Long before humans roamed the planet earth, insects were engaged in biological warfare. Parasitoid wasps feed on other insects by laying their eggs inside the host. Insects are not defenseless and may be able to fight off parasitoids with their immune system. Insects can avoid becoming a host to a parasitoid if they have immune cells capable of attacking and destroying the parasitoid eggs or developing larvae.
However, an immune system that can destroy the attacking parasitoids may not be enough. Some parasitoids such as Cotesia will inject polydnaviruses into their host along with eggs. The polydnavirus suppresses the immune system of the host. This leaves the eggs and developing wasp embryos free from attack by host immune cells. Polydnavirus can turn a non-host into a host. Interestingly, the polydnaviruses are of viral origin but were fully incorporated into the DNA of the wasp some 100 million years ago.
Some insects fight back against parasitoids by forming “mutual defense pacts” with microorganisms. The pea aphid contains bacteria that can host viruses and produce viral toxins. The viral toxins defend the aphid by killing the parasitoids. The bacteria and virus have complex interactions. The virus is contained on a bacterial plasmid and easily lost.
Insects have developed complex relationships with microorganisms that include their use as biological warfare agents in interactions among insects. Studies of these microbial agents and their complex interactions may lead to new methods of insect control.