How to Manage Pests
Pests in Gardens and Landscapes
The hoplia beetle, Hoplia callipyge (family Scarabaeidae), is a common pest of roses and other plants in many parts of California, especially the Central Valley. Because it has just one generation a year, it is a problem only from late March to May when the adult beetles feed on light-colored blossoms.
The adult beetle is oval and about 1/4 inch long. The head and thorax are a dark, reddish brown, and the wing covers are dark to light brown. Most of the body is a beautiful, iridescent silvery green in sunlight. The larvae are small, crescent-shaped grubs that live in the soil.
The hoplia beetle is in the same family as the Japanese beetle and sometimes is mistaken for that insect. However, there currently are no known populations of Japanese beetle in California. The hoplia beetle also is often mistakenly identified as the rose chafer, a pest beetle of roses that does not occur in California. (See Pest Notes: Roses in the Garden and Landscape: Insect and Mite Pests and Beneficials listed in References for other insect pests in roses.)
Female beetles lay glossy, white eggs in the soil of alfalfa fields, pastures, and in other areas of undisturbed vegetation, such as along fences and ditches. The larvae feed on decaying vegetation and plant roots but don’t damage woody plant roots. They develop slowly, remaining in the larval or pupal stage throughout the winter. In early spring they complete development, and adult beetles emerge from the soil. The adults fly to gardens where they feed on roses and other flowers. Adults generally are active from late March to early May. After feeding for several weeks, adults fly back to their egg-laying sites. There is a single generation each year.
Hoplia beetle adults are especially attracted to light-colored flowers and chew round holes in the petals of white, yellow, apricot, and pink roses. Their chewing can destroy a rose’s early buds and flowers, but the beetles don’t feed on the leaves. Hoplia beetles also feed on the flowers of calla, citrus, irises, lilies, magnolia, olive, peonies, poppies, and strawberries and on the young leaves and fruit of almonds, grapes, and peaches.
One way to manage hoplia beetles in your garden is to regularly handpick them off the flowers they are feeding on and dispose of them in a bucket of soapy water, or shake them out of the blooms directly into the soapy water. You also can just clip off blooms infested with beetles and dispose of them. Regular handpicking can be an important way of reducing future beetle populations in the immediate area.
Another nonchemical control measure is to fill white, 5-gallon buckets with water and a few drops of detergent to break the surface tension. Place these buckets in several locations throughout the rose garden; they’ll attract the beetles, which fall in the buckets and drown. The effectiveness of this method hasn’t been tested by research, and like handpicking, it might serve only to reduce the beetle population somewhat.
When planting roses in an area where these beetles are plentiful, consider choosing darker-colored varieties such as red roses to help avoid problems with this pest.
Sprays generally aren’t recommended. It’s very difficult to obtain effective control with insecticides, because the blossoms protect the beetles and the spray must come in direct contact with the beetles in order for it to kill them. Chemical control of the larvae in most cases isn’t possible, because they can live in the soil outside the garden or in surrounding landscapes. Systemic insecticides aren’t effective against the adults, because concentrations high enough to be toxic don’t occur in the blossoms where they feed.
If chewing damage can’t be tolerated for the 2- to 4-week period the beetles are present, an insecticide such as carbaryl* (Sevin), malathion, or the pyrethroid cyfluthrin can be applied. These insecticides kill only those beetles present at the time of the spray applications. Don’t spray blooming plants where honey bees are present, because most insecticides are very toxic to them. Avoid using these insecticides when possible. They are harmful to natural enemies, and some materials have been found in urban surface water systems at levels that warrant concern.
*As of August 1, 2020, pesticides containing the active ingredient carbaryl are restricted use materials in California. A valid pesticide applicator’s license is required for their possession and use. For more information see the California Department of Pesticide Regulation website.
Chittenden, F. H. 1901. Some Insects Injurious to the Violet, Rose, and Other Ornamental Plants. Washington, D.C.: USDA Div. of Entomology.
Dreistadt, S. H., J. K. Clark, and M. L. Flint. 2004. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, 2nd ed. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3359.
Essig, E. O. 1913. Injurious and Beneficial Insects of California. California State Commission of Horticulture. The Monthly Bulletin: Vol. II, Nos. 1 and 2.
Flint, M. L. and J. F. Karlik. Aug. 2008. Pest Notes: Roses in the Garden and Landscape: Insect and Mite Pests and Beneficials. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7466.
Flint, M. L. and J. F. Karlik. 2009. Healthy Roses, 2nd ed. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 21589.
Author: E. J. Perry, UC Cooperative Extension (emeritus), Stanislaus Co.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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