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Maximizing vine crop yield through understanding pollination

Most of the commercially important
vine crops have separate male and female flowers, and thus require
pollinators to transfer the pollen.

Most of the commercially important vine crops have separate male and female flowers, and thus require pollinators to transfer the pollen. To successfully form the fruit, we must understand something about the behavior of the pollinating insect, as well as the biology of the plant. Fortunately, a lot of work has been done in this area in recent years, so our understanding is good.

The most common pollinating insects are honeybees, which can be either wild, nesting in trees, old unused buildings, etc. or raised in commercial hives. Bumblebees are active and important cucurbit crop pollinators as well. Both the above species are generalist pollinators, and will visit the flowers of the full range of cucurbit crops, as well as other flowers in the vicinity of the hive. For crops of the genus Cucurbita, which includes summer and winter squash, pumpkins and ornamental gourds, the specialist pollinator the squash bee has been recognized as important. Although the same size as the honeybee, this species is solitary and nests in tunnels they dig in the ground near or in fields. Both male and female squash bees visit the flowers of squash and pumpkin, and they are often there early in the morning, long before the honeybee becomes active. Squash bees are numerous and widespread throughout the pumpkin growing areas of the Northeast, and appear from our studies to be the main species pollinating these crops.

So if there is such an abundance of pollinating insects for pumpkin and squash, there should not be a need to introduce hives of bees into fields. Some recent research in Illinois indicates that augmenting the natural population of pollinators with beehives can still be profitable. These investigators found in four years of study that pumpkin fields that had supplemental bees during pollination time had increased yields of larger fruits than fields left to native pollinators. Apparently, increased bee visits resulted in fruits with more seeds, and hence increased fruit size.

The ability of the cucurbit plant to produce flowers is also an important factor in determining the success of pollination and fruit set. Again, to use pumpkin as an example, under summer field conditions, in moderate temperatures, the plant produces first male, then female flowers on the main stem. Farther up the plant, female flowers are produced at about every fifth node. If the weather gets hot, and/or the plants are crowded in the field, the frequency of open female flowers can be decreased to such an extent that fruit set and yield are significantly reduced.

The flowering habits of the other vine crops, and their methods of pollination are sufficiently different from that of pumpkins and squash that they need separate explanations below:

Cucumber: Traditional cucumber varieties have a flowering habit similar to that of pumpkin explained above. In the 1960’s plant breeders discovered new lines that produced only female flowers, and no males. These so-called gynoecious lines were used to develop new varieties that have a high frequency of female flowers that produce high early yields. Although most of these field-grown cucumber varieties produce some male flowers, seed companies mix in a small percentage of traditional cucumber to make sure that there will be enough pollen available for adequate fruit set. The European greenhouse cucumber, on the other hand, is all-female in its flowering, producing no male flowers. These plants have another special trait; they set fruit without the need for pollination, so male flowers are not needed in this case.
 
Watermelon: This crop has a flowering pattern similar to pumpkin, and requires pollinators. In recent years, seedless fruits have become popular. For these, pollination is still required, even though no viable seed is formed in the fruit. The pollen must come from a standard variety that is capable of forming seeds. Generally, the recommendation is to plant one row of this pollinator variety for every three or four seedless rows. More recently, seed companies have developed special pollinator lines that can be mixed in with the seedless variety, and whose fruits are easily distinguishable, and kept separate.

Muskmelon: Plants of this species develop male flowers first, followed by perfect flowers. The latter contain both male and female parts, but the pollen is sticky, and still requires pollinators to transfer pollen. As with all these crops, the fact that the first nodes have male flowers implies that the plant is able to grow to adequate size before becoming stressed by the developing fruit. Since we pay particular attention to melon fruits that have adequate size and sugar content, having sufficient leaf area on the plant to produce high quality fruit is of great importance.

H. Chris Wien is a professor and researcher of vegetable crop physiology at Cornell University.  This article originally appeared in the June 2006 issue of Vegetable IPM News.