Eastern Tent Caterpillars

Note: This show first aired June 11, 2016.

Early summer in Maine is astonishing. One moment the forest canopy is open, allowing glimpses of the newly arrived migrating warblers, a day or two later and the broad leaves have exploded from their buds, and any chance you had of seeing a tiny fast moving bird is gone. These rapidly growing fresh green leaves are not just a beautiful annoyance to bird watchers, but a nutritious salad bar for many of the insects that those warblers come here to eat. The young newly emerged leaves have a high water and nitrogen content to support their rapid growth, and generally lack the defensive chemicals that develop age, protecting the mature leaves. The high nutrient content combined with a lack of protection make them easy targets for developing insect larvae.
 

One type of larva that everyone has been freaking out about this year is the Eastern Tent Caterpillar, aka Malacosoma americanum. If you have been around any cherry or apple trees this spring, you have seen these. They are a remarkably social insect, emerging as tiny caterpillars in May, from a mass of eggs that overwintered glued to a branch on the host tree. The larvae congregate together and immediately start spinning a large three dimensional web in the crotch between two branches, that’s the tent. This web serves as home base for this community of caterpillars. As a native of north America, they especially like native cherry trees, but are also commonly seen in apple trees, though will occasionally end up on other species. The caterpillars emerge just as the new leaves are emerging on the host tree in the spring, perfectly timed to hatch just as their food source does. As they grow they continue to spin more and more silk, enlarging their tent as they themselves need more room. The tent provides a space to congregate when they are not feeding, and a temperature regulation mechanism. The tent is usually oriented to have maximum surface area towards the sun in the morning, so the caterpillars can warm up, become active and digest their food more efficiently. They go through a series of six instars, or caterpillar size classes, shedding their soft exoskeleton and expanding their body into a new larger skin. Generally they spend all of their time on their host tree, but at the end of their larval period, the group disbands, and individuals wander off, in search of good places to pupate on their own. When you see individuals wandering about over the landscape, this is what they are doing—looking for a good spot like a tree trunk or fence post on which to spin their cocoon and undergo the three week transformation into an adult.

One would think that these caterpillars would be sitting ducks for any and all predators—after all they live in big juicy groups in large easy to spot nests. As a result they have evolved several defensive mechanisms that keep them off the menu for most birds and parasitic insects, the organisms that would be their most likely predators. First they have a behavioral tic that causes them to thrash about wildly if threatened. This makes it especially hard for a parasitic wasp to successfully inject an egg into the caterpillar. Secondly, as is noted, they feed on cherry trees, and cherry tree tissue contains cyanide, so these caterpillars can sometimes release cyanide containing juices if threatened. Thirdly, and probably most obviously, the backs of these caterpillar are lined with hundreds of irritating hairs. Any animal that eats the caterpillar has to contend with the build up of these hairs in their throats and stomachs, the effects of which can range from uncomfortable to debilitating. The Black billed cuckoo is one local bird that has evolved the ability to eat these irritating caterpillars hair and all, the birds eat their fill and then cough up the lining of their stomach, growing a new one ready for the next meal of hairy tent caterpillars.

Seeing a tree, like the black cherry tree in my yard, totally stripped of leaves by hungry caterpillars is alarming, when all around lush green leaves are spilling out of twigs. The cherry tree looks dead, but trees are not that easy to kill, and it takes more than a denuding by tent caterpillars to do in a cherry tree. They, and many other broad leaf trees, have the ability to grow another set of leaves later in the summer, from buds that form in the spring (as opposed to the ones that over winter). Watch a tree attacked by tent caterpillars this summer, by August it should have sprouted new leaves, and not a tent caterpillar in sight. The caterpillars pupate in June and emerge as adults at the end of June or into July depending on location. The adult’s job, as is the job of so many adult lepidopterans, is simply to mate as soon as possible, lay eggs and die. Those eggs, once laid, wait many long months, until next spring, before the next round of tent caterpillars emerges, and we start all over again. 

