A scale outbreak can be triggered by garden stress caused by over or under watering, the overuse of nitrogen fertilizer or the use of pesticides that rid the area of natural insect predators of the tiny insect.
Though scale insects are only 1/12- to 1/5-inch long, they can soon cover a plant with a cottony mass of white or bumps that can be black, brown, gray or white. Rose leaves will turn dark and wilt, and leaves of other plants will turn yellow and drop off.
The waxy shelled scale is immobile and easy to spot on the plant. They can be scrapped off leaves with a dull knife. Rubbing alcohol applied with a cotton swab can also help eliminate scale if it is caught early enough.
Attracting predator insects to the garden is a natural way to keep scale from infesting valuable plants while causing no harm to the environment.
Attracting Beneficial Insects to Combat Scale
Attract green lacewing into the garden. The larvae of the lacewing are known to feed voraciously on scale. Egg or larval forms of lacewings can be found commercially. Approximately 5,000 larvae will take care of a 2,500 square foot area. Yarrow, oleander and Queen Anne's lace all have flowers that are rich in nectar and will encourage the lacewings to take up residency. Lacewing also like wild lettuce.
Both adult ladybugs and their larvae will devour scale on the infested plants. The ladybug will also take care of aphids, whiteflies and spider mites in the garden. Plant alfalfa, goldenrod, morning-glory, coffeeberry, angelica, yarrow or oleander to keep ladybugs happy and reproducing. For the gardener without allergies, ragweed is also a favorite of the ladybug.
Braconid wasp eggs can be purchased for areas where the garden are humid and maintain a temperature above 59 degrees Fahrenheit. The braconid wasp will lay eggs in the bodies of insects and other larvae, weakening and killing the host when the eggs hatch and grow.
Once the braconid wasp is in the garden, a small planting of fennel will encourage it to stay. Parasitic wasps also like daisy, strawberry and white clover.