How to Manage Pests
Pests in Gardens and Landscapes
An introduced woolly aphid (Shivaphis celti), sometimes called Asian woolly hackberry aphid, infests the widely planted Chinese hackberry (Celtis sinensis) and other Celtis species. This hackberry woolly aphid also occurs from Florida to Texas and northward to at least Illinois. It also occurs in much of Asia.
Often the first observed sign of a hackberry woolly aphid infestation is the sticky honeydew it produces. The aphids also secrete pale wax, which covers their bodies. These woolly aphids on shoot terminals and leaves appear as fuzzy, bluish or white masses, each about 1/10 inch or less in diameter. Winged forms have distinct black borders along the forewing veins and their antennae have alternating dark and light bands.
Check for the insects to confirm that the cause of honeydew is aphids and not the citricola scale. Citricola scale (Coccus pseudomagnoliarum) is the only other honeydew-producing insect that infests hackberry at annoying levels in California, and it is most common in the Central Valley on citrus and hackberry.
Citricola scale females and older nymphs (immatures) are brownish to gray, oval, slightly dome shaped, and occur on twig bark from fall through spring. Because the scales are immobile most of their life and their mottled gray to brown color blends in with bark, these scales are easily overlooked. In the spring female scales produce tiny flattened, orangish nymphs that settle and feed on the underside of leaves during spring and summer, then move in fall to overwinter on bark. For more information, see Pest Notes: Scales.
This aphid is a pest because it produces copious honeydew excretions upon which blackish sooty mold grows creating a sticky mess on leaves and surfaces beneath infested trees. However, no long-term or serious damage to hackberry trees has been found after years of aphid infestations. Insecticides apparently are not warranted to protect the health or survival of infested trees but are applied when honeydew excretions are intolerable to people.
Hackberry woolly aphid adults, either winged or wingless, give live birth to aphid nymphs during most of the season when hackberry leaves are present. Spring and summer adults are all reproductive females. In the fall, winged males are produced; the aphids mate, and females lay eggs that overwinter on branch terminals. Eggs hatch in the spring as the hackberry produces leaves. In the San Joaquin Valley, hackberry woolly aphid populations typically are highest in the spring and fall with lower numbers and less honeydew production during the summer. The insect has many generations per year.
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, apply insecticide only where annoyance from honeydew cannot be tolerated.
Water-sensitive cards can be used by landscape managers to efficiently monitor honeydew dripping from trees. This technique provides a consistent measure of changes in aphid abundance and helps you decide when to take control action and afterwards to evaluate the effectiveness of your actions. The effort required to monitor honeydew can be warranted on tall trees where the honeydew-producing insects may be located too high to easily observe, where there is a low tolerance for dripping honeydew, and when managing many trees, such as along city streets and parking lots.
Monitor honeydew with the bright yellow cards sold for evaluating insecticide spray coverage and calibrating sprayers. Cards develop distinct blue dots upon contact with honeydew or water droplets. These droplet monitoring cards generally must be mail ordered and are different from the yellow sticky traps used to monitor winged insects. Regularly monitor honeydew beneath plants, such as the number of drops during four hours on the same time of day once a week. Label and save the cards from each monitoring date, and visually compare the cards among sample dates. For more information on monitoring honeydew, see Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide.
Provide trees with appropriate soil moisture, avoid compacting topsoil, and prevent prolonged waterlogging of soil near trunks. Excess irrigation promotes certain root decay pathogens and may contribute to a malady of undetermined cause that has killed many Chinese hackberry in some locations in California. 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 small enough for hosing to be practical.
The convergent lady beetle, multicolored Asian lady beetle, syrphid flies, and lacewing larvae are common aphid predators. No parasitic wasps or other natural enemies specific to this pest species have been reported in California. Several parasitic wasps of the Trioxys species attack this aphid in Asia, and the introduction of these natural enemies is one long-term possibility for managing this pest.
In many California situations, natural enemies do not provide adequate biological control of this aphid. Nevertheless, avoid spraying hackberry with broad-spectrum, persistent insecticides that kill natural enemies; because the citricola scale, another major hackberry pest, is well controlled by parasites in some locations.
Before applying insecticide, make sure plants are receiving appropriate cultural care and take steps to conserve natural enemies. Completely read and follow the product label instructions for the safe and effective use of the insecticide. Insecticides can have unintended effects, such as contaminating water, poisoning pollinators and natural enemies, and causing secondary pest outbreaks.
Nonresidual, Contact Insecticides
Where trees can be sprayed, completely covering shoot terminals and the undersides of leaves with a nonpersistent, contact insecticide can provide partial control of hackberry aphids. Available nonpersistent, contact insecticides include narrow-range or horticultural oils (e.g., Bonide Horticultural Oil), insecticidal soap (Safer Brand Insect Killing Soap Concentrate II), and pyrethrin products, which are often combined with piperonyl butoxide (Ace Flower & Vegetable Insect Spray, Garden Tech Worry Free Brand Concentrate).
