How to Manage Pests
Pests in Gardens and Landscapes
A hackberry woolly aphid, sometimes called Asian woolly hackberry aphid (Shivaphis celti), was discovered in California in 2002. It infests hackberry, especially Chinese hackberry (Celtis sinensis), throughout the state. This aphid was accidentally introduced into the United States in the late 1990s and also occurs from Florida to Texas and northward to at least Tennessee.
Hackberry woolly aphids secrete pale bluish or white wax over their bodies. These fuzzy masses on leaves are each about 1/10 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.
Often the first observed sign of a hackberry woolly aphid infestation is the copious, sticky honeydew it produces. Check for the insects to confirm that the cause is aphids and not the citricola scale, Coccus pseudomagnoliarum, which is the only other honeydew-producing insect that infests hackberry at annoying levels in California. Citricola scale is most common in the Central Valley on citrus and hackberry. Citricola scales are brownish to gray, oval, slightly dome shaped, and remain immobile most of their life. Adult females are mottled gray to brown and blend in with bark, making them hard to spot. Flattened, orangish nymphs occur on the underside of leaves, then move in fall to overwinter on bark. In spring, egg-laying females produce tiny scale crawlers that feed on leaves. See Pest Notes: Scales for more information on scales.
Hackberry woolly aphid adults, either winged or wingless, give live birth to young aphids during most of the season when hackberry leaves are present. Spring and summer adults are all reproductive females. In fall, winged males are produced, aphids mate, and females lay eggs that overwinter on branch terminals. Eggs hatch in spring after hackberries produce leaves. The insect has many generations per year, so aphids can be very abundant by summer. Shivaphis celti probably occurs only on hackberry trees, which include several species in the Celtis genus. It has been reported on at least a dozen Celtis species, some of which occur only in Asia.
This aphid is a pest because its copious honeydew excretions create a sticky mess and promote the growth of blackish sooty mold on leaves and surfaces beneath infested trees. No long-term or serious damage to hackberry trees has been found after several years of infestations. Insecticides apparently are not warranted to protect the health or survival of infested hackberry but may be justified when honeydew excretions are intolerable to people.
Inspect leaves for waxy masses and insects to be sure that aphids are the cause of annoying honeydew. Citricola scale, described above, also produces honeydew and is easily overlooked. Because plant health apparently is not threatened, take action only where annoyance from honeydew cannot be tolerated.
Excess irrigation promotes certain root decay pathogens and apparently contributes to tree death from a malady of undetermined cause that has killed many Chinese hackberry in some locations in California. Providing trees with appropriate soil moisture is the single most important action to promote tree health. Avoid fertilizing hackberry unless nutrient deficiency has definitely been diagnosed. Excess nitrogen has been shown to increase aphid numbers on certain other plant species. A forceful stream of water will dislodge and kill some aphids and wash away honeydew on plants that are small enough for hosing to be practical.
Classical biological control through introduction of natural enemies is one long-term possibility for managing this pest. Several parasitic wasps in the genus Trioxys attack this aphid in Asia. No parasitic wasps or other natural enemies specific to this pest have been reported yet in the United States. Predators such as the convergent lady beetle, multicolored Asian lady beetle, syrphid flies, and lacewings feed on aphids. However, in many California situations they currently do not provide adequate biological control. Nevertheless, avoid spraying hackberry with broad-spectrum, persistent insecticides that kill natural enemies, in part because citricola scale, another major hackberry pest, is well controlled by parasites in at least some locations.
Limit any foliar spraying to short-persistent, low-toxicity materials such as insecticidal soap or oil. However, soaps and oils will give only partial control because it is difficult to obtain the required thorough spray coverage of aphids on the undersides of leaves, especially on large trees. The aphid's woolly wax also protects it from sprays. Applications may need to be repeated because these materials provide no residual control.
Thorough application of horticultural oil (supreme, superior, or narrow-range oil) to twig terminals during the dormant or delayed dormant season (after buds swell, but before leaves open) may be effective for killing some overwintering eggs on twigs; however, delayed dormant sprays are unlikely to give complete control of aphids and will not prevent aphids from nearby sources flying in to colonize trees after leaves have opened. Dormant oil sprays are an effective treatment for controlling citricola scale, and aphid reduction could be a side benefit.
Trunk Injection and Implantation
Avoid injecting or implanting pesticides or other materials into hackberry trunks or roots. Chinese hackberry are susceptible to an unexplained, tree-killing malady. The suspected cause is a vascular wilt pathogen, which may be mechanically spread by unsterilized tools that contact internal parts of multiple hackberry trees. It would be unfortunate if implants or injections to provide short-term control of an aesthetic or annoying honeydew problem killed trees by spreading the undiagnosed malady.
Soil Injection or Drenching
A single soil injection or soil drench using systemic insecticide has been shown to provide effective season-long control. The systemic insecticide imidacloprid (Bayer Advanced Garden Tree & Shrub Insect Control, Merit) is available to both homeowners and professional applicators. The home-use product (Bayer Advanced Garden Tree & Shrub Insect Control) is measured into a bucket, diluted with water, and poured onto soil near the base of the tree trunk, as directed on the label. Late winter to early spring (when the new leaves flush) is believed to be the most effective time for a soil treatment in California.
Summer application of the material may still be effective when applied to trees that receive regular irrigation, but studies on mid-season treatments have been limited. No treatments should be conducted during fall, because hackberry leaves will soon drop naturally. Consult Pest Notes: Aphids for more information on aphid management and Pest Notes: Scales for suggestions if citricola scale is infesting hackberry.
Rates for Professional Applications
Imidacloprid applied once early in the season at the low label rate controls aphids on hackberry for the entire season according to recent study. For example, the Merit Professional label may state that 1 to 2 oz of material will treat 30 inches of cumulative trunk diameter. In this case, 1 oz should be enough to control aphids on three trees that each has a 10-inch diameter when measured at breast height.
In this instance:
When calculating the amount of pesticide to use, the diameter of the tree can be difficult to estimate, but it is easily calculated from circumference.
Dreistadt, S. H., R. J. Gill, J. G. Morse, P. A. Phillips, and R. E. Rice. 2001. Pest Notes: Scales. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7408.
Flint, M. L. 2000. Pest Notes: Aphids. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7404.
Halbert, S. E., and P. M. Choate. 1999. An Asian Woolly Hackberry Aphid, Shivaphis celti Das (Homoptera: Aphididae). Fla. Dept. Agric. & Consumer Services Ent. Circ. 392.
Authors: A. B. Lawson, California State University, Fresno; and S. H. Dreistadt, UC Statewide IPM Program, Davis
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