Member Reports: Mason Bees (Osmia lignaria)

 Certified Master Naturalist, Ed Coleman, wrote an interesting article to share on the native Mason Bee (Osmia lignaria).  

Warm days, cool nights and a rich deep soil nurtured by consistent rainfall makes Virginia's climate ideal for growing many apple varieties, according to the Virginia Department of Agriculture. The department ranks Virginia 5th in the nation for apple production with an estimated 225 million pounds harvested in 2017, contributing an estimated $235 million annually to the state’s economy.  Virginia has over 10,000 acres in apple tree cultivation spread across 100 commercial orchards primarily in the Shenandoah Valley and counties on the eastern slopes of the Blue Ridge mountains. Apple trees are angiosperms whose ancestors co-evolved with animals during the Cretaceous period and depend on insects for pollination and birds and mammals for seed dispersal.  A fully pollinated apple will contain 10 seeds; however, a minimum of 6 to 7 seeds per apple will produce good fruit. Apples that do not receive adequate pollination may become malformed as they develop or drop early.  Apple blossoms have both male and female anatomy.  However, to ensure genetic diversity necessary for adaptation to varied climates, soils, pests, and diseases, pollen from the same apple variety is prevented from reaching the ovule in non-self-fertile varieties.  Insects must transfer pollen from the anthers of one variety to the stigmas of another variety for pollen to proceed down the floral tube to the ovaries. Once fertilization takes place, the apple seed and fruitlet began to grow. In Virginia, honeybees account for about 25-30% of apple blossom pollination, leaving the rest for native bees and other insects.  As the honeybee populations decline due to Varroa mites, Acarapis woodi mites, and other contributing factors to colony collapse, there is one little native bee that is up for the challenge to become the preferred pollinator for Virginia’s backyard and commercial orchardists.   Smaller than a honeybee or about the same size as a housefly, the metallic blue-green, black, mason bee (Osmia lignaria) gets its name from its nest-building habit of sealing the cells where it lays its eggs with mud, like mortar. 

The average blossoming cycle for an apple tree when pollination can take place is about nine days. Cool weather will extend bloom time, whereas warmer weather will shorten.  Fortunately for Virginia’s apple growers, mason bees are early springtime pollinators, emerging when the temperature is between 50-55 degrees Fahrenheit, around the same time as apple trees bloom.   Honeybees prefer 10 degrees warmer and will usually not work in the rain either. A single mason bee can visit 20,000 blossoms per day.  For apples, 250 female mason bees per acre (2-10 bees per tree) is sufficient for adequate pollination depending on tree size and planting density.  By comparison, it takes a minimum of one small hive or approximately 20,000 to 30,000 honeybees to pollinate an acre of apple blossoms.  The mason bee is more efficient at pollinating apple blossoms because of its erratic flight patterns.  They fly back and forth between trees providing good cross-pollination, whereas honeybees generally move systematically from blossom to blossom within a single tree before moving on.   Instead of pollen baskets like honeybees, mason bees have hairy abdomens with specialized hairs, called “scopa" for collecting pollen. As they belly-flop into the center of a blossom with their pollen-packed hairy abdomens, mason bees deposit significantly more pollen than honeybees. 

Because mason bees are unable to excavate nesting cavities like carpenter bees, they must find tunnels in which to lay their eggs. In the wild, mason bees lay their eggs in small natural tunnels such as woodpecker holes, insect holes, and hollow plant stems, but can easily adapt to artificial chambers. Once a nest is selected, mason bees usually do not travel more than 300 yards in search of pollen, collecting from the closest source regardless of plant species.  Females remove pollen from their hairy bodies to regurgitate it into a sticky pollen and nectar mass which they place at the far end of their nesting tunnel until they store enough food to support a single larva.  After laying an egg atop of the mass of pollen and nectar, the female seals the individual cell with mud and repeats this process until the entire tunnel is filled.  Once she has filled one tunnel, the female will begin filling another one. During her 4-6-week lifespan, a female lays approximately 34 eggs, after which she dies. Larvae begin to hatch within a few days after the egg is laid, consuming the pollen and nectar stored within their cell.  In about ten days, the larva spins a cocoon and pupates, where it remains throughout the winter, finally emerging in the spring. Eggs destine to become female are deposited at the back of the tunnel with males up front.  Males emerge first and wait for their future mates to emerge and the life cycle repeats.

If mason bees are 100 times more efficient at pollinating apple blossoms than honeybees, how can apple growers attract this exceptional pollinator? Rather than importing honeybee hives into the orchard each spring, orchardists can construct sustainable mason bee ecosystems inside their own orchards for less than the cost of an annual import.  Since mason bees will only travel about 300 yards in search of pollen, an entire community can be localized to a given orchard.   Once a mason bee population is established, orchardists can help perpetuate successive generations of mason bees by following the three P’s of pollinator stewardship – Prepare, Provide, and Protect. 

