Pollinators and beneficial insects
Which crops need insect pollinators, bees and wild pollinators, why they are declining, other beneficial insects, and a pollination value sum.
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Most talk about insects in agriculture is about pests, yet the great majority of insect species do crops no harm, and many are essential. Bees and other insects pollinate fruit, vegetable, nut and oilseed crops; predators and parasitoids keep pests in check; dung beetles and termites recycle nutrients; and silkworms, honey bees and lac insects provide products worth billions. Protecting these beneficial insects is part of good farming.
Pollinators, their value and their decline, and other beneficial insects are regular exam topics in entomology, apiculture and sustainable agriculture.
Pollination and crops
Pollination moves pollen from anthers to stigmas. Cereals such as wheat, rice, maize and barley are pollinated by wind or by themselves and need no animals. But many other crops depend, to some degree, on animal pollinators, mainly insects.
A. M. Klein and colleagues (2007) found that 87 of the 115 leading global food crops benefit from animal pollination, although these crops supply a smaller share of total calories than the wind-pollinated cereals. The global Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) estimated in 2016 that 5 to 8 per cent of global crop production is directly attributable to animal pollination, worth roughly US$235 to 577 billion a year. Pollinator-dependent crops also supply much of the vitamin A, vitamin C, folate and other micronutrients in human diets.
Good pollination also improves quality: well-pollinated apples, strawberries and cucurbits are larger and better shaped, because each fertilised seed stimulates the surrounding tissue to grow.
The pollinators
- Honey bee (Apis mellifera, and Apis cerana in Asia): the main managed pollinator, easily moved in hives to crops in flower. Almond orchards in California alone use well over a million hives each spring.
- Wild bees: about 20,000 species worldwide, including bumble bees (also reared commercially for greenhouse tomatoes, which they buzz-pollinate), mason bees, carpenter bees, stingless bees and many solitary ground-nesting bees. Many are more efficient per visit than honey bees for particular crops.
- Other insects: hoverflies and other flies, beetles, butterflies and moths, and tiny midges, which are the main pollinators of cocoa.
- Vertebrates: birds and bats pollinate some crops, such as durian and agave.
L. A. Garibaldi and colleagues (2013) showed, across 41 crop systems worldwide, that wild insects increased fruit set regardless of how many honey bees were present: honey bees supplement wild pollinators but do not replace them.
Why pollinators are declining
- Loss of habitat and flowers, as hedges, field margins and natural areas are cleared.
- Pesticides, especially insecticides applied to flowering crops or drifting onto flowers; neonicotinoid seed treatments were restricted in the European Union from 2013 and 2018 partly for this reason.
- Pests and diseases of bees, above all the parasitic Varroa mite and the viruses it spreads, which spread worldwide from Asian honey bees.
- Climate change, which shifts flowering and pollinator activity out of step.
- Invasive species and loss of genetic diversity in managed bees.
Protecting pollinators on farms
- Do not spray insecticides on crops in flower; if unavoidable, choose products less toxic to bees and spray in the evening when bees are not foraging. Follow label bee warnings.
- Tell beekeepers before spraying near hives.
- Provide flowers and nesting sites: flower strips, hedgerows, cover crops, undisturbed ground and dead wood.
- Rotate and diversify crops, so flowers are available over a long season.
- Place hives well, at the right density and at the start of flowering.
Other beneficial insects
- Predators: ladybird beetles, lacewing larvae, ground beetles, predatory bugs, hoverfly larvae, and dragonflies, which eat pests. Parasitoids: wasps and flies whose larvae develop inside or on pests, such as Trichogramma egg parasitoids. These are covered in the article on biological control of insect pests.
- Weed biological control: the moth Cactoblastis cactorum cleared invasive prickly pear from millions of hectares in Queensland and New South Wales in the late 1920s and 1930s.
- Decomposers and soil builders: dung beetles bury livestock dung, recycling nutrients and reducing pasture flies; termites and other insects break down plant material.
- Productive insects: honey bees (honey, wax), silkworms (Bombyx mori, silk), lac insects (lac resin and dye), and insects reared as protein for animal feed, such as black soldier fly larvae.
What is pollination worth on a farm?
An apple orchard yields 40 t/ha, sold at 0.50 dollars per kg, so its crop value is 40,000 × 0.50 = 20,000 dollars per hectare. If apples have a pollinator dependence ratio of 0.65 (65 per cent of yield lost without pollinators):
- Value attributable to pollination = 20,000 × 0.65 = 13,000 dollars per hectare.
Hive stocking. The grower rents honey bee hives at 2.5 per hectare for 12 ha, at 60 dollars per hive:
- Hives = 12 × 2.5 = 30; cost = 30 × 60 = 1,800 dollars, or 150 dollars per hectare, less than 1.2 per cent of the pollination value.
Real dependence ratios and stocking rates vary by crop, variety and region; almonds, for example, are usually stocked more heavily, often around 5 hives per hectare.
Key points
- 87 of 115 leading food crops benefit from animal pollination; pollination contributes 5–8 per cent of global crop production (IPBES 2016).
- Cereals are wind- or self-pollinated; cocoa, kiwifruit, melons and pumpkin depend almost entirely on pollinators; almond, apple and cherry depend greatly.
- Honey bees are the main managed pollinators, but wild bees and other insects add pollination that honey bees cannot replace.
- Declines come from habitat loss, pesticides, Varroa and diseases, and climate change.
- Other beneficial insects include predators, parasitoids, weed biocontrol agents, decomposers and producers of honey, silk and lac.
What the exams ask
- School-leaving and entrance papers: what pollinators do, examples of insect-pollinated crops, and useful insects such as bees, silkworms and lac insects.
- Degree and research-fellowship entrances: pollinator dependence, the value of pollination, causes of pollinator decline, bee-safe pesticide use, and beneficial insects in IPM.
- Professional examinations: pollination management, hive stocking, and pollinator-friendly farm design.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Do I need bees for a self-fertile variety?
Often yes, at least partly. Many self-fertile varieties still need a vector to move pollen within the flower, and insect visits usually improve fruit set and quality.
Are honey bees native everywhere?
No. Apis mellifera is native to Europe, Africa and western Asia, and was introduced to the Americas, Australia and East Asia. Each region also has its own wild bees.
Is it safe to spray when bees are not visible?
Bees may arrive later, and residues on flowers can remain toxic for hours or days. Spray outside flowering if possible, and check the label for residual bee toxicity.
Sources
- Klein, A.-M. et al. "Importance of pollinators in changing landscapes for world crops." Proceedings of the Royal Society B 274, 303–313 (2007).
- IPBES. The Assessment Report on Pollinators, Pollination and Food Production. Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, 2016.
- Garibaldi, L. A. et al. "Wild pollinators enhance fruit set of crops regardless of honey bee abundance." Science 339, 1608–1611 (2013).
- Potts, S. G. et al. "Global pollinator declines: trends, impacts and drivers." Trends in Ecology and Evolution 25, 345–353 (2010).
- Delaplane, K. S. and Mayer, D. F. Crop Pollination by Bees. CABI, 2000.
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