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Bee Conservation in the Southeast B 1164

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Authors

Keith S. Delaplane, Extension Entomologist

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This publication was reviewed on Apr 30, 2013.

Summary

This publication is for people who want to know how to make land parcels 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.

Publication Full Text

Bee Conservation in the Southeast

Keith S. Delaplane, Extension Entomologist

Be Art by Keith Delaplane Keith Delaplane

Why Conserve Bees?

As bees visit flowers to collect pollen and nectar as food, they transfer pollen from flower to flower in a process called pollination. Pollination helps seeds and fruits 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 bees they think 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 $14 billion added value to American food production annually1 and more than $215 billion worldwide2. They can live in colonies managed by beekeepers and 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 surveys3,4. Honey bees, both wild and managed, are dying from a variety of reasons including exotic diseases and parasites, deteriorating habitat quality, and pesticide exposure — both environmental and intentional — as part of remedial action against bee parasites. The complex issue of honey bee decline is a matter of intense interest among scientists and agriculturists.

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.

A pollination vacuum occurs as bees of all kinds decline. 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 publication is for people who want to know how to make land parcels 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 Bee Pollinators in the Southeast

Honey Bees

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

Honey Bees Figure 1. Honey bees form long-lived colonies and nest in hollow trees or beekeepers' hives.

Bumble bees

These are large, fuzzy bees (Figure 2). Although they are social, 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 singlehandedly 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.

Bumble bee on flower Figure 2. A queen bumble bee. These large, fuzzy bees nest in grass hollows or abandoned rodent nests. Their colonies live for only one season.

Soil-nesting bees

This group includes thousands of species. Three important pollinating soil-nesters in the Southeast are polyester bees, Southeastern blueberry bees and squash bees. Polyester bees and Southeastern blueberry bees pollinate blueberry crops, and squash bees pollinate cucurbit crops. These bees are solitary. This means individual females emerge in the 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 in which the immature bees develop and spend the winter (Figures 3-4).



Polyester bee nest Figure 3. Polyester bees are solitary bees that nest in tunnels in soil. Their name comes from the biological polymer they produce to line their subterranean brood cells.
Squash bee nest Figure 4. Squash bees, another solitary soil nester, pollinate squash, pumpkin and gourd. Their nest entrances are visible as small mounds of soil, or tumuli, in areas of mixed grass and exposed ground.

Mason bees

These solitary bees 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 you assess the conservation value of your 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. This section discusses the most important principles of bee nesting biology and the practices that you can use to put the knowledge to work.

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.

Practice

Focus your habitat conservation efforts on sunny, open, undisturbed meadows (Figure 5), field margins, sun-drenched patches of bare soil, roadsides, ditch banks and woodland edges. Undisturbed areas like these can increase the abundance of bee nesting sites and diversity of flowering plant species on a farm. 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).

Bumble Bee nest Figure 5. Bumble bees prefer old rodent burrows in areas of un-mown grass; in these photos the observer is exposing a subterranean bumble bee nest to reveal the irregular wax combs and some of the bees.
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.

Principle 2

The richness of plant and bee species increases with time in undisturbed fallow fields. 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.

Principle 3

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

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 destroying active bumble bee colonies is less likely. A light mower is preferable to a heavy tractor-mounted implement that may crush the nests of overwintering soil-nesting bees.

Principle 4

Even managed pastures can be made more hospitable to bees.

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. Do not allow over-grazing because it promotes invasion of fast-growing grasses that crowd out nectar-yielding herbaceous plants. Herbicides can similarly reduce the number of pasture plant species. It is also important to not cut forage plants before they bloom — as is commonly done for making hay or silage — because this makes the pasture useless to bees.

Principle 5

Bees need nesting materials.

Practice

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

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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, which will 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 can be good, especially since non-honey bees tend to nest near where they were reared the previous year.

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.

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. Producing workers requires energy, so a colony's queen output ultimately hinges on season-long food availability.