These insecticides have low toxicity to people and pets and relatively little adverse impact on the populations of pollinators and natural enemies and the benefits they provide. These products will give only partial control because aphids’ woolly wax protects them from sprays; and it is difficult to obtain thorough spray coverage of aphids, especially on large trees. Because these insecticides provide no residual control and aphids have several generations each year, more than one application per season is generally needed.
Thorough application of horticultural oil (sometimes also called supreme, superior, or narrow-range oil) to twig terminals during the dormant or delayed dormant season (after buds swell, but before leaves open) can kill overwintering eggs on twigs. Delayed dormant spraying is unlikely to give complete control of aphids and will not control aphids that fly in from other hackberry trees after leaves have opened. However, dormant oil spray provides good control of citricola scale, and aphid reduction could be a side benefit if treatment is needed for scales.
Systemic insecticides are absorbed by one plant part (e.g., trunks or roots) and moved (translocated) to leaves or other plant parts. In comparison with systemics that are sprayed onto foliage, products labeled for soil drench or injection or for trunk injection or spraying minimize environmental contamination and may be more effective. Trunk application of systemic insecticides can provide relatively rapid control. There is a longer time delay between soil application and insecticide action. Properly applied, one application can provide effective control for the entire growing season. If applying systemic insecticide, use soil drench or injection or a trunk spray whenever possible.
Systemic neonicotinoid insecticides for use on landscape plants include imidacloprid (Bayer Advanced 12 Month Tree & Shrub Insect Control, Bonide Annual Tree & Shrub Insect Control Concentrate), which can easily be drenched into soil around the tree trunk using the mix-and-pour method as directed on the product labels. The neonicotinoid dinotefuran is also available for aphid control by home users (Green Light Tree & Shrub Insect Control with Safari 2G and Ortho Tree & Shrub Insect Control Granules). Dinotefuran (Safari) and imidacloprid (Merit) are also available to professional applicators.
Late winter is an effective time to apply imidacloprid. However, this is before you can know whether aphids will become abundant enough to warrant the application. Spring and summer application of a neonicotinoid can be effective on trees that receive regular irrigation, such as those growing in lawns. Delaying the application and monitoring regularly allows you to limit insecticide use to situations where aphids or their honeydew are becoming too abundant. No treatments should be conducted during the fall, because hackberry leaves will soon drop naturally.
Systemic insecticides can cause spider mite outbreaks and foliage sprays of them can be toxic to beneficial insects that contact spray or treated leaves. Systemics can translocate into flowers and have adverse effects on natural enemies and pollinators that feed on nectar and pollen. Although hackberry’s tiny flowers are wind pollinated, do not apply systemic insecticides to plants during flowering or shortly before flowering; wait until after plants have completed their seasonal flowering unless the product’s label directions say otherwise. With soil application, when possible, wait until nearby plants also have completed flowering, as their roots may take up some of the soil-applied insecticide.
Injection and Implantation
Avoid injecting or implanting pesticides or other materials into hackberry trunks or roots. With trunk injection and implantation, it is difficult to repeatedly place insecticide at the proper depth. These methods also injure woody plants and can spread plant pathogens on contaminated tools. Chinese hackberry are susceptible to an unexplained, tree-killing malady, which may be a vascular wilt pathogen that could 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.
When injecting or implanting into multiple plants, scrub any plant sap from tools or equipment that penetrate bark and disinfect tools with a registered disinfectant (e.g., bleach) before moving to work on each new plant. At least 1 to 2 minutes of disinfectant contact time between contaminated uses is generally required. Consider rotating work among several tools, using a freshly disinfected tool while the most recently used tools are being soaked in disinfectant. Avoid methods that cause large wounds, such as implants placed in holes drilled in trunks. Do not implant or inject into roots or trunks more than once a year.
Residual, Foliar Sprays
Foliar sprays of broad-spectrum insecticides with residues that can persist for weeks are not recommended for aphid control in landscapes and gardens. Pesticides to avoid include carbamates (carbaryl* or Sevin), organophosphates (malathion), and pyrethroids (bifenthrin, fluvalinate, permethrin). These are highly toxic to natural enemies and pollinators and can cause outbreaks of spider mites or other pests. Because their use in landscapes and gardens can run or wash off into storm drains and contaminate municipal wastewater, these insecticides are being found in surface water and are adversely affecting nontarget, aquatic organisms.
*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.
Dreistadt, S. H., J. G. Morse, P. A. Phillips, and R. E. Rice. 2007. Pest Notes: Scales. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7408.
Flint, M. L. 2013. Pest Notes: Aphids. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7404.
Halbert, S. E., and P. M. Choate. 2013. Asian Woolly Hackberry Aphid, Shivaphis celti Das (Insecta: Hemiptera: Aphididae). Gainesville: Univ. Florida Extension Publ. EENY288.
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. Div. Agric. Nat. Res. Publ. 3359.
Pest Notes: Hackberry Woolly Aphid
Authors: A. B. Lawson, California State University, Fresno; and S. H. Dreistadt, UC Statewide IPM Program, Davis.
Produced by University of California Statewide IPM Program
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