Some of the Vineyard’s wild species deal with winter by leaving – many of our nesting birds, for example, head south, as far as South America in some cases. Warm-blooded critters that stay here year-round – birds and mammals – all rely on some combination of curtailed activity (to conserve energy), exploitation of food sources, such as seeds and berries, that persist into winter, and shelter (to prevent heat loss) in order to survive the colder months. It’s easy to observe some of these species as they go about their daily business of survival.
But what about cold-blooded animals, such as insects? Interestingly, a few of these migrate, just like birds. The American Lady butterfly, for example, is generally common here in the summer but can’t survive sub-freezing temperatures at any stage in its life cycle. Each spring, our region is recolonized by this species as waves of individuals move north starting around mid-April.
For insects that stay here through the winter, the secret of survival is larger a matter of chemistry. Many species produce chemicals that work, quite literally, like the antifreeze in your car, lowering the freezing point of the fluids in the insect’s body tissues. If these fluids never freeze, the insect faces no risk from damage to its tissues caused by ice crystals.
Another important winter strategy for insects is overwintering in an immature state. Insect eggs and larvae are generally much simpler in structure than adults. And this simplicity makes them much less susceptible to damage due to freezing. Very few of our year-round resident insects overwinter as adults. But a little careful snooping around your yard may turn up examples of insects overwintering in a dormant, immature state. You might find, for example, the papery egg cases of a praying mantis stuck to the stems of plants in your garden. Spider egg cases can often be found stuck to the undersides of the eaves of a house. Or, on a warm, sunny winter day, you might stir up some tiny grasshoppers out of the leaf litter; these are likely to be nymphs of the Northern Green-striped Grasshopper, which differs from most of our other grasshoppers in that it overwinters as an immature nymph rather than as an egg.
Some invertebrates do overwinter here as adults, and again, the alert observer can sometimes spot them. Most prominent may be the Mourning Cloak butterfly, which is about two inches across, mostly black with a yellow border to its wings. Mourning Cloaks spent the winter as adults sheltering under flake of bark, in wood piles, or in similar sheltered locations. They become active in early spring or even in late winter (there is at least one January record for the Vineyard), on days when daytime temperatures warm up to about 50 degrees or higher. You can also probably find adult pill bugs (actually isopods, not insects) concealed under leaf litter in your yard or garden, along with various species of beetles.
Not all of these overwintering invertebrates will live to see the spring. Some simply run out of energy reserves and die. Others may succumb to fungal infection or other diseases. And many will be eaten by birds, many species of which are quite deft at finding and eating eggs, larvae, and pupae. But enough will survive to start the next generation, ensuring the survival of the species as a whole.