Entomology: UGA Honey Bee Program: Bees, Beekeeping, and Pollination

Bee Conservation in the Southeast

Update of circular 1164 of the Cooperative Extension Service of The University of Georgia and Ft. Valley State College, the U.S. Department of Agriculture and cooperating counties of the state of Georgia, July, 1998.

Keith S. Delaplane, Professor, Dept. Entomology,
University of Georgia, Athens, GA 30602 USA

Why Conserve Bees?

Bees visit flowers to collect pollen and nectar as food, and as they do this they transfer pollen from flower to flower in a process called pollination.  Pollination helps seeds and fruits to develop.  Many row crops and garden crops require bee pollination.  Good pollination makes higher yields, larger fruit, faster ripening fruit, and better tasting fruit.

When most people think of Abees@ they are thinking about the familiar honey bee, Apis mellifera.  This remarkable insect is the source for honey, beeswax, and a variety of other health and nutritional products. As important as these products are, their value pales in comparison to the value of honey bees as crop pollinators.  Honey bees are responsible for $2-9 billion added value to American food production annually [i], [ii].  They can live in colonies managed by beekeepers, and they can live as wild colonies in nature.  Both managed and wild honey bees are valuable pollinators.

American crop growers and home gardeners are concerned about declining numbers of wild honey bees.  There is evidence for this decline from scientific surveys [iii], [iv].  Wild honey bees are dying because of two exotic parasites that were introduced into North America in the 1980sCthe tracheal mite and varroa mite.  Managed honey bees survive reasonably well because they are routinely treated with miticides to control these parasites.  However, treatment is expensive and even treated bees sometimes die.  The situation is worse for wild honey bees because they are never treated.

Non-honey bees are also threatened.  These include wild bumble bees and solitary bees that nest in thick grass, soil, wood, or tunnels in wood.  These different types, or species, of bees are easily overlooked because they are rarely kept in hives, do not make surplus honey, and do not form large colonies.  Their nesting sites and food plants are frequently destroyed by human activities.

As bees of all kinds decline, that leaves behind a pollination vacuum.  And less pollination means lower food quality and higher food prices.  Thus, large bee populations are in everyone's best interest.  Anyone who grows or uses plant products is a stakeholder in bee conservation.

This bulletin is for anyone who wants to know how to make a property more bee-friendly.  The goal is to increase the number of bees foraging and nesting on one's property which will lead to improved pollination of row crops or garden crops.  A healthy bee population needs long-lasting nesting sites and plants that produce nectar and pollen during bee nesting season.  These facts are the foundation of any bee conservation program.

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Some Important Be Pollinators in the Southeast

Fig. 1

Honey Bees. These are the most well-known bee (Fig. 1).  They are social bees which means they live together in large colonies.  Honey bees thrive in man-made hives in which they can reach populations as high as 60,000 individuals.  The beekeeping industry is an important part of the U.S. agricultural economy.  The beekeeping industry in the Southeast produces honey and beeswax and provides crop pollination services.  A specialized branch of the industry, concentrated in south Georgia, raises bees and queens for sale to beekeepers around the world.  Honey bees are historically the most important crop pollinator.

Fig. 2

Bumble bees. These are large, fuzzy bees (Fig. 2).  They are social like honey bees.  However, the life cycle begins with a solitary overwintered queen.  She emerges from hibernation in early spring and finds a nest site, usually a cavity in thick grass or an abandoned rodent nest and single-handedly forages for nectar and pollen.  She raises a batch of worker bumble bees who help her forage and care for more young.  Eventually the worker population increases enough that the queen can stay at the nest and concentrate on laying eggs.  The colony population peaks at a few hundred individuals.  In mid- to late-summer the colony stops rearing workers and begins rearing new queens and males.  New queens mate and overwinter to start the cycle over again.  Workers, males, and the old queen die at the end of summer.

