Insects, spiders and other arthropods on green roofs are not uncommon. Actually, long term studies show that arthropod colonization on green roofs, whether one-story tall or 20-stories tall, mimics ground level colonization, sometimes with even greater diversity. Arthropods are a vital component for any healthy ecosystem and their diversity in an ecosystem keeps pesky and damaging arthropods in check.
However, infestations on green roofs do occur. Last Fall, we helped Swarthmore College diagnose and treat a white grub infestation on one of their green roofs. By recognizing the infestation signs early, the college’s professional horticulture team was able to control the grub problem through organic pest control methods.
Recently, a red spider mite infestation was reported in a school in the UK with a 12-month old green roof. The infestation on the green roof was allowed to go unchecked for so long that the bugs made their way into the school via air ducts. When children began complaining of bites the school had to close down for several days to fumigate.
Naturally, all green roofs are susceptible to infestation. Red spider mites are a common household and nursery pest. Spider mites thrive in hot, dry conditions. Being aware of their preference for hot, dry conditions and the effect they have on infested plants is vital information all professional green roof maintainers must possess.
It is important to remind installers that all nursery plants arriving on a project site should be checked for possible disease and pest infestation. Green roof installers and maintainers must be able to recognize the early signs of infestation and know how to control their populations first through non-chemical methods and finally with a targeted plant-based, non-toxic pesticide if necessary.