Caterpillars are the larval stage of butterflies and moths. Throughout Ontario, multiple species thrive. Some of these are harmless, while others are considered pests. The tableau of species in Ontario is made up of caterpillars that vary widely in coloration, size and other features. These caterpillars are seen throughout the year, though individual species may only make appearances during certain seasons.
The imported cabbage worm is an emerald caterpillar with a slim yellow stripe running down its back. More broken stripes run down the caterpillar’s side. The forest tent caterpillar, in comparison, shows how different caterpillar colorations can be in Ontario. This caterpillar is thin and dark blue or black in color all over with small yellowish spots running along its back. Gypsy moth caterpillars are also black, but are covered in hair-like extensions, unlike the imported cabbage worm and the forest tent caterpillar.
Different species are active at different times of the year. Forest tent caterpillars emerge from eggs at the start of spring and are cocooned in time for adults to emerge in June. Other caterpillars that emerge and feed in spring include the gypsy moth caterpillar, the leafroller and the green fruitworm. The fall webworm, on the other hand, is active from July through September. In addition to being seen at different times of the year, certain caterpillars also experience population booms cyclically. The gypsy moth caterpillar, for instance, is seen en masse about every 10 years.
Caterpillar habits also vary depending on the species. Leafrollers get their name from their habit of rolling larger leaves around them, where they hide when not feeding on younger, budding leaves. Forest tent caterpillars emerge from massive groups of eggs, numbering between 150 and 200. While they eat, they create silken thread mats on trees, where they rest. Some caterpillars, like the imported cabbageworm, feed quickly and then move into the pupa stage. In this case, it takes only a few weeks. Others, like the satin moth caterpillars, feed for a longer period, then overwinter before feeding again.
Many of Ontario’s caterpillar species are considered pests. Imported cabbage worms, for instance, feed on crucifer crops and can be a problem for agricultural workers who grow them. Forest tent caterpillars are responsible for defoliation of sugar maple, birch, aspen and other deciduous trees. This action can kill trees over time or make them more open to other pests. Satin moth caterpillars also cause damage to trees, feeding on native Populus species, back cottonwood, trembling aspen, silver poplar and Lombardy poplar.
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