Researchers have found a way for growers and homeowners to save money and time by knowing when and how much insecticide to apply to control the Asian hackberry woolly aphid.
The aphid showed up on Chinese hackberry trees in Davis and other Sacramento Valley cities in 2002. First found in Florida and Georgia in the late 1990s, it has spread rapidly across the south and into California.
The fuzzy white insects found on the tops and bottoms of leaves cause the leaves to drip sticky liquid all over everything beneath them. This sugar excretion draws ants, and a black mold may grow on surfaces covered with it. Even large populations of aphids do not kill the trees, but the amount of sticky goop is a major nuisance in many urban settings.
Hackberry woolly aphids secrete pale bluish or white wax over their bodies. These fuzzy masses on leaves are each about one-tenth inch or less in diameter. This waxy covering usually obscures the insect's gray, green, or yellow body. Winged forms have distinct black borders along the forewing veins. Their antennae have alternating dark and light bands.
Entomologist Andrew Lawson, Plant Sciences, California State University, Fresno, and Pam Geisel, University of California Statewide Master Gardener Coordinator, found that applying treatments of systemic imidacloprid in the spring after aphid populations are established is effective on well irrigated trees. Study results indicate that soil injections of imidacloprid (Merit 2F, Bayer Environmental Sciences) applied three weeks after trees leaf out when aphid infestations can be confirmed are as effective as applications made before bud break when treatments may be applied unnecessarily to uninfested trees.
Treatments can be applied after populations have been confirmed through monitoring. Checking leaf samples and using water sensitive cards to measure honeydew are reliable monitoring methods to determine the number of Asian hackberry woolly aphids that are present. Users can then see if the aphid population is high enough to require insecticide treatment. Researchers monitored aphid and predator populations for three seasons, and aphid densities were found to be highest in the spring and fall with lower densities during the mid-summer months. No treatments should be conducted during fall, because hackberry leaves will soon drop naturally.
Imidacloprid treatments are applied as a systemic insecticide, meaning that it is taken up by the roots of the plant and spreads through all the tissues of the plant. The material may be mixed in a bucket of water and poured around the trunk, or it may be injected into the soil using special equipment that places the material 6 inches below the soil surface. Both application methods have been shown to be equally effective.
In addition, soil injections of imidacloprid (Merit 2F, Bayer Environmental Sciences) at one-eighth of the low label rate (0.0125 oz/ inch DBH or 0.002675 oz AI/inch DBH) were shown to be as effective at reducing aphid populations as treatments applied at the regular low label rate (0.1 oz/ inch DBH or 0.0214 oz AI/inch DBH). Rate refinement allows users to reduce the cost of application by decreasing the amount of material required for pest control.
Lawson and Geisel took the initial steps to find a long-term, sustainable solution to controlling the hackberry aphid. With Kent Daane, UC Cooperative Extension specialist in biological control at UC Berkeley, they have contacted cooperators in China, and a foreign exploration trip is planned to search for natural enemies that may eventually be imported to help control the aphids.
The results of this two-year project provide municipalities, agencies, landscape professionals, and homeowners with valuable information regarding the lifecycle, monitoring, and control of the hackberry aphid. The UC Exotic/Invasive Pests and Diseases Research Program, and the Elvinia J. Slossen Endowment provided funding for this project.
