Q.This Year All My Hydrangea Stems Are Covered In Small White Fluffy Substance Which I Think Are Mealybugs ? How Do I Control Them A
nd what damage do they do ? All the hydrangea plants are affected : both large well established plants and those in pots. I have never had this before. After a while some seem to fall off onto the ground. Picture enclosed.
Looks like either white scale, Cottony Maple Scale or mealy buds (a cousin of scale). Anything that kills one will help with the others. Due to their life cycle, they are very hard to get rid of completely in just one growing season (translation: they may return in 2022).
Most people give them a blast of water from the hose to kill them. You have to aim the hose at the upper and lower parts of leaves and stems that are infected. Repeat again after a few days if necessary and be ready to detect them next year in case they come back again. A good shot can dislodge as many as 70 percent of these insects from plants. Keep in mind that a blast of water can also injure the plant itself so be careful. Other organic solutions: you could squish them by hand, release beneficial insects such as lady bugs or apply an insecticidal soap like Safer's Ultra Fine Oil. But do not use both organic and chemical controls at the same time because the insecticide will kill the beneficial insects.
You can also use an insecticide with the active ingredient imidacloprid but, if the pests have already left then the insecticides may not do much since they "left the nest" already. Still, I personally would apply it once any way this year. When the infestation is severe enough, some people have opted to destroy the plant. You could also try some Safari systemics. Note: some of the insects that fall down if you touch them may just be empty shells (they left already). Someone also recommended using an insecticide with the active ingredient dinotefuran (commercial name Safari, etc.) and said to apply it twice a year AND for two years in a row.