Fig. 6

Soil-nesting bees. This group includes thousands of species.  Three important pollinating soil-nesters in the Southeast are polyester bees (Fig. 3), southeastern blueberry bees (Fig. 4), and squash bees (Fig. 5).  Polyester bees and southeastern blueberry bees pollinate blueberry, and squash bees pollinate cucurbit crops.  These bees are solitary.  This means individual females emerge in spring and, mate, forage, and singlehandedly rear the next generation of offspring.  Females dig simple tunnels in soil in which they lay their eggs and the immature bees develop and overwinter (Fig. 6).

 

Fig. 7

Mason bees. These are solitary bees (Fig. 7) that nest in pre-existing tunnels such as old nail holes, beetle tunnels, or soda straws.  They are called mason bees because they seal their tunnels with mud or finely-chewed leaf material.  They are important pollinators of many early spring-blooming plants.

 

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Habitat Conservation

The information in this section can help one assess the conservation value of lands and identify steps for improving them for bees.  The immediate goal is to increase the density and species diversity of flowering plants and the density of good bee nesting sites.  In this section we will discuss the most important principles of bee nesting biology, then follow each principle with a practice one can use to put the knowledge to work.

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Principle 1

Bees thrive best in open, sunny habitats with an abundance and diversity of flowering food plants rather than in flower-poor, shaded woodlands [v].

• Practice
  

  Fig. 8

  Focus your habitat conservation efforts on sunny, open undisturbed meadows, field margins, sun-drenched patches of bare soil, roadsides, ditch banks, and woodland edges (Fig. 8).  Undisturbed areas like these can increase the abundance of bee nesting sites and diversity of flowering plant species on a farm [vi].  A farm can have large areas of such idle land, and using it for a bee sanctuary costs next to nothing and involves mostly a willingness to leave it undisturbed for the long term.  "Undisturbed" means no draining, plowing, or compacting with heavy machinery.  Periodic mowing, however, is required (see Principle 3 below).
  
  
• Practice
  Avoid heavily wooded areas for bee sanctuaries.  The only exception is those areas with nectar-producing understory and margin plants such as bramble, gallberry, and palmetto.  Certain tree species, for example red maple, sourwood, and tulip poplar, are good pollen or nectar sources, but even with these types of forest the bees are more likely to nest at the forest margins that have sun and a variety of nesting sites and flowering plants.

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Principle 2

The richness of plant and bee species increases with time in undisturbed fallow fields [vii].  As the diversity of flowering plants increases, so does the diversity of bee species.  A large diversity of bee species is good insurance for crop pollination.

• Practice
  Plan bee sanctuaries for the long term.  In time you can expect increasing numbers of plant and bee species in these undisturbed sanctuaries.  But one catastrophic event, plowing for example, can undo years' worth of progress.

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Principle 3

The most effective bee sanctuaries are mid-successional plant communities with an abundance of herbaceous perennials and few or no invading trees [viii].

• Practice
  Biannual mowing is advisable to keep a sanctuary from succeeding into shaded woodlands or scrub lands. It is best to mow in winter when it is less likely to destroy active bumble bee colonies.  A light mower is preferable to a heavy tractor-mounted implement that may crush nests of overwintering soil-nesting bees.

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Principle 4

Even managed pastures can be made more hospitable to bees [v].

• Practice
  The older the pasture, the more likely it is to have suitable bee nest sites and numerous plant species.  This means it is best to keep pastures more-or-less permanent.  Temporary pastures, such as those grown in crop rotation, have very low plant diversity even though the cover crop (such as clover) may be a rich bee resource for one season.  It is important to not allow overgrazing because it promotes invasion of fast-growing grasses that crowd out nectar-yielding herbaceous plants.  Herbicides similarly can reduce the number of pasture plant species.  It is also important to not cut forage plants before they bloomCas commonly done for making hay or silageCbecause this makes the pasture useless to bees.

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Principle 5

Bees need nesting materials.

• PracticeThis applies mostly to mason bees that need mud to seal their nests.  Make sure there is a source of mud near nest holes where mason bees are active.

