Blackberries offer abundant harvests to both commercial growers and home gardeners. Maintaining a healthy, productive patch requires careful attention to pest management, as various insects and mites can significantly reduce yield and plant vigor. Successful growing relies on accurately identifying these physical threats and implementing timely, targeted control measures to protect the plant’s foliage, canes, and developing fruit.
Identifying the Primary Threats
Threats to blackberries are often categorized by the part of the plant they damage. The Spotted Wing Drosophila (SWD), a type of fruit fly, is particularly damaging because the female uses a saw-like ovipositor to lay eggs in healthy, ripening fruit. Infested berries appear soft and sunken, and the presence of small, white larvae inside the fruit is the definitive sign. The fruit quickly breaks down, sometimes developing a fermented or sour smell.
Other pests target the structural components of the plant, specifically the canes. Cane borers, such as the Red-necked Cane Borer, cause noticeable symmetrical swellings or galls on the canes. The larvae tunnel inside the cane, which weakens the structure, reduces nutrient transport, and can cause the cane to die before the fruit matures. In contrast, the Raspberry Crown Borer bores into the crown and lower canes, causing wilting or “flagging” of individual canes as the larvae feed over a two-year life cycle.
Foliage-feeding pests like spider mites and aphids attack the leaves, leading to a decline in overall plant health. Spider mites are tiny arachnids problematic in hot, dry conditions, feeding on plant sap and causing characteristic stippling—tiny yellow or white dots—on the leaves. Heavy infestations result in fine webbing on the undersides of the leaves, which can cause them to drop prematurely. Aphids cluster on new growth, sucking sap and causing leaves to curl and distort, while also leaving behind a sticky residue called “honeydew.”
Foundational Cultural Practices
Preventative measures create an environment less hospitable to infestation. Sanitation begins with the immediate removal and destruction of old, spent canes (floricanes) after harvest, as many pests overwinter in this dead material. Promptly removing any dropped or overripe fruit from the ground is important for managing Spotted Wing Drosophila, preventing them from completing their life cycle.
Proper pruning and trellising significantly enhance air circulation and light penetration. Training canes onto a trellis system keeps them off the ground and reduces the humidity that favors pests like spider mites. Increased airflow also allows for better spray coverage if treatments become necessary. Pruning out low-hanging branches minimizes the chance of pests crawling onto the plant from the soil.
Careful site selection and maintenance contribute to pest resilience. Blackberries should be planted in well-drained soil and away from wild brambles, which can harbor pests and viruses. Maintaining plant health through proper fertilization and watering ensures the canes are vigorous and resilient to minor pest pressure. Avoiding excessive nitrogen fertilization is beneficial, as lush, soft growth can attract certain pests, including spider mites.
Direct Treatment and Management
Direct treatment focuses on controlling the active infestation with targeted products, often starting with organic options. Horticultural oils and insecticidal soaps are contact-based treatments effective against soft-bodied pests like aphids and spider mites. Application must thoroughly cover the pest, including the undersides of leaves. These products work by suffocating the insects or disrupting their cell membranes, but avoid applying them when temperatures exceed 90°F or on water-stressed plants to prevent injury.
Biological control involves introducing or encouraging beneficial insects that prey on the pests. Lady beetles and green lacewings are natural predators that effectively manage populations of aphids and spider mites. Predatory mites can also be introduced to control pest mite populations, offering a long-term, non-chemical solution. For pests that are larvae or caterpillars, such as certain fruitworms, the biological insecticide Bacillus thuringiensis (Bt) can be used, as it targets these specific stages without harming beneficial insects.
Chemical applications require careful timing and adherence to safety guidelines, particularly concerning the Pre-Harvest Interval (PHI)—the time required between application and harvest. For Spotted Wing Drosophila, insecticide applications must begin as soon as the fruit starts to ripen and continue throughout the harvest period, often requiring a zero-day PHI material.
For cane borers, treatments are structural: infested canes showing galls or wilting must be cut out a few inches below the damaged area and destroyed to remove the larvae before they emerge as adults. When using any product, select one labeled specifically for blackberries and apply it only at the correct growth stage to minimize impact on pollinators.
Distinguishing Pests from Diseases
Growers often confuse the symptoms of a pest infestation with those of a disease, which require fundamentally different management strategies. Damage from a pest results from physical activity—chewing, boring, or sucking sap—leaving characteristic signs like holes, tunnels, galls, stippling, or webbing. For example, a cane borer leaves a physical swelling from internal tunneling, while spider mites cause distinct yellow speckling on the leaves due to cell feeding.
Disease symptoms are characterized by discoloration, tissue death (necrosis), or abnormal growth caused by a pathogen. Fungal diseases like Anthracnose cause silvery lesions on canes or brown, shrunken fruit drupelets. Viral infections often present as mottled or streaked leaves, stunted growth, or leaf curl not accompanied by aphids or honeydew. If the problem involves cankers, spotting, or a powdery coating, the issue is likely a disease, requiring research into common blackberry diseases like Orange Rust or Cane Blight.