Woolly whitefly has been an especially bad problem this year on my citrus. It is a small fly that lives under citrus leaves. Symptoms of a woolly whitefly problem is when a leaf has fluffy white spots under the leaves that look like pieces of cotton. The fly secretes a sticky substance that the honey bees then collect which makes a poor grade of honey. This sticky substance also falls on the tops of leaves. On the top of the leaf a black sooty mold grows on the sticky substance, and this blocks the sun light from reaching the leaves. The answer to the problem: control with an oil water mixture.
Woolly Whitefly on a grapefruit tree.
This oil-water mixture is made with agricultural oil that contains a surfactant (soap type chemical) which allows the oil to mix with water and not float on top. They come in different weights or boiling points. The best strength for woolly white fly is 435°F oil. Unfortunately, they do not list their boiling point. So you just have to try a 2% mixture by volume in water and see if that will drown the fly. The oil plugs the breathing holes of the fly. It also loosens the sooty mold so that it can be washed off the leaves. Most oils are made from petroleum, but there is one on the market which is an oil mix made from canola oil for the people who want to be totally “organic.” This oil spray helps control a lot of pests that attack citrus such as mites, citrus leaf miners, and aphids. Do not spray oil mixes if the temperature is going to be over 95°F or close to 32°F. Both conditions can cause leaf burn.
Typical oil spray.