Black locust trees are common in many landscapes. Despite their resilience, they can be susceptible to insect infestations that compromise their health and structural integrity. Understanding these threats is important for maintaining the vitality of black locust trees.
Understanding the Black Locust Borer
The “black locust bug” often refers to the black locust borer (Megacyllene robiniae), a beetle that targets black locust trees. This longhorned beetle is about one-half to three-quarters of an inch long. It has a black body marked by bright yellow bands across its wing covers and thorax, which can lead to it being mistaken for wasps.
The black locust borer’s life cycle spans approximately one year. Adult beetles are most active from late summer to early fall (August through October), feeding on goldenrod pollen. During this time, females lay eggs in bark crevices of black locust trees, favoring stressed or newly planted ones. Once hatched, the small, creamy-white, legless larvae, or grubs, bore into the tree’s inner bark and sapwood.
The larval stage causes the most significant damage. Grubs tunnel deeper into the heartwood, creating extensive galleries within the trunk and branches. They spend winter in these tunnels, pupating in spring. Adult beetles emerge from the tree in late summer, completing the cycle. This tunneling disrupts nutrient and water flow.
Impact on Black Locust Trees
Larval tunneling compromises the structural integrity of infested trees. As grubs bore into the wood, they create tunnels that weaken the trunk and branches, making the tree vulnerable to breakage. This internal damage can lead to a decline in the tree’s health and stability. Disruption of the vascular system, which transports water and nutrients, causes visible symptoms.
Noticeable signs of infestation include wilting leaves, particularly on specific branches, indicating disrupted water uptake. Branch dieback is also common, where limbs or sections of the canopy wither and die. Swollen or gnarled areas can develop around larval tunnels. Fine, sawdust-like frass, larval excrement and wood shavings, often extrudes from small holes on the bark surface.
These symptoms lead to a general decline in the tree’s vigor. Infested trees may exhibit stunted growth, reduced foliage, and an unhealthy appearance. Young or newly planted black locust trees are particularly susceptible to severe damage, which can lead to their death. Older, established trees might tolerate some borer activity, but severe infestations weaken them, making them prone to breakage during strong winds or ice storms.
Management and Prevention
Maintaining the health of black locust trees is an important preventative measure against borer infestations. Healthy, vigorous trees are more resistant to pest attacks than stressed trees. Proper watering during dry periods and adequate fertilization help trees develop robust defenses. Selecting healthy nursery stock and avoiding bark wounds also contributes to their resilience.
Cultural controls manage existing borer populations and prevent outbreaks. Pruning infested branches during late fall or winter removes portions of the borer population. Proper disposal of infested wood, such as burning or chipping, prevents adult beetles from emerging. Avoiding mechanical damage to the bark, such as from lawnmowers or string trimmers, reduces entry points for egg-laying females.
Chemical controls are generally not the primary recommendation for established infestations due to the difficulty of reaching larvae deep within the wood. Insecticides are most effective when applied preventatively or at specific times to target newly hatched larvae or adult beetles before they lay eggs. This often involves systemic insecticides or bark sprays by a certified professional, typically for high-value young trees at high risk.
Precise timing, usually late summer or early fall when adults are active, is crucial. For widespread or severe infestations, or for large, mature trees, consulting a certified arborist is advisable. An arborist can assess damage, recommend strategies, and implement treatments.