Pollination efficiency of bumblebee, honeybee, and hawkmoth in kabocha squash, Cucurbita maxima, production in Kagoshima, Japan

Tsunashi Kamo,Shigeki Kishi,Aoi Nikkeshi,Yoshihiro Tanaka,Shoko Nakamura,Tomoya Tawaratsumida

doi:10.1007/s13355-021-00766-2

Abstract

Kabocha squash, Cucurbita maxima Duchesne, in delayed-start culture is one of the major cultivation systems in southwestern Japan. Honeybee introduction or hand-pollination has been practiced in this crop system, but the species composition, abundance, and pollination efficiency of major pollinators remain unclear. In the current study, we investigated which insect(s) accomplish pollination of kabocha squash. Among the total visitations to female flowers, honeybees, mostly Apis mellifera L. (Hymenoptera: Apidae), dominated at 94.4% and 85.3% at the experimental field in 2019 and 2020, respectively, followed by the bumblebee Bombus diversus diversus Smith (Hymenoptera: Apidae) and the hummingbird hawkmoth Macroglossum pyrrhosticta Butler (Lepidoptera: Sphingidae). The number of pollen grains on the body surface of B. diversus diversus individuals was significantly greater than that on honeybees, such that a single visit by B. diversus diversus deposited 5–6 times more pollen grains on a stigma than a honeybee visit, meaning that the bumblebee is a more efficient pollinator at the individual level. However, visitations by numerous honeybees compensated for their lower individual efficiency and resulted in fruit set. In addition, we report for the first time that a lepidopteran can pollinate squash: our data indicate that pollen grains deposited by several visits of M. pyrrhosticta were likely to exceed the estimated minimum number of pollen grains required for fruit set. Our survey at the experimental field and at three private farms revealed that far more pollen than is required for fruit set was deposited on the stigmas by wild pollinators and wintering honeybees. These findings indicate that kabocha squash production in delayed-start culture in this region is practicable without pollination enhancement effort, such as honeybee introduction or hand-pollination.

Highlights

Both wild insects and the domesticated European honeybee, Apis mellifera L. (Hymenoptera: Apidae) significantly contribute to the pollination of many crops (Breeze et al 2011; Potts et al 2010)
The other flower visitors were identified as Aulacophora nigripennis Motchulsky (Coleoptera: Chrysomelidae), Nitidulidae sp. (Coleoptera), Stomorhina obsoleta (Wiedemann) (Diptera: Calliphoridae), B. diversus diversus (Hymenoptera: Apidae), Lasioglossum mutilum (Vachal), Lasioglossum occidens (Smith) (Hymenoptera: Halictidae), C. annulata annulata, Megacampsomeris schulthessi (Betrem), Scolia decorata ventralis Smith
The current study revealed that B. diversus diversus was the most efficient pollinator per visit in delayed-start culture of kabocha squash in Kagoshima, Japan

Summary

Introduction

Both wild insects and the domesticated European honeybee, Apis mellifera L. (Hymenoptera: Apidae) significantly contribute to the pollination of many crops (Breeze et al 2011; Potts et al 2010). Though recent studies have emphasized the importance of wild pollinators in the production of many crops (e.g., Garibaldi et al 2013; Mallinger and Gratton 2015), the domesticated honeybee is still a reliable partner considering the fluctuating visitations and variable foraging traits of wild pollinators all over the world (Breeze et al 2014; Decourtye et al 2010). The larger the number of pollinator visits to a female flower, the greater the probability of the fruiting becomes, and more seeds the fruit has (Artz and Nault 2011; McGrady et al 2020; Vidal et al 2010; Winsor et al 2000). Aside from the squash bees, which are major pollinators of squash plants in origin, bumblebees and honeybees contribute to the production worldwide (Bomfim et al 2016; Knapp and Osborne 2019)

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Results

Conclusion