Division of Labour and Foraging in Bombus agrorum Fabricius

Abstract

The behavior of workers marked individually was studied in four observation nests. Both large and small workers foraged and undertook nest functions, however small workers begin to forage about 15 days after emergence as against 5 days for the large workers. By manipulation of colonies it was found that not the absence of foraging bees nor absence of nectar, but ontogeny caused the small house-bees to begin foraging. Longevity of the workers was variable, 69 days being the maximum recorded. On the average 29% of the bees died every 5 days. House-bees lived slightly longer than foragers, particularly in a bad season. About two-thirds of the total bee population were house-bees and one-third foragers at any particular time. No trend in labor with age was observed in the house-bees, foragers, when in the nest, performing a variety of functions. The larger foragers collected pollen and nectar, 75% of pollen loads being accompanied by nectar. The smaller foragers tended to collect nectar only. In all, 665 foraging trips were timed, the averages for two nests being 17.5 and 15.3 min. Pollen loads with or without nectar took longer to collect than nector loads alone and loads were collected faster in damp weather than in dry. Out of 120 pollen loads, 67 were mixtures of from two to six plant species. The pollen collection of individual workers indicated moderately fixed foraging habits. Normally the workers behaved indifferently towards one another but on a very few occasions attacks were observed in which the attacked bee was forced to disgorge liquid or leave the nest. The attacker was always a house-bee, the attacked a forager. A comparison with the dominance order in Polistes wasps is made.