Suicide attacks by stingless bees protect colony

Research shows that worker bees of the genus Trigona persistently "bite" during attacks and die rather than disengage from intruders.

By Elton Alisson

Agência FAPESP – In contrast with honey bees (Apis mellifera), stingless bees (Meliponini) lack a functional stinger and are often considered inoffensive and unable to defend themselves from attacks by predators or nest robbers.

A study by researchers at the University of Sussex in the UK, in collaboration with colleagues affiliated with the Ribeirão Preto School of Philosophy, Science & Letters (FFCLRP) and the Luiz de Queiroz College of Agriculture (ESALQ), both of which belong to the University of São Paulo (USP) in Brazil, shows that stingless bees do in fact resort to a range of defense mechanisms, even though they cannot sting like Apis mellifera because they have only vestigial (atrophied) stingers.

One such mechanism is “biting” an intruder so tenaciously that the bee dies rather than disengage during an attack, sacrificing itself to protect the nest from predation by other animals or to stop “robber” bees from stealing the colony’s food.

The study was carried out as part of a project supported by FAPESP. Its findings have been published in the journal Behavioral Ecology and Sociobiology.

“This is the first time suicidal defensive behavior has been described in social bee species, except for Apis mellifera, which has a serrated stinger that lodges in the victim and tears loose from the bee’s abdomen, causing death,” said Denise de Araujo Alves, a researcher at ESALQ and one of the authors of the article.

“Some species of stingless bees attack by biting. They often end up dying because they remain attached to the victim for a long time. In this way they repel intruders to defend their nesting colony containing a brood [offspring], the queen, food stores [honey and pollen], and architectural materials such as resin,” she told Agência FAPESP.

The researchers performed three field experiments with 12 species to study the defense mechanisms used by stingless bees.

In one of the experiments, small black felt flags were waved near the colony entrances of different species in an attempt to provoke the bees. The researchers measured how long it took the bees to begin an attack, the duration of the attack, and the number of attacking bees.

Some bee species reacted little or not at all when the flag was placed close to the colony entrance. However, workers of three Trigona species – T. hyalinata, T. fuscipennis and T. spinipes – as well as Partamona helleri bees attacked the flag in groups, very aggressively and for a period lasting nearly an hour.

Bees of the species T. hyalinata, for example, suffered fatal damage, such as separation of the head from the body, leaving the mandibles clamped to the flag instead of disengaging after the attack.

“Bees of these Trigona species attacked the flag en masse as soon as it was waved near the colony entrance,” Alves said. “Just the fact that an object passes in front of the colony is enough to represent a threat for them.”

Sharp “teeth”

To measure the level of pain that each of the stingless bee species studied inflicts when it attacks, the researchers acted as “guinea pigs,” allowing bees guarding the colony entrance to bite their forearms.

Species were ranked into pain categories on a scale of 0-5, ranging from those that could not be induced to bite or caused mild pain to those capable of breaking skin if persistent.

On tabulating the results, the researchers discovered that the stingless bee species with the most painful bites were those of the Trigona genus, which attacked most aggressively and left their jaws clamped to the flags in the first experiment.

“Their bite is far less painful than an Apis mellifera sting,” Alves said. “But if you bear in mind that the bite of an insect measuring only a few millimeters in length is capable even of piercing skin and that they attack en masse, the pain is considerable and the intruder must retreat from the colony.”

One of the reasons that a bite from a bee of the Trigona species is more painful than a bite from other stingless bee species is that Trigona bees have serrated mandibles with five sharp “teeth,” the researchers found.

This mandibular morphology is presumably what enables Trigona to cause more pain and damage to predators and would-be robbers of their colonies. According to the researchers, the bees’ serrated mandibles may therefore be a defensive adaptation.

“Other stingless bee species that don’t defend themselves so aggressively as Trigona have more rounded mandibles and lack very sharp teeth,” Alves said. “Workers of Apis mellifera, which defend themselves with their stingers, also lack mandibles with sharp teeth like those of Trigona.”

Suicidal tendency

The researchers performed a third experiment to test the willingness of stingless bees to suffer lethal damage and die during a biting attack.

The experiment’s first step was presenting the black felt flag to the bees for 5 seconds and passing a small paintbrush over the bodies of bees that attacked the flag, without causing them any physical harm.

Next, the bees that held on to the flag had their wings tugged using forceps. The possible responseswere either to let go of the flag, allowing them to be released and fly away, or to retain their hold, suffering damage to their wings to such an extent that they could no longer fly back to the colony.

The experiment showed that workers of six comparatively aggressive stingless bee species displayed a willingness to suffer fatal damage and die rather than disengage.

The largest proportion of “suicidal” stingless bees was from the most aggressive species, T. hyalinata: 83% of the individuals in the experiment were pulled apart by the forceps rather than let go of the flag.

According to the researchers, this behavior is comparable to that of Apis mellifera, which self-amputates its sting apparatus from its body and dies after an attack. “The study shows that Trigona species stingless bees are particularly defensive and even suicidal,” Alves said.

The stingless bee species that displayed the most aggressiveness and willingness to sacrifice themselves during biting attacks were those with the most populous colonies.

Colonies of T. spinipes, for example, can have as many as 180,000 bees, whereas colonies of Melipona quadrifasciata, a stingless bee species that does not attack, have approximately 1,000 bees each.

“The cost of a suicide attack by a group of worker bees from species with more populous colonies is much lower than the cost for species with small colonies,” Alves said. “It’s much more worthwhile for species with larger colonies to invest in this defense strategy because the loss of workers isn’t as great in comparison with less populous colonies.”

Suicide attacks by stingless bees involve older workers so as not to endanger the continuity of the colony.

“There’s a certain logic in recruiting older worker bees to perform dangerous tasks like nest defense and foraging, in order not to lose the younger members of the colony,” she said.

The attack is begun by a small group of worker bees. They then release pheromones to call other workers to assist them in their mission.

“This is the first time anyone has reported and quantified the behavior of stingless bee species that allocate part of their colonies to defense, showing that many ultimately die because the gain from defending the colony and dying is greater than if they allowed it to be attacked and died [along with the colony],” Alves said.

The article “Appetite for self-destruction: suicidal biting as a nest defense strategy in Trigona stingless bees” (doi: 10.1007/s00265-014-1840-6), by Shackleton et al., can be read online in Behavioral Ecology and Sociobiology at link.springer.com/article/10.1007%2Fs00265-014-1840-6#.