White Waxy Bumps on a Holly Tree

Scale Description and Life Cycle

• Both soft and armored scale species may attack a holly. The damaging adult females of both types of scale appear as small, immobile bumps that measure less than 1/4 inch in diameter. Armored scales have a flattened, plate-like cover that you can scrape off. Soft scales are smooth, waxy or cottony and their covering is actually part of their body and cannot be removed. The immobile adult female feeds and lays eggs under its protective covering. Eggs hatch into crawlers, or mobile scale nymphs, that crawl around on plants and are blown onto other plants before settling.

The Scale of the Damage

• A healthy holly tree can readily withstand and recover from a minor scale infestation. Heavy feeding can weaken a holly, slow growth and cause branch dieback. Often most problematic, however, is that soft scales produce a sugary, shiny-looking substance called honeydew as a byproduct of feeding. An unsightly black fungus, sooty mold, grows on honeydew. The honeydew also attracts ants which then protect the soft scales from their natural enemies.

Nonchemical Treatment

• Providing the holly with good cultural care that includes adequate irrigation often allows a holly to withstand minor scale activity. If the scale infestation is concentrated on a few branches on the holly or isolated leaves, pruning off the heavily-infested parts offers some control. Scales have many natural enemies including parasitic wasps, lacewings, lady beetles and predatory mites that often control the scales well unless they are disrupted by broad-spectrum, persistent pesticides, dust or ants protecting their source of honeydew.

Honeydew and Ants

• If you observe ants moving up and down the holly trunk and on branches where scales are feeding, the ants are gathering honeydew and interfering with the beneficial predators and parasites of the scales. To permit scale enemies to curb this pest's presence, you have to control the ants. Prune back any holly branches that touch the ground or other structures or plants to force ants up the holly stem. Wrap a band of fabric or heavy paper around the trunk, then apply petroleum jelly or a commercially-available sticky material to the band. Ants will become trapped in this material, which you need to inspect and stir with a stick regularly to break up any debris that would create a bridge for ants. If implementing a sticky barrier is not feasible, you can also place various types of ant baits in ant trails around the base of the holly.

Oil for Scales

• Where an infestation is heavy or a holly is particularly visible or prized, chemical treatment may be warranted. For effective control, apply a horticultural oil in spring when the scales are in their active crawler stage. Obtain or prepare a mixture that contains 2 percent horticultural oil and use a garden sprayer to thoroughly cover the scale-infested holly. Apply the oil spray on a small, inconspicuous section of holly and monitor that section for injury for 48 hours before treating the entire holly. Do not spray drought-stressed hollies or use oil when it is foggy, rainy or windy and if temperatures are below freezing or above 90 degrees Fahrenheit. A second oil application about 10 days after the first spray helps to ensure thorough treatment.