Worldwide, the number of insect pollinators has declined sharply in recent years. Beekeepers have experienced heavy loss of honey bee colonies; in the United States, the number of managed colonies is half of what it was sixty or seventy years ago (USDA). The number of wild pollinators has also fallen. In China, insect pollinators are so scarce that apple blossoms must be pollinated by hand, using brushes! (Goulson, 2012 in Chinadialogue; www.tinyurl.com/ps4cvou) Though numbers are difficult to quantify, Dr. Baldwyn Torto at ICIPE (African Insect Science for Food and Health, previously the International Center for Insect Physiology and Ecology) has also commented on a decline in the number of bees in East Africa. He pointed out that Kenya, formerly a producer of indigenous honey, now imports honey (Coordination Team, 2013).
Insects pollinate three quarters of flowering plant species—and more than half of human food crops—so this decline in insect pollinators is a sobering reality. Pollinators affect the diversity, abundance and quality of food crops (FAO, 2008).
Honey bees (Fig. 1) tend to have the biggest profile as pollinators. In a TED talk titled “Why Bees are Disappearing,” Dr. Marla Spivak highlights a number of interacting reasons for the precipitous decline in numbers of honey bees:
Vast fields planted with a single crop are called monocultures. Bees in a monoculture field lack variety in their diet, which is problematic even if the monoculture crop is one that is good for bees. Aside from the short window of time that the monoculture crop is flowering, bees find no nectar or pollen in the vicinity, making it essentially a flowerless landscape—a desert—for most of the year. However, monocultures present a feast for plant-eating pests, often resulting in the heavy use of pesticides by farmers. In much the same way that our own immune systems would be weakened by eating only one kind of food and by exposure to harmful chemicals, bees in this kind of environment become more susceptible to diseases and parasites.
What can we do? Spivak encourages a two-pronged approach that will benefit honey bees but also other pollinators: avoid pesticide contamination and plant bee-friendly flowers.
Avoid Pesticide Contamination
Pesticides’ impact on insect pollinators is multifaceted. Herbicides often destroy plants that are important sources of nectar and pollen for pollinators. Insecticides can expose bees, and other pollinators, to toxins in various ways. Direct exposure occurs when toxins are absorbed through the exoskeleton or ingested in nectar. Indirect exposure occurs as contaminated pollen and/or nectar is brought back to the nest. Bees may be killed on contact with insecticides, but exposure to toxins can also disrupt the ability of bees to navigate, and/or render them unable to fly.
Reducing pesticide contamination is an important action to take on behalf of pollinators. “Pesticide Considerations for Native Bees in Agroforestry,” a publication by the USDA National Agroforestry Center, shares helpful considerations and suggestions regarding pesticides and native bees, summarized in the following section.
Non-chemical methods of pest reduction. Agroforestry practices such as windbreaks, hedgerows and alley cropping can reduce the effects of pesticides, or need for pesticides, in a number of ways. They can reduce pesticide drift (either onto or off of a farm). If windbreaks and hedgerows are kept pesticide-free, they can provide a refuge for pollinators while crops are being sprayed. They can also provide nesting sites and food for insects that prey on or parasitize pest insects.
[Other creative and cost-effective alternatives to pesticides have been studied and implemented. For example, stemborers in eastern and southern Africa have been controlled using the ‘push-pull’ system designed by ICIPE. This system utilizes 1) trap crops, planted around the perimeter of a field, to ‘pull’ stemborers away, and 2) repellent plants, intercropped in the field, to ‘push’ the pests away. See EDN 77 and EDN 116 for more details.
In other cases, manually removing larval insect pests from plants makes economic sense, as has been found with pod borer larvae on pigeon peas in India (see EDN 84-4).]
Least-toxic chemical approaches. Clearly, the best option for encouraging pollinators is to not use any pesticides at all on crops. However, it may be unrealistic to completely avoid the use of pesticides. In situations where pest control is needed, look for the least-toxic approach. The U.S.-based National Sustainable Agriculture Information Service (ATTRA) has an online Ecological Pest Management Database that can help users find “biorational” pesticides, which specifically target a particular insect, and/or break down to become non-toxic. The database can be searched by pest type, pest name, pesticide trade name, and active ingredient/beneficial organism. You can find the database at https://attra.ncat.org/attra-pub/biorationals/.
