Symbionts of bumblebees

Predation occurs when one animal eats other animals and kills them. The animal that is killed is called the prey and the animal that does the killing is called the predator.

Symbiosis. Bumblebee Symbionts means living together, and there are three kinds of symbiotic relationships between species:

  • Parasitism. Where the parasite receives some benefit from the host and by doing so harms the host.
  • Commensalism. Where one partner receives some benefit from the other without harming or benefiting the other partner.
  • Mutualism. Where both partners benefit from the relationship.

Bumblebee Parasites

Cuckoo bumblebees, Psithyrus spp. The cuckoo female enters the bumblebee nest and lays her eggs, the bumblebee workers then rear these eggs as if they were their own sisters and brothers. The cuckoo bumblebee has no pollen baskets on her rearmost legs, and does not secrete wax for nest construction. There is no worker caste, and all cuckoo bumblebee eggs hatch as reproductive males or females.

Wax moth, Aphomia sociella (see right) . In northern Europe the wax moth is regarded as the most serious enemy of the bumblebee, and is found only in bumblebee nests. In some areas in southern England as many as 80% of nests can be destroyed. It is more commonly found in the above ground nests.

The adult moth flies from June - August at night. the female enters the bumblebee nest and lays her eggs. At first the caterpillars feed on nest debris, but as they grow they switch to feeding on the wax food cells, food stores and even larvae. Normally this destroys the nest. The fully grown caterpillar leaves the nest to overwinter as a pupa in a sheltered spot, It spins a strong, brown cocoon in autumn.

In the UK bumblebee boxes have become popular in recent years. If your box is invaded by the wax moth than I'm afraid there is little you can do for the nest as by the time you have discovered it it is too late. All you can do is clear out the nest contents and get it ready for the next year.

Invasion of the nest by the north American wax moth Vitula edmandsii does not always lead to the destruction of the nest as it does not feed on the larvae.

Brachicoma (Brachycoma) devia in Europe and Brachicoma (Brachycoma) sarcophagina in north America are flies that look a little like the common house fly. The female fly enters the bumblebee nest and lays larvae (this fly does not lay eggs) among the bumblebee larvae. The fly larvae attach themselves to a bumblebee larva and wait. Once the bumblebee larva has spun its cocoon the fly larvae start feeding on it and suck it dry. When they are fully grown they (one bumblebee cocoon can support 4 parasitic larvae) leave the bumblebee cocoon and pupate in the bumblebee nest. A heavy infestation will lead to the death of a colony.

Bumblebee nest destroyed by wax moth caterpillars

Ephestia kuehniella, a flour moth (see right), the a world wide distribution has been reported in North America as invading bumblebee nests. (Davidson, A. (1894). On the parasites of wild bees in California. Ent. News. 5: 170-172.

Conopid flies. The female fly waits on flowers till a bumblebee comes to feed, then she jumps on the bee and quickly pierces the bee's body laying her egg inside. The egg hatches inside the bee and feeds off its abdominal contents till it more or less fills the entire abdomen. The bumblebee dies after about 10 days and the fly larva pupates inside the husk of the dead bumblebee body emerging as an adult fly next summer. This parasite can change the behaviour of the bumblebee. Infected workers tend to stay outside the nest more than other workers, often remaining out all night, and they tend to favour colder microclimates, also before the bumblebee dies it attempts to bury itself. In Europe bumblebee queens are not usually parasitised as they emerge before the adult fly emerges. In the U. K. Conopids are usually active from June - August.

Mites. Locustacarus buchneri lives in the air sacs of the bee where it feeds on the haemolymph. It will not kill a colony, but it will weaken one. The mites hibernate inside new queens.

Ephestia kuehniella, the flour moth

Varroa sp. which has led to the destruction worldwide of numerous honeybee hives does not affect bumblebees. However there are some species of mites that live in bumblebee trachea and air sacs and others that live off their blood.

Parasitic wasps. These small wasps lay numbers of eggs inside the adult bumblebee and the wasp larvae eat the bumblebee from the inside.

Sphaerularia bombi (right), a nematode (tiny worm). This parasite is only found in queens and affects their behaviour. The bumblebee queen is infected by an adult female worm while she hibernates. It is believed that the nematode enters through her mouthparts. Around 12% of queens are infected, and this can rise to as much as 50% for late emerging queens. In the spring when the queen emerges from hibernation the worm begins to grow, then it turns its whole reproductive system inside out. The uterus grows and grows till it is between 1-2 centimetres long, while the rest of the worm is only a thin thing of a few millimetres. In a normal queen a hormone would be released and her ovaries would start to develop stimulating her to start building a nest, but somehow this does not happen in an infected queen. Meanwhile the worm releases up to 100 000 eggs into the blood of the queen, these eggs hatch and develop, moving into the gut and reproductive system. During this time the queen feeds only for herself, she makes no attempt to find a nest site, and her ovaries do not develop. Often she returns to her hibernation site, here the worm larvae are discharged with faeces into the soil. The mature worms mate, and wait for another queen to use the site to hibernate.

Ants. Various species will invade bumblebee nests if they find them and eat stores, eggs and grubs, this often destroys the colony.

Sphaerularia bombi

Bumblebee Commensals

Certain hoverflies, e.g. Volucella bombylans. The adult female fly enters the bumblebee nest and lays her eggs. The fly eggs hatch and the larvae feed on nest debris, doing no apparent harm. The fly overwinters as a pupa in the empty bumblebee nest. This hoverfly is also a bumblebee mimic, and it even buzzes if handled.

Certain mites, e.g. Parasitus fucorum. These mites are often seen attached to the thorax of bumblebees, whom they use as transport between nests. They scavenge on nest debris, then attach themselves to young queens and hibernate with them. These mites often have even smaller mites living on them!
"A flea hath smaller fleas that on him prey;
and these have smaller fleas to bite 'em,
and so proceed ad infinitum"
Jonathan Swift

Certain beetles, e.g. Antherophagus spp. These feed on nest debris and use adult bumblebees for dispersal.

Bumblebee Mutualists

Flowers which require pollen by an insect vector. It is believed that many species of flower co-evolved with insects and became so specialised that they need each other to survive. A good example of this is the monkshood flower which, in the UK, can be successfully pollinated only by a bumblebee.

Humans. Though we may also be the bumblebee's greatest enemy with our habitat destruction, introduction of non-native competitive honey bee, and over-use of highly toxic pesticides there is an increasing number of gardeners who don't use insecticide. So we get our flowers pollinated, the bumblebees get food in a safe environment. And those who raise crops in glasshouses would be unable to do so commercially without commercially reared bumblebees. For more about what bumblebees do for humans see the economic importance page.

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