Habitat Improvement with Installed Bee Pastures

Bee conservation can go beyond passive habitat  preservation to active habitat improvement by installing permanent bee pastures.  Bee pasture is a permanent planting of flowering annuals or perennials designed to attract bees over many weeks or months. The goal is improved bee nutrition to encourage high bee numbers, either by attracting them to the area, increasing the number nesting in the area, or by increasing their reproductive output.  Long-term payoff of perennial pastures may be good, especially since non-honey bees tend to nest near where they were reared the previous year [ix].

Candidate bee pasture plants should be rich in nectar and pollen, easy to grow, cost-effective, non-invasive, long-blooming, and not bloom at the same time as the crop and thus compete with it for pollinators.  Here are some principles and practices for bee pasture plantings.

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Principle 1

Bees reproduce better in habitats that have an uninterrupted season-long succession of bloom.  This is best illustrated with bumble bees.  The number of queens a colony can produce depends partly on the number of workers it can produce in the weeks leading up to the queen production period in late summer [x].  Producing workers requires energy, so a colony's queen output ultimately hinges on season-long availability of food.

• Practice
  

  Fig. 9a

  Fig. 9b

  In planning a bee pasture, it is important to choose a collection of plants that will produce an unbroken succession of bloom throughout the season.  Local beekeepers, county extension agents, and horticulturists are good sources of information about the important bee plants in an area and their historic bloom times.  This information can help you identify dearth times in the natural bloom calendar.  Your county extension agent, horticulture specialist, or the Appendix at the back of this bulletin can help you select bee pasture plants that bloom during those dearth times (Fig. 9).  Avoid installing pasture plants that bloom at the same time as the crop or else you run the risk that bees may prefer the pasture flowers over the crop flowers.

Here is a seed blend of eleven annuals that provides long-blooming bee pasture for set-aside farmlands.  Although this list was developed in Germany [xi], these plants are available as seed in North America.

40% phacelia (Phacelia tanacetifolia)	3% red radish (Raphanus sativus)
25% buckwheat (Fagopyrum esculentum)	3% cornflower (Centaurea cyanus)
7% white mustard (Sinapis alba)	3% mallow (Malva sylvestris)
6% coriander (Coriandrum sativum)	2% anethum (Anethum graveolens)
5% calendula (Calendula officinalis)	1% borage (Borago officinalis)
5% black cumin (Nigella sativa)

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Principle 2

Fig. 10

Perennials are better bee pasture plants than annuals (Fig. 10).  Although some annuals provide quick and relatively abundant bee forage, perennial herbs and shrubs are superior bee forage plants and deserve special attention by bee conservationists [xii], [xiii].  Compared to annuals, perennials are generally richer nectar sources.  Because of their longevity, perennials provide bee populations a more-or-less dependable food source year after year and encourage repeated nesting in the area.  This partly explains why the number of bee and plant species increase together over time in undisturbed meadows.

• Practice
  When possible, plant perennials for bee pasture.  Considering the repeated labor and inputs required for annuals, perennials are a cost-effective, low-maintenance choice for bee conservationists.

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Principle 3

Bee nesting and foraging activities center on flower-rich habitats.  Bumble bee queens prefer to nest in flower-rich meadows [xiv], and most bee species prefer to forage close to their nests [x].   The foraging range of non-honey bees is probably smaller than that of honey bees [v].

• Practice
  Place bee pastures as near as possible to the crop of interest.  This increases the chance of bees nesting near to, and foraging on, the crop.

Bigger is Better

As conservationists think of bee sanctuaries and pastures, they need to think big.  The diversity of bee species is highest in large, continuously-connected areas of suitable habitat.  Unfortunately, farming and urbanization do the exact opposite break up habitats into small fragments or "islands."  When there are many edges to a species's natural habitat, the edges may increase invasion of competitors, parasites, and predators, decrease the species's dispersal ability, and increase chance of inbreeding.

