Pollination and Honey Bees

27 April 2005

The following is a chapter by R. D. Fell on pollination, reprinted with permission from Mid-Atlantic Orchard Monitoring Guide, published by NRAES, 152 Riley-Robb Hall, Ithaca, New York 14853-5701. (607) 255-7654.

Good pollination is essential to the production of many fruit crops such as apples, pears, cherries and plums. Both yield and quality of the fruit are dependent upon the intensity of pollination. However, efforts to insure good pollination are often not given sufficient attention, especially during the busy spring season. Obtaining good pollination is not difficult, if a few basic rules and guidelines are followed.

In its simplest sense pollination involves the transfer of the male gamete, pollen, from the anther (part of the male structure of the flower) to the stigma, the receptive female structure of the flower. If the pollen is viable and compatible with the female tissue, it will produce a pollen tube that grows down into the ovary where fertilization of the ovule occurs, leading to the formation of a seed. This transfer of pollen from one part of a flower to another may be accomplished in several different ways, but in most orchard situations the primary agents of transfer are bees, especially honey bees. Good pollination, however, involves more than just the transfer of pollen from one flower to another. Several plant related factors must also be met. If pollination is to be successful: the pollen must be viable, the stigma must be receptive and there must be compatibility between the pollen and the female portions of the flower. If these conditions are not met satisfactorily or if pollen is not transferred between blossoms, little or no fruit will be set. Furthermore, insufficient pollen transfer can lead to poor fertilization of ovules, non-symmetrical fruit, and high rates of fruit drop. Many of these problems can be avoided by placing honey bee colonies in the orchard during the bloom period. The proper use and placement of honey bee colonies will help insure maximum benefits.

Fruit Crops and Varietal Requirements for Pollination

One of the basic requirements for setting fruit is an adequate amount of compatible pollen. With most tree fruit crops, the need for cross pollination is recognized, although considerable variability may exist. Most apple varieties will not set fruit by self-pollination and require cross pollination from another, compatible variety. Plums cultivars, whether European or Japanese, vary from complete self-incompatibility to complete self-compatibility, with some varieties that are cross-incompatible. Pollinating insects are necessary for fruit set on all cultivars, and most cultivars will benefit from cross pollination. Recommendations as to the best pollenizers for different fruits can be found in many nursery catalogs (e.g. Hilltop Orchard and Nursery Catalog). One important point to note, however, is that under general conditions the closer a tree is to a pollenizer, the better the set will be.

Number of Colonies:

The recommended number of hives to use on a per acre basis varies for different crops. This variation is due to a number of factors, including the particular fruit crop, the varieties to be pollinated, the arrangement and size of trees within an orchard, the population of wild bees and the weather conditions likely to be encountered during the bloom period. This variability makes it almost impossible to provide accurate recommendations for all crops and conditions. In most areas of Pennsylvania, West Virginia and Virginia the use of one honey bee colony per two acres of orchard (0.8 hectare) will provide for adequate pollination under most conditions. Exceptions to this general rule may be found in the following situations.

Apples: Pollination problems often occur with `Red Delicious' due to self-incompatibility, short ovule life and flower structure (bees can steal nectar from the side of the blossom without pollinating the flower). One colony per acre or per one and half acres (0.4 - 0.6 hectare) may be necessary for good set.

High density plantings will also increase the need for honey bees. As tree numbers increase over 260 trees per acre (642 per hectare), increase colony numbers to one per acre (0.4 hectare). Higher colony numbers may needed for very high density plantings.

Peaches and Nectarines: Honey bee colonies are rarely required for pollination in peach or nectarine orchards. Most cultivars bloom fairly early in the season when there is little competition from other plants. Peach flowers produce nectar and pollen and are attractive to many pollinating insects. Pollen is also produced when the stigma is receptive and most varieties are self-compatible.

Pears: Pollination can be difficult since the nectar produced by pear blossoms is of low sugar content (10-20%) and most pears are partly to entirely self-incompatible. One to two colonies of honey bees per acre (0.4 hectare) will help insure good crops.

Plums: Honey bee colony requirements will be dependent upon factors such as variety, the arrangement of pollenizers, and tree density. One colony per acre (0.4 hectare) will provide adequate pollination in most situations.

