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Who pollinates cucurbits?

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Content Author:
Lexi Gauger

Pollination concerns can be a struggle for producers, because sufficient pollination is crucial for getting a good yield. Most of our cucurbit crops have unisexual flowers, meaning that separate male and female flowers exist on each plant. These crops require pollination to occur to produce fruit. For pollination to occur, pollen must be transported from a male flower to a female flower. Male flowers protrude off of long stalks from the plant and contain pollen at their center. Female flowers are attached more closely to the plant and have a swollen bulb at the base of the flower. This is the ovary of the plant, which ultimately becomes the fruit of the plant. The centers of female flowers are also larger compared to the males.

muskmelon flowers
Image 1. Muskmelon flowers from front and side. Photos by Lexi Gauger.
squash flowers
Image 2. Acorn squash flowers from front and side. Photos by Lexi Gauger.

Most cucurbit flowers are only open for a few hours, so time is of the essence in pollination. Cucurbits cannot be pollinated by wind or self-pollinated. While hand-pollination is an option, it is much less labor-intensive to rely on insects to pollinate your field. Bumble bees and squash bees are the most effective pollinators of cucurbits. Many species of bumble bees exist in the wild, but the most common in the eastern United States by far is the Common Eastern Bumble Bee, Bombus impatiens. Hives of this species of bumble bee can also be purchased and added to your fields to increase pollination. Squash bees are native bees in two genera, Peponapis spp. and Xenoglossa spp., that are specifically adapted to pollinate cucurbits. They look similar to honey bees, but are slightly larger in size, less hairy on the abdomen, and have lighter-colored stripes than honey bees. Both bumble bees and squash bees have hairy bodies and frequent cucurbit flowers, making them ideal pollinators of cucurbits.

Image 3. A bumble bee (left) and a squash bee (right) visiting cucurbit flowers. Photos by Lexi Gauger.
bee bee

Honey bees are also good pollinators of cucurbits because they visit flowers frequently and move between flowers quickly. A variety of species of native bees also can pollinate cucurbits. Native bees exist in a wide range of shapes, sizes, and colors, but most are small in size and black or metallic in color.

Image 4. Examples of Kentucky native bees. Photos by Lexi Gauger.
bee bee bee

Bees are the most reliable pollinators in the insect world, but many other types of insects visit cucurbit flowers. Further research is needed to discover if they are actually capable of pollination, but it is definitely possible that these insects could transfer pollen between flowers. Wasps and ants are also frequent visitors of cucurbit flowers. Some types of flies visit flowers, especially hover flies. Beetles can be found crawling in and around flowers, such as soldier beetles. Cucumber beetles are a major pest of cucurbits, but they could also be serving the function of pollination because they are frequently found inside the flowers. Finally, moths and butterflies visit flowers to drink nectar and may transfer pollen on their legs in the process.

bees
Image 5. Pollinators of cucurbits. Infographic by Lexi Gauger.

After the female flower is successfully pollinated, the flower will close and wither, and the ovary will swell and grow into the mature fruit. The fertilized eggs from the ovary become the seeds within the fruit. Lack of pollination in a cucurbit field can lead to fruits not developing fully and being aborted by the plant, or otherwise misshapen and undesirable fruits. Aborted fruits will turn yellow and fall off the plant, and as they rot they can attract bacteria and pests and become a contaminant in the field. Successful pollination will help ensure healthy plants and an abundant yield.

squash
Image 6. An aborted acorn squash vs. a healthy mature fruit, and an aborted young muskmelon vs. a healthy young muskmelon. Photos by Lexi Gauger

In the mesotunnel systems used by the Current Cucurbit project, the netting systems that exclude pests from the crop can also keep out pollinators. We have developed a few options to allow for pollination while still reducing pest pressure, such as opening the ends of the mesotunnel, removing the netting at flowering, or adding bumble bee hives to an enclosed mesotunnel. Read more about these options here. While the vast majority of cucurbit crops require pollination, there are a few exceptions. Parthenocarpic cucumber varieties have female flowers that will develop into a fruit without being pollinated, so no insect pollinators are necessary. Despite the exception of Parthenocarpic varieties, pollination by insects is overall crucial to obtaining a high yield in your cucurbit crops. If utilizing insecticides in your crop fields, minimize harm to pollinators by not spraying during peak bloom, or early morning hours when pollinators are most active. Following the recommended strategies to promote pollination and identifying the various insect pollinators in your field can help ensure that you have adequate pollination and a high yield from your crops.