In late may, you take a stroll through an aspen woodlot and think: Uh-oh, those leaves is looking kind of skeletal; what’s up? An outbreak of forest tent caterpillars has started here, perhaps? Not necessarily. Before spraying, have another look at the larvae, misidentifying them could do more than waste your time; it could hurt the local ecosystem.
The graphic above explain which species to expect at different times of the year. It also shows their host plants and how to tell each one apart by its markings. It transforms a confusing problem into an exercise in ID management.
How to Identify Caterpillars Correctly
There’s a lot of diversity in the larval life stage (eight hundred-plus species of moths and butterflies in Alberta!). Because it coincides with the post-snow-melt phase of new plant growth (late May to August), that’s peak time of year for activity. Many caterpillars has short periods of feasting, lasting just a couple of weeks or so, then they enter their pupae stage. This means their active but less harmful times are good times to see them around.
The visual guide reveals how physical features aren’t the sole determining factor; habitat has an effect, too. Even if you see two caterpillars with same shade of color, say, they may be different creatures depending on where they grow: up north in spruce forests, versus on some prairie clover down south.
Caterpillars of the sphinx moths are another good illustration of what we commonly get wrong: They’re big and smooth skinned, with a horn-like projection on the end of their body that makes many mistakenly think they’re harmful invaders. The one with purple edged stripes is the wild cherry sphinx; the one with white diagonal stripes is the poplar sphinx (both feed on cherries and willows. Not vegetable-garden fare). Knowing their IDs mean no need to intervene.
Unlike these guys, for instance, the tent caterpillars spin a communal nest in a tree fork. That’s the hairy-sided, blue-spotted forest tent caterpillar. Its populations causes massive outbreaks every ten or so years in the more northern boreal parts of its range. If you cut down the tents early, you’ll prevent further infestation from reaching other nearby trees.
Be careful with certain hairy ones. Some of the hairy caterpillars will make your skin itch if handled roughly, such as those in the family Arctiidae (tiger moths and woolly bears). These include larvae of species such as the banded woolly bear, which has a reddish brown center band and is black at both ends. These larvae feed on grasses and plantain. Those foodplants mean they won’t damage any garden structures.
The white-marked tussock moth is not so safe because its yellow tufted hairs and bright red head are irritating to touch. Gloves are best for handling any unfamiliar caterpillar; don’t rub your eyes or face after handling either. Some have stinging hairs that can absorbs an allergic reaction.
The reference list is good because it saves time and risk. Caterpillar ID is not always simple: Butterfly larvae has their tricks, too. For instance, the Canadian tiger swallowtail begins life looking like a bird dropping to deter predators. Later, it turns bright green with fake eyespots. These spots confuse people who think it should of stayed one color all along. Milkweed-feeding monarch caterpillars only occur in late summer here, passing through on their way north; this is why you never see more than a moment of them per year. They migrate.
That explains the chart. It shows all those other shifts from season to season and what to look for. The host plant adds more clues. Knowing where something lives narrows the list; most (though not all) caterpillars is specialists on particular plants. A looper on an apple tree is likely a cankerworm, while one on milkweed is probably a monarch. That kind of background makes that unclear sighting suddenley recognizable.
It doesn’t require expertise to make your way through, just attention and using what you’ve got at hand. And the infographic provides some rapid references to help you know whether it was red or green or blue with dots or stripes or lines, and whether it does what it does, too. Keep it handy next time you spot something crawling in your garden, and let the details guide your response different than guesswork.
So knowing how to spot the larvae means turning a possible annoyance into an exercise in education. It’s a way of seeing that the forest is a living ecosystem where life tries to survive, and grow. Next time you’re walking among the aspens, pay attention to the leaves; maybe you’ll spot something hanging from a web or resting on the bark.