Bees & Carrots

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Bees and the crops they pollinate are at risk from climate change. Climate change will exacerbate the risk of extinction for bees and other pollinating insects already under threat from pesticides and habitat loss.

Bees essential to pollinate British crops face increased risk of extinction because of climate change, a major UN report is expected to warn.

Changes to habitats and to behaviour of different species as a result of warmer weather will exacerbate the danger to bee species already facing numerous other threats, according to scientists.

Some species could face extinction while declining numbers would harm harvests of British crops such as apples, raising fears from businesses such as cider-makers that their livelihoods could be at risk.

Bees pollinate more than 1 billion worth of crops in the UK each year including fruits and vegetables such as carrots, cabbages, apples and pears. (Intergovernmental Panel on Climate Change (IPCC) report 2016 - summary here)

Alfalfa Leafcutter Bees (full USDA report here)

The domesticated honeybee may get more glory, but when it comes to pollinating carrots, one tiny alfalfa leafcutter bee can do the job of 20 of its larger, noisier, more irritable cousins, says a U.S. Department of Agriculture researcher.

Alfalfa leafcutter bees are about 8cm long, and black with bands of white hair on the abdomen. "They prefer to spend spring and summer days gathering nectar and pollen from the purple blooms of their favorite plant, alfalfa," said entomologist Vincent J. Tepedino of USDA's Agricultural Research Service.

Species: M. rotundata

Genus: Megachile

Subgenus: Megachile (Eutricharaea)

Family: Megachilidae

Some references and further reading about bees and carrot pollination here.

Alfalfa leafcutter bees are among the species scientists call "solitary bees." These do not live in a communal nest or hive as do the familiar honeybee. Instead, as the insect's name suggests, the female alfalfa leafcutter cuts alfalfa leaves and uses them to build a nest of snug, cell-like compartments to house offspring.

"Domesticated honeybees can get irritable when large numbers of them are confined in small spaces," said Tepedino. "That intimidates people. Alfalfa leafcutter bees are gentler. And they tend not to buzz around your face or land on you like the meat flies some plant breeders now use."

Tepedino and colleagues at the Logan laboratory investigate native bees that can supplement or replace domesticated honeybees in fertilizing crops that depend upon insect pollinators.

Tepedino's study was the first detailed comparison of the domesticated honeybee and alfalfa leafcutter bee's performance as carrot pollinators in screened enclosures. It also was the first to show that alfalfa leafcutter bee offspring are well nourished by carrot pollen and carrot nectar. This bee typically grows up on rations of alfalfa pollen and alfalfa nectar.

"Unlike domesticated honeybees, alfalfa leafcutter bees don't mind working in screened enclosures or greenhouses needed for producing new kinds of carrots," said Tepedino, based at the ARS Bee Biology and Systematics Laboratory here. "About 150 alfalfa leafcutter bees working in screenhouses or greenhouses would do the job as well as 3,000 domesticated honeybees."

The pollination job is an essential but underappreciated part of a plant breeder's task of producing the new, more colorful or more nutritious carrot varieties that growers and consumers want. Tepedino said his study shows the leafcutter bee's efficiency may reduce pollination costs, as well as ease the stress on breeders and technicians who might otherwise work in cages with several thousand honeybees.

Alfalfa leafcutter bees are already raised and sold commercially to pollinate not only alfalfa but also carrots and onions in screened cages, Tepedino noted.

To produce new carrot varieties, pollen must be ferried from one variety of parent carrot plant to another, a process known as hybridization. That means carrot breeders must enclose the carrot plants--and the pollinating insects--in screenhouses or greenhouses to keep out other, unwanted pollen.

Tepedino ran his experiment with both species of bees using six screened cages, each containing 70 parent carrot plants. He found that yield of live seed from plants pollinated by either insect was approximately the same.

Tepedino used 9,000 adult honeybees and 450 adult alfalfa leafcutter bees for the study. He did the work with help from Asgrow Seed Company at Twin Falls, Idaho.

Solitary bees, Tepedino said, aren't attacked by two kinds of mites that have decimated many commercial honeybee colonies in this country in recent years. Also, the solitary bees can't mate with Africanized honeybees. So solitary bees--again unlike domestic honeybees--can't pick up the Africanized bees' trait of extreme defensiveness and increased likelihood of stinging people and animals.

Some references:

Getting to know Alfalfa Bess - Lancaster University (pdf) here

The Alfalfa Leafcutting Bee, Megachile rotundata: The World's Most Intensively Managed Solitary Bee January 2011Annual Review of Entomology 56(1):221-37 DOI: 10.1146/annurev-ento-120709-144836 SourcePubMed

Limited Cross Plant Movement and Non-Crop Preferences Reduce the Efficiency of Honey Bees as Pollinators of Hybrid Carrot Seed Crops -  February 2019

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