Bats: nice with rice?

Scientists believe a species of bat that preys on a major rice pest in Thailand could boost food security by preventing paddy losses
SciDev theguardian.com 16 Apr 14;

Scientists believe wrinkle-lipped bats could prevent annual paddy losses of nearly 2,900 tons, enough to feed 26,000 people for a year. Photograph: Barbara Walton/EPA

Bats that prey on a major rice pest in Thailand could save paddy harvests worth millions of dollars and help contribute to better food security, claim scientists in a paper published in Biological Conservation.

Using data from a previous study and their own field survey, the scientists put a value on the wrinkle-lipped bat's predation on the white-backed planthopper, a migratory insect pest in Asia that feeds on rice shoots.

The scientists calculated that each wrinkle-lipped bat (Tadarida plicata) consumes about 1,130 white-backed planthoppers (Sogatella furcifera) daily. With a population of almost 8m, the bat species may prevent annual paddy losses of nearly 2,900 tons – enough to feed 26,000 people for a year – with an export value of $1.2m (£716,000).

Thomas Cherico Wanger, lead author of the paper and a tropical ecologist at the University of Göttingen in Germany, suggests that Thai rice 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," says Wanger.

Bats have yet to be considered as a significant pest control agent in agriculture, according to Wantana Srirattanasak, senior entomologist of Thailand's department of rice. "Our farmers have never thought of using bats as biological control agents," she says.

A year-long survey in 2005 reported that planthoppers made up almost 30% of the diet of wrinkled-lipped bats. 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 estimate the number of white-backed planthoppers consumed by all the wrinkled-lipped bats in Thailand. They then estimated the amount of rice harvest saved due to the predation of these planthoppers.

But models simplify nature based on assumptions that might be wrong, Wanger warns, adding that it is crucial to "compile good data" and "to indicate the level of error that comes with an estimate".

"The model has merits as a thought experiment," says Geoff Gurr, applied ecology professor at Charles Sturt University in Australia.

But Gurr, who has been working on the biological control of planthoppers with arthropod predators, notes that only one field survey was used to estimate the bats' predation on planthoppers. "It is not a substantial base on which to extrapolate too widely," he says.

Another caveat is that the amount of rice saved by the bats – 2,900 tons – is only a tiny portion of the 25-30 million tons 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 says his team understands the limits of their model. But he argues it is crucial to test predictions of the model against field experiments and more data analyses. He says their modelling code, published with their paper, was made "as transparent as possible" to help others test their predictions.