The Cranberry rootworm beetle (Rhabdopterus picipes) is one of several leaf-feeding beetles and weevils that consume foliage of woody landscape plants in South Carolina. These nocturnal-feeding adult beetles are shiny, dark bronze-black, about 1/4-inch long, and 1/8-inch wide. These beetles are seldom noticed because they hide in the landscape mulch during the daylight hours, and the foliar damage appears similar to that of hail damage. The adults feed on landscape plants for several weeks in the late spring and early summer. Their feeding damage results in curved, C-shaped, and elongated holes in leaves of azalea, rhododendron, camellia, blueberry, hollies, roses, redbud, oakleaf hydrangea, and other shrubs. After feeding, the female adults lay eggs on the soil. Upon hatching, young beetle larvae move into the soil to feed on the roots of the host plant. They feed throughout the summer until fall, and then move deeper into the soil to over-winter. The larvae pupate within the soil during early spring, and the adults emerge in late spring to begin foliar feeding and repeat the one-year life cycle.
Cultural Controls: Keep landscape plants as healthy as possible to tolerate the damage. If there is inadequate rainfall, irrigate plants weekly during the growing season providing 1-inch of water per week.
A soil analysis test is recommended to determine which nutrients are needed in the soil to improve plant growth and to determine if lime is required. In the absence of a soil test, fertilize plants with slow-release, tree and shrub fertilizer, such as a 12-6-6, in early spring and again 6 weeks later at a rate of 1 pound per 100 sq ft. However, in the coastal counties of Beaufort, Charleston, and Horry, there is typically sufficient phosphorus that naturally occurs in the soil. Therefore, in these areas, use a 15-0-15 fertilizer around the shrubs during the spring. For proper nutrient management in landscape beds, have the soil tested the following fall or winter.
Although mulch provides a hiding place for the adult beetles, it is quite beneficial to landscape plants in conserving soil moisture, regulating soil temperature, and suppressing weeds. Apply a 3- to 4-inch deep layer of mulch around woody shrubs.
Chemical Controls: Once flowering is over, shrub foliage should be sprayed with spinosad (a natural insecticide), bifenthrin, lambda cyhalothrin, or permethrin as soon as feeding damage is detected. These are contact insecticides to control the adults feeding on the foliage. Also, spray to saturate the mulch or leaf litter beneath the shrubs, as during the daytime, the beetles hide in the mulch near the plants. To protect pollinating insects, do not spray during bloom. See Table 1 for examples of products labeled for foliar pest control on shrubs.
Alternatively, control can be obtained with soil drenches using a product containing imidacloprid. This a systemic insecticide that is taken up at the base of the shrub and moves upward into foliage and flowers. It may take one week for imidacloprid to translocate throughout smaller shrubs, or up to a few weeks for larger shrubs. It is always best to apply imidacloprid after flowering to reduce risk to pollinating insects. Be sure the plants are well-watered the day before application (to enhance insecticide uptake), then drench the soil around the base of the shrubs with a solution of imidacloprid. The amount of product to use is determined by the height of the shrub in feet, so follow the label directions for mixing with water. Systemic insecticide products generally protect shrub foliage for a year and are best applied in the spring. Because soil applied insecticides, which are drenched around the base of the shrubs, do not move further outward into the root system, they will not kill the grubs feeding on roots nearby. See Table 1 for examples of products containing imidacloprid.
Table 1. Insecticides to Control Cranberry Rootworm Beetle on Shrubs.
|Insecticide Active Ingredient||Examples of Common Insecticide Products Labeled for Use on Landscape Ornamentals|
|Bifenthrin||Ferti-lome Broad Spectrum Insecticide Concentrate
Hi-Yield Bug Blaster Bifenthrin 2.4 Concentrate
Monterey Turf & Ornamental Insect Spray
Up-Star Gold Insecticide Concentrate
Bifen I/T Concentrate
Talstar P Concentrate
|Imidacloprid||Monterey Once A Year Insect Control II
Ferti-lome Tree & Shrub Systemic Insect Drench
Martin’s Dominion Tree & Shrub Drench
Bayer Advanced 12 Month Tree & Shrub Insect Control Landscape
Bonide Annual Tree & Shrub Insect Control w/ Systemaxx
|Lambda-Cyhalothrin||Spectracide Triazicide Insect Killer for Lawns & Landscapes
Martin’s Cyonara Lawn & Garden Concentrate
|Permethrin||Bonide Eight Insect Control Vegetable Fruit & Flower Concentrate
Bonide Total Pest Control – Outdoor Concentrate
Tiger Brand Super 10 Concentrate
|Spinosad||Southern Ag Conserve Naturalyte Insect Control
Bonide Captain Jack’s Deadbug Brew
Bonide Colorado Potato Beetle Beater Concentrate
Ferti-lome Borer, Bagworm & Leafminer Spray
Natural Guard Spinosad Landscape & Garden Insecticide
Monterey Garden Insect Spray Concentrate
Ortho Insect Killer Tree & Shrub Concentrate