The Importance of Solitary Bees
Fruit, vegetable, and seed crops are directly dependent on animal-mediated pollination as are many crops providing human medicines, fibers, fuels, and livestock forage crops. Bees are specialized pollinators covered with branched hairs that facilitate the transfer of pollen. Bees require pollen as a source of food particularly during the larval growth phase. Bees are attracted to flowers for nectar, pollen, and essential oils. As they forage for these necessities, they inadvertently transfer pollen from plant to plant increasing the fruit and seed set as well as the genetic diversity of the plants they visit. Parallel declines between pollinators and the plants that depend on their pollination services have been identified. The fates of agricultural and natural plant communities are therefore closely linked to the fate of pollinators.
The European honey bee, Apis mellifera, is the most commonly managed pollinator and is in decline due to pests, pathogens, pesticides, and the recent phenomena termed colony collapse disorder. Managed honey bee colonies in North America have declined from 5.9 million hives in the 1940s to 2.7 million as of 2023. Managed honey bee colonies in North America have declined from 5.9 million hives in the 1940s to 2.7 million as of 2023.The plethora of problems facing the European honey bee emphasizes the importance of our native bee communities. The European honey bee is, after all, only 1 of an estimated 20,000 species of bee worldwide. In North America, there are approximately 4,000 species and over 1,200 species have been identified in CA alone. Over 1,200 species have been identified in CA alone.
Research has shown that some bees may be better at pollinating certain crops than the European honey bee. Many of these bees fly in conditions that honey bees would not. Bumble bees for instance have been known to fly in rainy weather and on days when there is snow on the ground. Bumble bees also use "buzz pollination" or "sonication" to vibrate the pollen from the anthers of some flowers which do not readily release their pollen. Some bees fly earlier and later than honey bees. Some bees have shorter or longer tongues than the honey bee allowing them to gain access to flowers unavailable to the honey bee.
Unfortunately, only a few alternative bees have been successfully managed as pollinators to date. In the 1880s four bumble bee species were introduced into New Zealand from England, increasing the production of red clover seed as intended. Bumble bees (Bombus terrestris, B. lucorum, B. occidentalis, B. ignitus, and B. impatiens) have since become available commercially through intensive laboratory rearing creating a multi-million dollar industry. There are concerns over transporting them to non-native areas as they can carry and transmit pests and pathogens to local bees. The hornfaced bee, Osmia cornifrons has been studied for over 60 years and is now being used to pollinate ~80% of orchards in Japan. Millions of leafcutter bees, Megachile rotunda, are raised annually to pollinate alfalfa in North America. The alkali bee, Nomia melanderi, is another bee that supports pollination in the Midwest.
An estimated 80% of native pollen bees nest in the ground. The only ground-nesting bee currently being managed (albeit at low levels) is the alkali bee (Nomia melanderi). The alkali bee is generally found in soils with a thin layer of salt at the surface level that regulates the moisture of the soil- a very specific habitat. The alkali bee looks like a small honey bee but has iridescent yellow or green bands on its abdomen. The alkali bee is an excellent pollinator of alfalfa and onion seed. The female collects pollen and nectar and then digs a hole in the soil about 10-20 cm deep. She then constructs a small cell and provisions it with a pollen-mass, lays an egg on the pollen-mass, and then seals the hole. The alkali bee needs a very specific soil with high salinity on the surface to trap the moisture content.
The chimney bee (Anthophora abrupta) is a gregarious, yet solitary, ground-nesting bee. These bees nest in burrows conspicuously adorned with chimney-like turrets. The neighboring nests are clustered like a small village, but the bees are solitary because there is no overlap of generations and each female cares only for her own nest and future offspring. They are not aggressive or even defensive of their nests and do not sting. When roughly handled, they can defend themselves by biting but are otherwise docile and should not be considered a threat. While still unmanaged, they are in fact beneficial pollinators and have been recorded on a variety of flowers.
Many solitary bees and wasps nest in pre-existing cavities in plant material such as hollow reeds, stems, branches, and in holes created by wood-boring insects. Wood-nesting, solitary bees locate an appropriate hole, collect pollen and nectar, provision the hole, lay an egg on the pollen/nectar ball, and then create a cell by sealing off the egg with its provision. They continue this process until the hole is filled with cells, at which point they search for another hole.
Wood-nesting, solitary wasps do not provision pollen and nectar for their progeny; instead, they collect small arthropods such as caterpillars, crickets, and spiders. The wasp paralyzes the prey by sting and carries it to a preexisting hole or straw that it has selected for a nest. The wasp then lays an egg on the provision, seals the chamber, and continues the process much the same as the bee. This behavior is beneficial in that the wasp may help control herbivorous insect pests such as the armyworm or the grasshopper. Many other solitary wasps also visit flowers for nectar and to hunt prey and therefore may also contribute to pollination.
Dr. Krombein investigated the potential of artificial cavities as trap-nests to study the natural history and biodiversity of wood-nesting bees and wasps. In doing so, he added a wealth of knowledge to the scientific community including largely unknown data on 43 bees, 75 wasps, and 83 parasites or predators associated with wood-nesting bees and wasps. Since this seminal work, several investigators have used trap nests to evaluate population dynamics, biodiversity, and habitat use by wood-nesting bees and wasps. These studies show that the diversity and habitat use of wood-nesting bees and wasps differ by location, habitat, and landscape context.
Managing alternative bees for pollination adds value to agricultural production and helps to insure our food sources. Large agricultural operations need a higher level of pollination than is available from the local population of native bees. A “pollination industry” has arisen to supplement this service as beekeepers and commercial bee suppliers sell or lease bees to growers. The European honey bee, our most commonly managed pollinator, is in decline due to pests, pathogens, pesticides, and the recent phenomenon termed colony collapse disorder. The plethora of problems facing the European honey bee may also be affecting these lesser-known native bee species. Monitoring these native bees in backyard habitats will hopefully provide a deeper understanding of these unsung heroes.
