In this Issue:


Apple Pollination


Vegetable Insect Update

Time to Watch for European Corn Borer Flights


Grape Berry Moth


Strawberry Sampling Data from MDA and Grower Cooperators

What Fungicide Do I Choose for Disease Control in Strawberries?


Apple Scab Infections

Weekly Trap Counts

Insect, Pest Fact Sheets

Vol 2 No.4   May 30, 2005

Apple Pollination

Marla Spivak, Univ. of Minn., Assoc. Professor & Extension Entomologist - Apiculture, 612-624-4798, spiva001@

Bee Pollination Leads to Better Apples

Pollination is the most critical event in the yearly production cycle of apples. When apple flowers are well-pollinated, they set 6-10 seeds and produce large, symmetrical apples.

The main pollinators of apples are honey bees, Apis mellifera, although other native bees such as orchard-mason bees (Osmia lignaria) also are very effective pollinators. Pollen is the sole source of protein for all bee species, and they are highly adapted to collect and disperse pollen on their foraging trips. Honey bees are considered the most important pollinators of apples because they have large populations that forage over large areas and they are easily transported into orchards for pollination purposes. Honey bees must obtain significant amounts of pollen to sustain the nutritional needs of their large colonies. To do so, they have a remarkable system of communication within the hive which helps them recruit a large number of foragers in a short time. In addition to the protein the bees derive from the pollen, apple blossoms produce abundant nectar, a sugary reward that honey bees store as honey in the hive.

Ideally, a variety of bees should be available for maximum pollination. The number of native bees has been reduced due to the use of pesticides and the destruction of nesting sites by modern agricultural practices. In the last decade, the number of honey bee colonies has diminished due to the introduction of two highly destructive parasitic mites specific to honey bees. Many home gardeners and growers of large commercial crops have noticed the lack of honey bees and have taken an interest in renting or purchasing honey bees and orchard-mason bees to increase pollination.

Honey bees do not fly in the rain or when temperatures are below 50° F. Orchard mason bees will forage in light rain and slightly colder temperatures, providing some pollination when honey bees do not. This year in MN, we had extended periods of rain during apple bloom, which limited the time all bees could fly. Between the cold temperatures, the lack of "bee-flying" weather, and reduced number of honey bee colonies available for pollination, there will probably be a significant reduction in fruit set this year in some orchards.

Pollination Requirements of Apples

Most apple cultivars require multiple visits by bees to set seed. Check seed set by slicing your apple crossways, and counting the fully developed seeds in the 5 seed pockets. An apple can have a maximum of 10 seeds (2 per pocket), and the largest, sweetest, and most symmetrical apples have over 7 seeds.

The king bud is the first flower to open and produces the choicest fruit. If the king bud fails, lateral blooms open a day or two later and also produce fruit. The optimum temperature for pollen tube growth and fertilization occurs between 65-80° F, and growth is slowed or stopped below 55° F.

Some cultivars may be partially “self-fruitful” -- they produce some fruit when pollinated with their own pollen. Most cultivars are “self-unfruitful” -- they produce fruit only when pollinated with pollen by another cultivar called a “pollinizer.” Several decades ago, numerous apple cultivars were grown in the same orchard, and cross-pollination was not a problem. Now growers tend to plant only a few cultivars, and suitable pollinizer varieties have to be interplanted in the orchard. For the best pollination, it is important to interplant cultivars that have approximately the same blooming times and that are equally attractive to bees. The difference in attractiveness is due to the amount of nectar the flowers secrete, which is the flower’s way of attracting and rewarding the bees that visit them.

There are a variety of recommendations for interplanting pollinizers. They range from planting two rows of the pollinizer between four rows of the main variety, to planting a pollinizer every third tree in every third row (one in nine trees). Honey bees tend to forage along a row, rather than across rows, unless the branches intermingle between rows. Bumblebees and orchard mason bees work less methodically and will tend to go between rows and across cultivars more readily. Because honey bees are the main pollinator, interplanting pollinizers within rows is probably the best idea, despite the problems it may present during harvesting.

How Many Bee Colonies?

The most common recommendation is to place 1-2 honey bee colonies per acre of apple orchard. In a home garden, one honey bee colony located within 1/2 mile of the garden should provide enough pollination. There are no set recommendations for how many bumblebee and orchard mason bee nests are needed per acre; however, it is best if they are used as a supplement to honey bee colonies, not as a substitute.

Honey bees should be moved into the orchard when the king blossoms are just opening. If the bees are moved in too soon, they will find other sources of nectar (such as dandelions) and will not work the orchard properly. If they are moved in too late, they will set up too much of the late and less vigorous bloom.

Care must be taken to mow the dandelions under the apple trees to prevent bee poisoning due to pesticide residues which may accumulate on the floor of the orchard.

If a grower wants to rent bee colonies from a commercial beekeeper for pollination, it is important that a pollination agreement be drawn up before the bees are brought into the field. The agreement will specify the rental fee and payment arrangements, where the colonies will be placed in the field, and when the bees should be delivered and removed. When determining the fee, the beekeeper will consider the following things: Can the bees be unloaded with a forklift? Can they be placed in a row along the edge of the field? Will the bees need to be given supplemental feedings? Colonies should be placed within 100 yards of the field, and if possible, at 500 ft intervals. The beekeeper must agree to provide populous colonies, and to demonstrate the strength of the colonies as selected by the grower. Finally, it is very important that the grower not use pesticides on the crop when the flowers are blooming.

Where to get bees?

For large acreage of apples, honey bee colonies can be rented from a beekeeper. Contact the Minnesota Honey Producers Association for names of beekeepers in your area: (763) 689-1065. Or, you could learn to keep bees yourself:



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Co-Editors: Bill Hutchison (, Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (, Department of Entomology, University of Minnesota

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jeanne Ciborowski, 651-297-3217, , MDA, 90 W. Plato Blvd., St. Paul, MN 55107-2094. You can access the Newsletter at the U of MN web site in htm format at: and at the MDA web site in pdf format at:

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