Pollination of Vine Crops

Agdex#:	256/26
Publication Date:	1987
Order#:	87-043
Last Reviewed:	Not Available
History:
Written by:	Jody Bodnar - Fresh and Ethnic Vegetables Specialist/OMAF

Table of Contents 

1. Introduction
2. Flowering Habits of Vine Crops
   • Watermelon and Cucumber
   • Pumpkin and Squash
   • Muskmelon
3. Timing Bee Introduction
4. Colony Numbers
5. Colony Distribution
6. Pollination Requirements
7. Pesticide Hazards to Bees
8. Pollination

Introduction

Economic success in the production of vine crops (cucurbits) such as cucumber, muskmelon, watermelon, pumpkin, squash and gourd ultimately depends upon large yields of quality fruit. For vine crops, large yields are primarily dependent upon adequate pollination.

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Flowering Habits of Vine Crops

Generally, vine crops produce male and female flowers at different locations on the same plant. The male flowers (staminate) have only male parts and produce pollen whereas the female flowers (pistillate) produce the fruit. Where both staminate and pistillate flowers occur separately on the same plant, the plants are referred to as being monoecious. However, some plants have perfect flowers (also known as complete or bisexual flowers) i.e. each flower contains both male and female parts, and these plants are referred to as being hermaphroditic. Most muskmelon cultivars bear staminate flowers first and bisexual flowers later on the same plant, and are referred to as being andromonoecious.

Cucurbit flowers are usually open for only one day. Male flowers appear first, produce pollen, and drop off. As well, unpollinated female (fruit-producing) flowers abort and drop off. Developing fruits will, however, temporarily inhibit the development of other fruit-producing flowers on the same runner and sometimes on the same vine. A large combined drop of male and female flowers does not necessarily mean a poor crop. Estimated crop yields should be assessed by counting developing fruit and not from the number of blossoms.

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Watermelon and Cucumber

Both watermelon and cucumber are normally monoecious, an exception being that the seedless European (greenhouse) cucumber is parthenogenic-meaning that fruit development occurs without requiring the normal process of pollination (i.e. the transfer of pollen from the male flower part to the female flower part). As well, the triploid or seedless watermelons do not require pollination.

Cucumbers will generally have staminate flowers borne in clusters of 5 at leaf-bearing nodes, with each flower being formed on a slender penduncle or stem. The pistillate flowers are usually borne singly on a stout penduncle at the nodes on the main stem and at the nodes of branches arising from the main stem. As with other cucurbit crops, the pistillate flower is easily recognized by the large ovary at the base of the flower (see Figure 1). In watermelon, the staminate flowers are borne singly in the leaf axils, while the pistillate flowers are formed at every seventh node.

The ratio of staminate to pistillate flowers will occur in varying proportions depending on plant growth, vigor and environmental conditions. Staminate flowers usually appear about 10 days before the pistillate flowers, and usually out-number pistifiate flowers by 10:1; however, this ratio has been reported to be as high as 100:1; in addition, there are seasonal variations to this ratio.

In an effort to increase yields of cucumber crops, allfemale lines (gynoecious plant types) were developed which have no male flowers at all, and of course would not produce any fruit if planted alone.

Figure 1. Female flower (left) and male flower (right) of Hubbard squash. Note ovary at the base of the female flower.

These gynoecious cultivars are pollinated and made productive by introducing seed (5 to l5%) of the normally-occurring monoecious cultivars at the time of planting.

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Pumpkin and Squash

Bush-type pumpkin and vining-type squash bear staminate flowers singly in the axils of leaves near the base of the plant first. Female flowers are also borne singly, and are interspersed with male blooms along the main stem and lateral branches. However, there is a tendency toward femaleness along the main stem of the plant progressing from the base towards the tip.

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Muskmelon

Muskmelon and related melons (e.g. Casaba, Crenshaw, Honey Dew, Persian) produce all-male flowers, as well as bisexual or perfect (hermaphroditic) fruit-producing flowers on the same plant. These melons differ considerably from other vine crops in flower development. The more-popular cultivars produce an abundance of male blooms (clusters of 3 to 5) throughout the plant. Complete or perfect flowers (and fruit) are produced singly only on short lateral branches at the first or second nodes. Even though perfect flowers in muskmelon have both male and female parts they do not pollinate themselves adequately without the aid of bees. This is not to say that the hermaphroditic flowers of muskmelon are not self-fertile, but only that they are not self-fertilizing.