References:

http://www.maine.gov/dacf/php/gotpests/bugs/factsheets2/tent-cater-wisc.pdf

https://entomology.ca.uky.edu/ef423

http://www.maine.gov/dacf/php/gotpests/bugs/tent-caterpillars.htm

http://web.cortland.edu/fitzgerald/Easterntent.htm

http://northernwoodlands.org/outside_story/article/of-cuckoos-and-caterpillars

http://www.nytimes.com/2016/05/15/nyregion/when-caterpillars-move-in-the-yellow-billed-cuckoo-feasts.html?login=email&_r=0

http://biologylabs.utah.edu/coley/ColeyPubs/2006Coley_cats_Oikos.pdf

https://extension.tennessee.edu/publications/Documents/W227.pdf

Spring Phenology

Note: This program first aired on March 19, 2016.

Years ago when I was in college a friend commented on the phenomenon of March break. He said that you leave for break and it is winter, and when you come back to school, it is spring. That was certainly the case this year, I went away for a week and when I returned I found the seeds germinating in the green house, wood cocks calling in the evening and elderberries in my yard breaking bud. The next day out walking in the spring like sunshine I came upon alders shedding pollen from their male catkins, another of my favorite early signs that spring. It may get cold again, and snow, but the mechanisms of spring are now underway, there is no turning back. I’ve been paying attention for enough time now that I know soon I’ll hear wood frogs on sunny afternoons with no wind, and after that will come the spring peepers. After the alders will come the blooms of beaked hazelnut, and after that red maple. The birds will return, one day I’ll look up to see turkey vultures and broad winged hawks, hear phoebes in the morning, and hermit thrushes in the evening.

Keeping track of the dates of all of these spring occurrences is a hobby I came to on my own. Do it for a few years and you have a living record of the seasons in your home ground, it becomes addictive. I can look back and see that last year I didn’t hear a wood cock at my house until April 1, this year I heard them on March 12, about the same time I heard them in 2013. Last year the alders started shedding pollen on April 14, this year they started a full month earlier, on March 13. I can make notes on the winter weather, but there is nothing as telling as seeing how early or late the first signs of spring arrive. Little did I know when I started that I was participating the time honored naturalist tradition of observing and documenting the timing of seasonal events in personal journals. It even has a name—phenology: the study of the timing of recurring natural events.

The neat thing is, phenology has made the big time. No longer is it just the pet of introverted nature nerds (like myself) everywhere, quietly rejoicing in the first bloom of shadbush, noting the emergence of the coltsfoot or watching for amphibian egg masses in local vernal pools. No, phenology is now big science. Climate scientists have discovered that naturalists’ records are a treasure trove of data related to how ecosystems are adapting and changing as climate warms. Just as arctic researchers are pouring over the journals of arctic explorers and sailors to document the changes in sea ice over the past two centuries, climate focused biologists and ecologists are reading the journals of 18^th and 19^th century naturalists to establish how the timing of natural events has changed in the industrial era. Looking at a large number of records, over a long period of time is best. If we just had to go on my ten years of data, what would we make of hearing the wood cocks in early March this year, not till April last year, and in mid March the year before that? We really couldn’t make much of these data, other than to broadly state that woodcocks call in the early spring, which we already know. Reading and compiling naturalists’ seasonal observations is one way scientists are finding data, another is the growing citizen science movement. Now you don’t have to simply keep a meticulous nature journal that you hope someone will find useful after you die, you can participate in a state or national phenological programs and start sending useful data to scientists immediately. The Signs of the Seasons program is based at the University of Maine and has New England as its focus area. The USA National Phenology Network collects similar information at a national level. Both have well developed online presences and I encourage you to check them out and consider volunteering.

Here in the 21rst century, I’ve inadvertently discovered another way of keeping records of spring phenological events. Looking back through Facebook posts I see that I have excitedly posted every year the day I first heard woodcocks peenting in the marsh beyond my house. Ditto for wood frogs. And nature oriented friends are doing the same. This past winter wasn’t long or particularly hard, but it was weird, and people are ready for the predictability of the signs of spring. It is refreshing that so many people find something as simple as hearing a woodcock so exciting. I think we’ve finally found what Facebook is good for.

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References:

Read all about UMaine’s Signs of the Seasons Citizen Science New England Phenology program, find out about training sessions and sign up to participate! https://umaine.edu/signs-of-the-seasons/

The USA National Phenology Network collects citizen science data at a national level https://www.usanpn.org/

Using Thoreau’s journals to track climate change: http://loe.org/shows/segments.html?programID=12-P13-00018&segmentID=4

A more science oriented article about Thoreau’s journals, including a researcher from Acadia National Park https://www.elsevier.com/connect/tracking-climate-change-with-the-help-of-henry-david-thoreau