Insects invade Taj Mahal again leaving green and black stains on marble walls. The issue has become a permanent problem as filth in the Yamuna river provides a perfect breeding ground for insects to breed, according to the Archaeological Survey of India (ASI). The black and green patches appear due to excreta deposited by a particular species of insect (Goeldichironomus) as per ASI experts. They further add that these patches can damage the intricate designs and floral mosaics and the marble surface of the monument.