Fig 1: Examples of parasitic cuckoo bees
You heard right, some species of bees—known as cuckoo bees—have lost the ability to provide for their offspring, leading them to parasitize the food provisions collected by other bee species. Cuckoo bees are brood parasites, or species with offspring that grow and feed off of the food gathered by another host species to feed their own young. Obviously, this has led to intense competition between the cuckoo bees (who need to parasitize the hosts’ nest to provide for their young) and the host bees (who need to deter cuckoo bees as they will kill the hosts’ offspring) As a result of this competition, both bee species are constantly evolving and perfecting their defense mechanisms against the other in what is known as an coevolutionary arms race. Currently, however, many cuckoo bees are able to parasitize a host bee species without being discovered. How do these cuckoo bees avoid detection from their hosts and feed off of their food provisions?
Over time, the cuckoo bees have evolved numerous morphological, physiological and behavioral mechanisms that allow them to successfully parasitize host nests under the radar. In general, solitary bees build a nest with compartments (brood cells) that hold a mass of pollen and nectar that the egg is eventually reared on. Once the female bee has deposited the egg in the brood cell, the larval instars—baby bees—feed on the pollen in their brood cell until they metamorphize and exit without coming into contact with the adult female.
Fig. 2: Two examples of pollen masses and the host-egg within a host species nest and cell.
The parasitic cuckoo bee uses olfactory and visual cues to locate and deposit an egg into one of the hosts’ brood cells. Often times, nesting host females secrete compounds that make brood cells waterproof during the final stages of construction. These secretions also provide a cue to parasitic bees who eavesdrop on those signals to locate a suitable nest. Female parasitic bee will also observe the activities of the nesting host and wait until the right moment to attack and enter the nest. Once inside, the host larvae are killed, allowing cuckoo larvae to eat the pollen left by the host female for her own eggs. The process in which the female cuckoo bee infiltrates the host nest and deposits her eggs can occur through three strategies. In the first case, the female cuckoo waits until the host bee is finished making her nest and has departed. Then, she chews through the nest’s resin entrance, destroys the host eggs and larvae, and deposits her own eggs on each pollen mass while rebuilding the cell partitions she previously destroyed. Finally, she rebuilds the resin door on her way out, leaving no trace with her eggs safe to grow inside.
Fig. 3: Sharp mandibles in various species of hospicidal, or host-killing bee larvae.
The second strategy is similar to the first in that the adult cuckoo bee enters the sealed nest and deposits her eggs, however, in this case, the parasitic larvae are the ones who kill the host larvae instead of the adult cuckoo bee—these are known as hospicidal larvae. These host-killing larvae hatch faster than host eggs, allowing them to seek out and destroy host larvae using sharp, sickle-shaped mandibles.
The third strategy allows the parasitic female to enter the host’s nest before it is sealed. Instead, the cuckoo bee waits until the host has left to forage before entering the open hole, depositing her hospicidal eggs, and leaving unbeknownst to the host bee who returns and completes her nest on schedule.
But how does the cuckoo bee conceal evidence of her visit and hide her eggs from the host during and after infiltration? Female cuckoo bees implement visual tactics, chemical mimicry, and egg-hiding in order to fly under the radar. For one, many parasitic bees have evolved morphological traits that protect them from non-parasitic bees during an attack. For example, many parasitic bees have a thickened cuticle, spine, lamellae, and carinae that protect vulnerable areas. These traits have evolved as non-parasitic bees who find the brood parasites in action are dangerous and aggressive, often dragging the parasite from the nest.
Fig 4: Example of a solitary host bee sealing her nest before departing.
Another mechanism that the cuckoo bees employ to avoid detection is through chemical mimicry. Chemical mimicry is more common in cuckoo bees attacking open nests as they need to conceal any olfactory evidence that may be left once the female host has returned. To achieve chemical mimicry using extraneous materials, parasitic bees can chew the edges of leaves that host bees use to construct their nest, gather the consecutive plant juice, and rub it on their entire body. They can also remove pollen from a host nest and rub it on themselves to create olfactory camouflage. That said, some cuckoo bees can also achieve chemical mimicry through producing their own compounds that mimic the odor of the host species and her nest. In one example, male cuckoo bees secreted and transferred compounds during mating to females that resembled the chemical secretions released by their host species during nest construction. Secreting these compounds that are identical to the host species may aid in olfactory camouflage and protect the female cuckoo bee from attacks.
The final way in which cuckoo bees avoid detection from their hosts is through egg hiding. Again, this occurs mainly in parasitic bee species that enter an open nest that is still under construction by the female host. Once the host is off foraging, the cuckoo bee deposits especially small eggs in slits perpendicular to the wall of the host’s nest cell so that the tip of the egg matches the inner lining of the cell. Sometimes, cuckoo bees can secrete waterproof compounds to the tip of the egg peeking out from the cell wall. This way, the space around the egg is filled in, a waterproof layer forms around the egg, and it is further hidden from the host bee species.
Overall, this paper describes the various ways in which parasitic bee species use their sensory modalities to successfully parasitize host bee species and deposit their eggs without detection. These evolved traits are also seen in other species of brood parasites such as wasps, birds, and fish, indicating that a remarkable level of convergence is taking place under heavy pressure for brood parasites to avoid the detection of their hosts.
