There are many little tricks to finding caterpillars and some of those tricks are specific to just a single species or a small group of species. For the Viceroy butterfly, Limenitis archippus, the method for searching for them also changes with the season.
The first generation of Viceroy caterpillars that we see each spring (March to early May here in New England) emerge from their overwintering shelters, called hibernacula, as third instars and shed their skins within a day or two. The caterpillars grow remarkably quickly on their host of willow or poplar catkins and young leaves and I often find large final instar caterpillars before the chill of April in NH has passed.
To begin your hunt, position yourself in ideal habitat. Viceroy enjoy open fields and wet meadows. Power line cuts are excellent, as are the edges of ski hills, untamed farm fields, and especially, wet sedgy floodplains. Vacant city lots with host growing between pavement cracks can also be very productive!
Next, target your host plants. Viceroy specialize on willows and poplars (family Salicaceae) but they don't just randomly select any old plant. They seem to vastly prefer small saplings, repeatedly mowed over plants, and low trunk and root suckers. They also seem to select plants somewhat removed from edge, often isolated in fields or in overgrown but still overall grassy and sunny spots. Take a look at some of the habitat and host plant pictures in this post - caterpillars were found on each of the pictured willows, within two feet of the ground.
Search for caterpillars. My eyes don't pick up on Viceroy caterpillars very easily. They are earthy-toned, bird-dropping mimics and are easy to overlook. But I can spot used overwintered hibernacula much more easily! Scanning small, leafing out, willows and poplars for the characteristic dead, curled, and silked up leaf hibernacula can quickly help you narrow down a search. Most often if a Viceroy caterpillar is present on a plant it will be within inches or feet from its used and abandoned winter home.
Part of being effective at finding caterpillars is finding ways to focus your search down from endless habitat to specific host plants to particular parts of a host. Beyond that keep in mind that the caterpillars you are targeting may well be more easily tracked by the signs they leave behind over just searching for the caterpillar itself.
In their subsequent spring and summer generations Viceroy create no hibernacula by which we can spot them. So our strategies must adapt and change with the season. We will highlight hunting summer time Viceroy in another post later in the year, but rest assured there are tricks for finding them as well.
Go find some caterpillars and tell us what you see!