If you notice small insects that look similar to caterpillars near a garden pond or other water source, they may actually be periwinkle bugs, also known as caddisfly larvae. Eventually these insects will grow up to become caddisflies, which are relatives of butterflies and moths. As larvae, periwinkle bugs are harmless garden creatures whose intriguing behaviors can be interesting to observe.
Periwinkle bugs, or caddisfly larvae, are long and thin, and can grow up to 1 1/2 inches long. They have six legs on the middle of their bodies and one set of prolegs with hooks on their abdomens. Periwinkle bugs also have hard heads, midsections and legs while the rest of their bodies are soft. Periwinkle bugs do not move to land until they are adults, so you will only see them near small garden ponds, often under rocks, until they reach adulthood.
Periwinkle bugs are water-dwellers, and can be found at the bottom of lakes, ponds, streams and other freshwater sources. They need little oxygen for survival. Different species of periwinkle bugs have different preferences for particular water temperatures, amounts of sunlight and levels of minerals.
An intriguing behavior of periwinkle bugs is their ability to create movable cases or homes that they use for protection and camouflage. The larvae gather pebbles, stones, leaves, sand and twigs, and use silk to weave them into elaborate shelters. The periwinkles then use their abdominal hooks to hang onto the cases and transport them. Each family of periwinkle bugs creates cases with designs that are different from those of all other families. The larvae live and grow in the cases until they reach adulthood.
Most periwinkle bugs eat aquatic vegetation including algae and leaves. Despite their small size, groups of them can be helpful in limiting algae and debris in small ponds. A limited number of larvae are carnivores. They create silk nets that they use as a filter to entrap insect prey. Still other periwinkle bugs are scavengers and will eat dead organisms.
When periwinkle bugs hatch, they are encased in a thick, slimy mass but quickly leave it to begin constructing their cases. Winter usually arrives during this stage, and periwinkle bugs become trapped in ice for up to six months. Although they are equipped to survive winter, minimal growth occurs during this hibernation period. In the later stages of development, which are longer, larvae feed heavily as they prepare to become pupa. During this final stage, they seal off their case or create a new one, called a pupal case. Here, they wait to transform into adult caddisflies. In two to three weeks, pupa chew their ways out of the pupal case. Adults, which look similar to butterflies or moths, have a lifespan of one year.