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Timing Bee Introduction

Adequate numbers of honey bees must be provided at the right time to avoid losses from unpollinated flowers. Native populations of bees cannot be depended upon for adequate pollination and maximum yields. The complete dependence on wild bees leaves pollination to chance. Bee colonies should be introduced into the field or on its border as soon as flowers appear. This will ensure that adequate pollination and fruit set occur as soon as female flowers appear and before they abort due to a lack of pollen transfer.

Early pollination and subsequent fruit set will give rise to early harvests and thus premium prices, particularly for the earlier melons. In addition, the muskmelons produced from crown flowers will be sweeter and larger, of uniform size, and may reduce the number of harvest picks by about one-third.

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Colony Numbers

The number of bee colonies needed for adequate vine-crop pollination (assuming strong broods) is influenced by the bees' attraction to surrounding crops and blooming weeds. Cucurbit flowers usually provide a good source of concentrated nectar. However, the low number of flowers per hectare often produces less total nectar than other floral sources. Consequently, honey bees may forage for more-productive sources. For example, a field of muskmelons will only provide about 1% as much nectar as does an alfalfa field of similar size.

It has been suggested that one bee per 100 flowers is needed for adequate pollination. In addition, it is generally necessary that each fruit-producing flower be visited at least 10 to 12 times for adequate pollination (assuming there are viable pollen grains on the visiting bees).

One strong colony per acre, containing 30,000 to 50,000 bees, or three strong colonies per hectare, is a minimum recommendation for all vine crops. To accommodate this number of bees, the hive should have at least 2 brood chambers or stories, and have 6 to 12 full frames of brood in all stages of development.

Increasing the number of hives per hectare to 7 or 8 will result in a shorter blossom-setting period of a week or more. This in turn should give a more-uniform fruit set and crop development to the point of advancing the harvest period by a week or more, and shortening the overall harvest period. Moreover, yields will be increased and the number of pickings reduced by up to one-third.

If in doubt to there being enough bees in a cucumber field (for example), walk into the field on a clear day; there should be at least 30 to 40 bees within a 5-m radius, or a very noticeable hum. If either of these two situations are not apparent, then likely more bees are required.

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Colony Distribution

Honey bees placed at the edges of a field and well distributed will provide twice as many visits to flower blossoms compared to a single apiary clustered at one end of a field. However, colonies located within the field will provide again twice as many visits to the blossoms compared to the same number of colonies located at the edges of a field. Colonies should ideally be shaded during very hot weather. If trees are not convenient to the field, then artificial shade can be provided. In addition, during hot dry periods, it is helpful to provide sources of water for the bees. Wire mesh or sticks located in these containers will prevent the bees from drowning.

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Pollination Requirements

Cucumbers for hand harvest and bush squash (e.g. zucchini) will need bees from early bloom until 3 to 4 days before the last harvest. High-population cucumber plantings for once-over mechanical harvest will require bee activity for only 5 to 7 days following bee introduction, provided good bee-flying weather prevails. For other vine crops, the bees should remain in place until blooming diminishes or sufficient fruit has been set. Generally, this will be about 2 to 4 weeks.

Ideal bee-flying conditions include: temperatures greater than 21°C (70°F), a humidity of less than 75%, wind speeds below 25 km/hr (15 mph), and few or no clouds in the sky.

The most effective time for pollination (particularly for cucumbers) is between 10:00 a.m. and 3:00 p.m. There seems to be no preference between either male or female flowers by the bees, as both flowers contain nectar. However, due to the short-lived nature of vine-crop flowers, and the fact that the stigma of the female flower should ideally receive between several hundred to one-thousand pollen grains, it is essential that enough bees are available to carry out this process. Pistillate flowers should be visited by bees at least 10 to 12 times for satisfactory fruit set. However, the more pollen transferred, the greater the number of seeds produced, and the bigger the fruit.

For muskmelons, a high correlation exists between the number of seeds and fruit size, i.e. the more seeds, the larger the fruit. In addition, increased bee visitation is associated with a greater number of seeds. Muskmelons containing fewer than 400 seeds are usually so small that they are considered culls. At least one viable pollen grain must be deposited on the stigma by a bee, and the subsequent fertilization of an ovule must take place in order that one seed be formed.

