A new pollinator in town

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

In 2005, the Northwest Michigan Horticultural Research Station (NWMHRS) was given a sizable donation of Osmia cornifrons, horn-face bees (HFB). Much of the initial research was conducted on Balaton ® cherry, a Hungarian variety that produces disappointing yields compared with the traditional Montmorency variety, especially in years when cool conditions persist during the bloom period. We hypothesized that fruit set would be improved if more pollen was transferred to the pistil more quickly after flower opening, thus increasing the chances for successful fertilization. Under Michigan’s variable spring conditions, we proposed using HFB to increase fruit set and ultimately help produce more cherries.

Osmia cornifrons is a pollinator that is native to Japan where they pollinate over 80 percent of Japanese apples and are considered as successful as honeybees for pollinating apples. These bees also demonstrate a distinct preference for Rosaceous plants. HFB are solitary and because they are not part of a social colony like honeybees, all females are capable of reproducing and each female must forage for its own offspring. This intensive foraging behavior is necessary to supply provisions for their larvae, and this foraging activity makes them desirable as pollinators in orchard settings; O.cornifrons visit approximately 4,050 flowers per day compared to honeybees that visit only 729 flowers in that same time. HFB adults are active for six to eight weeks, April through June, which are the peak pollinating months. These bees are easy to manage, reproduce without difficulty, and do not sting. They also do not require intensive management year-round because after pollination, the offspring develop in their nesting boxes and adults are not seen outside the colony until the following spring. There is only one generation of HFB per season.

With grower-funded support, we conducted our initial investigation into the effectiveness of HFB in Balaton ®. Preliminary observations suggest that HFB fly at cooler temperatures than honeybees. This species will also fly under slightly cloudier conditions and higher wind speeds than honeybees. Our foraging behavior study showed that HFB forage for a significantly longer amount of time per flower than honeybees, and HFB visit significantly fewer flowers per tree and for fewer flowers per minute than honeybees.

Horn-face bees have been shown to increase apple fruit set in Japan; fruit set by HFB was 82x that of honeybee fruit set. In order to quantify HFB’s pollinating capability in Michigan, nesting buckets were placed in Balaton ® orchards, two-three days before cherry bloom. Each orchard block was divided in half, and one side was stocked with HFB while the other half contained honeybees. Three orchards had significantly higher fruit set with HFB, while the remaining five orchards did not. Balaton ® yields were collected at harvest, and data suggest HFB may have the ability to pollinate as well as or better than honeybees, but only one orchard showed significantly higher yields with HFB. As most of our results are preliminary but promising, further research is warranted and needs to be expanded to other cropping systems. Determining the optimal timing to deploy HFB in orchards is the key to managing the bees and improving yields. Much of the 2005 data suggest that HFB were placed into the orchard too late to be most effective.

Recommendations based on preliminary results in Michigan

Number of bees/acre: Based on work from Batra, we are recommending approximately 250 female HFB for a 1-acre block of cherries and apples. If each 6-inch straw contains eight bees, four to five of those are males and the remainder of the bees is female. Currently, we have no good way of detecting how many bees are actually in the straws, but if there is mud filled out to the end, we can take a stab at six to eight bees per tube. Therefore, depending on how many bees per straw, we can suppose that there needs to be approximately 70 full straws per bucket per acre to meet the recommended rate.

Placement of bees in 1-acre blocks: HFB nesting buckets should be hung in a visible location as observational data suggests these bees use visual cues to locate their nesting sites. At the NWMHRS, we are placing the buckets into apple boxes tipped on their sides, such that the buckets can be hung from the side slats of the wooden apple box. Last year, we wired buckets to the tops of the tipped up apple box with standard wire at the most horizontal angle possible. HFB in the wild use old beetle holes in trees for nesting, and because of the nests position in the wild, horizontal nesting sites are conducive for bee entrance. We want to mimic the wild system, so buckets placed horizontally will work better than buckets placed at an angle. However, make sure the bucket tilts slightly downward to prevent rain from collecting in the bucket. Also, make sure to tie the buckets tight so they do not swing in the wind.

We recommend placing one bucket in the middle row if you do not have many bees. If you have enough bees, I would recommend splitting the 70 full tubes into two buckets, 35 full tubes per bucket, with the remaining tubes with no bees. Buckets should be placed a third of the distance into the orchards from both ends, either north or south or east or west. These bees do not forage as far as honeybees in terms of distance, approximately 200 m, so placing the 250 females (70 full tubes) in the middle of the 1-acre block may be beneficial. Splitting those full tubes into two buckets to try to spread the bees out through the orchard may be even more advantageous.

Timing of placement into orchard: Based on 2005 data and more recent literature, we recommend placing HFB into the orchards 1.5 to 2 weeks prior to bloom. Last year, we recommended putting them into the orchard three to four days before bloom, but we think this timing was much too late for optimal foraging activity (aka. pollination). The literature suggested that they needed a food source or they would “take off.” We did not see this behavior last year as their “homing” behavior was much stronger than their desire to depart to look for a food source. The major behavior we all noticed last year was their need to mate before major foraging activity. Males chew through the mud first, as they are laid toward the opening of the tubes. The males feed a bit, but they are more concerned with emerging females. Emerging and mating took longer than anticipated last spring.

Although the following information will vary based on temperature, males usually emerge within three days of placement into the orchard (as long as temperature is above 55°F), and females will emerge by day 6. Again, they will need a few days to mate and feed. Females must collect pollen and nectar for their offspring, and these females will forage excessively for their brood; hence, the majority of pollinating potential comes from these egg-laying females. In order for females to reach the rigorous foraging stage, they must emerge, find a mate, mate, feed and then she will begin to look for food. This timing is not well known for Michigan orchards, but our current recommendations based on last year’s results are to place the buckets into the orchard 1.5 to 2 weeks before bloom. Again this timing will be dependent on temperature as they emerge based on temperature—the warmer the day, the faster they will emerge and vice versa. Additionally, these bees do not live long, approximately 35 days, so we want to time emergence close to fruit bloom in order for these bees to use fruit pollen for a food source rather than other pollen from other blooming plants.

The need for mud: HFB females use mud to separate the cells of their nest (tube). Therefore, somewhere near the hive, a mud source is needed, especially in a dry year. The recommended mud source is a 1 to 1.5 ft deep trench, approximately 20 yards from the nesting sites; this trench provides bees with a low angle to approach the nest. The soil in the trench should be kept moist, but there should be no standing water. The bees are capable of locating their own source of mud, but in the event of a droughty season, a mud trench is a good idea.

Propagation/new nesting: The only thing needed to propagate more O. cornifrons is a 6- to 8-inch depth hole with a 5/16 inch diameter. There are many methods of creating these “nesting sites:” drilling holes into wooden blocks of wood, pvc pipes, cardboard tubes and potentially many more. However, the key point to remember is that these bees will need enough empty holes to increase the size of the colony, and each female is capable of producing two to four nests each year. Therefore, if we put out 250 females per acre, a minimum of 750 empty nesting tubes are needed.

Another observation from 2005 is that the emerging bees prefer to reestablish the tubes from which they emerged. They will not start creating nests in an empty bucket of tubes unless the “used” bucket is completely full. Therefore, we are recommended setting up your buckets with the minimum number of occupied tubes (about 70 tubes with 250 females) and the remainder of the tubes should be empty. We have used old square “cherry” buckets with wide screens to hold the tubes in the bucket, but any bucket would probably work.

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