Seeing damaged raspberry leaves is a common experience for growers. When leaves start disappearing or become riddled with holes, the first step is to correctly identify the culprit. The specific pattern of feeding provides the most significant clue to the pest responsible. Pinpointing the source of the damage is the only way to select a successful management strategy.
Common Pests and Their Damage Patterns
One distinctive type of leaf damage is skeletonization, where an insect consumes the soft green tissue but leaves the tougher vascular structure, creating a lacy appearance. The primary cause is the larva of the Raspberry Sawfly (Monophadnoides geniculatus). These light-green, bristle-covered larvae feed on the underside of leaves in late spring and early summer. In severe infestations, only major leaf veins may remain.
A different, often more complete, skeletonization is caused by the adult Japanese Beetle (Popillia japonica). These metallic green and coppery-brown beetles are active during warm, sunny days, congregating on the upper parts of the canes. They are particularly damaging because they also feed on flowers and fruit, boring into ripening berries.
Large, irregular holes and ragged edges often point to generalist feeders like caterpillars, slugs, or earwigs. Caterpillars cause severe defoliation, confirmed by dark, pellet-like droppings called frass. Slugs create irregular holes with smooth edges and leave behind a shiny, silvery slime trail.
Nighttime feeders, such as slugs and earwigs (Forficula auricularia), chew through leaves under the cover of darkness, making them difficult to spot during the day. Earwigs are reddish-brown insects with a prominent pair of pincers that chew small, ragged holes in younger and older leaves. To confirm a nocturnal pest, inspect the plants after dark using a flashlight.
Non-Chemical Control Strategies
The most immediate non-chemical method for managing larger pests is handpicking, which is effective against Japanese beetles and sawfly larvae. Japanese beetles can be flicked off the plant and into a container of soapy water to drown them. Sawfly larvae and caterpillars should be inspected for on the underside of the leaves, where they prefer to hide.
Cultural practices play a significant role in reducing pest populations by disrupting their life cycles. Removing leaf litter and debris from around the base of the canes eliminates overwintering sites for sawfly larvae and other pests. Pruning out old, infested canes and ensuring adequate air circulation further discourages pests and limits the spread of issues.
Physical barriers offer a preventative approach using insect netting or floating row covers. These covers must be completely sealed around the base of the plants to block adult insects, such as Japanese beetles, from landing and laying eggs. This strategy is useful during the short period when highly mobile adult beetles emerge and are actively feeding.
For slugs and earwigs, traps and barriers offer targeted relief near the soil level. Earwigs can be trapped using rolled-up damp newspaper or pieces of garden hose, which they crawl into for shelter during the day. Slugs are deterred by barriers of diatomaceous earth or copper strips. They can also be trapped using low-sided containers filled with beer or a sugar-water solution.
When Targeted Treatments Are Necessary
If a leaf-eating infestation is severe, targeted treatments may be necessary, starting with biological controls. Bacillus thuringiensis subspecies kurstaki (Btk) is a naturally occurring soil bacterium that acts as a stomach poison. It is only effective against caterpillars. The pest must ingest Btk, which releases a toxin that paralyzes the digestive system, causing it to stop feeding and die.
Btk is not effective against sawfly larvae, beetles, or slugs, as those pests are not susceptible to the specific toxin. Application must be timed precisely when caterpillars are young and actively feeding. Since the product breaks down quickly in sunlight, it will not harm beneficial insects like ladybugs or bees.
Organic contact sprays, such as insecticidal soaps and Neem oil, offer a broader application for soft-bodied pests. Insecticidal soaps work by penetrating the insect’s outer membrane, causing cellular disruption and dehydration. Neem oil, derived from the neem tree, works as a repellent and disrupts insect feeding and growth.
When using these sprays, thorough coverage is important because they only kill pests on contact, and reapplication is often necessary after rain. Stronger chemical sprays, such as those containing pyrethrins, are considered a last resort for overwhelming infestations, offering a faster knockdown effect. If using stronger treatments, follow the pre-harvest interval instructions listed on the product label to ensure the berries are safe for consumption.