It could be either aphids or docimillia mites. Aphids are easily treated with a fertilizer. Docimillia mites are a whole different matter, try mixing water with sodium bicarbonite, the mixture should be 80/20, then generously spray your rhubard plants. It may take two or even three sprayings to kill them. Spray the plants when the sun is at high noon, docimillia mites do not enjoy hot weather, but will be happy to eat the organic poison when the weather is cooling off. By the way, its an instant kill. The second spraying to to assure that the eggs don't hatch. -Birdy