice would you give to folks who would like to learn more about practical ecological restoration and land management techniques? Any other book recommendations? Conferences? Professional organizations? Thank you
Well, I wrote the book precisely because I was having trouble finding published materials on those topics. I think there is a lot of local knowledge, but it’s often not easily accessible by people other than by finding local experts. I’m hoping others will chime in here with suggestions.
In some parts of the country, there are local clubs/associations (Prairie Enthusiasts, Wild Ones, Audubon Chapters, etc.) that do fantastic work and have lots of knowledgeable people to learn from. Volunteering for a local non-profit or government agency who is doing the kind of work you’re interested in is another great way to learn. The North American Prairie Conference is always a great one to go to if you’re looking for non-technical (as well as technical) information and advice. If you tell me where you’re from I might be able to help connect you with some local expertise.
John Helzer asks: Where do you see this blog in another 10 years?
Thanks for asking. In ten years, this blog would like a position with more responsibility and a considerably higher salary. It enjoys its current role, but feels like it could contribute more to the organization if it was given a broader reach, and maybe a few interns to help cover basic tasks. It thinks its track record so far has shown that it can handle additional responsibility. As an aside, this blog would like to inquire about its 401k options. The writer of this blog, however, is very happy in his current job and would like to continue splitting his time between research, advising land stewardship, and communication work.
Lisa asks: When I attended University of Nebraska many years ago, I took a class from Paul Olson called The Literature of Agriculture. We read Willa Cather, Mari Sandoz, Ole Rolvaag, Wendell Berry, and others. Are there books of fiction or non-fiction that have particularly influenced your work or your interest in Midwestern landscapes and culture? Or your philosophy of land management?
I’ve always appreciated Willa Cather’s accurate and effective portrayals of prairies in her writing. She put the right plants in the right places and had them blooming at the right times of year, which is impressive. When I was college age, I read most of the authors you listed there, as well as William Least Heat-Moon and some others. I would say, though, that Aldo Leopold is and was the most influential in terms of my overall approach to conservation – especially his writings about a land ethic.
Having said all that, I’ve found that I’ve learned most from direct interactions with people who have been thinking about and working in land management for a long time. Riding around in a truck and walking side by side through prairies with people like Bill Whitney, Gerry Steinauer, and Al Steuter, in particular, has taught me more than any book.
Anna Helzer asks: Who is your favorite child? (Other than your square meter photo book)
On a less serious note, do you have a favorite blog post or conversation from the last decade?
My favorite child would never ask that question. Also, did I tell you about what the square meter book did when we were at the park the other day? It was adorable…
The river otter running gag has been fun – I probably need to revisit that again soon. I also love photographing, learning about, and sharing images and information about insects most people have never heard from. I really enjoy it and don’t imagine I’ll ever run out of material for that kind of post.
Apart from that, I hope one of my more productive messages has revolved around the idea that prairies are more resilient than many people give them credit for. I’ve advocated for the importance of habitat heterogeneity and the idea that it’s ok to manage in ways that temporarily set back the vigor of plants, including the ones with pretty flowers, in order to create that suite of habitat conditions. I know I’ve made people uncomfortable by expressing opinions on that topic, but I hope that discomfort has led to helpful conversations and experimentation. Local expertise should absolutely drive how prairies are managed, but I’m proud that I’ve helped push people consider a broader set of options as they make local decisions.
Bruce Morrison asks: I just discovered your blog this past winter/spring – you do a great job! I live on a very small remnant (3 acres of pasture), surrounded by crop and other pasture ground. We have found many native plants here – both grasses and forbs. We try and burn parts of the small pasture ground we own and leave parts for 2-3 years between. We do worry about invertebrate diversity and I have seen changes over the past 18 years here, for instance we used to see 2 different species of Argiope spiders fairly regularly, but have not seen any the past 6 years. They even used to be in our gardens and yard, but no longer. We do not spray any gardens or orchard trees – never. However our neighbors spray their crops which surround us on all sides. I worried about our burning frequency causing invertebrate population drops – but suspect the crops are a larger issue. Is breaking a tiny space into 4 sections and burning one or two sections every 2-3 years still too much for invertebrate pressure in your opinion?
Thanks very much, Bruce. It sounds to me like your approach to reducing impacts to invertebrates is sound. Have you seen changes in the plant community over those 18 years? I wonder if the invertebrates are potentially responding to plant community changes, though there are a lot of reasons you could be seeing fewer Argiope spiders, using your example.
Pesticides could certainly be an issue, but so could isolation/fragmentation of habitat. If the other pastureland around you has decent species diversity, maybe it’s not an issue, but having only 3 acres of isolated habitat can make it hard to maintain healthy communities, despite your best efforts. If a species has one bad year, either because of something you’ve done or for other reasons, it could easily blink out and never return, especially if there is no connection to other good habitat.
Regardless, I think what you’re doing makes sense. You might consider augmenting your fire regime by creating habitat heterogeneity with some small patch mowing. That could be a way to give inverts and other small animals some habitat choices. It could be as simple as mowing a variety of patches at different times of year and at different heights.