Of all the jobs that busy bees and other social insects — ants, wasps and termites — have, none is more demanding than undertaker. Disposal of insect corpses is pretty much a 24/7 task for workers charged with doing it. A colony of 5 million eastern subterranean termites (Reticulitermes flavipes), for instance, can lose 70,000 members a day. A leafcutter ant colony can turn over a couple of million workers every four months. Keeping all those bodies from piling up and spawning disease that could destroy their colony is the task of workers with specialized behavior that reacts to olfactory cues from the corpse. These chemical signals enable insect undertakers to recognize dead colony mates and even tell them how to dispose of the remains, depending on the circumstances of death.
Despite a half century of studies, insect undertaking behavior still is mysterious to humans. Research into such behavior has increased recently, coincidently with the COVID-19 pandemic, due to the concept advanced by many scientists of the colony as a superorganism. This superorganism concept has served as a model useful in human epidemiology and disease control.
(A word about terminology: scientists now prefer the term “eusocial” to describe insects that, collectively, are working parts of a community essential to their survival.)
INDUSTRY IMPLICATIONS. Insect undertaking behavior — also called corpse management — varies among honey bees, ants and especially termites, according to the circumstances of death, such as how and when it occurred. Insect undertakers use a variety of methods to dispose of the deceased, including removal from the nest, burial on the spot or elsewhere, walling off in a tunnel and, quite often, eating them.
The more scientists learn about undertaking behavior, ramifications for the pest management industry become more apparent. “Undertaking behavior directly interferes with our current pest management practices,” says Dr. Xuguo “Joe” Zhou of the University of Kentucky’s entomology department. He’s an expert on how behavior governs the social homeostasis — or equilibrium — of termite colonies.
Efforts to control termites biologically using the Metarhizium entomopathogenic fungus work because of undertaking behavior, says Zhou. Past research shows, for example, that Formosan subterranean termites (Coptotermes formosanus) avoid contamination with nest-mates killed by some termiticides.
Zhou is lead author of a paper on termite corpse management published in January 2021 in the Annals of the Entomological Society America. Although behavioral research is basic, he says, it does have practical implications. Disrupting the corpse management of a colony and its equilibrium, says Zhou, could be a powerful strategy against termites since undertaking behaviors that include avoidance of contaminated nestmates help survival. “Undertaking behaviors respond to the fungus-infected individuals or insecticide-contaminated corpse to maximize the existence and survival of termites at a colony level,” says the researcher. “Maintaining the equilibrium of social homeostasis is critical for the overall fitness of the colony.”
Killing individual termites, he adds, “may not be necessary to eliminate the colony.” A better strategy, he suggests, would be to weaken the colony’s overall fitness, upsetting its equilibrium and causing colony collapse, similar to what has afflicted honey bees.
A COMPLEX JOB. Social insects are the only animals other than humans that have a complex set of behaviors for disposal of their dead. Insect undertakers initiate corpse management when they sense odiferous chemicals given off just before and after death.
The most common method of handling corpses by ants and bees and, to some degree, wasps, is to haul them away, far away from the nest. Corpse management in honey bees is pretty straightforward. An undertaker bee typically examines the dead bee with its antennae, grasps its appendages with its mandibles, transports it outside and drops it outside the hive. Simple enough, but when done efficiently, the process protects the colony against diseases and the hive-destroying varroa mite (Varroa destructor).
Ants commonly lug dead corpses faster and farther than when they are simply keeping the nest tidy by carrying off debris. It makes sense, given that corpses can be a safety hazard. Red imported fire ants (Solenopsis invicta) and the common red ant (Myrmica rubra) are among the species observed carting corpses outside the nest and dropping them on refuse piles. Leaf-cutter ants (Atta) move corpses to special chambers that serve as tombs.
Burial is one of the disposal methods most frequently seen in termites, which have a complex set of corpse management behaviors that can depend on various circumstances. The burial also depends on whether the dead termite belonged to a reproductive, worker or soldier caste, according to Zhou and his colleagues. The researchers placed termites in a simplified laboratory substitute for a nest and examined the behavioral responses of the workers towards the corpses.
TERMITE TOMBS. Termite undertakers cannibalized corpses of all castes up to 64 hours after death, but not after that point. It makes sense from a survival standpoint because, say the researchers, “When left to decompose further, the corpse would become too depleted of nutrients or risky to be recycled.” Moreover, for a termite, a fresh body is like a vitamin supplement. “Given that wood is notorious for its imbalance of carbon/nitrogen ratio, consumption of freshly dead individuals would provide an important nitrogen resource for wood-feeding termites,” the researchers said.
It is also common for termites to bury fungi-killed nestmates and old corpses to physically isolate them from healthy individuals. When challenged with the fungus Metarhizium anisopliae, the Formosan termite displays undertaking behavior that appears to be shaped by the density of corpses. When the level of mortality is low and corpses few, they are eaten. When mortality increases, the insect undertakers tend to bury rather than consume.
When burial was the method, undertakers interred dead workers immediately while soldiers were either buried or walled off. This burial behavior mirrors how workers wall off ingress points when soldiers are killed while defending the colony. Research conducted several years ago on the Formosan subterranean termite, published in the Journal of Economic Entomology, indicates that the behavior of termites to insecticides must be taken into account when evaluating control methods. Termites exposed to cultures of the fungus Metarhizium anisopliae transfer disease to previously healthy individuals.
By sealing off a corpse infected with a transmissible control agent, termites enable the living to avoid them and potential hazards they pose with little effort, much easier than having to relocate the nest or abandon a large part of its area. Some termites have been observed sealing off entire sections of a nest that have been treated with insecticides.
Walling off termites killed by an insecticide may be a response to its repellency or to the chemical signals given off by the dead. Undertaking behavior performed by subterranean termites can sometimes circumvent a soil termiticide barrier treatment, says Zhou. To bury a corpse, undertakers cover it with feces and chewed food mixed with saliva, both of which have antifungal properties.
THE COLONY SURVIVES. Undertakers responded faster to dead workers than corpses of other castes. The researchers suggested that because in sheer numbers, workers are so dominant, their bodies deserve fast attention to maintain the termite colony’s fitness.
For undertakers to react, they must be able to distinguish the dead from living members of the colony. Zhou and his fellow researchers profiled the different chemical signatures that enable termite undertakers to make corpse management decisions. The chemicals released by the corpses are fatty acids, including 3-octanol and oleic acid (a sign that death was recent) and 3-octanone, which is produced later. These are among the chemical compounds believed to be signals by which eusocial insects recognize death and how to process the insect remains.
As evidence of the different responses of undertakers to various chemical signatures, worker corpses gave off higher amounts than soldiers of 3-octanone and 3-octanol, released at death and then diminishing over time, reports the study. Neither of these chemicals was detected in nymphs of various types destined to reproduce, although the absence could be caused by limits on detection of trace amounts during the experiments.
The termite reproductive nymph has two versions, one which becomes winged male and female alates that will fly off to found new colonies and another, which remains behind and produce eggs within the colony to supplement those of the queen. Oleic acid, which builds up slowly after death and signifies that the corpse has lain around for a while, was detected in all castes, although accumulated more slowly in soldiers.
Oelic acid — sometimes called the “death pheromone” — is the only chemical cue signaling death found across the board in all eusocial insects. The automatic reaction to oelic acid and other undertaking behaviors are evolved adaptations to promote survival of the colony.
Given the risk posed by dead termites that may be contaminated, it can be a dangerous job, but as the old saying goes, for the welfare of all, somebody has to do it.
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