Parasitism is the most common lifestyle on Earth. Parasites target hosts across the spectrum of life, from single cells to individuals to societies. Hosts fend off assaults via constitutional defenses, physiological responses (immune systems), and behaviors.
Colonies are especially attractive, as they contain rich resources by their high densities of hosts. Some social insects evolved to parasitize other social insects.
Social parasites are especially common among ants, where they appear in 3 categories: slavers, resident inquilines, and provisional inquilines. Permanent inquilines coexist with their host colony. Slave-making ants raid other colonies to capture and enslave their brood.
Temporary inquiline queens invade a host colony, kill its queen, initiate egg-laying, and rely upon the brood care behaviors of host workers to succeed. Parasitic success spells the loss of the host colony’s reproductive output. Social parasite queens exhaust a colony after a generation – enough time to produce parasitic prodigy which proliferate to other nests.
A provisional parasite queen needs to be able to enter the host colony, be accepted by host workers, and ensure that the host workers rear her offspring. The first line of defense is detection of an intruding queen, either by looks, behaviors, or chemical cues. Evolved camouflage and mimicry can defeat the initial opportunity by hosts to avert a dire fate.
Once a parasite queen is established, targeting the alien offspring is the only way a host colony may recover. The onus falls upon host larvae, who can save the colony by eating the eggs of the parasite queen.
Larvae participate in post-infection defence by selectively removing parasite eggs, thus acting as a 2nd line of defence. ~ Finnish evolutionary zoologist Liselotte Sundström et al
Adult ants smell with their antennae, which larvae lack. How larvae discriminate between the eggs of sisters and those that pose to a threat to colony survival is not known, but a critical element is precocious knowledge: innately understanding the situation and knowing what to do.