Scale insects are often associated to fungus. Females secreting honeydew trigger sooty mold growth and harm the host-plant. Induced undesirable sooty mold on plants makes scale insects some of the most damaging phytophagous insects. For instance, Cryptococcus fagisuga, a felt scale insect living on beech trees is associated to a fungus called Neonectria coccinea that will grow from the holes created by the insect’s puncture hole from which it feeds. Ultimately, the mold growth becomes more lethal to the tree than the insect. This is so true that I remember my Master mentor telling me that beech trees have almost disappeared from Paris because of this species!
Sooty mold are also harmful to scale insects, as the female secreting honeydew are sessile and they would literally die in their own moldy poop. This is when ants comes into play as essential mutualistic animals: by profiting from scale insect honeydew droplets as a food source, ants naturally clean their secretive waste, preventing sooty mold growth.
In an extraordinary situation, a species of scale insects from the mealybug family (Pseudococcidae) has taken advantage of mold and developed a unique symbiosis with a fungus species. Orbuspedum machinator, a newly described species from bamboo twigs of Thailand, has legless females and uses hyphae of a fungus Capnodium species to make herself a roof. In exchange, the mealybug secretes honeydew and feed the fungus.
A win win situation.
Open access: Gavrilov (2017) An amazing symbiosis between an animal and a fungus in a new species of legless mealybug (Insecta: Pseudococcidae)