Bombus spp. (Hymenoptera: Apidae) as Pollinators of Male-Sterile Upland Cotton on the Texas High Plains1

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Abstract
Efficient pollination of male-sterile (MS) cotton, Gossypium hirsutum L., remains a key factor for the commercial production of hybrid cotton seed. A study was conducted to investigate the importance and potential use of Bombus species as cotton pollen vectors on the Texas High Plains. A survey of 13 counties showed that Bombus pennsylvanicus (DeGeer) was the predominant species. Bumble bees visited cotton flowers throughout the blooming season, and they foraged for both nectar and pollen. More foragers were present on MS than isoline male-fertile (MF) flowers, but visitation to the MS flowers was restricted primarily to morning hours. Nectar and pollen foragers collected in MF flowers carried large amounts of cotton pollen on their body (5,586 to 16,235 grains per forager). Both types of foragers also visited MS flowers, although the quantity of pollen they carried was significantly less (82 to 4,138 grains per forager). Bumble bees carried more cotton pollen than honey bees on MS and MF lines. Wooden domiciles provided an acceptable means for moving bumble bee nests into an area needing pollination, but subsequent visitation to the target fields by the imported bees was low. All colonies in domiciles were parasitized by Psithyrus variabilis (Cresson) during the season. In a cage study, cotton bolls harvested on MS lines had more seeds per boll when Bombus was used as pollinators than when honey bees, Apis mellifera L., were used. This indicates that nests of Bombus species could be used effectively to pollinate small crossing blocks of MS cotton isolated under cages. It also suggests that, on an individual basis, bumble bees are more efficient pollinators of MS cotton than honey bees, which is consistent with the greater cotton pollen load found on Bombus.

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