Working with beekeepers for optimal pollination for fruit production - Updated for 2009
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.
Note: This was originally printed in the March 18, 2008 Fruit CAT Alert. Updates have been made to reflect current situations.
The decline of honey bees, a national trend
Despite the importance of honey bees, the beekeeping industry has been in decline since two parasitic mites, varroa (Varroa descructor) and tracheal mites (Acarapis woodi), invaded the United States in the 1980s. Varroa mites have nearly wiped out the feral (unmanaged) honey bee population in the United States (Kraus and Page, 1995) and managed honey bee colonies have been declining mainly due to more complicated management because of the mites. For example, in Michigan alone, the total number of honey producing colonies has decreased from 95,000 in 1988 to 65,000 in 2006. This is a loss of almost one-third of managed bee colonies during the last 16 years. About 30,000 of these colonies are “migratory,” whereby beekeepers move their bees to southern states (e.g. Florida, Georgia) to overwinter their colonies, and come back in April for fruit tree pollination. Varroa mites continue to be the major threat to our honey bees. Overwintering mortality seems to be not as bad as last year. At a MSU apiary, we lost about one-third of our colonies compared to two-thirds last year.
Colony collapse disorder (CCD)
The most recent crisis in honey bee population is called CCD (colony collapse disorder), which was reported in the national news during 2007. Colonies affected by CCD show three symptoms. First, bees “disappear” from their colonies with no dead bees inside or near the hive. A colony, seemingly healthy in September will have no bees, or a handful of bees, with a queen, around October/November. The colony will have brood (eggs, larvae and pupae). This suggests adult bees left or died outside the colony quite rapidly. Usually, the colonies are not invaded right away by opportunistic pests: small hive beetles, wax moths, and other honey bees (Bees will take honey from a nearby hive, a behavior called robbing) for two to three weeks. This disorder was large in scale; it has been reported in 33 different states (CCD Working Group, 2007a). The disorder is also severe with large beekeepers (5,000 to 9,000 colonies) losing up to 90 percent of their colonies (CCD Working Group, 2007b).
In Michigan only about 6,000 colonies (out of a total of 65,000 colonies) were reported to be affected in 2006. But another Michigan beekeeper recently reported in February 2008, another loss of 2,300 colonies while pollinating almonds in California. The cause of this disorder is still unknown and honey bee scientists all over the country are studying the problem. There was one paper (October 2007, Science) reporting that Israeli Acute Paralysis Virus (IAPV) was highly correlated with CCD occurrence and the virus probably came to United States through package bees imported from Australia, which began in 2005. A more recent study (December 2007, American Bee Journal) determined that IAPV was in this country as early as 2002, therefore it did not play a major role in causing CCD. Mainly because of this paper, the embargo against Australian package bee importation never went into effect in 2008, to the relief of the almond growers.
In 2009, Australian package bees were also imported for almond pollination in California. Currently, most scientists think it might be a combination of several stresses. Bees are stressed by pesticides applied inside for controlling mites and other pests, or outside for controlling pests on crops and brought back by bees. Other stresses are migratory transportations across several time zones, novel pathogens (A new nosema disease, Nosema ceranae was found to be present in this country for over 10 years, yet we only learned that it was here because of the CCD crisis.) by the varroa mite, which suppresses the immune system of bees, and by the many types of viruses the mite transmits. IAPV is just one of the 20 viruses bees can be infected with; other common viruses include acute bee paralysis virus (ABPV), black queen cell virus (BQCV), chronic bee paralysis virus (CBPV), Kashmir bee virus (KBV), and sacbrood bee virus (SBV). A very recent article in Scientific American (April 2009) gives a more in-depth discussion of CCD.
In light of these problems, growers are urged to work even more closely with beekeepers to ensure a good pollination result. We feel that the following steps can help growers to optimize their fruit and vegetable pollination.
Understand basic honey bee biology and behavior
Understanding some basic bee biology and beekeeping will facilitate your inspection of the hives, gauging of quality/strength of the hives, and help maximize the use of bees for your pollination.
Honey bees are social insects and only the sterile female workers do all the in-hive work (cleaning, drying nectar into honey, feeding young) and outside work (foraging for water, pollen, nectar and propolis, and colony defense). The queen’s only job is to lay about 2,000 eggs per day and release queen mandibular pheromone to let the workers know that she is present and healthy. The males’ (drones) only job is to mate with queens and are produced only during May to August. A typical colony of bees has about 30,000 workers, one queen and a few to hundreds of drones. About a third of these workers are foragers. Foragers show flower constancy so that they tend to focus on flowers of a single species, resulting in more efficient pollination.
Internal factors affecting foraging behavior
To provide adequate pollination, honey bee colonies must be of sufficient strength, free of diseases, and have a laying queen with enough brood. A newly installed package bee colony with two pounds of bees, would have about 9,000-11,000 workers and is considered on the weaker side. Such a colony would concentrate heavily on brood rearing and only have about 1,000-2,000 foragers. Only stronger colonies would send out about 30 percent of bees as foragers. A typical median-strength overwintered colony would have about 30,000 workers and can send out 10,000 foragers. If you are comfortable checking bees, check for the presence of chalkbrood, American foulbrood and varroa mites. In general, three to five frames of solid brood suggest a fertile queen and a healthy colony.