Cherries: For sweet cherries one colony per two acres (0.8 hectare) should provide adequate pollination for most cultivars, provided sufficient cross pollinizers are present in the orchard. For tart cheery varieties one colony per 2-3 acres (0.8-1.2 hectares) should suffice.

Placement of Colonies for Maximum Pollination

The placement of colonies in an orchard is important to maximizing pollination benefits. Colonies should be distributed throughout an orchard in small groups of four to eight hives. Ideally no trees should be more than 100 to 150 yards (91-137 m) from a small group of hives. Hives should be placed in sunny locations that are protected from the wind. They should not be set in low lying areas where moisture or moisture laden air will settle. Hives should also be set on low stands (6-12 inches; 15-30 cm) and not on the ground. The selection of good hive sites and the use of hive stands will increase flight and help to insure foraging activity under marginal weather conditions. Cool temperatures (below 70-75 degrees F; 21-24degrees C), wind and rain will all reduce the flight activity of honey bee colonies. Decreases in both numbers of bees visiting blossoms and the distance from the hive at which bees forage occur with a decrease in temperature. Figure 3 shows the effect of temperature on foraging activity (numbers of bees foraging) and distance from the hive at which bees visited blossoms. These data emphasize the importance of locating hive sites around the orchard for maximum pollination.

Colony Condition for Pollination

Honey bee colonies used for pollination should be 1 1/2 to 2 story hives and have a bee population of 25,000 to 30,000 bees. However, estimating colony strength is difficult, even for many beekeepers. The general recommendation used for evaluating colony strength is based on the amount of brood (developing bees) in a colony. Hives used for pollination should have at least 6 frames with brood and a sufficient number of adult bees to properly care for the brood.

When bees are rented for pollination, specify that all of the colonies meet this basic strength requirement. Also request permission to inspect 10 to 15% of the hives, at your selection, for proper condition. If any questions arise as to colony condition, this provides the opportunity to inspect the hives or have them examined by an experienced beekeeper or other professional. Determining whether a hive meets the basic strength requirement requires that it be opened and inspected. Simply looking at the entrance for bee activity will not provide an accurate estimate of strength. However, the behavior of bees around the entrance can provide an indication of whether the colony is alive and functional or dead or very weak. Look for numbers of returning foragers with pollen loads on their hind legs (yellow or orange colored pollen balls). Foragers with pollen indicate a live, functioning colony that is rearing brood. A dead or weak colony may have activity at the entrance caused by bees attracted to the nest in an attempt to rob honey, but it will not have large numbers of returning pollen foragers.

Estimating Bee Numbers in the Field

One question that frequently arises is how one can determine if he or she has sufficient bees in an orchard for good pollination. Accurate determinations are not really possible, but an estimate can be made by observing bee activity on trees. On a warm, sunny day when the trees are close to full bloom, one should be able to observe a minimum of 8-12 bees working on a tree in a 30 second period. By watching bee activity on several trees and in several areas of the orchard a reasonable average activity can be computed. In making such counts be sure to include trees that are farthest from the hives. If the average number falls below 8 then there may not be enough hives present. Note especially counts on trees farthest from the hives, low numbers of bees on these trees, but not on trees closer to the hives, indicate that the hives are not well distributed throughout the orchard.

The best guide to pollination success is fruit set and fruit quality. Unfortunately, these factors can only be assessed after the bloom period, but an evaluation of pollination success can be made several weeks after bloom and used as a basis for future decisions on pollination needs. Simple estimates of the numbers of developing fruit on trees at different distances from the sites where hives were located will indicate if more hives or better hive distribution is needed in the future. In apple orchards, young fruit can be collected, cut open and examined for seed number. In orchards with good pollination the majority of the fruit will have more than 6 seeds developing.

Figure 3. The effect of temperature on honey bee foraging activity on apple trees. At cooler temperatures fewer bees are observed and most bees forage close to the hives (out to a distance of 6 trees). An increase of as little as 6ยก F in temperature increases bee numbers almost 8 fold and causes a more even distribution at greater distances. (Adapted from: Free, J.B. and Y. Spencer Booth. 1963. The foraging areas of honey bee colonies in fruit orchards. Jour. Hort. Science 38:129-137.)

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