Summer World_ A Season of Bounty - Bernd Heinrich [42]
I searched under the same tree and other poplar trees and within an hour or two had picked up 246 identically rolled-up leaves. Most of them had a similar caterpillar inside, about 0.3 to 0.4 inch long. Leaf-rolling by caterpillars is common, but finding rolled-up leaves on the ground under the tree is not. Every one of the rolled leaves that contained a caterpillar was missing most (though not all) of its stem—proof that the tree had not spontaneously aborted the leaves. Petioles are tough; they do not tear or break in a storm. The connection between the petiole and the twig would break first, and therefore it was not the tree that was getting rid of its caterpillars but vice versa.
The caterpillars had rolled themselves up by grabbing an edge of the leaf, attaching silk that stuck to the outer leaf edge, pulling in and attaching the other end of the (non-stretching) silk to the inner leaf surface, and repeating the process until they gradually rolled the leaf into a tube. After finishing that, they had reached through the top of the tube and gone to the considerable effort, and maybe risk, of sticking their neck out to chew through the petioles so that they and their leaf tube, or roll, would fall to the ground. They then stayed in their leaf rolls, and eventually pupated inside them. I kept each of the clipped-off leaf rolls with its enclosed caterpillar, which molted into a pupa, and in the first week of July little gray moths emerged.
Why did the leaf-rolling caterpillars clip off the leaves they were in? This leaf clipping was much different from what I had observed before. The other caterpillars stayed on the tree, where there were always other leaves to feed from. These, in contrast, were isolating themselves from the tree, and as a consequence seemed to be restricting their food supply. Why would that be advantageous? Are they safer on the ground than on the tree? To find out, I took 200 of the freshly dropped leaf rolls containing caterpillars, divided them into five groups, and distributed them to five different locations on the ground. One week later all the leaf rolls were still in place. So far, so good—the ground seemed safe. But maybe the caterpillars are equally safe on the tree. That would be difficult to know, though, if they normally don’t stay there. I wondered, however, what would happen if I unrolled them from their apparently safe little houses after they had become grounded and then put them back up into the tree.
There were two young poplar trees in the clearing by my cabin, and I released numerous of the unrolled caterpillars onto their branches. They seemed not well suited to hanging on, especially to poplar leaves, which vibrate wildly in even the slightest breeze as though designed to shake off caterpillars, and many did immediately fall off. However, others hung on, and within a day I found thirty newly made leaf rolls. That is, my caterpillars almost immediately made themselves new homes. Two days later, however, seven of the thirty rolls had been clipped off. I saw no sign of predation, but it seemed that using tree saplings in a field was not a fair test to examine predation when their natural habitat is the crowns of large trees in the forest.
Making the observations was fun, especially if I could continue them in the top of a tree. So I carried a supply of leaf rolls that I had gathered on the ground up into the crown of an old aspen tree, perched comfortably on the branches, made sure that they had no existing leaf rolls or petioles from previously clipped-off leaves, and then unrolled one leaf at a time and released caterpillars onto these cleared, marked branches.