Holly (Ilex spp.) is a deciduous evergreen tree or shrub. With many different species to choose from, hollies are known for their deep green, glossy leaves and bright red ornamental berries that appear during the winter months. Thriving in U.S. Department of Agriculture plant hardiness zones 5 through 9, the holly tree is pyramidal in shape and stands up to 50 feet tall. Proper cultural care is required to keep your holly tree free from common pests.
Different areas of the country have different species of native leafminers. Leafminers are the larval state of black flies, which make holes and lay eggs within the leaves of the holly tree. These eggs hatch within the leaf and the leafminer eats its way through to the surface of the leaf, causing holes and general deformations in the leaves. American holly trees (Ilex opaca) are the most affected by leafminers. They become especially prevalent if natural predators like birds or wasps do not keep populations in check. Small infestations are easily controlled by removing the affected leaves by hand. Otherwise, a, organic horticultural oil or insecticide is needed to prevent leafminers from harming the aesthetic value of your tree. Horticultural oils are only effective if the temperature is between 40 and 85 degree Fahrenheit. A 3 to 4 percent solution is effective when the tree is dormant, while a 2 to 3 percent solution is effective during periods of active growth. This means mixing between 2 and 5 ounces of horticultural oil with 1 gallon of water; however, products may differ, so be sure to follow your product's specific mixing instructions. Use a sprayer to apply the oil to both sides of the leaves.
The most common mite species to affect hollies is the Southern red mite. This spider-like pest feeds on your holly, sucking in and stealing nutrients from the leaves. The result is discoloration and yellowing on the underside of holly leaves. The Southern red mite is a small pest that is red to brown in color and only 1/50-inch in length. However, it leaves behind a calling card of whitish gray webbing on the underside of your leaves, which lets you know that mites are causing the problem. Mites appear in large numbers when the weather is particularly hot and dry. They are easily controlled by spraying your tree with water, using a garden hose. Insecticidal soap may be needed if populations are large and out of control. As mite populations increase, they begin to infest both sides of your tree leaves, as well as the branches of the tree. Mix a 2 percent solution of insecticidal soap and water. This ensures a high enough concentration to kill heavy mite infestations, although mixing instructions may vary by product. Apply the solution to the underside of your holly leaves with a sprayer.
Several species of scale that infest holly trees, including soft and armored scale species. Scale is a small insect that does not have legs, and therefore, is unable to move or crawl. Scale is generally found on the underside of holly leaves, and look like bumps or spores attached to the leaf, although some scale is found on holly branches as well. Like mites, scale feed by sucking water, sap and other nutrients from the leaves of your holly. Always remove leaves or scrape away scale if they appear in small numbers. However, if scale is a persistent problem or populations are high, treat your holly tree with organic horticultural oil early in the spring before new growth occurs. At this time of year, you are more likely to wipe out pest populations, as the oil is able to kill both adult scale and eggs. Mix a 2 to 3 percent solution by combining 5 to 7 tablespoons of horticultural oil with 1 gallon of water. Products may vary, so check your label instructions. Apply the oil with a sprayer on the underside of your holly leaves where scale is present.
4. Cultural Care
Proper cultural care for your holly tree means providing the best growing conditions to increase health and vigor. When a tree is healthy and vigorous, it is less likely to suffer invasion from pests and diseases. Hollies are full sun trees, but also tolerate partial shade. They are adaptable to many different soil conditions, as long as the soil is well draining. Holly requires a regular watering schedule, especially during hot weather and dry spells. It is best to add a layer of mulch around the base of your holly tree to improve moisture retention and to keep the tree’s soil insulated in the winter and cool in the summer. Always prune away dead diseased or unhealthy branches, as this is an invitation for pests. If you notice pests on your holly tree, address the problem immediately; do not wait for populations to get larger, as they have the potential to grow quickly and out of control.
- University of California Davis Agriculture & Natural Resources: Holly -- Ilex spp.
- Arbor Day Foundation: Holly, American Ilex opaca
- Clemson Cooperative Extension: Holly Diseases & Insect Pests
- Cornell Cooperative Extension: Horticultural Oils
- Bonide: All Season's Horticultural and Dormant Spray Oil Concentrate
- Washington State University Extension: Guidelines for Insecticidal Soap
- Jupiterimages/Photos.com/Getty Images