When a tree appears “covered in flies,” the visible insects are usually a symptom of a deeper, hidden infestation. This common horticultural issue is caused by small, sedentary plant pests that feed on the tree’s internal fluids. Flies, wasps, and ants are attracted to a sweet substance left behind by these pests, indicating a significant population of sap-sucking insects is at work. Understanding the true culprits and the substance they produce is the first step toward restoring the tree’s health.
Identifying the Primary Sap-Sucking Pests
The insects most responsible for this problem are members of the order Hemiptera, which use specialized mouthparts to extract sap from the tree’s vascular system. These pests include aphids, scale insects, and mealybugs. They feed on the phloem, the tissue that transports sugars produced by photosynthesis. Because these pests must ingest large volumes of sap to obtain sufficient protein, they excrete the excess sugar-rich fluid as waste.
Aphids are small, soft-bodied, pear-shaped insects that cluster densely on new growth and the undersides of leaves. They can be green, black, yellow, or pink. Their populations can explode rapidly, and a close inspection will reveal these tiny insects firmly attached to the tissue.
Scale insects often appear as immobile, waxy bumps or shell-like coverings on the stems, branches, and leaf surfaces. They are protected by this armor, making them resemble part of the plant rather than a living insect. Soft scale varieties are most associated with this issue, as they feed on the phloem and produce copious amounts of waste.
Mealybugs are another common sap-sucker, characterized by their slow movement and a distinctive white, cottony wax coating. These insects typically gather in protected crevices of the tree, such as where leaves meet the stem. They often look like small, fuzzy cotton masses.
The Role of Honeydew and Sooty Mold
The sticky material coating the tree and attracting flying insects is called honeydew, which is the sugary excretion of the feeding pests. Sap-sucking insects pierce the phloem to access the nutrient stream. Since sap is primarily water and sugar with low protein content, they must process a large volume, excreting the surplus sugar water. This clear, sweet liquid drips onto the foliage, bark, and surfaces beneath the tree, leaving a glossy, sticky residue.
Flies, wasps, and ants are highly attracted to honeydew as a readily available food source. Ants often “farm” the pests, protecting them from natural predators in exchange for the sugary excretion, which encourages the infestation. The presence of these other insects is a reliable sign that honeydew-producing pests are actively feeding above.
Honeydew also provides a perfect growth medium for sooty mold, a specific type of fungus. These fungi are non-parasitic and do not directly infect the tree tissue. Instead, they grow superficially on the honeydew, creating a dark, powdery, soot-like layer that turns leaves and branches black. A heavy coating of sooty mold reduces the leaf’s ability to absorb sunlight, impeding photosynthesis and leading to plant stress.
Effective Strategies for Management
The most effective management plan involves eliminating the primary pest population and then cleaning the affected areas. Early detection is important. Initial action can involve using a strong jet of water to physically dislodge pests from the leaves and branches. This mechanical removal is particularly effective for aphids and also washes away some of the sticky honeydew.
For active infestations, non-chemical treatments such as insecticidal soap or horticultural oil are recommended. These work by smothering the insects and have a low impact on beneficial species. Horticultural oils, including neem oil, can be applied to coat and suffocate the pests, though timing is important to avoid plant burn in hot weather. Systemic insecticides may be necessary for severe cases, as these products are absorbed by the plant and poison the pests as they feed.
Once the pest population is controlled, the honeydew and sooty mold will eventually weather away, but the process is slow. To restore the tree’s aesthetic appeal and photosynthetic function quickly, the residue can be washed off with a mild solution of soapy water. For hard-to-reach areas, a consumer-grade pressure washer with a broad tip can be used carefully to dislodge the remaining residue.