Batmen (may) save your bank and put rice on your plate

[Another version of this article was first published on SciDev.net]

Bats that prey on a major insect pest of paddy in Thailand could help ensure food security and save paddy harvest worth millions of dollars, according to a paper published in Biological Conservation.

A team of German and Thai scientists modeled the predation of the wrinkled-lipped free-tailed bat (Tadarida plicata) on the paddy insect pest, white-backed planthopper (Sogatella furcifera). Extrapolating their model across Thailand, they calculated that the bats—each estimated to eat  1,130 planthoppers daily—are preventing paddy losses of 2,892 tonnes per year, which is worth USD$1.2 million and enough to feed 26,000 people for a year.

A wrinkle-lipped free tail bat (Tadarida plicata)

A wrinkle-lipped free tail bat (Tadarida plicata)

Thomas Cherico Wanger, lead author of the paper and tropical ecologist at University of Göttingen, suggests that Thai farmers can recruit bats in their fields by providing roosting boxes.

“The model shows that 300 bats in each roosting box can protect almost 700kg of rice per year.”

White-backed rice planthoppers (Sogatella furcifera) are one of the top rice pests in Thailand.

White-backed rice planthoppers (Sogatella furcifera) are one of the top rice pests in Thailand.

The white-backed planthoppers are one of top rice pests in Thailand, says Geoff Gurr, applied ecologist at Charles Stut University, Australia. Gurr has been working on biological control of planthoppers with arthropod predators. Bats however, were not considered in their work.

“Our farmers never think of using bats as biocontrol agents,” says Wantana Srirattanasak, senior entomologist of the Thailand Department of Rice.

Yet, a year-long survey in 2005 reported that planthoppers made up almost 30% of the items found in wrinkled-lipped free-tailed bats’ diet. Working from this survey, Wanger and his team built a model to “quantify the amount of rice that bats protect when they feed on planthoppers.”

Combining data from the literature with their field observations, the scientists used the model to first estimate the number of white-backed planthopper consumed by all the wrinkled-lipped free-tailed bats in Thailand, and then estimate the amount of rice harvest saved due to the predation of these planthoppers.

“Modeling is a tool to test a priori formulated hypotheses and scientific questions,” says Wanger. Models however, simplify nature based on assumptions which might be wrong. Hence, it is crucial to “compile good data” and “to indicate the level of error that comes with an estimate.” Wanger’s model ran 10,000 iterations, which allowed Wanger to “quantify the level of error in his predictions.”

“The model has merits as a thought experiment,” says Gurr of the study, but notes that only one field survey was used to estimate the bats’ predation of the planthoppers. “It is not a substantial base on which to extrapolate too widely.”

Another caveat is that the amount of rice protected by the bats, 2,892 tonnes, is only a tiny portion of the 25-30 million tonnes of rice produced annually in Thailand. Bats might not be irrelevant, says Gurr “but they are a very small portion of the mortality that would be required to control the planthoppers.”

Wanger’s team understands the limits of their model, and emphasizes that it is crucial to test predictions of the model against field experiments and more diet analyses. Their modeling code, published with their paper, was made “as transparent as possible” to help others test their predictions.

Wrinkle-lipped bat emerges from their caves to feed.

Wrinkle-lipped bat emerges from their caves to feed.

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