At ECHO’s farm in southwest Florida, we use “soft” chemical controls whenever possible. Andy Cotarelo, ECHO farm manager, defines “soft” chemicals as ones that are pest target specific, are non-persistent (readily metabolized in the soil) and have a low toxicity for the applicator and for non-target organisms. He commented, “Even with the use of “soft” chemical controls, we limit their use to times when the population threshold of pests rises to the point that it becomes necessary. We do not spray for insect pests preventively, but in reaction to the population levels. We do this by surgically spraying for pests instead of blanket applications. We do not spray restricted use pesticides on the farm except in rare cases to knock out a critical pest, especially on plants intended for seed production.” One example of such a critical pest is white fly on tomatoes; sometimes a stronger pesticide is used on the farm so that Tomato Yellow Leaf Curl Virus (TYLCV) is not spread by the flies early in the season.
Andy added, “Most of the harder chemistry is limited to perennials because of the benefit received from the labor of spraying.” When it comes to diseases (in contrast to pests), he said, “You have to foresee the conditions that will promote the growth of disease and spray preventively. If you spray after the plant is infected or when you see the disease, most of the time you are mitigating the effects of the disease, not curing it. It is very difficult to spray vegetables for a disease curatively.”
Some examples of chemicals used regularly on ECHO’s farm are commercial neem products, Bacillus thuringiensis, soap, oil, garlic and potassium bicarbonate. We use these products in a system in which we employ beneficial organisms, plan crop timing to reduce hosts for pests, and choose varieties that resist pests naturally.
|Neonicotinoids are a class of pesticides that have been in the news in 2014. These are systemic chemicals, meaning that “they are absorbed by the plant and are transferred through the vascular system, making the plant itself toxic to insects.” The chemicals persist long after application (even for months or years in the soil). In plants that have been treated with these chemicals, neonicotinoids are present in the pollen and nectar that bees collect. Some metabolites (the compounds that result when plants metabolize neonicotinoids) are at least as toxic to honey bees as the original compounds. Exposure to neonicotinoids may make bees more susceptible to parasites and diseases. (Information from http://nas-sites.org/pollinators/about-pollinators/). The European Commission has placed a two-year ban on three types of neonicotinoids.|
Some pesticides can be made from plants for a less-toxic chemical approach. Homemade neem spray is a common one. Another plant-based pesticide (good for the softer-bodied nymphal stage of many insects) is made from a pinch of cayenne pepper (or ground dried chili with seeds) and one teaspoon of liquid dish detergent in one liter of water. Sometimes a small amount of oil is also added. The mixture is sprayed on the insects. For more ideas of pest control without using purchased pesticides, see Lowell Fuglie’s book Producing Food without Pesticides: Local solutions to crop pest control in West Africa, reviewed in EDN 84-5.
“Pesticide Considerations for Native Bees in Agroforestry” (Vaughan and Black 2007) contains helpful information for reducing the impact of insecticides when they are used. If at all possible, avoid ‘broad-spectrum’ insecticides, which are toxic to insects in general. Apply insecticide on a calm day to minimize drift, and do not apply right to the edge of the field. Avoid use of micro-encapsulated insecticides (especially broad-spectrum ones), because forager bees will collect and bring them back to the nest. If using an insecticide toxic to bees, never apply it to a crop in bloom. If applying a less-toxic insecticide to a flowering plant, do so in the late evening, when bees are no longer foraging. Do not use more insecticide than the product’s label recommends.
