No getting by these Mariola soldiers without a fight as they guard the entrance to the colony. Photo credit: Paula Shrewsbury
A trip to the rainforest in Central America would be incomplete without a visit to a hive or two of stingless bees. Here in the US our notion of bees is usually honey bee-centric, visions of rectangular white boxes packed with racks of honeycomb, busy workers gathering pollen and nectar, and a queen producing legions of brood, all guarded by fearless female warriors capable of a delivering a fierce sting. Tropical stingless bees are a bit different. They typically nest in tree hollows and earthen crevices but with invasion of their realm by modern man, cracks and crevices in cinder block and masonry walls have become popular nesting sites. Often these hollows have rather large openings and to limit access to the colony and facilitate defense, voids are narrowed to trumpet-shaped entrances constructed with a sticky substance called propolis, a mixture of wax and other materials. This defensible portico may be to help stingless bees repel ants and other creatures that would love to raid the colony and plunder the honey, pollen, and baby bees inside. On several occasions local residents reported these beguiling bees to be “Mariolas” and their cultivation is known as Meliponiculture, the domestication of stingless bees. In addition to being delicious, honey produced by Mariolas is thought to be medicinal and to aid in healing many disorders.
Masonry walls with internal voids are a favored spot for Mariolas to build their nests. The nest is protected by hovering soldier bees ready to attack flying nest raiders and a phalanx of warriors protecting the tubular entry to the colony. Aerial guards swirl slightly out of focus around the entrance while workers returning to the colony with nectar and pollen zoom directly into and out of the nest.
Although stingless bees lack the ability to sting, they are by no means defenseless. Each nest of Mariolas I observed was guarded by soldiers hovering in the air just outside the nest’s entrance while others lingered near the circular opening to the nest. Recently, scientists have discovered that these soldiers are a unique caste, some 30% heavier than their nest-mates. Their job is to guard the colony from marauding enemies, including species of robber bees that commonly raid stingless bee colonies. Their coup de main involves grabbing an invader by an antenna or wing and refusing to let go, thereby thwarting an attack. In a previous episode we reported on a larger darker Belizean cousin of the Mariola that employed a different defense. In this instance, the nest entrance was guarded by several workers that watched carefully and mounted a surprising attack when people ventured too close. The assault consisted of dozens of workers flying into faces and hair of on-looking humans. They seemed to pay special attention to eyes, noses, and ears. Although they lacked stingers, their annoying bites were very persistent, forcing the intruders to vacate the premises near the hive. In his book The Insect Societies, E.O. Wilson describes accounts of stingless bees attacking human intruders. Some species eject an irritating liquid that causes a burning sensation to skin. This trick has earned them the local name of cagafogos or “fire defecators” in Brazil. So potent is this defense that it may dissuade very aggressive attackers like army ants from entering nests.
The first clip shows a stingless bee visiting a blossom as intended by Mother Nature, a way that might transfer pollen. Small holes nibbled at the base of the flower provide access to nectar by nectar robbers, including sometimes sneaky Mariolas. Watch as a bee shuttles back and forth to nectar-bearing holes at the base of the flower.
Mariolas bees were common visitors to many kinds of flowering plants in the Costa Rican rainforests. Some clearly were carrying out the well-designed plans of their flowering partners gathering nectar and pollen from the reproductive structures of the flower. Others engaged in hijinks known as nectar robbing at the blossom of a plant with an unusually long corolla. Nectar robbing involves chewing a hole through the petals of a flower near its base and drinking the nectar from the bottom of the corolla rather than entering from the natural opening in the front. While disappointing for the hopeful plant that depends on pollinators to transport pollen while stopping for a drink of nectar, this tiny bee with a short tongue appears to have found a clever and somewhat sneaky way to obtain the nectar necessary to help sustain its colony.
Acknowledgements
References used in this episode include the following: “A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee” by Christoph Grütera, Cristiano Menezesb, Vera L. Imperatriz-Fonsecab, and Francis L. W. Ratnieksa; and the fascinating book "The Insect Societies" by E.O. Wilson. We thank Lee Hellman and the hearty crew of BSCI 339M: Tropical Biology in Belize and our fearless Costa Rican guides, Mario Cedeño Castro, Don Abelino, and Doña Wendy, for providing the inspiration for this Bug of the Week.