The sight of damaged foliage on a blackberry plant is a common issue. These brambles are resilient, but their leaves are appealing to a variety of garden pests. Understanding the precise cause of the damage is necessary for implementing an effective solution that protects both the plant and the developing fruit. This guide focuses on diagnosing and managing the creatures responsible for consuming blackberry leaves.
Identifying the Signs of Damage
Different feeding behaviors leave distinct clues, which can help pinpoint the specific culprit attacking the plant. One of the most common forms of damage is known as skeletonization, where the soft tissue of the leaf is consumed, leaving only a lacy network of veins intact. This pattern is often the result of small larvae or certain beetles feeding systematically across the leaf surface.
Circular holes or irregular notches chewed out of the leaf margins suggest larger caterpillars or adult beetles that consume the entire leaf layer. Another sign is the presence of leaves that are rolled, folded, or tied together with silken threads. This indicates a larva is creating a protected shelter while it feeds inside.
Tiny pinpricks, or stippling, accompanied by a subtle yellowing or bronze discoloration, point toward sap-sucking pests like spider mites or aphids. These pests draw fluids from the cells rather than chewing the leaf material.
Cleanly severed cane tips or missing entire shoots suggest a larger animal is grazing in the area. Deer and rabbits typically leave sharp, angled cuts on young primocanes. If the damage is localized high on the plant and involves a complete removal of tender new growth, it is often a sign of mammalian browsing.
The Primary Pests Targeting Blackberry Leaves
The Japanese Beetle (Popillia japonica) is recognized by its metallic green body and copper-brown wing covers, measuring about half an inch long. Adult Japanese Beetles are gregarious, feeding in groups during the summer months. They cause severe skeletonization by preferring leaf tissue between the veins, which reduces the plant’s ability to photosynthesize and makes the damage instantly recognizable.
Another common pest is the Blackberry Sawfly, whose larvae are responsible for much of the early-season leaf damage. The larvae resemble small, pale green caterpillars with fine bristles and feed openly on the underside of leaves, chewing holes or causing skeletonization. Sawflies typically have only one generation per year. The larvae feed for about two to three weeks before dropping to the soil to overwinter.
Various caterpillars, including species like the Red-humped and Yellow-necked caterpillars, also target blackberry leaves, often feeding in clusters. These larvae consume substantial amounts of foliage, particularly on the tender new primocanes, in their final growth stages.
Cane borers are a more insidious pest. The adult beetles lay eggs whose larvae tunnel inside the canes, causing the leaves on the affected cane to suddenly wilt and die.
Immediate and Non-Chemical Control Methods
For managing small, localized infestations, handpicking remains an effective control method. Japanese Beetles, for instance, are sluggish in the cool morning hours and can be easily shaken off the foliage directly into a bucket of soapy water, which kills them. This mechanical removal is effective when populations are first detected.
For soft-bodied pests like sawfly larvae or aphids, a direct spray of insecticidal soap or horticultural oil offers immediate control. These substances work by coating the pest, leading to suffocation or disruption of their cell membranes, and are safe for use on edible crops. Neem oil, derived from the neem tree, acts as a repellent and disrupts the feeding and growth cycles of various insects without harming beneficial species when applied correctly.
Physical barriers can prevent access by both insect and mammalian pests. Fine mesh netting or floating row covers placed over the blackberry canes can exclude Japanese Beetles and other flying insects during their peak feeding period.
Cultural practices are also a form of control, such as promptly pruning out and destroying any canes that show signs of cane borer tunneling or severe leaf damage, which reduces overwintering sites for pests.
When to Use Targeted Chemical Controls
When pest populations are severe and non-chemical methods have not provided sufficient control, targeted treatments may be necessary to protect plant health and fruit yield. The choice of product must be specific to the pest and approved for use on food crops. For caterpillar infestations, including those from sawfly larvae or leafrollers, an application of Bacillus thuringiensis subspecies kurstaki (Btk) is a selective option.
Btk must be ingested by the caterpillar to be effective, making it harmless to beneficial insects and mammals. It is important to apply Btk when the larvae are small, as it becomes less effective against larger, mature caterpillars. For Japanese Beetle outbreaks, some pyrethroid-based insecticides offer rapid knockdown, but these are broad-spectrum and require careful timing.
Any application of chemical treatments must follow the label instructions regarding dosage, application timing, and the pre-harvest interval (PHI). It is particularly important to avoid spraying any broad-spectrum insecticides during the blackberry’s bloom period to prevent harm to pollinating insects like bees. Dormant oil sprays applied in late winter can also help reduce the populations of certain overwintering pests before the new foliage emerges.