e-Megachile rotundata. This is a European bee that has been introduced to other regions . As a solitary bee species, it does not build colonies or store honey, but is a very efficient pollinator of alfalfa, carrots and some other vegetables.
The importation of the species to North America was to assist in the pollination of food crops, and it has now widespread. The species is also found in Queensland, Australia, but not recorded in other states. The vernacular in Australia is Lucerne leafcutter bee.
Female alfalfa leafcutter bees have stingers, but both sexes will use their mandibles as a defensive mechanism, usually only defending themselves when squeezed or antagonized. Therefore bee suits, such as those required with honey bees, are not really necessary when dealing with these bees. The ratio of males to females is generally one to one.
Alfalfa Leaf Cutting Bee. Image Courtesy of USDA.
This species of leafcutter bee is cultivated to pollinate alfalfa grown for seed, a function that it does far more efficiently than honeybees. These leafcutter bees are provided with predrilled "bee boards" that they use for nest construction. At the end of the season, the nest cells with developing bees are collected and carefully stored, to be released the subsequent season when alfalfa blooms.
Figure 53. Alfalfa Leaf Cutting Bee Pollinating Alfalfa.
Commercially produced bee blocks, consisting of a wood block drilled with a series of dead-end holes, are now widely available. These types of bee nests were initially developed in the 1960s by alfalfa seed producers in the western United States to attract and manage large numbers of the non-native alfalfa leafcutter bee (Megachile rotundata). Their sole purpose is to attract wild populations of these bees.
Bee Block for Attracting Large Population of Alfalfa Leaf Cutting Bees.
More recently they have been modified to manage the blue orchard bee (Osmia lignaria), a bee that is active only in the spring and will not pollinate later-flowering fruits and vegetables. Consequently, all of the nest tunnels are a uniform size and depth, which may be either too large or too small for many other species. Nest blocks with a greater diversity of hole sizes and depths are necessary to attract a variety of bees that are active throughout the year.
Under the best circumstances these nests can attract large numbers of cavity-nesting bees and boost their local populations. However, because these nests concentrate bee populations in unnaturally large numbers in a small space, they can become infested with parasites and disease spores after several seasons.
Without regular sanitation or the phasing out of nest materials, these parasites and diseases threaten long-term pollinator health wherever they are used. Because contaminated nest blocks left unattended in the landscape continue to attract wild bees from the surrounding area, they have the potential to do harm. Only with proper management can these nests maintain healthy bee populations indefinitely.
Alfalfa flowers provide an example of native bee efficiency. The stamen (the structure holding the anthers) of alfalfa flowers is held under tension by two flower petals, and springs forward with force when released by a visiting bee. This triggering discourages many bees, including honey bees, which learn to avoid being hit by the stamen by approaching the flower from behind, where they can gather nectar but not pollen. The alkali bee (Nomia melanderi), a native ground-nesting megachilid bee, is not discouraged by this unusual flower structure and is a major pollinator of alfalfa seed in some western states. It has been used as an effective pollinator of alfalfa when grown for seed in the western United States. In this case large areas of barren soil are provided for the nesting of this bee,
Mason Bees. These are a gentle beneficial insects that has potential as a pollinator of apples, cherries, and other tree fruits. It is found throughout most of North America, particularly in wooded areas but often around homes in towns and cities.
Mason Bees. Image C