CROP POLLINATION BY BEES
by K.S. Delaplane & D.F. Mayer

Cranberry

Flowering

Cranberry (Vaccinium macrocarpon) is native to North America. The flower is 1/4 - 1/3 in (0.6-0.8 cm) long and contains 5-8 stamens tightly compacted to form a tube surrounding a single style. The flower hangs downward, and when it opens the four petals bend back to fully expose the sexual column. For the first 2 days the anthers release pollen while the stigma remains dry, unreceptive, and hidden inside the ring of stamens. But the style keeps growing and by the time the flower has shed its pollen the stigma has become exposed, sticky, and receptive. Thus, the flower's behaviour discourages selfing. There are nectaries at the base of the flower and bees probing for nectar at young flowers are dusted with pollen. Cross-pollination occurs when bees visit older flowers that have receptive stigmas. The pollen is heavy and not wind-borne, thus bees are the primary pollinator. Each pollen grain can generate up to four pollen tubes, and each ovary has 24-36 ovules; thus, it takes only a few pollen grains to set a fruit. Petals of newly opened flowers are white or slightly pink. Petals may stay attached for up to three weeks and change to a rosy pink if the flower is not pollinated. A rosy hue to a field indicates inadequate pollination.

For bees, cranberry is a marginal source of pollen and a poor source of nectar. Yet nectar is crucial to the pollination process and without it bees would not visit the older flowers that have atrophied anthers and receptive stigmas. The variety 'Stevens' in New Jersey produces 25-35% more nectar sugar per flower than either 'Ben Lear' or 'Early Black', and the nectar secretion rate of 'Stevens' was not improved by a two-year fertilization programme (Cane and Schiffuauer, 1997). This study suggests that nectar production is at least partly under genetic control and, therefore, a viable character for crannberry breeding programmers.

Figure 36.1 Flower of cranberry, Vaccinium macrocarpon. (Source: Darrell Rainey.)

Cranberry Pollination Requirements

Because pollen release and stigma receptivity are separated in time, insects are necessary in order to move pollen from active anthers to receptive stigmas in different flowers. Wind is not an important pollinator of cranberry. Cranberry typically sets no more than 40-50% of its flowers (Marucci and Moulter, 1977). Although the authors attribute this to 'natural attrition or overproduction of blossoms', they also say that high bee concentrations help plants attain their maximum potential fruit-set. Thus, insect pollination may be a limiting factor in cranberry production and growers should encourage large bee numbers to ensure maximum yields.

Cranberry may benefit from cross-pollination among varieties. Fruit-set is relatively high (~73%) in New Jersey, USA, bogs in which vines of different varieties grow alongside each other, and cross-pollination increased fruit-set, seed number, and fruit size in more controlled tests (Marucci and Moulter. 1977).

Cranberry Pollinators

Many species of bees visit cranberry, but few occur in large numbers. The diversity of bee species is high in cranberry in southeastern Massachusetts, USA, but only honey bees and bumble bees (mainly B. impatiens and B. bimaculatus) occur in large numbers (MacKenzie and Averill, 1995). Bumble bees are numerous in abandoned and natural bogs, whereas honey bees are numerous only in cultivated bogs in which bee hives are imported. Native solitary bees are more abundant and species-rich in abandoned and natural bogs than in cultivated areas -a trend also found by MacKenzie and Winston (1984) in British Columbia, Canada.

Honey bees and the bumble bees B. mixtus, B. occidentalis, and B. sitkensis are the most numerous cranberry pollinators in Washington, USA (Patten et al., 1993). Honey bee visitation on cranberry peaks at 3 p.m.. whereas bumble bees are more constant throughout the day.
About 25 species of insects visit cranberry in cultivated bogs in Ontario, Canada (Kevan et al., 1983). The bumble bees B. terricola and B. vagans are abundant and avidly work cranberry flowers.

The native solitary leafcutting bee Megachile addenda has shown promise as a cranberry pollinator in southern New Jersey (Cane et al., 1996). The females forage for cranberry pollen with which they provision their subterranean nests, and they are efficient pollinators. The daily foraging activity of one female may account for 1291-1440 berries. Commercial management of these bees, however, is limited by high rates of nest parasitization.

