Seed Trade News. October, 1995, Vol. 116, No. 9, p.14.
For over 30 years the alfalfa leafcutter bee has been used to pollinate alfalfa seed in the Pacific Northwest and Canada. In those regions, dormant and semi-dormant varieties of alfalfa are grown predominantly. In California, where non-dormant varieties are grown for seed, the honeybee has been the pollinator of choice. Recently though, growers are increasingly turning to the alfalfa leafcutter bee in California. Growers are buying leafcutter bees because they are rapid and effective pollinators and because honeybees are often high priced or unavailable. This is largely because of the impact of tracheal and Varroa mites on honeybee colonies. With Africanized bees now appearing in southern California, many growers expect honeybee availability to worsen in coming years. In addition, there is the looming question of grower liability for stinging incidents involving these Africanized colonies.
Darrell Silveira, a grower in Mendota, California, has been using leafcutter bees since 1989. He pollinates 1800 to 2000 acres per year using leafcutters. Darrell feels that he gets double the yield using these bees. “The only disadvantage of the leafcutters is management,” said Darrell. He feels that in his area, it is very difficult to successfully pollinate alfalfa seed with honeybees alone. International Pollination Systems, Inc. (I.P.S.), has been working with a number of growers in the San Joaquin Valley. Stephen Peterson, California Manager, explains, “There are certain challenges in managing bees in this environment, but the feedback from the industry has been extremely positive and we are gearing up to meet the demand.” These challenges include working around the growers irrigation and spray schedules, and the higher temperatures seen in the San Joaquin Valley.
Growers of alfalfa seed have found that leafcutter bees can shorten the pollination season by several weeks. This means they can make fewer late season irrigations and pesticide applications. Pollinating early helps growers avoid higher Lygus bug, mite and worm pressure that typically plague the late season in California. This was especially true in 1995, when Lygus bugs were extremely difficult to control late in the season.
For those unfamiliar with leafcutter bee biology, the alfalfa leafcutter bee is a solitary bee that makes its nests in holes. Unlike the honeybee, the leafcutter bee has no queen or division of labor. The leafcutter bee is smaller than the honeybee, about 3/8″ in length. Each bee fends for itself, though they are gregarious. The females use leaf pieces that they cut with their sharp mandibles, to line their nests. The bees collect pollen and nectar and mold it into a doughy ball in which to lay their eggs. While honeybees load pollen onto their hind legs, the leafcutter bees brush the pollen onto the underside of their abdomen where stiff hairs hold the pollen. The leafcutter bee does not make honey, therefore it’s utility is solely as a pollinator. The leafcutter bee typically has only one generation per year, but can have two or three generations in warmer climates, such as in California. The leafcutter bee overwinters as a mature larva inside a cocoon made of leaf pieces and silk spun by the larva.
In the spring, the cocoons need to be incubated in order for adults to emerge at the right time. It takes approximately 21 days at 86°F for adults to emerge. The bees can be incubated in the nesting material where they were deposited, or they can be stripped from their holes and incubated as loose cells. When the bees are ready to emerge from their cells, they are taken to the field and released in shelters placed at regular intervals throughout the field. In California, the most common shelter is a modified cotton trailer. Plywood sides are attached to the trailer and a tarp forms the roof. Nesting material is added to the shelter so that each female is provided at least one hole to nest in. The most common nest material is a styrofoam board 12″ long , 3.75″ deep and 20″ long. Each of these boards has 1500 holes. The holes are 1/4″ in diameter and 3.75″ deep. Wooden boards with drilled holes are also commonly used. The bees are stocked at the rate of 2 gallons (20,000 bees) per acre. Because only about 30% of the bees are female, this equates to 6,000 females per acre. Shelters typically contain 30 to 50 gallons of bees, or enough to pollinate 15 to 25 acres.
The leafcutter bee female has a stinger and will use it to defend herself. However, the leafcutter bee is not aggressive and does not defend it’s home, as the honeybee does. The sting of a leafcutter bee is about half as painful as a honeybees and they do not leave a stinger behind. Unlike honeybees, leafcutter bees do not die after stinging. No protective clothing is needed when working with leafcutter bees and the shelters do not pose any threat to the public, even when placed near homes. Sometimes though, leafcutter bees will cut semi-circles out of rose petals, which may annoy some neighbors.
Mike Rosso, pollination consultant for IPS, has found that after leafcutter bees have been working in a field, more pollen collecting honeybees can be found in the field. Mike feels that once the leafcutters have begun to pollinate a field, the honeybees switch over into a pollen collecting mode, making them more effective pollinators. Honey bees are usually brought into a field in mid May to begin pollinating the early bloom. Then leafcutter bees are brought to the field in late May to early June to intensify the pollination work through mid July. The leafcutter bees can then be removed while the honeybees finish up with the late blooming flowers. Shelters can be moved from field to field as long as all the bees are safely resting in their holes. This means that moving must be done at night or very early in the morning. It also tends to work best when the bees are moved at least 1/4 mile, otherwise the bees may fly back to the original site of the shelter. The adults live for about 6 weeks. Once the flight activity has diminished, the full nest boards may be removed from the fields. The cocoons are then refrigerated over the winter either loose or inside the nesting boards.
Leafcutter bees can also pollinate some vegetable seed crops. Cage tests have shown that the alfalfa leafcutter bee will pollinate onion and carrot seed. Although the honeybee is still the King (or shall we say Queen) of the pollinators, the diminutive alfalfa leafcutter bee is beginning to play a significant role in agriculture today. As more and more leafcutter bee shelters appear each year in California alfalfa seed fields, the future of the leafcutter bee in California looks promising.