Thus, bee sanctuaries and pastures should be as large as possible.  One large, connected bee sanctuary, ideally on a scale larger than that of an individual farm, is better than several small, disconnected sanctuaries.  One expert recommends that for a normally functioning agricultural landscape the area of land in cultivated fields or mowed meadows should not exceed 75 percent of the total area.  The remaining 25 percent should be left as bee sanctuary [vi].

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Bee Conservation and Plant Conservation

Altered natural habitats are a prime cause of species loss not only of bees but of native plants.  Plants whose habitats become fragmented are widely separated from each other and may have trouble attracting pollinators.  One can imagine the vicious cycle at work here: habitat fragmentation separates the plants from their pollinators; plant numbers decline for lack of pollinating bees; bee numbers decline for lack of food plants.

Some modern agricultural practices may also rob native plants of habitat and lure away their pollinators.  Large acreages of bee-attractive crops, such as canola, may lure all bees, native and exotic, away from native plants, depriving them of pollination and contributing further to their decline [xv].

Native bee conservation goes hand in hand with conservation of native plants that depend on them for pollination.  Without their pollinators, the colorful bee-pollinated plants that beautify our surroundings, control erosion, and increase our property values would decline with unknown effects on the wildlife that depends on them for food.  Thus, bee conservation is not just an issue for beekeepers and crop growers and home gardeners, although food production is by far the most important arena.  It is at the very center of plant production and conservation, and all who use and enjoy plant products are stakeholders.

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Appendix

Below is an incomplete list of wild and commercially-available plants that provide prolonged-blooming bee pasture in the Southeast.  It is important for bees, especially bumble bees, to have an unbroken succession of bloom all season to build up their local populations.  If you want to encourage large bee populations, consider growing an assembly of plants from this list so that bloom is more or less continuous.  It is important to choose bee pasture plants that are rich in nectar and pollen, easy to grow, cost-effective, non-invasive, long-blooming, and do not bloom at the same time as the crop.  Plants in the table are listed in chronological order of their average first month of bloom.

