Alkali bee

The alkali bee (Nomia melanderi, described by Cockerell in 1906) is a solitary, ground-nesting bee in the family Halictidae that lives in the arid basins of the western United States.^[1][2:1] It is the only ground-nesting bee that has been successfully managed as a crop pollinator, and it has become the most intensely studied ground-nesting solitary bee.^[2:2] Growers in the western United States keep it in purpose-built nesting beds to pollinate alfalfa grown for seed.^[3:1]

The bee is something of a paradox: it nests in hot, dry midsummer yet needs damp, salty soil.^[2:1] In Washington's Touchet Valley its numbers grew to 17 million nesting females in the early 2000s, the largest metapopulation of non-social bees ever recorded.^[3:1]

Description and life cycle

The alkali bee is about 7/16 inch (11 millimeters) long, dark in color with iridescent bands.^[4:1] It has one generation a year. Eggs hatch within three days, the larvae stay dormant for up to ten months (typically through the winter), and adult females are active for about a month.^[4:1]

Although each female nests on her own, nests cluster in dense aggregations. The largest known aggregation, on a 1.5-hectare site in the Touchet Valley, held 5.3 million nesting females at densities of up to 1,000 nests per square meter.^[3:1]

Nesting habitat

In nature the species uses crusted, salty soils of alkali flats. Artificial beds built for it need silty loam with no more than seven percent clay, soil moisture held between 8 and 35 percent, irrigation through standpipes, and surface salt applied at about 1 pound per square foot.^[4:2] Nests lie in roughly the upper 8 inches (20 centimeters) of the soil.^[4:2] Building a bed that works requires understanding how water moves through silty soil and how that interacts with particular mineral salts.^[2:1]

Some isolated populations survive in cold climates by nesting amid hot springs.^[2:3]

Foraging and pollination

Alkali bees are polylectic, meaning they collect pollen from many kinds of plants, and they forage over long distances. Their main wild floral hosts are either non-native or poorly known.^[2:4] Females visit about 12 alfalfa flowers a minute.^[4:1]

Because they range widely, alkali bees can stray into neighboring crops treated with insecticides.^[2:4] Growers' management therefore includes bee-safe pesticide protocols.^[3:2]

Management

In Washington's Touchet Valley, a census of all 76 past and present nesting beds found that, from 1999 to 2006, populations grew ninefold.^[3:1] Practices credited with sustaining them include subirrigation of nest sites, surface salting, weed management, adequate alfalfa bloom, and bee-safe pesticide use. Several sites kept robust populations for about 50 years.^[3:2]

Growers can estimate a bed's population without digging it up. In a study of 13 beds in the Walla Walla Valley, counts of emergence holes in 0.5 m² quadrats in July correlated strongly with the number of overwintering prepupae counted in soil cores in October (r² = 0.73 in 2010 and 0.94 in 2011). The nondestructive count lets growers decide whether to add supplementary pollinators such as the alfalfa leafcutting bee (Megachile rotundata).^[5:1]

The alkali bee is the second most valuable managed solitary bee for US agriculture, and perhaps for the world.^[2:3]

Parasites and associated species

Unlike most ground-nesting bees, the alkali bee is not plagued by kleptoparasitic bees (species that lay eggs in another bee's nest). It does host other parasitoids.^[2:3] The main one in the northwestern United States is the bee fly Heterostylum robustum, also called the bomber fly. It lays eggs near host nesting sites without distinguishing burrows from depressions in the soil. The first-stage larva finds a host larval cell and waits there until the host larva is full grown before feeding on it.^[6:1] A Pacific Northwest pest-management handbook says the flies typically destroy 20–40% of alkali bee larvae before winter. Its most effective control is walking the beds between 9 am and noon for two to three weeks, starting before the bees emerge, to kill newly hatched flies that cannot yet fly.^[7:1]

Managed beds also shelter other native bees. Another ground-nesting bee, Calliopsis personata, has been found nesting in alkali bee beds in eastern Washington. The one foraging female examined carried mainly willow pollen, which suggests it adds little to alfalfa pollination.^[1:1]

Genetics

A draft genome of the alkali bee is 299.6 million base pairs long and has 10,847 predicted protein-coding genes. About 37.5% of it is repetitive DNA.^[8:1] Females sampled from nesting aggregations in and around Touchet, Washington, showed no evidence of population structure, and the analysis indicated that the effective population size had declined within the last 10,000 years.^[8:2]