The parasitoid may alter the normal life of the host until its needs are met, but it only kills the host when the larval life history stage is complete, and the parasite is ready to move to the next stage in its life cycle. This partly is how a parasitoid differs from a parasite. Once the larvae of the parasitoid have developed to a point at which they no longer need the host, they burrow to the outside of the body and begin to spin a cocoon in which to pupate. Then the weakened host dies. This caterpillar of a Buck Moth (top) was a host to several small wasp parasitoids. Within three hours of the caterpillar being caught, larvae of the wasp began to burrow out of its body and spin cocoons (bottom). The most common parasitoids are tiny wasps, many of them in the family Brachonidae, and adults are only a few millimeters long. The parasitoids often are very specific about what organisms they can use for hosts, so they target particular species. For example, certain species of parasitoids specifically search for appropriate life history stages of gypsy moths (a non-native pest species), potato beetles, corn borers, cabbage worms, tomato hornworms, or aphids. These are species considered by humans to be pests due to their damage to agricultural crops. As a result, some of the parasitoid species are commercially available as biological control agents against specific crop pests.