Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Leaf Rollers

Thursday, October 11, 2012

I am on a mission.  My mission is to identify all of the butterflies that visit my backyard.  So far, I have identified 22 different species.  I am sure that I have missed some, because they are in the back yard much more frequently than I am.  Still, as time goes on, I expect to increase my sightings.
Of course, identifying the butterflies is just the beginning.  I also need to learn something about them: when they are active, their flight habits, how many generations per year, what the caterpillar looks like, the caterpillars’ food, which stage overwinters, etc. .  
Clearly there is so much more to nature study than identification.  So as I identify each new butterfly, I do a little research and record my findings on the computer.  I am building up a set of electronic butterfly flash cards.  Using these flash cards as the desktop of my computer, I am slowly but surely learning my butterflies.

My latest find was quite a surprise.  According to my butterfly book - Butterflies through Binoculars by Jeffry Glassberg - the long-tailed skipper is not supposed to be found in Massachusetts.  But there it was, flower-hopping in my back yard.  Take a picture.  Look it up in the book for positive id and do some research.  Sometimes, a little research uncovers some unusual behavior.  And so it happened that I discovered some unusual behavior for the long-tailed skipper as well as many other skippers.

Long-tailed skippers are called bean leaf rollers. The caterpillars feed on plants in the bean family and, like many skippers, they produce a shelter by making a couple of cuts in the leaf edge, rolling over the cut section and lining the inside with silk. From this shelter, the caterpillar hides during the day and ventures out at night to feed.

This shelter hides the caterpillar from prying eyes. If a predator can’t see it, it can’t find it. At least that is true for predators that find their prey by sight. However, a predator doesn’t have to see it’s prey to know of it’s existence. There are other clues that can give it away. One such clue to an animal’s presence is it’s poop, or frass as a caterpillar’s poop is called. One way to find a caterpillar is to look for the frass. Since caterpillars are eating machines - they eat constantly - they produce a lot of frass. The bigger the frass, the larger the caterpillar.

Studies have shown that ants are more likely to find caterpillars on plants if there is frass at the base of the plant. If they find frass at the base of the plant, they climb the plant in search of the caterpillar. If there is no frass, they look elsewhere.
Other predators can also cue in on the frass to locate the caterpillar. Paper wasps, a major predator of many caterpillars, can use the odor of the frass to locate their prey. So will parasitic wasps. Rather than eating the caterpillar, they sting it, laying their eggs inside the body of the caterpillar. There, the eggs hatch and feed on the caterpillars body fluids.

Somehow this just doesn’t seem fair. A caterpillar must eat to survive. However, if it eats, it must poop. If it poops, it might just be leading its predators right to it. There must be a way to get around this problem. And of course, there is.

The remedy is to fling the poop. Rather than just let its frass fall where it may, the long-tailed skipper, and many other skippers, fling their frass away from their shelter. They build up the force for ejection by raising their blood pressure. The frass is held in place by a comb until the pressure is enough to force open the comb and fling the frass from the caterpillar. Depending on the size of the caterpillar, the frass may be ejected up to 3 feet. Quite a feat for a caterpillar just over 1 inch long!

One question comes immediately to mind. How did this apparatus evolve? There has to be a series of steps leading up to the finished product. It doesn’t happen overnight. A caterpillar doesn’t just realize the need for an ejection apparatus and develop one. Instead, a butterfly lays a clutch of eggs. These eggs hatch into caterpillars that are ever so slightly different from the last generation. If these differences are to the caterpillar’s advantage, they grow to butterflies that lays the next clutch of eggs. These hatch into caterpillars that are ever so slightly different from the last generation. It will take many generations of caterpillars with ever so slight differences for an apparatus like this to be developed.
Truly, the most amazing thing about this behavior is that it has developed in the first place.