Alfalfa Bees Prove Their Carrot Competence

LOGAN, Utah, Aug. 20--The domesticated honeybee may get more glory, but when it comes to pollinating carrots, one tiny alfalfa leafcutter bee can do the job of 20 of its larger, noisier, more irritable cousins, says a U.S. Department of Agriculture researcher.

Alfalfa leafcutter bees are about a quarter-inch-long, and black with bands of white hair on the abdomen. “They prefer to spend spring and summer days gathering nectar and pollen from the purple blooms of their favorite plant, alfalfa,” said entomologist Vincent J. Tepedino of USDA’s Agricultural Research Service.

Tepedino and colleagues at the Logan laboratory investigate native bees that can supplement or replace domesticated honeybees in fertilizing crops that depend upon insect pollinators.

Tepedino's study was the first detailed comparison of the domesticated honeybee and alfalfa leafcutter bee's performance as carrot pollinators in screened enclosures. It also was the first to show that alfalfa leafcutter bee offspring are well nourished by carrot pollen and carrot nectar. This bee typically grows up on rations of alfalfa pollen and alfalfa nectar.

"Unlike domesticated honeybees, alfalfa leafcutter bees don't mind working in screened enclosures or greenhouses needed for producing new kinds of carrots,” said Tepedino, based at the ARS Bee Biology and Systematics Laboratory here. “About 150 alfalfa leafcutter bees working in screenhouses or greenhouses would do the job as well as 3,000 domesticated honeybees.”

The pollination job is an essential but underappreciated part of a plant breeder’s task of producing the new, more colorful or more nutritious carrot varieties that growers and consumers want. Tepedino said his study shows the leafcutter bee’s efficiency may reduce pollination costs, as well as ease the stress on breeders and technicians who might otherwise work in cages with several thousand honeybees.

"Domesticated honeybees can get irritable when large numbers of them are confined in small spaces,” said Tepedino. “That intimidates people. Alfalfa leafcutter bees are gentler. And they tend not to buzz around your face or land on you like the meat flies some plant breeders now use."

Alfalfa leafcutter bees are already raised and sold commercially to pollinate not only alfalfa but also carrots and onions in screened cages, Tepedino noted.

To produce new carrot varieties, pollen must be ferried from one variety of parent carrot plant to another, a process known as hybridization. That means carrot breeders must enclose the carrot plants--and the pollinating insects--in screenhouses or greenhouses to keep out other, unwanted pollen.

Tepedino ran his experiment with both species of bees using six screened cages, each containing 70 parent carrot plants. He found that yield of live seed from plants pollinated by either insect was approximately the same.

Tepedino used 9,000 adult honeybees and 450 adult alfalfa leafcutter bees for the study. He did the work with help from Asgrow Seed Company at Twin Falls, Idaho.

Alfalfa leafcutter bees are among the species scientists call "solitary bees." These do not live in a communal nest or hive as do the familiar honeybee. Instead, as the insect’s name suggests, the female alfalfa leafcutter cuts alfalfa leaves and uses them to build a nest of snug, cell-like compartments to house offspring.

Solitary bees, Tepedino said, aren't attacked by two kinds of mites that have decimated many commercial honeybee colonies in this country in recent years. Also, the solitary bees can’t mate with Africanized honeybees. So solitary bees--again unlike domestic honeybees--can’t pick up the Africanized bees' trait of extreme defensiveness and increased likelihood of stinging people and animals.

Scientific contact: Vincent J. Tepedino, ARS Bee Biology and Systematics Laboratory, Logan, Utah 84322-5310. Telephone: (801) 797-2559, fax 797-0461, andrena@cc.usu.edu