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Bombus ashtoni

By Sara Diamond and Jeanette Cruz

Geographic Range

Bombus ashtoni is found in tropical and temperate zones, most notably in North America, north of Mexico. (Carpenter, 1997)

Habitat

Bombus ashtoni parasitize closely related species, such as Bombus affinis and Bombus terricola, and reside in the nests of these bumblebees (Fisher, 1984). Bombus nests are found in the ground and in deserted bird and mouse nests (Carpenter, 1997). (Carpenter, 1997; Fisher, 1984)

Physical Description

Bombus ashtoni is terrestrial and polymorphic (indivuduals may be of different sizes). Females are smaller than their host queens. (Fisher and Sampson, 1992).

Bombus ashtoni has two pairs of membranous wings with reduced venation. The hind pair is smaller than the front pair, and both pairs of wings are joined by a row of hamuli or tiny hooks (Krombein, 1997).

Mouthparts are formed for biting. Mouthparts consist of paired mandibles and a labiomaxillary complex formed from membranous connections between the maxillae and the labium. The mandibular, salivary, Dufour's, and venom glands are long and round. In females, venom glands are extremely long and convoluted. B. ashtoni have larger mandibles than their hosts, which are shortened but broader at the apex, and lacking a basal keel (Fisher and Sampson, 1992).

Sternites are thickened, and females have no corbiculae (the pollen baskets formed by long curved hairs, on their hind legs). The female ovipositor is longer and broader than that of the host, and it is strongly recurved.

Bombus ashtoni eggs are smaller than host eggs and are narrowed towards the middle (Fisher and Sampson, 1992). (Fisher and Sampson, 1992; Krombein, 1997)

Development

Bombus ashtoni undergoes complete metamorphosis and passes through egg, larval, and pupal states. All members of this species have reproductive capabilities.

Reproduction

Males are reared before females, especially since males establish flight territories and are long-lived. This means that males are capable of mating more than once. The sex ratios of these social parasites are found to favor females (Fisher, 1987).

Reproduction is short in duration. Males are produced from unfertilized eggs and females are produced from fertilized eggs (Krombein, 1997). Females use wax from the destroyed host egg cells to construct their own egg cells. Eggs are laid near the center of the comb and are distinguishable from host eggs by their rough edges. There is no worker caste (Fisher, 1987). (Fisher, 1987; Krombein, 1997)

Bombus ashtoni bees rear no workers, so they rely on host workers to assist them in rearing offspring. Because of this, females have decision-making processes similar to their hosts regarding the number of workers needed and how best to maintain reproductive control over them. The methods by which they determine how many workers are needed and when to reproduce are poorly understood.

The earlier B. ashtoni are introduced into a host nest, the longer they will wait before laying their eggs. The eggs are laid during the worker growth phase of colony development. The result is a reduced number of workers reared in the parasitized nests. Replacement of host eggs with parasite eggs is a gradual process, so an overlap between colony investment in Bombus workers and Psithyrus reproductives exists. (Fisher, 1987)

Females guard egg cells until the eggs hatch. They do this by pushing host worker bees away from the cells and by mauling host bees. Mauling occurs when the host queen has lost dominance or is removed. The host bee is grasped from above, held close to the underside of the parasite's abdomen, and is released. The ability to guard the eggs is decreased as the number of parasite eggs increase. The result is the loss of parasite brood. B. ashtoni females attempting to maintain a dominant egg-laying position after the queen has lost dominance face animosity by the laying workers. Also, females do not guard the developing larvae. (Fisher, 1987)

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • female
  • pre-independence
    • protecting
      • female

Behavior

Bombus ashtoni is an obligate social parasite of bumble bees (Bombus spp.) These bees invade their hosts' nests, and if successful, are able to monopolize the existing reproductive biology of their hosts.

Bombus ashtoni has a relatively passive nest invasion strategy. The invading females do not fight the host queen, nor do they aggravate the worker bees (Fisher, 1984). Female B. ashtoni gain control and become dominant egg-layers within a colony through some unknonwn mechanism. They are incapable of displacing the host queen, which apparently suppresses worker egg development (Fisher, 1987). (Fisher, 1984; Fisher, 1987)

Communication and Perception

Little is known about their communication habits.

Food Habits

Females eat the eggs laid by the host queen (Fisher, 1987). Bombus ashtoni get carbohydrates from the host resources, presumably stored nectar and pollen. (Fisher, 1987)

  • Animal Foods
  • eggs
  • insects
  • Plant Foods
  • nectar
  • pollen

Ecosystem Roles

To the extent that these animals interfere with the food supply and reproduction of their hosts, they impact those species of Bombus negatively.

Species Used as Host
  • *Bombus*

Economic Importance for Humans: Positive

There is no known economic importance of this species.

Economic Importance for Humans: Negative

There is no known economic importance of this species.

Conservation Status

This species is in no danger and has no special status.

Contributors

Sara Diamond (author), Animal Diversity Web.

Nancy Shefferly (editor), Animal Diversity Web.

Jeanette Cruz (author), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor.

Glossary

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

delayed fertilization

a substantial delay (longer than the minimum time required for sperm to travel to the egg) takes place between copulation and fertilization, used to describe female sperm storage.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

References

Carpenter, F. 1997. Pp. 231-239 in McGraw-Hill Encyclopedia of Science and Technology, Vol. 9, 8th Edition. The Lakeside Press: R.R. Donnelley and Sons Company.

Fisher, R. 1984. Evolution and host specificity: a study of the invasion success of a specialized bumblebee social parasite. Canadian Journal of Zoology, 62: 1641-1644.

Fisher, R. 1987. Queen-worker conflict and social parasitism in bumble bees (Hymenoptera:Apidae). Animal Behaviour, 35: 1026-1036.

Fisher, R., B. Sampson. 1992. Morphological Specializations of the Bumble Bee Social Parasite Psithyrus ashtoni (Cresson) (Hymenoptera: Apidae). Canadian Entomologist, 124: 69-77.

Krombein, K. 1997. Pp. 704-713 in McGraw-Hill Encyclopedia of Science and Technology, Vol. 8, 8th Edition. The Lakeside Press: R.R. Donnelley and Sons Company.

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