CROP POLLINATION BY BEES
by K.S. Delaplane & D.F. Mayer

Squash, Pumpkin, and Gourd

Flowering

Squash, pumpkin, and gourd (Cucurbita species) plants are normally monoecious which means that there are both male and female flowers on the same plant (Fig. 46.1). Male flowers outnumber female ones by 3.5: 1 to 10: 1. Each flower is about 3 in (7.6 cm) wide. Each male flower occurs at the end of a slender stem and has three anthers. Each female flower occurs at the end of a short peduncle and has a thick style and a two-lobed stigma; the swollen ovary occurs at the base of the corolla and is divided into 3-5 sections. Male flowers produce nectar and pollen, and female flowers produce nectar. Female flowers produce more nectar and attract more bees compared to male flowers (Nepi and Pacini, 1993). The pollen grains are large and well suited to insect transport. Flowers open early in the morning and close around noon of the same day, never to reopen (Skinner and Lovett, 1992; Nepi and Pacini, 1993).

Figure 46.1. Flower of squash, which is representative of other Cucurbita species such as pumpkin or gourd. The dissected female flower is on the left and the male on the right. (Source: Darrell Rainey.)

Squash, Pumpkin and Gourd Pollination Requirements

Because these plants have separate male and female flowers, some agent -usually bees -is necessary to transfer pollen from male to female flowers. Squash plants caged to exclude pollinators produced no fruit in west Tennessee, USA (Skinner and Lovett, 1992). Wind does not pollinate Cucurbita. Ovules are fertile only during the flowering period or on the day before in the squash variety 'Greyzini' in Italy. Pollen viability in a newly opened male flower is about 92% but by the time it closes that same morning the viability will have dropped to 75%, and by the next day it will be only 10% (Nepi and Pacini, 1993). Thus, it is important for a female flower to be pollinated as early as possible on the day it opens while pollen is still viable.

Early bee activity is even more important on hot days when flowers close early. Rate of fruit-set is similar in selfed or crossed fruit, but cross-pollination produces heavier fruit as shown in squash in India (Girish, 1981).

Different varieties of the same species, and even different species, can cross each other in Cucurbita (Table 46.1). Even with incompatible species the pollen of one may stimulate parthenocarpic (seedless) fruit development in another; this phenomenon does not result in contaminated seed, but it does reduce total seed yield (Free, 1993). Thus, different varieties and species of Cucurbita must be isolated from each other in order to produce large quantities of pure seed.

Squash, Pumpkin and Gourd Pollinators

Squash bees and honey bees are the most important pollinators of Cucurbita species. Squash bees (Peponapis pruinosa) are excellent pollinators of squash and pumpkin. Compared to honey bees, they make more contact with reproductive parts of a flower, work faster, and work earlier in the morning. However, squash bees are no more efficient than honey bees at setting fruit in spite of their desirable behaviours (Tepedino, 1981). Nevertheless, growers should always encourage wild squash bee populations. Supplemental colonies of honey bees are unnecessary where squash bees occur in high numbers

Honey bees work Cucurbita flowers and effectively pollinate the crop (Table 46.2). They are the most numerous bee visitor to summer squash in west Tennessee, and they freely visit male and female flowers. Honey bees with large pollen loads are often seen visiting female flowers (Skinner, unpublished report). However, honey bees are generalists and they readily move to any competing bloom that offers richer reward than Cucurbita. This is a problem in the southeastern US where wild gallberry, a rich source of nectar, blooms in competition with early squash. This is a situation where honey bee attractants may help focus bees on the crop; however attractants have a mixed record with Cucurbita species (Margalith et al., 1984; Loper and Roselle, 1991; Schultheis et al., 1994). Another way to deal with competing bloom is to increase the density of honey bee hives so that competition for nectar and pollen forces bees to forage on the crop. In Georgia, competition from wild bloom is less of a problem in fall plantings of squash.

