FRISON 1916: Recounts Sladen’s record (Can. Ent., Mar. 1915), which was the first
ever, of a Psithyrus inhabiting the nest of a Bombus in the new world (P. insularis, in
British Columbia); AND: adds his own record of P. variabilis (at Champaign, IL)
inhabiting the nest of B. pennsylvanicus.
FRISON 1917: Observes that the study of Bombus nesting is difficult because all but the
new queens die in autumn, so that, in early spring, the colony is so small as to be very
difficult to locate. Also, if you transfer a nest to an observation box, the queen almost
always abandons it, or if confined, dies shortly afterward.
MEDLER AND CARNEY, BBs OF WISCONSIN:
General:
Food supply: depends mainly on (1) time of year of flowering (queen in spring—willows;
workers through summer—clovers, alfalfa; and new queens/males late—asters,
goldenrod), and (2) bee tongue length (red clover, vetch, and thistle for long-tongued
spp.; aster, goldenrod, and sweet clover for short-tongued spp.)
Pollination effort: to produce one bushel of red clover seed would require more than 15
million visitations.
Natural enemies of the colony: mainly ants; also skunks
Psithyrus:
Externally similar to Bombus (often mistaken for them), but are actually parasitic on
Bombus. They have no worker caste; chief modifications involve loss of cobicula and
thick, toughened integument.
Queens first emerge later than those of Bombus, in order to search for established
Bombus nests. She kills all eggs/larvae/pupae of the existing colony and lays her own
eggs; her brood will be taken care of by the Bombus workers; no Psithyrus workers are
produced. The Bombus queen remains in the nest but stops laying eggs, and the colony
eventually dies off without producing males and new queens as it usually would.
B. fervidus apparently is able to ward off Psithyrus by covering the invading queeen with
honey. Plath (1927) reported an unsuccessful attempt by a psithyrus to enter a honey bee
colony.
Bombus:
Castes: (1) Queen; (2) Workers; (3) Males.
Valuable as pollinators; probably a major role in evolution of angiosperms.
Life Cycle:
Queeen hibernates, then starts colony in spring (Apr-May); bumblebees seen neactaring
at this time are queens. Earliest spp. (12-24 April) are bimaculatis, terricola, ternarius,
griseocollis, impatiens, and perplexis. Latest (4-28 May): americanorum, borealis,
rufocinctus, vagans. In general (i.e., in 3 of the 5 spp.), queens of Psithyrus appear much
later (June).
Queen first finds nest site; may be under, at, or above ground level, but abandoned mouse
nests (underground) are preferred.
Second, constructs a wax honey pot in which nectar is stored (esp. needed in bad
weather).
Third, constructs a wax egg cell, lays eggs.
Larvae hatch, spin, and pupate in only one week.
Once the workers hatch, they do all foraging; queen lays eggs, never leaves nest again.
Two means of feeding larvae:
(1) Pocket makers: construct pockets of wax alongside larvae; returning workers
drop pollen into pockets
(2) Pollen storers: pollen stored remotely from larvae, and nurse bees take pollen
to larvae, transferring it via their mouths (regurgitation)
BUT: In some spp., only larvae destined to become workers are fed by pocket method,
whereas those to become queen or male fed via regurgitation. And, within female larvae,
queen versus worker can be determined by differential feeding.
In late summer, males and new queens are produced; old queen dies. New queens and
males do mating flights, but only the inseminated new queens overwinter; males and
workers die off by winter.