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One of the most common complaints from homeowners during the growing season is the appearance of a black, soot-like coating on leaves, stems, patios, vehicles, and outdoor furniture beneath landscape plants. While many assume this black growth is a plant disease, the real culprit is often an insect infestation. Sooty mold is a fungal growth that develops on honeydew, a sticky, sugar-rich waste product excreted by sap-feeding insects. Common honeydew-producing pests in Texas landscapes include aphids, whiteflies, scales, mealybugs, and psyllids. As these insects feed on plant sap, they remove more sugar than they can utilize and excrete the excess as honeydew. Sooty mold fungi then colonize this sticky residue, forming a dark coating on plant surfaces. The mold itself does not infect plant tissues or directly damage leaves. However, heavy accumulations can block sunlight and reduce photosynthesis, leading to decreased plant vigor and aesthetic quality. In severe cases, leaves may yellow, growth may slow, and plants can become stressed, particularly when combined with other environmental challenges. Several landscape plants are commonly affected by sooty mold in Texas, including crape myrtle, gardenia, holly, hibiscus, magnolia, citrus, and many ornamental trees and shrubs. Crape myrtle bark scale, for example, is a frequent cause of heavy honeydew production and subsequent sooty mold development on crape myrtles. Likewise, whiteflies and aphids often contribute to outbreaks on ornamentals and flowering plants. Successful management requires addressing the underlying insect problem rather than focusing solely on the mold. Begin by inspecting plants carefully, especially the undersides of leaves and stems, where many sap-feeding insects congregate. Look for live insects, cast skins, sticky honeydew, or ant activity. Ants are often attracted to honeydew and may indicate the presence of hidden pest populations. An integrated pest management (IPM) approach is the most effective strategy. Encourage natural enemies such as lady beetles, lacewings, parasitic wasps, and predatory bugs that help suppress aphids, scales, and whiteflies. Avoid unnecessary broad-spectrum insecticide applications that may disrupt beneficial insect populations. When pest numbers exceed acceptable levels, select insecticides based on the target pest and follow all label directions. Once the insect population is reduced, honeydew production will decline, and existing sooty mold will gradually weather away through rainfall, irrigation, and normal leaf growth. In some situations, washing leaves with water can improve appearance, but this will not solve the problem if the insects remain active. When blackened leaves appear in the landscape, remember that sooty mold is usually a symptom rather than the primary issue. Identifying and managing the honeydew-producing insects will provide the most effective long-term solution and help maintain healthy, attractive landscape plants throughout the season.
Figure: Sooty mold on crape myrtle leaves (A) caused by crape myrtle aphid (B), and sooty mold on crape myrtle stem/bark caused by crape myrtle bark scale (C)
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When growers and landscape professionals think about insects, the first thought is often pests and the damage they cause. However, not all insects are harmful. Many beneficial insects play an important role in naturally suppressing pest populations in greenhouses, nurseries, and landscapes. These “good bugs” are valuable partners in Integrated Pest Management (IPM) programs and can help reduce reliance on chemical insecticides. Beneficial insects are a key component of Integrated Pest Management (IPM) programs. They help suppress pests naturally and can improve long-term pest management success when properly conserved or introduced into production systems. Two important groups of beneficial insects are predators and parasitoids. Predators actively consume pests throughout their life cycle. Common examples include lady beetles, green lacewings, minute pirate bugs, predatory mites, and hover fly larvae. These natural enemies feed on aphids, whiteflies, spider mites, scales, thrips, and other soft-bodied pests commonly encountered in Texas greenhouses and landscapes. Parasitoids are specialized insects, usually tiny wasps, that develop inside or on a host insect and eventually kill it. Several parasitoid species are commercially available and widely used in greenhouse biological control programs, especially for whitefly and aphid management. Parasitoids are specialized insects, usually tiny wasps, that develop inside or on a host insect and eventually kill it. Several parasitoid species are commercially available and widely used in greenhouse biological control programs, especially for whitefly and aphid management. Greenhouse environments often provide ideal conditions for biological control because temperature, humidity, and pest populations can be monitored more closely. In landscapes and nurseries, conserving naturally occurring beneficial insects can also provide significant pest suppression over time. Unfortunately, beneficial populations are frequently disrupted by unnecessary or repeated applications of broad-spectrum insecticides. One of the biggest challenges in pest management is recognizing when beneficial insects are already helping control a pest problem. In some situations, professionals may apply insecticides before evaluating predator or parasitoid activity. This can eliminate natural enemies and create secondary pest outbreaks or pest resurgence. In addition, repeated use of the same insecticide groups can contribute to pesticide resistance, making future pest management more difficult. Scouting remains one of the most important tools in preserving beneficial insects. Correct identification of both pests and beneficials allows professionals to make informed treatment decisions. When insecticide applications are necessary, selecting reduced-risk or selective products can help preserve natural enemy populations while still managing key pests. Habitat also plays a role in supporting beneficial insects. Flowering plants, reduced dust, proper irrigation management, and minimizing unnecessary pesticide exposure can all encourage beneficial activity in landscapes and production systems. Biological control is not always a complete replacement for insecticides, but it is an effective management tool when incorporated into a broader IPM strategy. As greenhouse and landscape industries continue to face challenges from invasive pests, environmental concerns, and pesticide resistance, beneficial insects should not be viewed as forgotten tools. Instead, they should be recognized as valuable partners in sustainable pest management programs. Photo credit: Rafia Khan, Ph. D. Assistant Professor and Extension Specialist Texas A&M AgriLife Research and Extension Center Overton, TX.  |
AuthorKim Cabrera, TNLA Marketing Manager Archives
July 2026
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