AGRA: Parts of the Taj Mahal's pearly-white exterior are turning green, this time even in winter.

According to the Archaeological Survey of India (ASI), the culprit is "Goeldichironomus", an insect species that deposits its droppings on the monument's marble surface.

To counter the problem and find a long-term solution, ASI has embarked on a study this month - which may take up to a year.

Superintending archaeologist Rajkumar Patel said on Tuesday that the insect problem has been persistent since 2015, except for 2020 when the pollution levels were low due to the lockdown.

This year, the insect has been present in large numbers and "is still breeding, despite a drop in temperature", said Patel."In previous years the insects disappeared by October-end.

These insects prefer to take short flights away from the water while they mate.

North wall is the worst-affected," he added.

First spotted in 2015, it leaves dark brown and green stains on the marble and the inlay work.

It breeds in the polluted waters of the Yamuna river, an ASI official pointed out.

"The ASI has been trying to get rid of the stains by washing them with distilled water and rubbing them with a cotton cloth.

But the insect keeps reappearing during its breeding season (between March and April and September and October)," the official added.

According to Patel, ASI's chemical department is conducting a detailed study on ways to stop the insects' growth on the monument's surface.

He said, "This is not a uniform problem.

It is restricted only to the Taj's riverside face."Agra-based doctor and environmentalist Sharad Gupta said, "The solution to this problem is to increase the water flow in the Yamuna and reduce the pollution levels in the river, which is giving these insects an ideal environment to breed behind the Taj."