Male flowers open about 10 days before female flowers and out-number female flowers at least ten to one in ordinary monoecious varieties. These varieties are planted at a rate of 5000-15,000 plants acre (12,350-37 ,000 ha). Harvesting the fruit stimulates the plant to produce more flowers and fruit, so several hand-harvests are possible. If fruit remains unharvested it inhibits the formation of new female flowers and fruit on the same vine.
Gynoecious varieties (with almost exclusively female flowers) were developed in order to provide a rapid succession of female flowers and a highly uniform crop suitable for one destructive, machine harvest. These varieties are grown in dense plantings of about 50,000 to 150,000 plants acre (123,500-370,500 ha).
Figure 37.1. Flower of cucumber, Cucumis sativus.
(a) male, (b) female. (Source: Darrell Rainey.)
All of the major varieties are interfertile. Each stigma should receive several hundred grains of pollen for best fruite set and quality (Seaton et al." 1936). Pollen grains do not necessarily need to be spread evenly on all three lobes of the stigma to form a good fruit, but it does take multiple bee visits to optimize fruit-set. The minimum number of required bee visits per flower is probably variety-dependent and has been recorded as 8-12 (Connor, 1969; Stephen, 1970; Lord, 1985), ³ 18 (Stanghellini et al., 1997), and six (Gingras et al., 1999).
About 30% of the flowers in gynoecious plantings should be male in order to optimize pollination and economic return (Connor and Martin, 1971). This means that gynoecious varieties require a supplemental source of male flowers. One way this is done is by pre-mixing commercial seed with around 10% monoecious seed.
Cucumbers produced from early flowers on young plants often have fewer seeds and inferior shape compared to fruit produced without competition on more mature vines (Connor and Martin, 1970). In unpublished studies these authors found that older vines produce larger female flowers with more ovules. Thus, fruit quality may be improved if pollination could be delayed until vines are more mature. Connor and Martin tested this idea with selected monoecious, gynoecious, and gynoecious hybrid varieties. Delaying pollination (with cages to exclude or include bees) for up to 11 days after the first appearance of female flowers increased the number of fruit per plant and the dollar value per acre. The increase was likely due to stronger vine and root growth in more mature plants.
It is difficult to realize the benefits of delayed pollination under field conditions. The only way to delay pollination in monoecious varieties is to delay importation of honey bee hives, but one cannot practically keep wild bees from visiting the plants. With gynoecious varieties, one can interplant male-bearing monoecious pollenizer varieties a few days after planting the gynoecious variety. That way, male flowers become available to bees only after gynoecious plants have reached a suitable level of maturity. Connor and Martin (1969a,b) recommended alternating a 40-ft (12 m)-wide strip of gynoecious variety with a narrow strip of monoecious pollenizer planted a few days after the gynoecious variety. The strip of monoecious pollenizer should probably be no wider than 3 ft (1 m) (Free, 1993). Delayed pollination in gynoecious varieties is further complicated by the fact that even gynoecious plants produce a few male flowers.
In contrast to the results of Connor and Martin (1970), delaying pollination in high-density plantings of the variety ‘Napoleon’ in Texas did not increase yield (Underwood and Eischen, 1992).
Cucumbers are grown in Europe that set fruit parthenocarpically without pollination. Pollination is undesirable in this context because it results in misshapen, devalued fruit. Growers exclude pollinators with modified glasshouses and government-enforced restrictions on beekeeping (Free, 1993).
Bees collect pollen on cucumber in early morning and switch to nectar later in the morning. Pollen foraging in Maryland, USA is highest before 10 a.m. and decreases dramatically in the afternoon (Tew and Caron. 1988b). Over 80% of honey bee visits in Michigan occur between 9 a.m. and 2 p.m. (Collison, 1976). The stigma is most receptive to pollen in the morning. The first honey bee to visit a flower coats a large portion of the stigma with pollen. and over 40% of flowers receiving one bee visit will set fruit. Although there is less receptive surface on the stigma with each succeeding visit, multiple bee visits per flower (within the range of 1-20) increase fruit-set and average number of seeds per fruit (Collison. 1976).
Large bee populations are especially important in gynoecious plantings where there is an enormous density of female flowers requiring bee visitation.
Cucumber growers are often interested in chemical bee attractants because honey bees are easily distracted away from cucumber to more attractive forages. Unfortunately. the research record for attractants in cucumber is not encouraging. Bee-ScentÒ attractant in North Carolina cucumber did not increase honey bee visitation, yield, or monetary return (Schultheis et al., 1994).
Bumble bees pollinate field cucumber effectively in North Carolina (Stanghellini
et al., 1997). The number of aborted fruit decreases as the number of flower
visits by either bumble bees or honey bees increases. The rate of abortion
is lower with bumble bees when compared at an equal number of bee visits.
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| Increased yield in plots caged with bees > 400% compared to plots without bees |
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| Increased yield in plots caged with bees > 160% compared to plots without bees |
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| Increased yield 37.5% and 47.5% in two fields with supplemental hives compared to fields without supplemental hives |
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| Increased weight of fruit per plant > 340% in glasshouses with bees compared to glasshouses without bees |
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| Increased yield per plant almost 3 times higher in plots with bees compared to plots without bees |
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Atkins et al. 1979. U Calif. leaflet 2253.
Collison 1973. Ms Thesis. Michigan State.
Collison 1976. PhD Thesis. Michigan State.
Connor 1969. Ms Thesis. Michigan State.
Connor & Martin 1970. Jour. Amer. Soc. Hort. Sci. 95:456-458.
Connor & Martin 1971. Hort Science 6:337-339.
Free 1993. Insect Pollination of Crops. 2nd Ed. Academic Press.
Gingras et al. 1999. Jour. Econ. Ent. 92:435-438.
Gustafson 1939. Amer. Jour. Botany 26:135-138.
Jenkins 1942. Proc. Amer. Soc. Hort. Sci. 40:411-412.
Le Masson 1987. Rev. de l’Agric. 40:915-924.
Levin 1986. Using honey bees to Pollinate Crops. USDA leaflet 549.
Lord 1985. Amer. Bee Jour. 125:623-625.
McGregor 1976. Insect Pollination of Cultivated Crops. USDA Agric. Handbook 496.
Schultheis et al. 1994. Hort. Sci. 29:155-158.
Seaton et al. 1936. Mich Agric. Exp. Sta. Spec. Bill 273.
Stanghellini et al. 1997. Amer. Bee Jour. 137:386-391.
Stephan 1970. Amer. Bee Jour. 110:132-133.
Tew & Caron 1988. Fruit crops 1987 (Ohio State Res. Circul.) 295:38-41.
Underwood & Eischen 1992. Amer. Bee Jour. 132:816-817.
Williams 1994. Agric. Zool. Rev. 6:229-257.