Late Spring, Summer
In outbreak years, tent caterpillars usually hatch in May and defoliate huge areas. They then pupate in June or early July. As the caterpillars grow, they increase their fat content, and also develop stiff hairs and calcium oxalate crystals that clog digestive tracks, which makes them unpalatable to most animals and birds. Bears eat the caterpillars at this time. No one knows how bears deal with the hairs and crystals as they make these caterpillars their main meal when they are at their fattest. Interestingly, although tent caterpillars provide food for bears in the spring, they reduce food for bears in the summer due to their defoliating berry bushes in June, which reduces berry production in July/August.