External factors affecting foraging behavior
Environmental factors also affect honey bee foraging. Bees do not work in the rain and work less on cloudy days. Foraging activity is positively related to temperature with a linear relationship from 60-90°F. Bees also slow down when it gets too hot (over 90°F). High winds above 20 mph, will inhibit flying activity. Bees tend to fly lower, near the orchard floor when winds are high. Bumble bees can forage at lower temperatures and lower light conditions.
Finding a beekeeper nearest to you
Zachary Huang has established a database of beekeepers that are willing to provide pollination services with over 420 beekeepers registered. The majority of them are from Michigan. Go to http://cyberbee.net, click beebase on the left, then click #2 “For beekeepers providing pollination services” and search beekeepers by area code, county, zip code or a last name. Once you have a working relationship with a beekeeper, it is best to keep working with the same one year after year.
Pest management during pollination
Do not apply broad-spectrum insecticides when flowers are open. Bee hives should be removed immediately after pollination if post-bloom pesticide applications are planned. By monitoring for pest problems carefully during bloom, growers can help minimize the need for pest control. If an insecticide application is necessary during bloom, the compounds that are least toxic to bees should be used, with careful observation of the pollinator-restrictions on the label. In general, dust form is more harmful to honey bees and morning or day applications are not as safe for bees as evening applications. Inform the beekeeper before a spray so that colonies can be shut down for one to two days with wetted burlap blocking entrances, if highly toxic insecticides have to be sprayed. This database lists the toxicity of various pesticides to honey bees: http://apiculture.com/databases/pesticides.htm.
Different strategies for different crops
Use the “early” strategy for tree fruits. For tree fruit crops, it is advantageous to have bees working the flowers as soon as they open. This provides multiple benefits. It improves the odds that fertilization will occur before the ovules start to lose vigor (this can happen in only three days on some crops). Flowers are more likely to receive the multiple visits needed to deposit enough pollen. In many crops, it is important to pollinate the first flowers (cherry) or “king blossoms” (apple) because they set the best fruits.
Use the “late” strategy for small fruit crops
Generally, flowers of small fruit crops are less attractive to honeybees than other flowers due to flower shape and less nectar, so the opposite strategy is used. Let the crop start to bloom before bringing bees in so that bees tend to forage more on your crop. If brought in too early, bees will learn to forage elsewhere and when crops bloom, they are not attractive enough to get the bees “back” to where you want them. Blueberry flowers have about three days to be pollinated after the flowers open, but you want the bees to stay in the field, so move bees into blueberry fields after five percent bloom, but before 25 percent of full bloom.
The “late” strategy is especially important for cranberries, which are not very attractive to bees. Luckily, cranberry flowers will stay open for a while if not pollinated, and the petals will turn to a rosy color if not pollinated in time. In cranberries, it is better to wait until 10 percent bloom in order to maximize the yield. If you see too many flowers turning rosy, this means you did not have enough pollinators, so make sure you increase the number of bee hives next year.
Hive density recommendations
Because Varroa mites had wiped most of our feral (unmanaged) honey bee populations, recommended rates for pollination prior to 1987 have to be increased to compensate the lack of “free” honey bees. The accompanying table lists recommended rates for hive density. From an economic point of view, it is best to start with the highest number of hives you can afford, then cautiously reduce it the following year to see if your yield is affected. An alternative method is to place different densities of honey bee colonies in separate orchards and determine if there is a difference in yield.
Table 1. Recommended density of honey bee colonies (per acre) for Michigan crops.
|Apple||1-3||Dwarf varieties need more hives.|
|Sweet cherry||1||Balaton may need more.|
|Pear, plum, peach||1|
|Blueberry||3||Cultivars vary in their dependence on pollination.|
- Honey bees as pollinators: http://cyberbee.net/column/pollinator/beepoll.pdf
- Bumble bees as pollinators: http://cyberbee.net/column/pollinator/bumblepoll.pdf
- Pollination and pesticides: http://cyberbee.net/column/pollinator/pesticides.pdf
- CCD Working Group. 2007a. Map of CCD distribution. http://www.ento.psu.edu/MAAREC/pressReleases/CCDMap07FebRev1-.jpg
- CCD Working Group. 2007b. Map of CCD distribution. CCD Frequently Asked Questions (FAQ). http://www.ento.psu.edu/MAAREC/FAQ/FAQCCD.pdf
- McGregor, S. E. 1976. Insect pollination of cultivated crop plants. USDA-ARS,
- Washington, D.C. available on line: http://gears.tucson.ars.ag.gov/book/
- Kraus, B. & R.E. Page, Jr. 1995. Effect of Varroa jacobsoni (Mesostigmata: Varroidae) on feral Apis mellifera (Hymenoptera: Apidae) in California. Environmental Entomology 24: 1473-1480