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Plants for prolonged-blooming bee pasture in the Southeast [xvi], [xvii], [xviii], [xix], [xx], [xxi]
Common Name	Scientific Name	Type	Availability	Resource (nectar orpollen)	Bloom Dates
Cajeput (Tea Tree)	Melaleuca quinquenervia	tree	feral	n,p	much of the year
Chickweed	Stellaria spp.	ann. or per. herb	feral	n,p	much of the year
Cucumber	Cucumis sativa	ann. herb	cultivated	n,p	much of the year
Pumpkin	Cucurbita spp.	ann.	cultivated	n,p	much of the year
Alder	Alnus spp.	tree	feral	p	January-June
Blueberry	Vaccinium spp.	shrub	cultivated, feral	n,p	January-June
Maple	Acer spp.	tree	feral	n,p	January-May
Cantaloupe	Cucumis melo	ann. herb	cultivated	n,p	February-August
Citrus	Citrus spp.	tree	cultivated	n,p	February-May
Dandelion	Taraxacum spp.	bien. or per. herb	feral	n,p	February-September
Dead Nettle (Henbit)	Lamium spp.	ann. or per. herb	feral, ornamental, sometimes invasive	p	February-October
Elm	Ulmus spp.	tree	feral	n,p	February-April
Groundsel	Senecio spp.	ann. or per. herb, shrub	feral, ornamental	n,p	February-May
Hawthorn	Crataegus spp.	shrub, tree	feral	n,p	February-June
Peach	Prunus persica	tree	cultivated	n,p	February-April
Pine	Pinus spp.	tree	cultivated, feral	p	February-April
Skunk Cabbage (Polecat Weed)	Symplocarpus foetidus	per. herb	feral, ornamental	p	February-April
Titi (Spring Titi)	Cliftonia spp.	shrub	feral	n,p	February-April
Willow	Salix spp.	tree	feral	n,p	February-June
Apple	Malus spp.	tree	cultivated	n,p	March-May
Ash	Fraxinus spp.	tree	feral	p	March-May
Blackberry	Rubus spp.	shrub	cultivated, feral	n,p	March-June
Black Locust	Robinia pseudoacacia	tree	feral	n,p	March-June
Cherry (cultivated and uncultivated)	Prunus spp.	tree, shrub	cultivated, feral	n,p	March-May
Cottonwood	Populus spp.	tree	feral	p	March-May
Flowering Dogwood	Cornus florida	tree	feral	n,p	March-April
Gallberry	Ilex glabra	shrub	feral	n,p	March-June
Mustard	Brassica spp.	ann. or bien. herb	feral	n,p	March-September
Oak	Quercus spp.	tree	feral	p	March-May
Persimmon	Diospyros virginiana	tree	cultivated, feral	n,p	March-June
Plum (cultivated)	Prunus spp.	tree	cultivated	n,p	March-April
Rape (Canola)	Brassica napus	ann. herb.	cultivated oilseed	n,p	March-May
Rattan Vine	Berchemia scandens	shrub	feral	.	March-June
Redbud	Cercis spp.	shrub, tree	feral, ornamental	n,p	March-May
Tupelo	Nyssa spp.	tree	feral	n,p	March-June
Vervain	Verbena spp.	ann. or per. herb	feral, ornamental	n,p	March-October
Alsike Clover	Trifolium hybridum	per. herb	cultivated forage	n,p	April-September
Bindweed	Convolvulus spp.	ann. or per. herb	feral, ornamental, sometimes invasive	n,p	April-September
Buckeye	Aesculus spp.	shrub, tree	feral	n,p	April-May
Buckthorn	Rhamnus spp.	shrub, tree	feral, ornamental	n,p	April-June
Catclaw	Acacia greggii	shrub, tree	feral	n,p	April-July
Coneflower	Rudbeckia spp.	ann., bien, or per. herb	feral, ornamental	n,p	April-September
Corn	Zea maize	ann.	cultivated	p	April-September
Crimson Clover	Trifolium incarnatum	ann. herb	cultivated forage	n,p	April-June
Elderberry	Sambucus spp.	shrub, tree	feral, ornamental	n,p	April-July
Holly	Ilex spp.	shrub, tree	feral, ornamental	n,p	April-June
Honey Locust	Gleditsia triacanthos	tree	feral	n,p	April-June
Honeysuckle	Lonicera spp.	shrub	feral	n,p	April-August
Horsemint (Bee Balm)	Monarda spp.	ann. or per. herb	feral, ornamental	n,p	April-October
Huckleberry	Gaylussacia spp.	shrub	feral	n,p	April-June
Johnson Grass	Sorghum halepense	per.	cultivated forage, feral, sometimes noxious	.	April-November
Marigold	Gaillardia pulchella	ann.	feral, ornamental	n,p	April-October
Mesquite	Prosopsis glandulosa	shrub, tree	feral	n,p	April-June
Pear	Pyrus spp.	tree	cultivated, ornamental	n,p	April-May
Pepper Vine	Ampelopsis spp.	vine, shrub	feral	n,p	April-August
Persian Clover	Trifolium resupinatum	ann. herb	.	n,p	April-September
Privet	Ligustrum spp.	shrub	feral, ornamental	n,p	April-July
Red Clover	Trifolium pratense	short-lived per.	cultivated forage	n,p	April-September
Sage	Salvia spp.	ann. or per. herb, shrub	ornamental	n,p	April-May
Sweet Clover (White, Yellow)	Melilotus spp.	bien. herb	cultivated forage	n,p	April-October
Thistles	Cirsium spp.	ann., bien., or per. herb	feral	n,p	April-October
Tickseed	Coreopsis lanceolata	per. herb	feral	n	April-June