**Table 46.1. Cross-compatibility of certain *Cucurbita* species {Whitaker and Davis, 1962). A '+' indicates species that will cross with each other, and a ‘-‘ indicates species that will not cross.**
	C. maxima (winter squash)	C. mixta (cushaw squash)	C. moschata (pumpkin)	C. pepo (summer squash)
C. maxima (winter squash)	NA	-	-	-
C. mixta (cushaw squash)	-	NA	+	-
C. moschata (pumpkin)	-	+	NA	-
C. pepo (summer squash)	-	-	+	NA

**Table 46.2. Effects of increasing bee visits to *Cucurbita* species flowers.**
Location	Bee(s) tested	Conclusion	Reference
Illinois, USA	Honey bee	As number of bee visits to a pumpkin flower increases from 1 to 12, fruit-set increases from 6% to 64% and number of seeds per fruit increases from 273 to 366.	Jaycox et al (1975)
India	Asian honey bee	As number of bee visits to a summer squash flower increases from 1 to 7, fruit-set increases from 30% to 100%.	Girish (1981)
Utah, USA	Honey bee, squash bee	One bee visit results in 22% fruit-set in summer squash, but flowers with numerous visits have 66% fruit-set.	Tepedino (1981)

**Table 46.3. Recommended bee densities for squash, pumpkin and gourd.**
No. of honey bee hives/acre (ha)	Reference
1 (2.5)	Hughes et al (1982)
0.04-3 (0.09-7.4)	McGregor (1976)
0.8-1.6 (2-4)	Goebel (1984)
1-2 (2.5-5)	Levin (1986)
0.4-3.2 (1-8)	Kevan (1988)
1-3 (2.5-7.4)	Scott-Dupree et al (1995)
1-2 (2.5-5)	Skinner (1995)
1.5 (3.8)	Literature average

Other bees
1 squash bee per 20 flowers	Derived from Tepedino (1981)

Skinner and Lovett (1992} found that bumble bees were more efficient than honey bees at promoting good fruit-set in western Tennessee. Six of eight (75%) single bee flower visits by bumble bees resulted in whole fruit, whereas five of 16 (31 %) single bee visits by honey bees resulted in whole fruit. However, in normal conditions and with adequate bee populations, every flower is visited many times, probably by more than one bee species, and fruit-set is very good.

Fruit-set and seed number increase as the number of bee visits to flowers increases (Table 46.2). This is due to the fact that seed formation increases as the number of pollen grains deposited on a stigma increases (Winsor et al., 1987). Thus, large bee populations help ensure maximum flower visitation, pollen deposition, and crop yield.

Other wild solitary bees will visit squash in Tennessee, namely Agapostemon virescens, Augochlora pura, Dialictus sp., Halictus sp., Triepeolus remigatus, and Mellisodes bimaculatus (Skinner, unpublished report). These bees probably contribute to squash pollination, but their relative efficiency is unknown.

References

Free 1993. Insect Pollination of Crops. 2nd Ed. Academic Press.

Girish 1981 MS Thesis Bangalore, India.

Goebel 1984. Aust. Beekeeper 85:166-174.

Jaycox et al. 1975. Amer. Bee Jour. 115:135-140.

Hughes et al. 1982. Pollination in Vine Crops. NC Agr. Ext. Ser. leaflet A6-84.

Kevan 1988. Pollination: Crops and Bees. Ont. Min. Agr. & Food Pub. 72.

Levin 1986. Using Honey Bees to Pollinate Crops. USDA leaflet 549.

Loper & Roseller 1991. Amer. Bee Jour 131:177.

Margalith et al. 1984. Jour. Apic. Res. 23:50-54.

Mc Gregor 1976. Insect Pollination of Cultivated Crop Plants. USDA Handbook 496.

Nepi & Pocini 1993. Ann. Botany 72:527-536.

Schultheis et al 1994. Hort Science 29:155-158.

Scott DuPree et al 1995. A guide to managing bees for crop pollination. CAPA

Skinner 1995. U Tenn. Coop Ext. Leaflet SP409-B.

Tepedino 1981. Jour. Kansas Ent. Soc. 54:359-377.

Winsor et al 1987. Amer. Nat. 129:643-656.

Whitaker and Davis 1962. Cucurbits. Leonard Hill, London.