The beauty of the intricate marble work at the Taj Mahal is being threatened, this time by insect droppings.

Insects breeding in the polluted Yamuna River near the 17th century monument leave behind green and black patches of waste on its walls, archaeological experts said on Monday. While workers scrub the walls clean daily, regular scrubbing can ruin the floral mosaics and the sheen of the marble, Bhuvan Vikram of the Archaeological Survey of India said. He added that marble panels of plant motifs on the walls or reflective tiles are “becoming disfigured”, The Associated Press reported.

Authorities are looking to find a permanent solution to the issue created by an insect, which is a type of elongated fly that resembles a mosquito. The insects are multiplying in great numbers in the Yamuna, which is polluted to the point that it no longer supports fish that once kept the insect count in check, environmentalist Yogesh Sharma said. According to Girish Maheshwari, who heads the Department of Entomology in Agra’s St John’s College, the growth of algae and phosphorus deposits from ash dumped by a nearby cremation ground “are the primary source of food for this particular species of insect”.

The Taj Mahal attracts millions of tourists to Agra every year. This makes damage to its structure a matter of serious concern. Uttar Pradesh Chief Minister Akhilesh Yadav on Sunday ordered an investigation into the discolouring of the World Heritage site. He directed senior officials, including those of the State Public Works Department, UP Pollution Control Board and archaeological department, to examine the monument to find the “real cause” of the problem and find a “quick solution”, The Hindu reported.

Earlier this month, the National Green Tribunal sent notices to the Ministry of Environment and Forest and the Uttar Pradesh government, among others, after hearing a plea filed by an environment activist from Agra. According to the petition, the dumping of waste in the Yamuna led to the stagnation of the river and the consequent “explosive breeding” of the insect, which is a “biological indicator of water quality and localised water pollution.” Alarmed by the phenomenon, Yadav vowed that his government “will not allow any damage to happen” to the monument.