Titi (Summer Titi)	Cyrilla racemiflora	shrub	feral	n,p	April-July
Tulip Poplar	Liriodendron tulipifera	tree	feral	n,p	April-June
Vetch	Vicia spp.	ann. or bien. herb	cultivated forage	n,p	April-September
White Clover (White Dutch, Ladino)	Trifolium repens	per.	cultivated forage	n,p	April-October
Yellow Rocket	Barbarea vulgaris	bien. or per. herb	feral, sometimes noxious	n,p	April-June
Alfalfa	Medicago sativa	per. herb	cultivated forage	n,p	May-October
American Beautyberry (French Mulberry)	Callicarpa americana	shrub	feral, ornamental	n	May-June
Aster	Aster spp.	per. herb	feral	n,p	May-November
Bermuda Grass	Cynodon dactylon	per. grass	cultivated forage	.	May-November
Bitterweed	Helenium amarum	ann.	feral	n,p	May-November
Carpet Grass	Phyla nodiflora	per. herb	feral, groundcover	n	May-frost
Catalpa (Catawba)	Catalpa spp.	tree	feral	n,p	May-June
Chinese Tallow Tree	Sapium sebiferum	tree	ornamental	n	May-June
Grape	Vitis spp.	per. vine	cultivated	n,p	May-July
Palmetto (Cabbage Palm)	Sabal spp.	palm	feral	n,p	May-July
Palmetto (Saw Palmetto)	Serenoa repens	palm	feral	n,p	May-July
Prickly Pear	Opuntia spp.	cacti, tree-like	feral, ornamental	n,p	May-June
Raspberry	Rubus spp.	shrub	feral	n,p	May-June
Smartweed	Polygonum spp.	ann. or per. herb	cultivated, feral, ornamental	n,p	May-November
Sorghum	Sorghum bicolor	ann.	cultivated	p	May-October
Sourwood	Oxydendrum arboreum	tree	feral, ornamental	n,p	May-July
Spanish Needles	Bidens spp.	ann. or per. herb	feral, ornamental	n,p	May-November
Sumac	Rhus spp.	shrub, tree	feral	n,p	May-September
Virginia Creeper	Parthenocissus quinquefolia	vine	feral, ornamental	n,p	May-August
Watermelon	Citrullus lanatus	ann.	cultivated	n,p	May-August
Anise Hyssop	Agastache spp.	per. herb	feral, ornamental	n,p	June-September
Balloon Vine	Cardiospermum halicacabum	ann. or bien. vine	feral, ornamental	.	June-August
Basswood	Tilia spp.	tree	feral	n,p	June-July
Blue Vine	Cynanchum laeve	per. herb	feral	n,p	June-September
Boneset (Joe-Pye Weed)	Eupatorium spp.	per. herb, shrub	feral, ornamental	n,p	June-November
Buckwheat	Fagopyrum esculentum	herb	cultivated	n,p	June-frost
Buttonbush	Cephalanthus spp.	shrub, tree	feral	n,p	June-September
Clethra (Sweet Pepperbush)	Clethra alnifolia	shrub	feral	n,p	June-September
Cotton	Gossypium spp.	ann. herb	cultivated	n,p	June-September
Cowpea	Vigna unguiculata	ann. herb	cultivated	n,p	June-September
Cranberry	Vaccinium macrocarpon	evergreen	cultivated, feral	n,p	June-July
Ironweed	Vernonia spp.	per. herb, shrub, tree	feral, ornamental	n,p	June-October
Lespedeza (Bush Clover)	Lespedeza spp.	per. herb, shrub	feral, ornamental	n,p	June-October
Lima Bean	Phaseolus lunatus	herb	cultivated	n,p	June-July
Loosestrife (Purple Loosestrife)	Lythrum salicaria	per. herb	cultivated, feral	n,p	June-September
Mexican Clover	Richardia scabra	ann. herb	cultivated, feral	n	June-frost
Milkweed	Asclepias spp.	per. herb	feral	n	June-August
Mint	Mentha spp.	per. herb	cultivated, feral, ornamental	n	June-September
Partridge Pea	Cassia fasciculata	ann. herb	feral	n,p	June-October
Prickly Ash	Aralia spinosa	shrub, tree	feral	n	June-August
Star Thistle	Centaurea spp.	ann., bien., or per. herb	feral, ornamental	n,p	June-October
Sunflower	Helianthus spp.	ann. or per. herb	cultivated ornamental and oilseed, feral	n,p	June-November
Vitex (Chaste Tree)	Vitex spp.	shrub, tree	ornamental	n,p	June-July
Broomweed	Gutierrezia texana	per. herb	feral	.	July-October
Goldenrod	Solidago spp.	per. herb	feral	n,p	July-November
Ragweed	Ambrosia spp.	herb	feral, often noxious	p	July-October
Snowvine	Mikania scandens	per. vine	feral	n,p	July-frost
Soybean	Glycine max	ann. herb	cultivated	n,p	July-October
Woodbine	Clematis virginiana	per. herb	feral, ornamental	n,p	July-September
Brazilian Pepper Tree	Schinus terebinthifolius	shrub, tree	feral, ornamental, sometimes noxious	.	August-October
Crown-beard	Verbesina spp.	ann. or per. herb, shrub, tree	feral	n,p	August-October
Matchweed (Snakeweed)	Gutierrezia  sarothrae	per. herb	feral	n,p	August-October
Prairie clover	Dalea spp.	herb, shrub	feral	n,p	September-October
Baccharis (Groundsel)	Baccharis spp.	shrub	feral, ornamental	n,p	October-November
Strawberry	Fragaria x ananassa	per. herb	cultivated,feral	n,p	December-May

