More about cotton:

Nectar is normally produced in five different areas on the cotton plant, although the reason why the nectar is secreted is not clear. Trelease (1879) made a detailed report on cotton nectar secretion and its possible purposes. Glover, Agricultural Report 1855, p. 234, mentions these glands - and their secretion of a sweet substance, which ants, bees, wasps, and plant bugs avail themselves of as food. 

Trelease believed that the floral nectaries were associated with pollination but that extra-floral nectaries were associated with attracting harmful insects away from the delicate flower parts. 

Cotton is usually referred to as a partially cross-pollinated crop, although many breeders have treated it as a completely self-fertile and self-pollinating crop except for accidental and unwanted cross- pollination caused by pollinating insects. Cross-pollination has been referred to as "natural crossing," and is considered detrimental because of the introduction of off-type plants into the progeny

Various breeders have reported the percentage of natural crossing in their area and commented upon its detrimental effect. Balls (1912) reported 13.3 percent natural crossing of cotton in Egypt, and proposed the isolation of plants under mosquito netting to exclude bees. He noted, however, that some strains "resent this treatment and refuse to hold their bolls"

In contrast to bumble bees and Melissodes bees, honey bees show a preference for the extra-floral nectaries of cotton and often seem reluctant to enter the cotton flower. When a honey bee enters a cotton flower, it may emerge coated with pollen, then alight on a leaf, and comb much of the pollen off without attempting to pack it in the pollen baskets on the hind legs. However, all of this pollen is not removed, and a familiar sight, where bees are working cotton, is their incoming at the hive entrance coated with cotton pollen.

Pollinating bees are an obstacle to most cotton breeders attempting to develop pure lines. Each cotton blossom from which the breeder desires seed must be enclosed or isolated in some way, otherwise the pollinating insect may dilute the line by bringing pollen to it from another type of cotton plant. 

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