Adult mealy bugs are about 1/16th inch long and look like miniature balls of cotton. You can usually see the insects and their eggs. Like most insects, they don't like light so look for them down inside the plant where the leaf meets the stem and on the undersides of leaves.

Closeup view of Mealy Bugs

Mealy Bugs on Oncidium Leaves

Mealy bugs feed by piercing the plant surface with mouths that look like hypodermic needles. As they suck out the sap, they excrete a sticky substance that coats the leaves. Called "Honeydew", this excretion first looks like honey, but soon turns to a moldy black. This coating makes it hard for the plant to breathe and attracts ants.

Mealy bugs live for six weeks to two months. Because the adults are on the move most of their lives, they can cover a lot of plants in their short lives. Eggs hatch in 5-10 days, or wait longer until conditions are right.

Controls

Because mealy bugs are protected by a waterproof wax that resembles cotton, they can be difficult to eradicate. 

Method #1.

1. Dab each bug with a cotton swab that's been dipped in rubbing alcohol.

2. Rinse the plant in lukewarm water.

3. Repeat treatment whenever you see more insects.

Method #2

1. Spray plant with Neem Oil. Be sure to get under leaves and into the crevices between the leaves and stems.

2. Repeat treatment once a week for three weeks whether you see more insects or not because your getting the adults but not the eggs.