Volume 43 Number 4 Winter 1996 Volume 43 Number 4 Winter 1996
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Tai Roulston, Blair Sampson, and James Cane National publicity about honey bee declines has led to worries of inadequate pollination, although diverse crops may be enjoying adequate pollination provided by native, unmanaged bees. AAES researchers found plentiful pollinator activity on Alabama summer squash and pumpkins, despite the frequent absence of honey bees. The most important pollinators were squash bees on mid-season plantings of squash and bumble bees on pumpkins and late plantings of squash. Honey bees were brought to America aboard the ships of early European colonists for honey and wax production. They now range across most of the country due to the activities of bee keepers and the proliferation of feral colonies. Because honey bees are prolific, relatively easy to manage, and good pollinators of many crops, they have been extensively adapted for crop pollination. Recently, however, greatly reduced feral populations of honey bees and killed the hives of many bee keepers. This has led to concern that crops would receive inadequate pollination. But North America is home to thousands of native bee species, many of which are important pollinators of wild and cultivated plants, such as squash, pumpkins, and gourds.
AAES researchers conducted a census of honey bees and native bees visiting flowers at six squash and three pumpkin fields in East-Central Alabama. Most common visitors could be identified in flight. Three hundred flowers were surveyed at each pumpkin field, and 40-300 flowers at each squash field. At small plantings, most open flowers were examined. At larger plantings, rows of plants in full bloom were chosen and every flower in the row was checked until reaching a total of 300 flowers. The types of summer squash surveyed were zucchini, yellow straight-neck, and yellow crook-neck (both early and late plantings). The pumpkin varieties were Staff, Big Max, Spooktacular, and Spirit. While it might seem that crops as different as zucchini and pumpkin should have different pollinators, all squash and pumpkins have similar flowers and many are considered to be the same species. Since ornamental gourds are closely related, these results may also apply to their cultivation in Alabama. Findings from the census are sufficient to describe bee activity in Central Alabama, and they likely reflect pollinator populations throughout the state. The primary pumpkin pollinators were bumble bees. In August, bumble bees were abundant at all three pumpkin fields in the census -an average of nine bees per 100 flowers. They accounted for over 50% of total bee visitors. The remaining pollinators included honey bees, sweat bees, leaf-cutting bees, and squash bees. Two of the pumpkin fields had hives of honey bees nearby. In one of these fields, honey bees were found in similar abundance to bumble bees, but at the other field, none were found on pumpkin flowers. Bumble bees were also the predominant visitors at two late-season plantings of summer squash blooming at this time. The main pollinators of early plantings of summer squash, those flowering in June, were squash bees. Native squash bees predominated at all five squash fields - one bee for every two to seven flowers - and accounted for 91% of total visitors. They are clearly the primary pollinators of small acreages of squash in East-Central Alabama, as they are in many other parts of the country.
Squash bees (the genera Peponapis and Xenoglossa) are native to the Americas, occurring from Argentina north through much of the United States. Unlike honey bees, squash bees do not live in social colonies. Each female digs her own vertical tunnel in the ground, usually near the host plants, and spends the morning gathering nectar and pollen to feed her offspring. A squash bee's foraging activity is highly synchronized with the host plants' bloom: the bee emerges from her burrow near dawn as the flowers open, quickly gathers pollen and nectar, and ceases foraging by late morning when the flowers close. Males, in contrast, patrol host flowers all morning looking for unmated females, then crawl into a wilting flower to pass the afternoon and night. Where squash bees are present, males (which cannot sting) can be found by opening wilted squash flowers in the afternoon. In addition to being effective pollinators, squash bees, if present, will always visit squash, unlike bumble bees and honey bees which may find better rewards at other flowering species. Where squash, pumpkins, or gourds are not planted every year, squash bee populations can not build up, and other bees will be necessary for pollination. Generally, growers need to tend to their pollinaors only when there are indications of insufficient pollination, such as poor fruit set, early fruit abscission, and small, or misshapen fruits. Most insecticides are directly toxic to bees, or will kill them if taken up in nectar or pollen. However, insecticides may be used with care. Since squash and pumpkin flowers open only once and wilt a few hours later, usually by noon, bees are primarily at risk from insecticides in the morning hours. One market grower whose field was part of the AAES census reported spraying his 400 yellow summer squash plants weekly with either carbaryl or Malathion, but only at dusk. His crop enjoyed good pollination by a burgeoning population of squash bees (one bee in every six flowers). He brought nearly 4,000 pounds of summer squash to market. Overall, plentiful pollinator activity was found on Alabama summer squash and pumpkins. Crop management pracices that consider the life cycles and daily activity patterns of native pollinators will often ensure adequate squash and pumpkin pollination with wild bees and minimize the circumstances for which growers will need to hire honey bees. Roulston and Sampson are Graduate Research Assistants and Cane is an Associate Professor in Entomology. Debbie Folkerts, an Assistant Professor of Botany and Microbiology, produced the bee photographs used in this article. |
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