Scientists identify pesky midges

Two different species plaguing Bedok and Pandan reservoirs, but reason for outbreaks still not known
Grace Chua Straits Times 15 Dec 12;

THE researchers studying Bedok Reservoir's midge nuisance have solved a pesky ecological whodunnit.

They now know that two different non-biting species of the tiny insects are the ones responsible for the swarms that have plagued residents around Bedok and Pandan reservoirs for the past couple of years.

But they have not yet worked out the how or why, or how to prevent future outbreaks.

"The first thing you have to know is who's causing the problem, then you can start addressing the issue," said National University of Singapore biologist Rudolf Meier, who is leading a study of the midge problem. The three-year project is part-way through its first year.

The culprit at Pandan is one called Polypedilum nubifer, a common nuisance species that dominates ecosystems almost everywhere it is found.

But at Bedok, the culprit is a fly called Tanytarsus oscillans, a minuscule green species. It has previously been found in Sumatra, India and Japan - but is not known to cause problems anywhere else in the world, said Professor Peter Cranston, the Australia-based midge expert helping with the study.

Identifying the midges is not as simple as looking at stripes or spots, researchers said. It involves staring at the insect's rear end and mouth parts under a microscope to work out whether they really are different from other species.

Prof Cranston said of the Bedok issue: "We don't know what the environmental trigger is yet."

These midges live in the reservoir year-round, but their numbers explode only at the end and beginning of the year. Elsewhere, triggers for a midge swarm can be natural, such as water that goes from flowing to still. Or they can be man-made. For instance, nutrients flow into waterways from deforestation, construction or changing agricultural practices, Prof Cranston said.

But water agency PUB said the water quality in Bedok Reservoir, which is more than 25 years old, is the same as it was before the midge swarms started last year.

Seven ponds in the area collect rainwater from Bedok, Tampines and Tanah Merah, and flow into the reservoir.

Another explanation is that the small fish that feed on midge larvae might have been eaten by larger non-native fish that live in the reservoir.

At Pandan Reservoir, plant roots that dangle from floating wetlands provide a place for these small fish to hide, said PUB biologist Michelle Sim.

Now, the research team breeds midges in the lab and collects data on environmental variables like temperature and water quality to work out possible causes.

Three times a week, the PUB also counts the overall number of midge larvae (of all species) in the water; if it gets above a threshold number, it knows a swarm of adult midges is about to occur.

At Bedok, general preventive measures such as fogging larvicides and scrubbing algae off rocks where midges might lay their eggs are used, and these target all midge species, said Mr Goh Chong Hoon, a deputy director of catchment and waterways at PUB.

In the past few months, there have been only isolated complaints, he said. And he said that currently, there are no warning signs yet of another midge outbreak. "So far, we've been able to keep the numbers low."