Biological control is the use of predators and parasitoids to reduce the population of pests. When pests are threatening a crop, these beneficial insects can be released to eat the pest and prevent further damage. In some cases, insects that eat certain weeds can be released to keep the weed from spreading.
There are some differences between predators and parasites and parasitoids.
Predator: Require more than one host to complete their development and they kills host when they feed on it. Example: Ladybug.
Parasite: Require only one host to complete their development but they rarely kill the host. Example: Lice.
Parasitoid: Require only one host to complete their development but almost always kills the host in the process (which is similar to predators, but predators require many hosts).