The Spotted Lanternfly (Lycorma delicatula) is an invasive planthopper from Asia that threatens agricultural and ornamental plants across the United States. This pest feeds on the plant’s vascular system, using its piercing-sucking mouthpart to extract nutrients from the phloem tissue of over 70 host species. This feeding activity, rather than structural damage, stresses the host and reduces its vigor. Identifying these damage signs is important for early detection and management.
Direct Indicators of Feeding Activity
The most immediate signs of a Spotted Lanternfly infestation are related to the insect’s feeding process on woody plants. As the nymphs and adults insert their mouthparts into the bark, they create small, open wounds. These feeding sites often start to “weep” as the plant’s sap leaks out.
This sap leakage forms moist, dark spots on the trunks and branches, sometimes leaving a gray or black trail down the bark. The oozing sap may ferment, giving off a distinct, sour, or vinegar-like odor that can be noticeable near heavily infested trees. The large number of insects feeding in one area, particularly on the trunk of a tree, can result in a heavy loss of the phloem tissue.
Nymphs and adults often aggregate in large numbers to feed, leading to concentrated stress on specific branches or shoots. When a small branch or shoot is subjected to this intense feeding, it may quickly lose vigor, causing the leaves to wilt, curl, or prematurely drop off. This rapid decline of a branch, known as “flagging,” is a direct visual cue that the plant’s vascular system has been compromised by the pests. The creation of these wounds also provides entry points for other pathogens or opportunistic organisms.
Secondary Signs: The Development of Sooty Mold
As the Spotted Lanternfly consumes large quantities of sugary sap, it must excrete the excess liquid, which is known as honeydew. This waste product is a sticky, glistening substance that coats the plant surfaces below the feeding insects. In high-density infestations, this honeydew can be so abundant that it feels like a light rain falling from the canopy.
The sugary residue serves as a growth medium for a specific type of fungus called sooty mold. This mold appears as a black, powdery, or crusty layer on the leaves, branches, and trunks of the infested plant. It is also commonly found on surrounding objects, such as decks, patio furniture, and vehicles parked beneath infested trees, creating a significant nuisance.
The presence of sooty mold is a strong indicator of a current or recent infestation, although the mold itself does not directly infect the plant tissue. Its main harm comes from physically covering the leaves, which blocks sunlight and reduces photosynthesis. This reduction in energy production weakens the host plant. The honeydew also attracts other insects like wasps, bees, and ants.
Assessing Plant Injury and Specific Host Reactions
The severity of Spotted Lanternfly damage varies significantly depending on the host plant species and the intensity of the infestation. While the insects feed on a wide variety of plants, including maples, walnuts, and stone fruits, the most severe outcomes are seen in a few specific hosts. For most established hardwood trees, the feeding is considered a stressor that can cause localized dieback, defoliation, and reduced aesthetic value, but rarely results in the death of a mature tree.
In contrast, grapevines are highly susceptible, and heavy, continuous infestations can lead to vine decline and death. High populations, sometimes exceeding 100 insects per vine across multiple seasons, cause winter hardiness loss, reduced fruit yield, and the demise of the entire vine. Research suggests that populations of 70 to 200 lanternflies per vine significantly alter the plant’s carbohydrate and nitrogen dynamics necessary for long-term health and fruit production.
Nymphs tend to feed more on the actively growing shoots and herbaceous plants, while adults primarily target the woody tissue of trunks and branches. This difference in feeding location means that damage signs on younger, non-woody plants may appear as general wilting and stunting.
On mature trees, the signs are concentrated on the main trunk and older limbs. The invasive Tree-of-Heaven (Ailanthus altissima), a preferred host, is particularly vulnerable, with widespread dieback and death observed under heavy pressure. Severe dieback results from chronic feeding stress, which depletes the plant’s stored starches and makes it more vulnerable to secondary issues like drought or disease.