The term “occasional invaders” includes those pests that may occur in buildings at some stage in their life cycle but which do not usually complete the entire life cycle within the structure. Most of these pests live outside of buildings but may come inside, usually after an increase to epidemic proportions. Buildings are generally entered by chance while the pests are looking for harborage, water, or food. Although they may enter in large numbers, they usually do little damage and are a nuisance because of their presence. Millipedes, centipedes, sowbugs and pillbugs are all occasional invaders.

Centipedes have multiple body segments with 1 pair of legs per segment, and may reach 3 inches long. They are grayish-tan with long antennae and extremely long legs. When disturbed by light, they move very quickly. They prefer moist environments and feed on other small insects such as roaches and houseflies. They are really beneficial insects, and unless they become extremely abundant they should not be killed.

Millipedes are somewhat similar to centipedes except that they have 2 pairs of legs per body segment and their legs are very short. Their bodies are typically round and their antennae are short. They generally feed on decaying vegetable matter and tend to be found in mulch beds. They tend to move into homes through doorways and in house plants that are brought inside.

Sowbugs and pillbugs are not insects but are more closely related to crayfish. They have round, slate-gray colored bodies with distinct segmentation and seven pairs of legs. Both can roll up into a ball shape. They tend to feed on decaying plant material and plant roots and are often found in mulch beds or compost heaps. They may move into homes via house plants that are brought inside.