When pesticides are used, certain management practices can help to reduce the risk to pollinators. Avoid using pesticides on weeds unless they threaten crop production; weeds can be an important source of nectar and pollen. For example, Spanish needles (Bidens pilosa), common in much of the tropics and subtropics, is a nectar source for honey bees and may have other uses besides (Morton, 1962). For invasive weeds, spot treat them rather than broadcasting an herbicide, so that other plants will still flower and produce pollen and nectar for pollinators. Consider planting a green manure/cover crop that will flower; at ECHO’s farm in Florida, bees are often seen on pigeon pea flowers. Finally, consider mulching to reduce weed growth without the need for herbicides.
Plant Bee-Friendly Flowers and Other Plants
Planting flowers is another concrete step to help encourage the presence of pollinators. Yards and roadsides are two places where flowers can be grown. ECHO Asia Notes #18 (September 2013) included an article about flowers that attract beneficial insects, including pollinators. The article mentioned several members of the daisy family that are known to attract beneficials, including cosmos, marigolds, sunflowers, coreopsis and coneflowers. The ECHO Asia Seed Bank offers seed of Cosmos sulphureus; African marigold (Tagetes erecta); and Zinnia elegans. Some of the crops featured in our ECHO Florida Seed Bank also attract pollinators (see the “From ECHO’s Seed Bank” section of this issue).
Farming practices can also provide habitat and a food source for pollinators. As mentioned in the previous section, some kinds of cover crops (such as clover and alfalfa) provide nectar for bees. Hedgerows can help break up a “pollen desert” caused by a single crop. Flowering brassicas (e.g. cabbage and broccoli) also attract an abundance of bees. When planting brassicas, consider allowing a few of the plants to flower.
In an article titled “Growing Insects,” Richard Conniff described a blueberry farm in Michigan that was planted with rows of wildflowers between the rows of berry plants. As a result, the farm became home to lots of pollinator and predatory insects—and the farmer experienced less need for insecticides.
Finding the “right mix” of wildflowers to attract pollinators can be tricky. “Plants for Native Bees in North America,” (Shepherd 2008) an Invertebrate Conservation Fact Sheet from the Xerces Society for Invertebrate Conservation (www.xerces.org), shares the following:
“To help bees and other pollinators—like butterflies—you should provide a range of plants that will offer a succession of flowers, and thus pollen and nectar, through the whole growing season. Patches of foraging habitat can be created in many different locations….Even a small area planted with the right flowers will be beneficial, because each patch will add to the mosaic of habitat available to bees and other pollinators.”
The Xerces Society gives the following general guidelines for selecting good bee plants (reprinted from their website):
- Use local native plants. Research suggests native plants are four times more attractive to native bees than exotic flowers.
- Choose several colors of flowers. Flower colors that particularly attract bees are blue, purple, violet, white and yellow.
- Plant flowers in clumps. Flowers clustered into clumps of one species will attract more pollinators than individual plants scattered through the habitat patch. Where space allows, make the clumps four feet or more in diameter.
- Include flowers of different shapes. Bees are all different sizes, have different tongue lengths, and will feed on different-shaped flowers. Consequently, providing a range of flower shapes means more bees can benefit.
- Have a diversity of plants flowering all season. By having several plant species flowering at once, and a sequence of plants flowering through spring, summer, and fall, you can support a range of bee species that fly at different times of the year.
As you think about which plants to incorporate to help attract pollinators, also consider flowering perennials, especially indigenous species. At ECHO’s farm, several perennials are regularly visited by bees when they are in flower. These include Yellow Poinciana (Peltophorum pterocarpum), Clerodendrum spp., Jatropha integerrima, and Firebush (Hamelia patens).
Although honeybees have the largest profile as pollinators, they are certainly not the only insects important for pollination. According to a document by the FAO (2008), “…of the slightly more than 100 crop species that provide 90 percent of national per capita food supplies for 146 countries, 71 crop species are bee-pollinated (but relatively few by honeybees). Several others are pollinated by thrips, wasps, flies, beetles, moths and other insects.”
The National Academies (http://nas-sites.org/pollinators/about-pollinators/) shares the following information about pollinators and their favorite flowers:
- Bees prefer blue or yellow flowers and those that are sweet-smelling.
- Butterflies like flowers that are red, yellow or orange. Scent doesn’t matter; butterflies rely more on vision and less on scent to find nectar.
