Examination of ‘pollen balls’ in nests of the alfalfa leafcutting bee, Megachile rotundata

SUMMARY

Nests of Megachile rotundata, the alfalfa leafcutting bee, were removed from alfalfa fields in the early autumn. X-radiography was used to analyse nests and revealed that 20–50% of the bee cells contained so-called ‘pollen balls’, a term commonly used in the USA to indicate cells that are provisioned but lack healthy eggs or developing larvae. Dissection of these cells showed that most contained pollen-nectar provisions with collapsed eggs or early, pre-defecating larvae, and a moderate proportion of them had a provision onto which no egg had been laid. Other cells contained incomplete or no provisions along with older dead larvae, beetle larvae and their faeces, or saprophytic fungi (as hyphae). Most provisions were at least somewhat moist, with 25% being very dry. A significant difference in the moisture condition and in the proportions of cell content categories was found among the bee populations examined. These results help direct future studies into understanding why bee cells are provisioned with nectar and pollen but fail to produce mature larvae, and why there are differences in the types of failure between commercial populations. X-ray analysis is inadequate for determining the true moisture condition and content of pollen ball cells. The use of the term pollen ball should be reserved for categorizing data from X-rays of bee cells, but because of its ambiguous nature should not be used to implicate bee mortality.

Keywords:

• Megachile rotundata
• X-ray analysis
• saprophytic fungi
• immature bee mortality
• alfalfa pollination
• cell provisions