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Pesticide Hazards to Bees

To minimize bee poisoning, bees should be placed in the field only at a time when needed. As soon as the crop has set, bees should be removed-usually 3 to 4 weeks. However, once bees are placed in a field, all pesticides toxic to bees should be applied only in late evening or at night during cooler temperatures when the bees are inside the hives. Pesticides non-toxic to bees may be applied during the day when bees are visiting flowers. It is safer to use sprays rather than dust since dust particles can inadvertently be taken back to the hive and fed to the immature bees.

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Pollination

Pollination is the transfer of pollen from the anther to the stigma. The transfer of pollen to any flower on the same plant or clone is self-pollination, (or selfing); the transfer of pollen to a flower on a different plant is cross-pollination. Self-pollination is usually accomplished by gravity or by the actual contact of the shedding anther with the sticky stigmatic surface. In cross-pollination wind and insects are the important agents of pollen transfer. Most plants both self and cross-pollinate naturally to varying extents, depending on functional or structural features of the flower or on genetic incompatibility. Plants are referred to as self-pollinated when the amount of cross-pollination is less than about 4%, and as cross-pollinated when cross-pollination is predominate. The latter is almost entirely true of cucurbit plants.

To know which cucurbit crops can cross-pollinate with one another may be important for saving seed which will produce true-to-type plants the following year. Although there are distinct visual differences between pumpkins, squashes and gourds, they are not important regarding cross-pollination.

All vine crops belong to the Cucurbitaceae family, but only the pumpkins, squashes, and most gourds belong to the genus Cucurbita. However, cross-pollination will depend upon the species in this genus (See page 3). Watermelon and citron both belong to the same genus (Citrullus) and therefore will cross-pollinate each other. Muskmelons and Casaba melons will cross, since they are both in the same genus (Cucumis) and also the same species (melo). Cucumbers belong to the genus Cucumis and thus will not pollinate with pumpkins, squashes, gourds and watermelons, since they are of a different genus. In addition, cucumbers will not cross with other melons of the same genus, since cucumbers belong to a different species known as sativus.

Members of the genus Cucurbita can be further classified for purposes of cross-pollination. Basically, all of the summer squash, winter squash, pumpkins, and gourds belong to one of four species. These are: C. pepo, C. moschata, C. mixta, and C. maxima.

Examples of cultivars within each species are as follows (Table 1):

Summer Squash

Most of the summer squash belong to C. pepo, and the fruit is primarily used in the immature stage while the rind is very soft. Generally, summer squash is comprised to 3 main groups: 1) 'Crooknecks', 2) 'Patty Pan' or 'Bush Scallop' types, and 3) the zucchini types. Both the zucchini types and crooknecks can be further classified as being either green elongated or yellow elongated.

Winter Squash

This group of squash is used mainly when mature with hard rinds. They store well and are suitable for baking. The flesh is generally finer and milder than that of pumpkins.

Pumpkins

Although pumpkins are similar to winter squash botanically, they are classified by their external appearance. The fruit of pumpkins are also used when mature. Their internal flesh is generally considered coarse and strong in flavor, and thus not generally served as a baked vegetable.

Gourds

Gourds are derived primarily from two species of the Cucurbitaceae family known as Lagenaria and Luffa. These are grown for their use either as ornaments or utensils.
Species of Lagenaria include 'Birdhouse', 'Trumpet Gourd' or 'Calabash', which generally require a longer growing season than we can provide. Gourds are not well adapted to cool regions, except for those cultivars from Cucurbita pepo.
The Luffa-type gourds do not cross-pollinate with other gourds and squashes. Luffas are also known as the dish-cloth gourd, Luffa sponge or vegetable sponge.
Within the Cucurbitaceae family, plants of the same species will cross-pollinate. However, there can also be some cross-pollination between selected species. (See Figure 2. below)

Figure 2. Crossing possibilities between code groups (see Table 1). An x indicates the two groups cross readily; 1-mile isolation is a minimum--preferable, 1-1/2 to 2 miles. A diagonal broken line indicates that crosses between the two groups have been reported, but they are of little or no significance for contamination in seed production; in these cases, 1/4-mile isolation is enough. This chart is developed from data in Whitaker, Thomas W., and Glen N. Davis, The Cucurbits (London and New York: Leonard Hill, Interscience Publisher, 1962).

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