Rational Management of Solitary Bees of the Genus Xylocopa and Centris for Pollination in Agricultural Areas
Breno Magalh„es Freitas
Solitary bees of the genus Centris and Xylocopa have proven efficient pollinators of some tropical crops in Brazil. Usually, these crops are not attractive to honey bees, such as West Indian cherry (Malpighia emarginata) and "murici" or wild cherry (Byrsonima crassifolia) that show flowers which secrete oils instead of nectar; or grow io hostile environments where honey bees are not abundant, such as cashew (Anacardium occidentale); or their flower morphology do not allow Apis bees to pollinate efficiently, such as passion fruit (Passiflora edulis); or have open flowers rich in rewards attracting a range of flower visitors, such as guava (Psidium guajava). The former three crops are well pollinated by Centris species, while the latter two crops are by Xylocopa bees.
Efforts are being made in Brazil to rear and use Centris and carpenter (Xylocopa) bees as crop pollinators. Studies with Centris bees are still mainly restricted in identifying species which are efficient pollinators and in using trap nests to study their nesting biology. A relative success has been achieved to species that nest in wood cavities, but those species nesting in the ground have shown much more difficult to rear in trap nests. Main difficulties to build large populations of these bees have shown to rely in the facts that (i) they are not gregarious making difficult to get large number of nests in small areas, (ii) they produce small number of offspring per nest, (iii), offspring do not hibernate and show short egg-adult cycle and (iv) they are active all year round nesting at any time of the year. Now, management recommended aims to stimulate nesting and presence of natural populations of Centris bees in the crop areas creating nesting conditions required by the bees, avoiding to spray the crop with pesticides or reducing its use o the non-flowering stages of the crop, refraining of ploughing the agricultural area to preserve ground nests, not removing wild herbs that flowers when the target crop is not flowering to fix bee population in the area, and preserving native vegetation on the surroundings of the cultivated area to furnish bees to the target crop.
Studies with carpenter bees have been more successful, probably because these bees are better investigated and show less reproductive and behavioural variability than Centris bees. They also show the same reproductive characteristics listed above as difficulties to build large populations of Centris bees, but carpenter bees are much bigger, need to visit more flowers and cover an area much larger than Centris does, needing smaller population in the target area than Centris. A nesting box made up of movable wooden frames and glass windows was successfully developed to rear Xylocopa bees. There, it is possible to monitor the gynes while they excavate nests and rear offspring, to swoop frames between nesting boxes, to move the nests in and out cropping areas, to estimate the number of adults living and foraging in the target crop, to reduce or increase Xylocopa population in the field, etc. Presently, some passion fruit growers have just started to use these rational nests and are achieving increments up to 92.3% in initial fruit set. Although rational management of these bees are now possible, practices to attract and fix natural populations such as the above mentioned to Centris bees, the use of wooden posts to support the vines and cultivating other plant species which blooms when passion fruit is not flowering are recommended.
Edited from the article Rearing solitary bees for crop pollination written by Breno M. Freitas, Júlio Otávio P. Pereira and José H. de Oliveira-Filho and available at the homepage of the Bee Research Group of Universidade Federal do Ceará (www.abelhas.ufc.br/articles.htm)