Cocoons Found in Soil
Insects have several stages in their life cycle. Most garden insects emerge from an egg early in the spring and spend the entire summer feeding in their larval form -- often, it is the feeding larvae that are frequently garden pests. At the end of the summer, if larvae have stored up enough energy, they make their way to the soil where the spin a cocoon underground for the winter. In spring, they emerge, mate and start a new generation of insects. Finding a cocoon in the soil is a natural process and does not indicate a pest problem; there are many insects that develop in cocoons underground. However, you should diligently monitor you garden plants for pests in spring.
Red Humped Caterpillar
The red humped caterpillar (Schizura concinna) is a large, common caterpillar found throughout North America. It feeds on a wide range of deciduous trees, but is considered a pest due to its preference for fruit trees such as apple, pear, cherry, plum and apricot as well as blackberry bushes. Fully grown larvae are about 2 inches long and they have, as their name suggests, a red head with an enlarged segment just behind. In large numbers, red humped caterpillars can quickly defoliate trees and cause significant problems in orchards. The larvae spin light brown cocoons under the leaf litter and topsoil beneath trees. Red humped caterpillars develop into common gray moths.
The fall cankerworm (Alsophila pometaria) is a pest of hardwood trees across North America. Cankerworms, more commonly referred to as inchworms, have one generation per year. Larvae are light green to dark brown and usually have two stripes running the length of their back. A large number of dark caterpillars may be indicative of an infestation, according to Penn State University. Infestations of cankerworm larvae can consume leaves as fast as trees develop them in the spring, causing stress and increasing the vulnerability of affected trees to secondary pests and diseases. Larvae lower themselves to the ground from the canopy by a silken thread and then spin a cocoon made of soil and silk. Adults emerge late in the fall, mate high in the tree and lay eggs that will hatch into the following year's population. Two seasons of intense cankerworm infestations can kill a mature hardwood.
Squash Vine Borer
Squash vine borers (Melittia cucurbitae) can be a serious pest in home gardens with cucurbits -- melons and gourds. Larvae have strong jaws that they use to chew through stems close to the ground and then feed on the nutrients inside the plant. In large numbers, borers can decimate cucurbit crops. The larvae develop through late summer then dig underground and spin a cocoon where they overwinter until May. In May, borers emerge as diurnal moths that resemble wasps. Adults are black with flashy red coloration on their abdomen. The adults mate, females lay eggs on the leaves or stems of cucurbits and the larvae bore into the plants and develop the same as their parents. However, larvae that develop early in the summer will spin a cocoon underground in the middle of the summer, emerge and lay another set of eggs. The final eggs will overwinter underground and start the life cycle all over.