Food for Thought

Janet Raloff
Food for Thought

The Buzz over Coffee

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Most people consider the continued spread of Africanized honeybees in the Americas as horrifying news. Nicknamed killer bees, these notorious social insects rile into stinging mobs with little provocation. But new research finds evidence that these irritable insects have been performing a hitherto unrecognized service for people around the world. They've helped keep down the cost of growing high-quality coffee.

Although coffee plants are self-pollinating, David W. Roubik of the Smithsonian Tropical Research Institute in Balboa, Panama, reports that coffee-bean yields skyrocket when the shrubs' flowers are visited by pollinating insects. And in much of the Western Hemisphere's coffee-growing neotropics, those pollinators have become Africanized honeybees.

Roubik was studying Panamanian coffee production in 1982, when the Africanized bees emerged in Panama's easternmost tip. By 1985, the bees were conducting regular sorties into coffee groves where Roubik was collecting data.

Growers and pickers weren't excited about the new arrivals because they kept getting stung. "I got attacked a few times, too," Roubik notes. However, the entomologist suspected that in addition to being nuisances, the invading bees might also be offering benefits. From the beginning, he had noticed that the insects were regular visitors to coffee flowers.

Two decades later, he has now quantified just how beneficial Africanized bees are. Berry yields of coffee plants increase by some 60 percent when the feisty bees take up residence on the edges of a coffee grove, according to Roubik's report in the June 13 Nature. Each berry contains two seeds, or beans, that are eventually roasted to produce brewable coffee.

Roubik told Science News Online that the findings from this study and a far larger previous one that's still in press show that Africanized bees' "free service" to growers accounts for roughly one-third of their earnings.

The busy bees

Prior to the arrival of Africanized bees, other insects had been doing the flower circuit in coffee groves. Most were stingless bees, insects that produce honey but tend to be smaller than European honeybees. A few big bees, such as bumblebees, also showed up for coffee.

However, Roubik observes, because the plants didn't require pollinators to set seed, most biologists discounted the importance of such winged visitors. That "was just bad botany," he says.

Coffee flowers emerge from nodes every 10 centimeters or so along a branch. As many as 200 blooms may develop at each node. To test the role of pollinators, Roubik enclosed some flowering branches in mesh bags with holes too small for insects to pass through. He then counted and logged the insects visiting unbagged branches. As fertilized flowers gave way to berries, he counted and weighed all fruit that survived their 8-month ripening period.

His new data show that unbagged coffee blooms saw a lot of bee traffic. The average flower lasts a day, yet over that time stands a good chance of being visited 40 times by honeybees. Stingless bees and other pollinators also dropped by, but nowhere near as frequently.

If every flower on a branch were fertilized and developed fully, a coffee plant wouldn't be strong enough to support the berries produced. "So, the plant actually makes decisions as to how many [fruit] it will bring to full term," Roubik says. Week after week, more unripened fruit drops off. By harvest, only about a fifth of the coffee berries that started from pollinated blossoms remains.

In his first study, of shrubs 6 to 8 years old and at peak production, Roubik and his colleagues tallied the activity of all pollinators. "I also looked at [the resulting] 27,000 fruit" and kept track of their survival through several tedious counts and recounts, says Roubik. He and his colleagues found that the unbagged flowers yielded almost 60 percent more fruit by weight.

In his second study, Roubik compared fruit production of bagged and open branches in a 5-hectare field of 2-year-old coffee shrubs. The researchers suspected that these younger plants, in their first flowering, were more likely to show variability in their fruit and bean yield.

On unbagged branches, which therefore had hosted insect pollinators, an average of about 200 baby berries eventually winnowed down to about 40 ripe ones. The latter figure is roughly 50 percent more than the eventual yield of bagged, self-pollinated branches, Roubik reports. Moreover, he notes that ripe berries on insect-pollinated branches were an average of 7 percent heavier than those on self-pollinated branches.

Bee-cause there were trees

Many U.S. farmers seasonally rent portable honeybee hives to help pollinate crops. European honeybees aren't available in the neotropics, and the Africanized bees that are there aren't amenable to such management. However, Roubik found that in Panama, the feral Africanized bees–which tend to inhabit the hollows of large, old trees–had no trouble finding his test plots. The reason: Panamanian coffee growers maintain shady forests in and around their fields.

Indeed, coffee evolved on the shady slopes of mountains. Only in the past couple decades have growers in many parts of the world begun cultivating varieties that can take full sun. In general, Roubik says, coffee drinkers find that the beans of full-sun plants don't make as good a tasting brew as shade-grown coffee does. Growers of coffee in sunny fields tend to crowd more plants into a hectare, and they use fertilizer, pesticides, and herbicides, says Roubik. Shade-coffee growers don't, and the price they pay is a lower crop yield.

Roubik speculates, however, that the cost savings in farming shaded coffee plants in pollinator-rich forested environments offset much of the yield difference between sun-grown and shade-grown coffee crops.

Over the past 5 years Vietnam has become one of the world's leading coffee producers. The high yield of Vietnamese coffee plants has helped produce a coffee glut on world markets and thus contributed to depressed prices, Roubik says. He adds that the Vietnamese product is generally considered to be of low quality.

On the other hand, Central American farmers tend to grow high-quality beans, which are suited to boutique coffee bars and other high-end retailers in the United States and Europe. However, the world coffee glut probably would price shade-grown beans out of the market, says Roubik, if it weren't for the free pollination services of killer bees.


David W. Roubik

Smithsonian Tropical Research Institute

Unit 0948

APO AA 34002-0948


Further Reading

Milius, S. 2001. Africanized bees rescue loner trees. Science News 160(Dec. 8):367. Available to subscribers at [Go to].

_____. 2000. Invaders can conquer Africanized bees. Science News 157(May 13):317. References available at [Go to].

Raloff, J. 1996. Growers bee-moan shortage of pollinators. Science News 149(June 29):406. Available at [Go to].

Weiss, R. 1989. Texans devise battle plan for Africanized honeybee invasion. Science News 136(Dec.16):389.

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