- Hummingbirds are attracted to red, orange or yellow flowers. Like most birds, hummingbirds do not have a highly developed sense of smell, so flower scent doesn’t matter.
- Bats like flowers that are large and white or pale in color. Some bat-pollinated flowers are open only at night and typically have a fermented, fruity or musky scent.
- Moths are attracted to sweet-scented flowers that are typically large and white or pale in color. Some moth-pollinated flowers are open only at night.
- Flies. In tropical regions, flies are often found on pale, dark brown, or purple flowers that stink of dung or carrion. In temperate regions, they can be found on flowers of many colors, usually those that have easy access to nectar.
- Wasps’ preferences are unknown.
- Beetles are typically attracted to flowers that are white or green and have a wide opening.
Many crops have very specific pollinators. The FAO, in collaboration with ICIPE, published an “Initial Survey of Good Pollination Practices” in 2008 (FAO 2008). The document contains information about “pollinator-friendly good agricultural practices,” in an attempt to document such practices before they are lost, and to base conservation activities on practices that already exist locally. It includes case studies from various locations around the globe, illustrating how valuable it can be to understand the biology and physiology of a crop and its pollinators.
For example, papayas exist as male, female, or bisexual (flowers with both male and female structures). Fruits develop on female and bisexual trees, with variations in fruit shape between female and bisexual plants (Morton 1987). On small farms in Kenya, as described in the first chapter of “Initial Survey of Good Pollination Practices” (FAO 2008), the most saleable papaya fruits are those from female-flowered trees. The male-flowered trees offer nectar and provide the pollen that is needed by the flowers on female trees—so it is important to retain some male trees, even though they are less productive in terms of fruit. Papayas produce white flowers that release their scent after sunset, suggesting pollination by moths or bats, and in fact several varieties of hawk moth are primarily responsible for pollination. In order for hawk moths to flourish in the area, they require nectar sources that can feed adult moths throughout the year. Food plants are also needed for the hawk moth larvae, to ensure that the moths will be able to reproduce and maintain a presence in the area. In the Kenyan location described in the chapter, morning glories that grow on hedgerows provide nectar and also are larval host-plants—so encouraging growth of morning glories can have a significant impact on papaya production.
Provide Nesting Sites for Bees
Minimizing use of insecticides and providing flowers for pollinators have been discussed. Another action to help insect pollinators is to provide nesting sites for honey bees and for native bees.
Honey bees produce delicious honey while they pollinate, so encouraging their presence can be doubly advantageous. ECHO’s Technical Note “Beehive Designs for the Tropics” contains descriptions and designs for several types of honey bee hives.
Nesting sites can also be made for other types of native bees. The Invertebrate Conservation Fact Sheet “Nests for Native Bees” (Shepherd 2012) presents ways to create nesting sites for native bees. These can be as simple as a bundle of reeds tied together and hung, or a wooden block drilled with a number of holes that vary in depth and diameter. Though geared toward North American native bees, the guidelines may be general enough to apply elsewhere as well. The Fact Sheet includes information about nests for wood-nesting and cavity-nesting bees, groundnesting bees, and bumble bees.
Throughout the tropics, stingless bees (Fig. 2) are important indigenous pollinators, especially in tropical America. These bees are in the family Meliponinae; there are hundreds of species. Individuals range in size from 2 to 13 mm, and colonies can include hundreds or tens of thousands of bees. In the wild, most stingless bees build nests in protective cavities using cerumen, a mixture of beeswax and plant resins. Stingless bees make small wax pots in which brood are raised, and somewhat larger wax pots in which they store honey. Historically, people have kept stingless bees in hollow logs, transferred from the forest.
If also raising stingless bees for honey (in addition to keeping them for pollination), other factors should be taken into consideration when providing artificial hive space. In an article in Bee World called “Beekeeping with Stingless Bees: A new type of hive,” Marinus Sommeijer (1999) describes the UTOB hive design, which largely separates the brood area and the honey-collecting area. This allows for minimal disturbance of the brood when honey is harvested, and also minimizes the likelihood that phorid flies will infest the brood. Though ideal dimensions will vary depending on the species of stingless bee, the basic design can be found at http://web.science.uu.nl/sommeijer/hive/hive.pdf.
