Snowflakes in August? In recent years, many Mississippians have been perplexed to see what look like snowflakes drifting by on a hot summer day. If you see such “snowflakes” this year, look for nearby sugarberry trees (We usually call them hackberry trees here, but technically they are sugarberries.) and you will probably find the source of this phenomenon. Asian wooly hackberry aphids are non-native insects that were first detected in the US around 1996. Since then, they have spread across the country. These aphids only occur on sugarberry or hackberry, Celtis spp. Populations begin building in the spring after trees leaf out and increase with each generation, occasionally building to high numbers by late summer and early fall.
Black sooty mold on or under sugarberry trees is another indication of Asian wooly hackberry aphids. Heavy aphid infestations produce a lot of honeydew, which in turn results in heavy accumulations of black sooty mold. The sooty mold accumulates on the leaves of the host trees and on plants growing near infested trees, as well as on vehicles and patio furniture located beneath infested trees. Hackberry aphid populations vary considerably from year to year, so trees that were covered with sooty mold one year may or may not have a problem the next. Lacewings and lady beetles, including the imported Asian Lady Beetle, are important predators of these aphids, but there are no significant parasites.
Control: Controlling insects on large shade trees in urban settings is always problematic because of safety issues, application issues, and costs. Consequently, the do-nothing-approach of simply living with the problem is the most common method of dealing with this aphid. Heavy infestations can make trees unsightly, at least for part of the year, but they do no long-term damage to the tree.
However, for trees growing over patios, parking areas, or other locations where accumulations of honeydew and sooty mold are not acceptable, the do-nothing approach may not be acceptable either. Asian wooly hackberry aphids can be controlled with soil-applied applications of systemic insecticides containing imidacloprid or other neonicotinoid products. Such treatments are costly but are relatively easy to apply and can provide season-long control when applied properly. For best results, treatment should be applied in late winter/early spring just before or as trees are leafing out, but treatments applied later into spring should also reduce late season aphid numbers and sooty mold accumulation. Keep trees properly watered after treatment to improve control. Avoid treating in late summer and fall when it is too late for treatments to have much effect before leaf drop. If you have heavily infested trees this fall, make a note to treat next spring to prevent the problem next year.
See Extension Publication 2369, Insect Pests of Ornamental Plants in the Home Landscape, page 5 and pages 39-41, for more information on aphids and aphid control.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
The information given here is for educational purposes only. Always read and follow current label directions. Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled. Mississippi State University is an equal opportunity institution.
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