Prepare for the next generation of mason bees by creating nesting communities.  Holes drilled in wood blocks, cardboard tubes, and hollow plant stems like reeds and small bamboo, are suitable choices for these tunnel nesters.  Artificial tunnels for mason bees should be at least 6 inches deep and 5/16 inches in diameter.  A female mason bee will lay up to 8 eggs in a 6-inch tube.  Although solitary, mason bees will nest in close proximity to one another.  This allows for efficient use of building materials for orchardists as the outer container or mason bee house can be constructed to hold numerous tubes. For example, a simple box container 4 inches wide, 4 inches high, and 6 inches deep can hold 100 tubes. Untreated pine is the most common material used for the outer container of a mason bee house, although cedar, cypress, or PVC may prove more durable when exposed to Virginia’s weather.  For those not gifted with wood working skills, there are many commercially options available for mason bee houses and for tubes as well.   The best location to mount a mason bee house is 6-7 feet above the ground, preferably under the eave of a garage or garden shed for protection from weather.  Mason bees require mud to seal their chambers, generally with a clay mixture. For those with sandy or loam soils, clay/mud mixtures can also be purchased, but must be kept moist.

 

Mason bee (Osmia lignaria) Photo credit: Wikimedia Commons

 

Provide adequate forage throughout the mason bee’s lifecycle. Early blooming fruit trees such as apples, pears, and cherries are great sources.  Orchards planted with early, mid, and late season blooming fruit tree varieties can extend the overall bloom season upwards of a month.   When it comes to attracting bees and providing forage, think purple, blue, and yellow flowers.  Native trees that attract early emerging mason bees include Eastern Red Bud, Red Maple followed later by Tulip Poplar, Black Locust, and Sourwood. Early spring flowering plants such as dandelions with their bright yellow flowers attract bees with little to no effort from the landowner, while growing creasy greens (Barbarea verna), beneath fruit trees is a tried-and-true old Appalachian practice of attracting bees to the orchard as both trees and greens generally bloom around the same time.  Consideration should be given to available forage when determining placement of a mason bee house.  Since mason bees collect pollen from the closest source, placement of the artificial nest site can significantly impact the female’s productivity.

 

 

 Mason bee house at Horne Creek Historical Farm.  Note untreated wood blocks 5/16” diameter 6” long, protected from weather by metal roof, protected from predators with poultry wire, south facing

Protect comes with its own three P’s as well – pesticide, predators, and parasites.  Protecting mason bees is essential to sustaining the population.   Pesticide dusts should be avoided during the mason bee nesting cycle as dust can be easily picked up on the bee’s hairy bodies, just like pollen.  Care should be taken to minimize pesticide drift by spraying when the wind is calm or when bees are not active such as early morning, late evenings, or overcast days.   Birds are the primary predators for mason bees, therefore securing poultry wire to the entrance of the mason bee house is an effective solution to protect the larvae.  While solitary bees generally have less problems with parasites than social bees like the honeybee, parasitic wasp such as Chalcid wasps will attack the mason bee larval stage.  Pollen mites, while not directly attacking, starve the developing larvae by consuming their food source.   Disposable tubes are better choice for nest material as these prevent the spread of disease and parasites since they are only used once. Removing tubes from the mason bee house in late October and storing them inside a cool area between 30-45 degrees Fahrenheit until early spring is also a way to protect larvae from both predicators and parasites.   However, removing the tubes too early in the season can dislodge the developing larva from its food source, causing it to die.

 

Given that mason bees are more effective at pollination and its populations are more easily and economically sustained than the honeybees, should Virginia’s apple growers simply abandon the honeybee and just rely on the mason bee for fruit tree pollination?  Surprisingly, the answer is no.  Orchardists need not repeat the mistakes of the past by relying too heavily upon a single species for pollination.  A diverse set of pollinators, including honeybees, provides the best chance for success.  While mason bees will clearly play a more prominent role in the Virginia’s orchards moving forward due to the decline in honeybees, there are other native bees such as mining bees and sweat bees that deserve recognition for their contribution to Virginia’s apple industry as well.  Like Mason bees and honeybees, these bees and other fruit tree pollinators would benefit greatly from the skillful application of the three P’s.

 

Mason bee tower with capacity to hold 2,000 6-inch paper tubes (12,000 – 16,000 larvae); Dimensions: 48” high, 9” deep, 8” wide; Construction materials under $100 (two 1” x 8” x 8’ cedar boards, one 4” x 4” x 10’ treated post,  exterior screws, vinyl protective mesh

 

About the Author:   Ed Coleman is a Virginia Master Naturalist in the Southwestern Piedmont Chapter and owns and operates an apple orchard in Floyd County VA