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References

i. Robinson, W.S., R. Nowogrodzki, & R.A. Morse. 1989. American Bee Journal 129: 411-423, 477-487

ii. Southwick, E.E. & L. Southwick, Jr. 1992. Journal of Economic Entomology 85: 621-633

iii. Kraus, B. & R.E. Page. 1995. Environmental Entomology 24: 1473-148

iv. Loper, G.M. 1995. American Bee Journal 135: 823-824

v. Osborne, J.L., I.H. Williams, & S.A. Corbet. 1991. Bee World 72: 99-116

vi. Banaszak, J. 1992. Agricultural Ecosystems and the Environment 40: 179-192

vii. Gathmann, A., H.-J. Greiler, & T. Tscharntke. 1994. Oecologia 98: 8-14

viii. Dramstad, W. & G. Fry. 1995. Agricultural Ecosystems and the Environment 53: 123-135

ix. Butler, C.G. 1965. Proceedings of the Royal Entomological Society of London (A) 40: 77-80

x. Heinrich, B. 1979. Bumblebee economics.  Harvard University Press, Cambridge, Massachusetts

xi. Bauer, M. & W. Engels. 1992. Apidologie 23: 340-342

xii. Parrish, J.A.D. & F.A. Bazzaz. 1979. Ecology 60: 597-610

xiii. Fussell, M. & S.A. Corbet. 1992. Journal of Applied Ecology 29: 451-465

xiv. Bowers, M.A. 1985. Ecology 66: 914-927

xv. Williams, I.H., S.A. Corbet, & J.L. Osborne. 1991. Bee World 72: 170-180

xvi. Ayers, G.S., & J.R. Harman. 1992. In The hive and the honey bee (J.M. Graham, ed.), Dadant & Sons, Hamilton, Illinois

xvii. Cane, J.H. 1993. In Proceedings of the Southeast Blueberry Conference. Tifton, Georgia

xviii. Delaplane, K.S. 1993. Honey bees and beekeeping.  University of Georgia Cooperative Extension Service Bulletin 1045

xix. Hortus Third. 1976. Macmillan Publishing Co., New York

xx. Krewer, G.W., K.S. Delaplane, & P.A. Thomas. 1996. HortScience 31: 750

xxi. Mitchell, T.B. 1962. Bees of the eastern United States.  North Carolina Agricultural Experiment Station Bulletin 15

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