Spring brings a flush of new green leaves to our landscape, in every shade and hue. But these young leaves do not keep their pristine shapes for long, because an army of hungry caterpillars race to eat them as fast as they can. The timing is no accident. It is an ongoing tit for tat between plants and insects that has driven evolution of both groups for millennia.
Watching young leaves unfold from their buds and expand into their full size has always amazed me. It allows me and others to see many of the small details of leaves before they become worn from attacks from insects and other herbivores, and wear and tear from winds and summer weather. Newly emerged leaves can highlight the pattern of veins, pubescence (small hairs) and spines that often fade quickly.
But it doesn’t take long to spot the work of caterpillars, either by seeing them directly, spotting the frass (droppings) they leave behind, but mostly by the holes chewed into the new leaves. For insects, it is a race to consume the rich nutrients in new leaves before other less palatable compounds develop. For plants, chemicals that help protect them from grazers like caterpillars, develop late in the process, when leaves are fully developed.
It is for this reason that eggs laid by moths and butterflies hatch as early as possible in spring so that the young caterpillars can get the jump on eating. Late hatching caterpillars grow more slowly because of the less nutritious older leaves that are also harder to chew and digest. The more of a delay in growth, the more likely you will become a meal for hungry predators, like birds.
In some species, like Eastern Tent Caterpillars, some eggs hatch earlier than others. If the weather holds, they beat the odds and grow quickly. If a late cold front kills these early hatching caterpillars, later hatching eggs will take their shot, ensuring that some survive.
But this explosion of growth of plants and caterpillars is also timed to nesting of most of our woodland birds. As Doug Tallamy documented in his book Bringing Nature Home, and subsequent related books, most of our common backyard birds feed their young a diet of soft-bodied caterpillars and spiders.
Based on his research, a pair of Carolina Chickadees work hard to deliver 350 to 570 caterpillars to their nestlings every day, for the 16-18 days from hatching to fledging. For a typical clutch of Chickadees (ranging from 3-10 chicks), that can total anywhere from 6,000 to 9,000 caterpillars.
The premise of Tallamy’s research was that native plants support greater numbers and diversity of caterpillars compared to non-native trees and shrubs. Oaks, for example can support hundreds of species of caterpillars. Common non-native species, like Chinese Tallow, support no more than two the three types of caterpillars. For a hard-working adult Chickadee, an urban area full of non-native species will not support its young.
So, the battle between plants and the insects that consume them can have winners and losers. In some years, most new leaves on a plant can be consumed by hordes of hungry caterpillars. And yet, even then, a healthy plant can recover. Early developing insects, like caterpillars, can produce large numbers of adults. In some years, however, poor weather can reduce their numbers. And yet, there is usually so many caterpillars around that birds can feed their young, especially in habitats dominated by native species.
Putting aside the war happening in your yard, looking for and watching the many types of caterpillars on your plants can be fun. Best to look, rather than touch, as many of these insects have urticating hairs that hurt. Their beauty is a sign to leave them alone. Have fun!
Hope to see you in our great outdoors!