IT’S A HAIRY, many-legged wriggling thing crawling across moist soil of your garden, where you’re down on your knees wielding a trowel. Panic may set in, because who knows whether it’s safe. Or whether it intends to munch its way through your veggies? You won’t know until you identify the beast. That moment of recognition can turn a possible pest into something interesting… Or it can tell you that what you’ve encountered is an enemy.
That is why it is so important to carry a good reference with you. The chart above breaks this down by grouping species into families, which saves you from wild guesswork. Learn the pattern; don’t try to memorize every name.
How to Identify Garden Caterpillars
That diagonal stripe tells you something: strength and speed. That tail horn? It’s pointing you toward a hawk-moth just about every time. Those are branched spines. That’s typically a butterfly larva telling you to leave him be. They’re visual clues. In other words, they have evolved over millions of years.
Once you begin to know where to look, it becomes more or less a trick of psychology. And then look at the size of those larvae. This one’s nearly as big as my thumb, but this other one is slim and harmless. Some of these larvae grows larger than your thumb. It is ninety millimeters long and is probably a member of the hawk-moth family (and probably eating privet or ash). And these are Britain’s giants, built to devour fast then change form. The other extreme is that tiny slug-like ones may be munching away on nettles or brassicas. Usually length isn’t the only clue to their identity, but it rules out many possibilities right away.
The thing that everyone gets wrong when they begin to notice nature is that we focus too much on color, not noticing structure that signals the species itself. Perhaps the best evidence of all is provided by host plants. When I see someone on stinging nettles, for instance, I know it’s probably either a tiger moth larva or a peacock butterfly (two very different-looking creatures), though they share the same menu. One is jet black, covered in white spots, a miniature spiked sea urchin of a caterpillar. The other is more like a fuzzy sausage along the pathway, its body covered in thick hair: the tiger moth larva. Knowing who eats whom allows me to anticipate their behavior… And the kind of damage they might do to my crop plants.
But timing is an extremely important factor in determining what you’re seeing, and identifying. Most caterpillars is around from May through September. The warmest months are the best time to see them because plants are growing at their fastest pace. That is generally when they will be out and about. And within those dates, the biggest, hungriest larvae of all, meaning most active… Will be in July and August. There are exceptions, of course. Some species completely defy the rules, like the winter moth, which gets around by emerging early (when the oak buds first open) and catching the gardener unawares. Knowing the seasons means knowing why certain pests appear suddenly and then vanish without a trace.
Practical matters include safety: A few hairy caterpillars have tiny bristles all over them that can make your skin itch (or even affect breathing) when touched. Oak processionary moths are one such creature; they crawl in a long line up tree trunks, dropping irritating dust into the air as they go. Don’t touch these kinds of larvae unless you know what they are. Better yet, keep your distance and learn more by reading a field guide, rather than ending up lumpy for days from an itchy rash. Be particularly careful with those that appear to be fluffy or otherwise seemingly harmless.
With time, and some patience, you will see things differently. Start spotting the body shapes (loopers really do arch their way through; there’s nothing subtle about it). Observe that those larvae with no bumps and ridges may be the larvae of a camouflaged moth called the geometrids. Their job is to look like leaves or twigs, not run from them, so they are more successful as still life than fast movers, and therefore use camouflage instead of speed to evade enemies.
The more you know about their defenses, the more you’ll understand larger ecosystem dynamics unfolding in your back yard. Knowing what’s underfoot makes all the difference in how you react to nature in general. Fear becomes respect; confusion, curiosity. And suddenly it’s not just a garden full of plants. It is a living thing, like a theater where each creature has a part to play.
Next time a wriggler pops up in your garden or among your foliage, take the time to get a good look at it before reacting. What does it have? Horns? Check. Host plant? Check. Season? Check. Armed with those bits of information, maybe the mysterious visitor will be far more interesting than originaly thought.