Pollination of Pumpkin and Winter Squash - Thanks to Bumble Bees! Dr. Kimberly Stoner Connecticut Agricultural Experiment Station New Haven

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1 Pollination of Pumpkin and Winter Squash - Thanks to Bumble Bees! Dr. Kimberly Stoner Connecticut Agricultural Experiment Station New Haven

2 Basics of Pumpkin and Squash Flowering and Pollination Separate male and female flowers About 3 weeks of female bloom Ratio of male to female flowers changes over the flowering period Each flower open for only one morning Most pollination happens early in the morning some studies say by 8 am! With good bee activity, pollen is gone from male flowers before10 am

3 Pollination and Fruit Set Some cultivars of summer squash can set fruit without pollination. Pumpkin and winter squash cannot. First fruit dominance after the first fruit is set, setting of additional fruit depends on plant resources Pollen deposition required for fruit set for pumpkins, in the range of 1250 to 2000 pollen grains per stigma, roughly 8-12 bee visits

4 Cast of Characters Honey Bees, Apis mellifera Bumble Bees, mainly Bombus impatiens Squash Bees, Peponapis pruinosa

5 Bees on Pumpkins or Winter Squash in CT Common name Scientific name % of visits to all flowers % of visits to female flowers % of visits to male flowers Common Eastern Bumble Bee Squash Bee Honey Bee Bombus impatiens Peponapis pruinosa Apis mellifera 66% 64% 68% 22% 16% 23% 11% 21% 9%

6 Honey Bees From BeeInformed.org Mostly seeking nectar in bottom of flower Often walk down the petals, avoiding stigma Have been shown to increase yield in some experiments, but not in recent study at Cornell Advantage sheer numbers at time and place of pollination

maximum size and producing queens in August Can be purchased Questions

7 Bumble Bees Common Eastern Bumble Bee, Bombus impatiens Most abundant visitor overall Numbers build over growing season - colonies at maximum size and producing queens in August Can be purchased Questions about health of commercial bumble bees Alejandro Chiriboga Michael Thomas, CAES

8 Adapted from: Bumble Bee Life Cycle In the early stages, the queen takes care of all nest duties As the colony grows, the workers take over Mated queens emerge And look for nest Site (Spring) At the end of the colony cycle, males and queens are produced

larvae only pollen from Cucurbita (pumpkins and squash) One

9 Squash Bees Ground-nesting solitary bees nest in cultivated fields and in surrounding grassy areas and crops Feed their larvae only pollen from Cucurbita (pumpkins and squash) One generation per year emerge in early to mid- July, numbers decrease in August

10 Life Cycle of Solitary Bees

11 Squash bee nest Kristen Bochu, Cornell

12 Squash bee nest aggregation Kristen Bochu, Cornell

13 Experiment Supplemental Mark all female flowers in the plot. 1/3 with natural pollination and stigmas removed to measure pollen deposition 1/3 with natural pollination, followed to fruit set and harvest 1/3 with supplemental pollination, followed to fruit set and harvest Pollination

14 Experiment Supplemental Pollination Carried out 7 times over 3 years on 3 experimental farms (13 total mornings) No significant differences in fruit set, fruit survival to harvest, fruit weight, seed number, or seed weight For all 13 mornings, average pollen count was over 2000 grains per stigma, ranging from 2,326 to 18,080

15 How do bee counts relate to pollen count on the stigmas? Once each hour, worker counted bees on 100 flowers, recording bee species and sex of flower At the end of the morning, workers collected stigmas from female flowers, and we analyzed for pollen in the lab

16 How do bee counts relate to pollen count on the stigmas? From 2012 to 2015, we have done this 80 times (80 mornings) on 20 pumpkin and/or winter squash fields in CT Only once did we find a pollen count below 2000 pollen grains per stigma (Griswold, 2012)! To our surprise, total bee count did not have a significant relationship to pollen deposition on the stigma So, we looked in more detail at the data.

17 Bumble Bees per Female flower vs. Pollen deposition R 2 = 0.21, p = Significant, explaining about 20% of variance Only sample with less than 2000 pollen grains per stigma

18 More research in progress After seeing this relationship, we focused much more on bee behavior on the flowers, particularly on female flowers We have 2 years of videos of female flowers, with observations of bee behavior, and stigmas harvested for pollen counts We have 1 year of videos of male flowers and stigmas and synandria harvested over the morning to measure pollen disappearance from male flowers and appearance on the stigmas of female flowers

19 Squash bee scraping pollen with mandibles

20 Take Home Messages So Far On any given sunny morning in CT, pollination is likely to be more than adequate for fruit set in pumpkins & winter squash Pollination happens early in the morning. You will still see bees in the field late in the morning, even in the afternoon, but pollination is over. Bee visits especially to male flowers may just be collecting food, not pollinating

21 Acknowledgements Funding from USDA-NIFA-SCRI grant # as the Specialty Crops Research Initiative, Pollination Security for the Northeast and Hatch grants to the CT Agricultural Experiment Station Assistance from Tracy Zarrillo, Morgan Lowry, and numerous seasonal workers: Krystian Madrid, Ellen Bulger, Meghan Stucke, Ghada Salah, Ben Gluck, Amelia Tatarian, Erica Carbone, Jess Gambel, Kelly Niland, Anna Petersen, Annalise Kieley

Box 1106 New Haven, CT 06504 Phone: 203.974.

22 Jess Gambel Kimberly Stoner Department of Entomology 123 Huntington Street P. O. Box 1106 New Haven, CT Phone: Website: Jess Gambel

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