Honey bees are not efficient pollinators of cranberry. Bumble bees work the flowers more rapidly and their pollen loads are less contaminated with pollen from other plant species. Bumble bees rarely rob flowers, that is, probe for nectar from the flower base while avoiding contact with the stigma (MacKenzie, 1994). In cultivated bogs in Ontario with imported honey bee hives, the number of honey bees foraging on cranberry was low regardless of the distance from the hives. and few honey bees were foraging on cranberry farther than 220 ft (200 m) from the hives (Kevan et al., 1983). Consequently, fruit yield and number of seeds per flower were unaffected by distance from honey bee hives. Honey bees were attracted to numerous competing nectar sources, and the authors suggest that honey bees may be more effective in very large bogs where densities of native bees and competing nectar sources are comparatively low.

Because bumble bees are good pollinators and occur naturally near bogs, cranberry production and bumble bee conservation are, conceptually at least, a perfect match. There are three ways to go about this -to leave idle land undisturbed for bumble bee nesting sites; to grow supplemental bee pasture plants in field margins; and to provide artificial nest boxes along bog edges (see Chapter 8, pages 67 and 69). Supplemental bee pastures made up of borage (Borago officinalis), bee plant (Phacelia tanacetifolia), catmint (Nepeta mussinli), and anise hyssop (Agastache foeniculum) grown alongside Washington cranberry bogs were attractive to bumble bees, but the long-term benefit of the effort was uncertain (Patten et al., 1993). In general, bumble bee conservation efforts are not measurably success- ful in Washington cranberry (D.F. Mayer, personal observation).

Honey bees are widely used for commercial cranberry pollination in spite of their relative inefficiency. Honey bee hives are easily moved into bogs, thus raising enormously the number of bees foraging on cranberry (MacKenzie and Averill, 1995). Honey bee densities in Washington at the one hive per acre (2.5 ha) rate resulted in 25-43 more barrels of cranberries than did densities at half that rate (D.F. Mayer, unpublished data). Although native bees, especially bumble bees, are good pollinators, they do not always occur in large numbers (MacKenzie and Winston, 1984). Thus, honey bees are the most practical way to ensure cranberry pollination.

Honey bee hives that are established and acclimated to a cranberry bog seem to outperform colonies imported just before the pollination season, unlike the pattern for most other crops. Caucasian honey bees seem to work cranberry better in cool weather than do Italian strains. Bees can set most cranberry flowers in about 4 days of good flight activity. Thus, beekeepers should leave bee hives on bogs for at least 1 week of good weather. It may take 3 weeks of real time in order to accumulate one week's worth of good weather.

**Table 36.1. Recommended bee densities for cranberry.**
No. of honey bee hives/acre (ha)	Reference
0.2-10 (0.5-25)	McGregor (1976)
3-4 (7.4-10)	Levin (1986)
1 (2.5)	Kevan (1988)
2 (5)	Macfarlane et al. (1994)
1 (2.5)	Scott-Dupree et al. (1995)
3 (7.6)	Literature average

Other bees
443 bumble bees/acre (1100 ha)	Hutson (1925)
451 Megachile addenda females/acre (1114 ha)	Cane et al. (1996)

References

Cane et al. 1996. Ann Ent. Soc. Amer. 89:361-367.

Cane & Schiffhauer 1997. Jour. Amer. Soc. Hurt. Sci. 122:665-667.

Kevan et al. 1983. Proc. Ent. Soc. Ontario 114:45-53.

Kevan 1988. Pollination: Crops & Bees. Ont. Min. Agr. & Food Pub. 72.

Levin 1986. Using Honey Bees to Pollinate Crops. USDA leaflet 549.

MacKenzie 1994. Apidologie 25:375-383.

MacKenzie & Averill 1995. Am. Ent. Soc. Amer. 88:334-341.

MacKenzie & Winston 1984. Canad Ent. 116: 965-974.

Macfarlane et al. 1994. Melanderia 50:13-19.

Marucci & Moulter 1977. Actn. Hort. 61:217-222.

McGregor. Insect Pollination of Cultivated Crops. USDA Agri. Handbook 496.

Pattin et al. 1993. Jour. Apic. Res. 32:73-79.

Scott-Dupree et al. 1995. A Guide to Managing Bees for Crop Pollination. CAPA leaflet.

Winston 1987. Biology of Honey Bee. Harvard Univ. Press.