Practice

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 end of this publication can help you select bee pasture plants that bloom during those dearth times (Figure 6). 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.

Seasonal nector availability top Figure 6. Most flowering and nectar production by plants in the Southeast is in early spring and autumn. Mid-summer is often a nectar dearth and a difficult time for bees (top). One goal of a managed bee pasture is to introduce plants that bloom during the natural dearth times (bottom).
Seasonal nector availability bottom Figure 6. Most flowering and nectar production by plants in the Southeast is in early spring and autumn. Mid-summer is often a nectar dearth and a difficult time for bees (top). One goal of a managed bee pasture is to introduce plants that bloom during the natural dearth times (bottom).

Here is a seed blend of 11 annuals that provides long-blooming bee pasture for set-aside farmlands. Although this list was developed in Germany5, these plants are available as seed in North America:

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

Principle 2

Perennials are better bee pasture plants than annuals (Figure 7).

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. 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.

Perennial Vitex trees Figure 7. Perennials are generally better bee pasture plants than annuals. Perennial Vitex trees bloom in mid-summer and are very attractive to bumble bees.

Principle 3

Bee nesting and foraging activities center on flower-rich habitats. Bumble bee queens prefer to nest in flower-rich meadows, and most bee species prefer to forage close to their nests. The foraging range of non-honey bees is probably smaller than that of honey bees5.

Practice

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

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Bigger is Better

Conservationists thinking of bee sanctuaries and pastures 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' natural habitat, the edges may increase invasion of competitors, parasites and predators, decrease the species' dispersal ability and increase chances 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.

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Bee Conservation and Plant Conservation

Altered natural habitats are a prime cause of species loss not only of bees but also 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: 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.

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.

Plants for prolonged-blooming bee pasture in the Southeast

Common Name Scientific Name Type Availability Resource
(nectar or
pollen)
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 saliva 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
n,p
Willow Salix spp. tree feral
n,p
February-June
Apple Mains 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
Common Name Scientific Name Type Availability Resource
(nectar or
pollen)
Bloom Dates
Flowering Dogwood Cornusflorida tree feral
n,p
March-April
Gallberry Ilex glabra shrub feral
n,p
March-June
Mustard Brassica spp. arm. 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 arm. herb. cultivated oil-seed
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 1
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
Cora 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
Common Name Scientific Name Type Availability Resource
(nectar or
pollen)
Bloom Dates
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
SOD.
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 dactvlon per. grass cultivated forage
May-November
Bitterweed Helenium amarum aim. 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
Common Name Scientific Name Type Availability Resource
(nectar or
pollen)
Bloom Dates
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
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
Common Name Scientific Name Type Availability Resource
(nectar or
pollen)
Bloom Dates
Prairie clover Dalea spp. herb, shrub feral
n,p
September-October
Baccharis (Groundsel) Baccharis spp. shrub feral, ornamental
n,p
October-November
Strawberry Fragariax ananassa per. herb cultivated, feral
n,p
December-May
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 ever-green 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

References

1 Morse, R.A. and N.W. Calderone. 2000. The value of honey bees as pollinators of U.S. crops in 2000. Bee Culture 128: 1-15

2 Gallai, N., Salles, J-M., Settele, J., and B.E. Vassiére. 2008. Economic valuation of the vulnerability of world agriculture confronted with pollinator decline. Ecological Economics 68: 810-821

3 vanEngelsdorp D., Hayes J., Underwood R.M., and J.S. Pettis. 2008. A survey of honey bee colony losses in the U.S., fall 2007 to spring 2008. PLoS ONE 3(12), e4071. doi:10.1371/journal.pone.0004071

4 vanEngelsdorp D., Hayes J., Underwood R.M., and J.S. Pettis. 2010. A survey of honey bee colony losses in the United States, fall 2008 to spring 2009. Journal of Apicultural Research 49(1): 7-14 doi:10.3896/IBRA.1.49.1.03

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


B 1164 | This publication was reviewed on Apr 30, 2013.
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