By Rex Graham
A U.S. ecologist has discovered that insect-eating birds devour the worst pest of Costa Rican coffee plantations, reducing damage to ripening coffee berries and increasing income to growers by up to $310 per hectare annually. Daniel Karp used sophisticated DNA tests to prove that Yellow Warblers and four other bird species gorge on 2-milimeter-long coffee berry borer beetles, the most damaging insect pest to the world’s $90-billion-a-year coffee industry.
Coffee plantation insect pest
The 2-millimeter-long beetle is native to Africa where coffee plantations were first developed. The beetle began spreading a century ago via the transport of coffee seed.
The beetle arrived in Costa Rica in 2000, and is now destroying up to 50 percent or more of the coffee berries on plantations in Central American. The same or higher damage is reported in Hawaii and nearly all other coffee-producing countries.
Karp’s study, published in the Aug. 27, 2013, online version of Ecology Letters, was the first to document the economic benefit to coffee growers of pest-control provided by insect-eating birds: they reduced coffee berry borer beetle infestation by 50 percent.
This study has implications beyond coffee, yellow warblers and Costa Rica. Rather than relying on feel-good pronouncements on the importance of biodiversity, the study offers credible, quantitative estimates of the value of a pest-removal “service,” provided by birds in this case, and the habitat that makes this service possible.
Coffee berry borer beetle
An adult female beetle (Hypothenemus hampei) nibbles a single tiny hole in a coffee berry and lays 35 to 50 eggs. Inside, the offspring grow, mate, and then emerge from the commercially ruined berry to disperse, repeating the cycle. Pesticides are mostly ineffective because the beetle juveniles are protected inside the berry nurseries, but they are vulnerable to predation by birds when they emerge.
Coffee pickers pass by the discolored berries and harvesting equipment screens out any that get through. Growers remove infested berries on bushes and on the ground. However, those efforts may be only slowing the spread of beetles.
When beetles emerge from infested berries, birds queue up for the bonanza like Starbucks regulars with free gift cards. (Karp reported that local bats tested hadn’t eaten enough beetles to be detected in their feces.)
Ornithology economic payoff
Karp is a lifelong birder and connoisseur of good coffee. He conducted the study as part of his Ph.D. thesis with a distinguished team of Stanford University scientists that combines rigorous field studies with analyses of the economic benefits of diverse habitats.
In Costa Rica, Karp focused on coffee bushes at a Rainforest Alliance-certified farm and a farm located in a deforested region. In both places, he grabbed the seventh branch from the top of coffee plants and examined 100 berries for borer entry holes.
He found that birds significantly reduced infestations at both plantations during two growth seasons. When birds were excluded from coffee bushes on both farms, the borer-infestation rate doubled.
Free pest control
His experiment was designed to test the theory that maintaining natural habitat improves coffee farm yields by supporting predators of coffee pests. The theory may sound plausible, but it was never rigorously tested.
To do that, Karp mist-netted birds amid dense coffee foliage. Forests adjacent to farms are thought to be important to insect-eating birds, Karp said, as a source of alternative prey, refuge during disturbances, and sites for nesting, hibernation and roosting. However, the bottom line for coffee growers is income. Do insect-eating birds deliver on that score? The study reported an eye-popping value for the avian pest control.
Win-win for biodiversity, farmers
“These pest-control services prevented $75 to $310 (U.S. dollars) per hectare, per year, a benefit per plantation on par with the average annual income of a Costa Rican citizen.” the study said. “Retaining forest and accounting for pest control demonstrates a win-win for biodiversity and coffee farmers.”
One of Karp’s mentors was ecology Professor Gretchen Daily, director of Stanford’s Center for Conservation Biology and co-author with journalist Katherine Ellison of “The New Economy of Nature: The Quest to Make Conservation Profitable.” Retail sales of coffee are about $90 billion a year. About 20 million households owe their livelihoods to the caffeinated crop.
Pests found in poop
Karp’s proved the pest-control service of birds by analyzing their poop for the presence of digested beetles. “We had the not-so-glamorous task of collecting the birds’ poop, and then taking it back to Stanford and looking through the DNA within it to learn which birds were the pest preventers,” Karp said in a Stanford news release.
The 5 beetle-eating birds
He was surprised to find beetle DNA in the digestive systems of five bird species:
Rufous-capped Warbler (Basileuterus rufifrons)
Yellow Warbler (Setophaga petechial)
Rufous-breasted Wren (Pheugopedius rutilus)
Buff-throated Foliage-Gleaner (Automolus ocrolaemus)
White-tailed Emerald (Elvira chionura)
All five species were more common in the plantation with nearby forests. (Karp said other species may be eating the beetles, but his experimental techniques didn’t prove it.)
All birds were difficult to see until captured in mist nets. Karp said he didn’t actually see a bird actually eating a borer beetle. The only way to tell was by DNA analyses of their droppings, collected in sterile bags during mist netting.
Assessing other pest control networks
The take-home message of Karpetal_EcolLett_2013 to coffee-growing countries is the importance of birds that eat borers. “Maintaining countryside forest elements on farmland may therefore represent a critical component of borer control strategy,” said Karp, Daily and their five other co-authors.
They also said in their study, that their approach should be applied to other predator-pest networks in other places. “Our results show that adjusting agricultural practices to conserve countryside forest elements, and associated biodiversity, may limit loses from the most damaging pest of one of the world’s most economically important crops.”