For more information about stingless bee biology, see David Roubik’s (2006) review article, “Stingless Bee Nesting Biology.” In his article, Roubik quotes Roger Morse: “It is possible to keep bees in a hive only because we understand their biology. Beekeeping is the application of our knowledge of bee behavior.” For online access to this and other documents mentioned in this article, see the references section below.
Though insect pollinators have been declining over recent years, strategies exist that can help bolster their numbers. Look for alternatives to pesticides and, when necessary, use less-toxic pesticides applied in a manner that is least likely to harm insect pollinators. Plant flowers and use farming practices that will provide nectar for pollinators. Where feasible, provide suitable habitat for nesting sites. Read and learn about pollinators for crops that are of particular interest; an understanding of specific pollinators’ biology and physiology can lead to targeted strategies for ensuring that a food source and nesting sites are available year-round.
Bicksler, Abram. 2013. New Seed Bank Additions. ECHO Asia Notes #18.
Conniff, Richard. 2014. “Growing Insects: Farmers can help to bring back pollinators.” Yale Environment 360. www.e360.yale.edu/feature/growing_insects_farmers_can_help_to_bring_back_pollinators/2735/
Coordination Team. 2013. Declining Bee Numbers: Measures to protect the pollinators in Africa and Asia. http://Capacity4dev.ec.europa.eu/article/declining-bee-numbers-measures-protect-pollinators-africa-and-asia
Dufour, Rex. Updated 2014. Biorationals: Ecological Pest Management Database. National Sustainable Agriculture Information Service. www.attra.ncat.org/attra-pub/biorationals/
FAO. 2008. Tools for Conservation and Use of Pollination Services: Initial Survey of Good Pollination Practices. Food and Agriculture Organization of the United Nations. www.internationalpollinatorsinitiative.org/uploads/SURVEY%20DEC%2008%20Small.pdf
Morton, Julia F. 1962. Spanish Needles (Bidens pilosa L.) as a Wild Food Resource. Economic Botany 16 (3), pp. 173-179.
Morton, Julia F. 1987. Fruits of Warm Climates. Julia F. Morton
National Academies. 2012. About Pollinators. Resources on Pollinators, National Academy of Science. www.nas-sites.org/pollinators/about-pollinators/
Roubik, David. 2006. Stingless Bee Nesting Biology. Apidologie 37(2): 124–143. www.apidologie.org/articles/apido/abs/2006/02/m6034sp/m6034sp.html
Shepherd, Matthew. 2012. “Nests for Native Bees.” Invertebrate Conservation Fact Sheet, Xerces Society for Invertebrate Conservation. www.xerces.org/wp-content/uploads/2008/11/nests_for_native_bees_fact_sheet_xerces_society.pdf
Shepherd, Matthew. 2008. “Plants for Native Bees in North America.” Invertebrate Conservation Fact Sheet, Xerces Society for Invertebrate Conservation. www.xerces.org/wp-content/uploads/2008/11/general_bee_plants_fact_sheet_xerces_society.pdf
Sommeijer, Marinus. 1999. Beekeeping with stingless bees: a new type of hive. Bee World 80:70-79 http://web.science.uu.nl/sommeijer/hive/hive.pdf.
Spivak, Marla. 2013. Why Bees Are Disappearing. TED Talk. www.ted.com/talks/marla_spivak_why_bees_are_disappearing
Vaughan, Mace and Scott Hoffman Black. 2007. “Pesticide Considerations for Native Bees in Agroforestry.” Agroforestry Notes #35. USDA National Agroforestry Center. www.plants.usda.gov/pollinators/Pesticide_Considerations_For_Native_Bees_In_Agroforestry.pdf
Berkelaar, D. 2014. Bringing Back the Pollinators. ECHO Development Notes no. 125