Dr. Frans van Alebeek on Mason bees and their parasites:
(Portion of a personal letter reprinted with permission of the author)
First some clarification may be necessary on which genus of bees we are talking. I looked up the name Mason bee in Richards & Davies, 1977. Imms' General Textbook of Entomology (Tenth Edition) (Volume II, pp 1256-1257). According to this text, Mason bees belong to the genus Chalicodoma. However, these bees generally construct their own nests from mud, resin, leaves or other materials, attached to walls, stones or barks of trees. However, the garden catalogues you mention offer blocks of wood with small holes drilled into it. That type of 'trap-nests' is more appropriate for the closely related genus Osmia, which normally construct cells with mud walls in any suitable hole or cavity in wood, mortar (hollow stems, keyholes, snail shells, empty galls, etc.). My own experience is mostly with (NW-European) Osmia and Megachile species (and several other wood-nesting Hymenoptera).
When I hung my first wooden trapnest (a block of wood full of drilling holes varying in diameter from 2 - 10 mm) on a sunny spot in my garden, it was immediately colonized by Osmia bees starting nests in there. I have been studying them, there fellow inhabitants, and parasitoids now for 5 years. These solitary bees are indeed notorious for NOT being aggressive. Of course, females can defend themselves, when seriously threatened, with their sting. But it takes much more provoctions to make them sting, and their nesting sites can very easily be studied from very close distances. In addition, in these solitary bees there is (as far as I know) no alarm pheromone as in honeybees, and thus no group-attack which can make honey-bees and yellow jackets so dangerous around their nests. I am not aware of any studies quantifying flower visit frequencies and ranges compared for honey-bees and Mason bees. Both groups collect pollen and nectar. Being solitary bees, they do not teach conspecific females the locations of rewarding flower patches as honey bees do. Thus, there is no group-learning and specialization on one flower species. However, some groups of solitary bees are specific to the plant genus (or even species) in the pollen they collect for their nests. But Osmia and Megachile seem less specific. Nevertheles, I have observed that they do have preferences for flowers in the Cosmopolits family (especially Cirsum, Cardius, Centauria, etc) with dense flowerheads where they can easily collect a lot of polen in one visit. Flower range for honey visits is mainly depending on tongue length, depending on bee species size.
One important difference with honey bees is that the latter species flies around the whole season (if weather allows for), whereas most Osmia and Mechachille species have much shorter flying periods. Some species fly very early in the season (March, April), and thus often forage under weather conditions when indeed honey bees are almost inactive. However, later in the season, those early bees are no longer around, and other species may have their flying period. I do not know whether solitary bees can completely replace honey bees in their pollination activity and in the range of flowers they visit. However, experts say that populations of wild bees decline when the number of honey bee hives increases in an area. This suggests that there is an important overlap in resource utilisation, and that solitary bees may be able to compensate for declining honey-bee populations.
Providing nesting sites (trap-nests) will almost certainly increase their numbers. However, I have also observed that after the first two years of very successful colonisation of the trap nests in my garden, the number of parasitoids attacking the nests is increasing dramatically. At this moment (5 years after establishment of Osmia, Megachille and other colonies in the garden), my Osmia population is virtually wiped out by a massive attack of a gregarious little fly (Caccoxenus sp.). And my leaf-cutting Mechagille species sustain heavily losses from attack by another dipteran parasitoid (Anthrax anthrax). In addition, several hymenopteran parasitoids (Chrysididae or gold-wasps, Ephialtus sp., several Gasteruption spp. and Hybomischos sp.) are attacking the nesting sites. This may reduce their effectiveness as polinators of course.
Finally, you can easily construct your own nesting sites for these solitary bees and wasps. They make terrific study objects, and offer excellent educational opportunities for youth interested in nature. Just take some piece of hard (none fibre-lose) wood (like oak or beech). Drill many (horizontal) holes into the block, as deep as your drill allows, but with a dead end. Do not drill all the way through, the bees need a closed end to start their nest. Drilling holes should be as smooth as possible, with little fibres on the sides. Holes can be anything in diameter between 2 and 10 mm, allthough the larger Osmia and Megachille species prefer sizes of 6 - 8 mm. Hang your block on the sunniest spots in your garden, under some cover to protect them from rain. The more sun (especially in spring) the more hours your bees will be active. Prevent blocks from becoming wet during longer periods; otherwise the brood will be destoyed by fungus. Hollow straws and pieces of bambou (with one end closed) also provide good nesting opportunity. I hope this information is of any help. Success.
Frans van Alebeek Dept of Entomology, Wageningen Agricultural University P.O. Box 8031, 6700 EH Wageningen, The Netherlands e-mail: <<<>