Pumpkins and Squash: What Are Their Pollination Needs?

All species and varieties of pumpkin are highly dependent on insect pollination because these plants have male (anthers and pollen) and female (stigma and ovules) structures located in different flowers (Figure 1). This means that without bees visiting these flowers, there is no transfer of pollen from the anthers to the stigmas, and therefore no fruits are produced. Here is information about the pollination needs of these essential seasonal crops as colder weather approaches.

In Pennsylvania, Cucurbita crops are visited by at least 38 species of bees. However, just a few of them account for over 95% of flower visits. These key pollinator species of Cucurbita crops in Pennsylvania include honey bees (Apis mellifera), bumble bees (at least 6 species of the genus Bombus), and specialist squash bees (Eucera pruinosa). While bumble bees can be easily identified by their larger body size and black and yellow color patterns on their abdomens, squash bees and honey bees are similar in size and coloration (Figure 2). Still, these bees can be easily differentiated in the field by observing their flight patterns. Squash bees are fast fliers, and their females actively collect pollen from Cucurbita flowers. In contrast, honey bees are slow and are only seen drinking nectar from flowers.

Figure 2. Left—honey bees, photo: Katy Evans. Center—squash bees, photo: Laura Jones. Right—bumble bees, photo: Margarita Lopez-Uribe.  

Cucurbita pollen is large, spiky, and heavy, so it is generally difficult to collect and digest by most bees. For example, bumble bees and honey bees do not actively collect pollen from these flowers because it has negative effects on their colonies. However, these generalist pollinators move pollen between flowers as they pick it up when drinking nectar. Among the three groups of key bee pollinators of pumpkin in Pennsylvania, bumble bees are the most effective pollinators on a per-visit basis compared to squash bees and honey bees (Figure 3). Optimal pollination is achieved after only two bumble bee floral visits, while five visits of a squash bee or a honey bee are required to achieve optimal yield.

Figure 3. Image from Biology and Pollination Services of the Squash Bee.

Generally, Cucurbita crops do not experience pollination problems in Pennsylvania because wild bee populations (both squash bees and bumble bees) are large and provide optimal pollination services. Still, pollination problems can occur and can be identified by low fruit set, small fruit with fewer seeds, and/or misshaped fruit. These pollination problems are often the result of low pollinator activity on crop flowers due to:

1. Low abundance of bee pollinators
2. Pollinators preferring alternative floral resources adjacent to the crop
3. Poor weather, such as rain, high winds, or lack of sun that limits pollinator activity during the time when flowers are open, generally from dawn to midday.

Other sources of poor pollination include low pollen viability due to high temperatures and low production of female flowers per plant.

Pollination services can be improved using enhancements to boost wild bee populations, introducing managed pollinators (e.g., honey bees, bumble bees), minimizing pesticide use, and reducing deep tillage. Here are some suggestions to protect pollinators in general and to enhance wild bee populations:

Enhance squash bee populations:  Squash bees nest underground at relatively shallow depths. To enhance squash bee populations, it is recommended to leave a margin of bare or disturbed ground around fields to ensure space for nesting bees. Disturb hard-packed soil to make it easier for bees to dig through, or add sand clay-heavy soil to provide promising nesting sites. Consider no-till systems or shallow tilling, particularly around nesting aggregations.

Enhance bumble bee populations:  Bumble bees nest in cavities underground in adjacent natural areas around the crops. Supporting natural habitats (such as native forests or grassland) around cucurbit crops helps support a higher density of nests and foragers. To support bumble bee colonies beyond the Cucurbita bloom period, consider integrating other plants near fields that bloom both before (e.g., penstemon, coreopsis) and after (e.g., goldenrod) to provide these generalist wild bees with sustained resources throughout their active season. Bumble bees can also be purchased for pollination services through companies like Biobest and Koppert. It is generally recommended to have one colony per acre (the cost of one colony is approximately $300). Keep in mind that purchased bumble bee colonies do not overwinter and can only provide pollination services for one season.

Introduce managed honey bees:  While it is common practice to supplement pollination with commercial honey bee colonies when pollination problems are observed, honey bees may not significantly increase Cucurbita crop yields because they prefer to forage on alternative floral resources. While honey bees collect nectar from flowers, they are unable to use Cucurbita pollen in the colony. Therefore, honey bee colonies often leave Cucurbita pollination contracts in a weakened state, which can increase their risk of overwinter mortality. Additionally, introducing managed bees for pollination can facilitate the transmission of pathogens from managed to wild bees. Therefore, limiting the introduction of managed pollinators is recommended when possible. Some additional management practices that can be considered to decrease pathogen transmission include: renting honey bee colonies that have low levels of Varroa mites and placing managed colonies in the center of the plantations where wild bee abundance is lower than at the edge of the fields.

Choose pesticide applications wisely:  Pesticide applications should be minimized because they can kill bees or reduce their ability to forage and perform basic functions. Because the bloom period of pumpkins and squash is long, it is recommended to limit neonicotinoids to treated seeds only (residues in pollen and nectar from treated seeds are 10–100 times lower than from soil-drench, transplant-drip, or foliar spray applications). If fungicide applications are necessary during bloom, choose effective products with the lowest toxicity ratings possible, as indicated on the pesticide label. Keep in mind that flowering weeds around crops can be important routes of pesticide exposure for bees. Removing blooming weeds such as dandelion or white clover is recommended before pesticide sprays rather than afterward.

For more information about pumpkin and squash pollination, check these other resources:

Biology and Pollination Services of the Squash Bee
Brochu KK, Fleischer SJ, and López-Uribe MM. 2021. Biology of the squash bee, Eucera (Peponapis) pruinosa. Penn State Extension.

McGrady CM, Troyer R, Fleischer SJ. Wild bee visitation rates exceed pollination thresholds in commercial Cucurbita agroecosystems. Journal of economic entomology. 2020 Apr 6;113(2):562-74.

Ullmann, K., Cane, J., Fleischer S., Treanore, E., McGrady, C. 2017. Integrated crop pollination for squashes, pumpkins, and gourds. Project Integrated Crop Pollination. ProjectICP.org

En Español: Polinización Integrada de Cultivos de Calabazas

#BeesofPA: What is active now?

Pollinator activity winds down as temperatures get colder. However, you may still see some bee species flying around during warm days in the month of October (Figure 4). For example, the green sweat bees Augochlora pura and the small carpenter bee Ceratina calcarata. Additionally, bumble bee (e.g., Bombus impatiens) queens may be seen flying out as they are looking for nectar sources before finding overwintering sites. Honey bee (Apis mellifera) foragers may also be out foraging when temperatures are above 55°F.

Figure 4.