Small Insect That Can Be Beneficial As Biological Controls of Pests

By Renn Tumlison,

 Internal parasites are organisms that get their nutrients by feeding inside of a ‘host’ organism, usually without killing – or even significantly harming – that host. If a parasite killed its host, the parasite would also lose its home, so it also would die. An organism called a parasitoid represents another level of this type of relationship that is less well known to most people. In some parasitoid life cycles, an immature life history stage of one species of insect develops within the body of another species of insect. To live inside of a host, the parasitoid obviously must be smaller than the host. It acts as a parasite by feeding on body fluids and organs but does not kill the host – yet.

Buck Moth Before and After

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.

The adult is a free-living form that may take on the role of a predator, which kills food before eating it (whereas the parasitoid stage ate food before killing it). In many species of parasitoids, the adult female searches for a host in which to lay her eggs. The females can be very good searchers and can locate the target species of host insect even when the host species is not abundant. Finding an appropriate host, she deposits her eggs into the host with a needle-like structure called an ovipositor. Within six days of forming the cocoons on the buck moth caterpillar, adult parasitoid wasps cut their way out of the cocoons and emerged (right photo). Because the parasitoid usually kills its host before the host species can reproduce, some insect parasitoids are used as biological controls. Those species of parasitoids are considered to be ‘natural enemies’ of the species they use as hosts.

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