As animal pathogens, fungi help to control the population of damaging pests. These fungi are very specific to the insects they attack; they do not infect animals or plants. Fungi are currently under investigation as potential microbial insecticides, with several already on the market. The mycorrhizal relationship between fungi and plant roots is essential for the productivity of farmland. Without the fungal partner in root systems, 80–90 percent of trees and grasses would not survive. Yeasts have been used for thousands of years in the production of beer, wine, and bread.