I was back at the Niobrara Valley Preserve last week to help with a little bison work and a board meeting. My wife was able to come with me, and we stayed an extra night so we could do some hiking Saturday morning before heading home.
Kim and I decided to hike up the bluffs north of the river where the 2012 wildfire transformed an overgrown savanna of pines and cedars into a burgeoning grassland/shrubland dotted with burned tree skeletons. Autumn is well established along the Niobrara River, and there have already been several hard freezes and some light snows. Despite that, we found plenty of color and texture to enjoy while we wandered, as well as a couple very pleasant surprises.
In last week’s Photo of the Week post, I mentioned that I’d spent part of a morning photographing white fluffy seeds in autumn prairie. (It’s not a bad life, all things considered.) As I walked that morning, I noticed how quiet it was. In fact, the only sounds I heard were those of my feet crunching through the dried grass. We’ve had three below-freezing nights in the last week or so (25, 27, and 28 degrees F), and those cold temperatures have eliminated most insects – and their sounds – from the prairie.
However, the prairie was not completely devoid of insects. As I was photographing seed heads of false boneset (Brickellia eupatorioides), I noticed that one of the flower stems seemed much thicker than it should have been. Upon closer inspection, it turned out to be a caterpillar. Anyone know what kind it is? I certainly don’t.
In fact, there were two caterpillars on the same plant. Did they survive the cold temperatures right there on the plant? Or did they drop down into the thick leaf litter below during those frigid nights and find enough warmth to avoid freezing? Are they larvae of a moth that overwinters as a caterpillar? Or are they the last reproductive gasp of a moth species that migrates north each year in the spring, produces multiple generations, and then finally succumbs to the winter cold?
The coloration of the caterpillars matches fall prairie foliage very well, which makes me wonder whether they are of a species that overwinters as a caterpillar – and has protective coloration to match late fall dormant stems. Maybe one of you will know some of the answers to my questions and can solve the mystery. That’d be great, but in the meantime, it’s just as much fun to speculate as to know!
The caterpillars weren’t the only insects in the prairie. Just a few steps away, I found the insect pictured below. Can you identify it from the front?
The photo below gives you a better look.
The assassin bug appeared to be waiting quietly for another insect to venture near enough to become a meal. Based on the scarcity of insects in the post-hard-freeze-prairie, that could be a long wait. Besides the assassin bug and the two caterpillars, the only other insect I saw was a ladybug, though I’m sure there were others that survived the first cold snaps – including wolf spiders. Long-time blog readers might remember a post from a couple years ago about a wolf spider I found running around on the ice on an 18 degree F day.
So how do insects do it? They’re cold-blooded, right? They should be particularly vulnerable to really cold weather…
Well, a good hard freeze does bring death to lots of insects. However, their species show up again the next year, so they clearly have strategies for getting through the winter. Many species overwinter as eggs, but others survive as larvae or even adults. Most of those that overwinter as larvae or adults seek shelter from the worst of the cold by burrowing underground or beneath deep leaf litter. Even so, they may have to withstand temperatures well below freezing. Some insects produce a kind of anti-freeze solution to protect themselves from freezing, while others change the way they store liquids within and between their cells so their cells don’t rupture when those liquids freeze and expand.
…and of course there are lots of other strategies. You can read a technical description of some of those in this research article, if you’re interested. Or you could read a more general post I wrote last year about winter survival strategies of a number of animals.
What’s happening to the insects in prairies near you? Have you already seen the big first freeze die off? Maybe you live in a climate where that never happens? I’d love to hear your stories.
During the winter, prairie becomes nearly monochromatic. The scarcity of color exposes the architecture of the plants. It’s as if the prairie has been deconstructed before our eyes, stripped down to its framework before being rebuilt for the next season.