The white-marbled walls of the Taj Mahal have been tainted yellow by air pollution and worn thin from the curious hands and shuffling feet of the millions of visitors that stroll through each year.

Now, its centuries-old facade is under attack from another foe: excessively defecating flying bugs.

And their hued poo is turning the mausoleum green.

Experts at the Archaeological Survey of India, the body responsible for conserving the monument, told the Associated Press on Monday that swarms of mosquito-like flies breeding in the nearby Yamuna River are to blame for the greenish-black patches blotting the Taj Mahal’s walls. The bug, from the genus Goeldichironomus, is attracted to the bright walls of the monument, reports ABC News.

This particular insect breeds near polluted waters, making the Yamuna River a perfect place to plot an invasion. Drought has dried up the river, environmentalist Yogesh Sharma told the AP, leaving behind nothing but stagnated water unfit to support the fish that once kept insects at bay. Ash from a nearby crematorium is dumped into the river, reports the AP, serving as the insects’ primary source of food, along with algae.

The greening marble is unsightly, and Taj Mahal tour guides like Shamshuddin Khan are concerned the problem could negatively affect tourism.

“This is like a fungus, growing onto the walls,” Khan told ABC News. “When they see this kind of thing they say ‘Taj is getting dirty!’ The dirtiness is increasing, that kind of green fungus is increasing day by day.”

More concerning, though, is that officials and conservationists are at a loss for how to fix it. Currently, workers wash the walls clean each day, but excessive scrubbing can damage the delicate flower mosaics and sleek marble surface, Bhuvan Vikram of the Archaeological Survey of India told the AP.

“A series of marble panels depicting plant motifs on the walls or reflective tiles used in this part of the monument are becoming disfigured,” Vikram said.

High-ranking officials have called the insects and their green waste a “serious concern.”

It seems the most productive solution would be to eliminate the insects’ river breeding grounds, though officials have yet to figure out how.

“Officials have been asked to investigate as why there is sudden increase in number of these insects and how to control their population,” Uttar Pradesh state spokesman Navneet Sehgal told the AP.

Efforts to preserve the Taj Mahal, built by emperor Shah Jahan to honor his late and favorite wife in the early 1600s, have spanned the centuries. The earliest recorded repairs date to 1652, reports DNA India, when cracks in the main dome and elsewhere were flagged. Most recently, conservationists have worked to decrease the harmful pollution in the Taj Mahal’s home city of Agra that was yellowing the pearly white facade. Hundreds of coal-burning factories have closed over the last couple of decades, and conventional cars are banned within a 1,640-foot radius of the structure.

Inaction surrounding the polluted river that runs by the Taj Mahal has frustrated activists for years.

“The monument gives glory to the city, and the city gives glory to the monument,” M.C. Mehta, an Indian lawyer, told Smithsonian Magazine in 2011. “This has taken more than 25 years of my life. I say: ‘Don’t be so slow! If somebody is dying, you don’t wait.’ ”

The reporter who wrote the Smithsonian piece, which explored how conservationists were trying to save the Taj, explained the river like this:

“The river, once such an integral component of the Taj’s beauty, is a mess, to put it mildly. I visited one of the city’s storm drains where it empties at a spot between the Taj Mahal and the Agra Fort, a vast sandstone-and-marble complex that was once home to Mughal rulers. In addition to the untreated human waste deposited there, the drain belches mounds of litter—heaps of plastic bags, plastic foam, snack wrappers, bottles and empty foil packets that once held herbal mouth freshener. Environmental activists have argued that such garbage dumps produce methane gas that contributes to the yellowing of the Taj’s marble.
When I stepped down to photograph the trash heap, I felt an unnatural sponginess underfoot—the remains of a dead cow. According to Brij (Khandelwal), who has reported on the subject for Indian publications, the bodies of children have also been interred here by people too poor to afford even a rudimentary funeral.”

While officials try to determine how to eliminate the river’s most recent pesky guests, experts believe the insects’ green excrement won’t permanently damage the famed marble.

“The deposit on the Taj is water soluble. We are trying to clean it with water. But cleaning the Taj Mahal with water will not solve the problem,” Girish Maheshwari, who heads the Department of Entomology at St. John’s College, told ANI. “We know where and how these insects grow, so if we solve the problem at the basic level, we can stop them from growing in numbers and there will be no marks on the Taj.”