The raspberry sawfly (Monophadnoides geniculatus) is a common pest of cane fruits that causes significant defoliation in home gardens. While the adult resembles a small, non-stinging wasp, the larval stage chews holes in raspberry leaves. Larvae typically emerge in late spring and early summer, quickly reducing the plant’s photosynthetic capacity. Understanding the sawfly’s life cycle and recognizing damage signs are the first steps in effective management. This guide provides practical methods for identifying this pest and employing targeted control strategies.
Identifying the Raspberry Sawfly and Damage Signs
The most noticeable sign of infestation is the distinctive feeding damage on raspberry leaves. Larvae chew scattered, irregular holes, and heavy feeding can skeletonize the foliage, leaving only large leaf veins. Damage usually begins in May and June, often starting lower on the cane before progressing upward, negatively impacting plant vigor and berry yield.
The sawfly larva is a pale green, caterpillar-like insect, growing up to two-thirds of an inch long. A defining characteristic separating it from true caterpillars is the number of fleshy, leg-like prolegs on its abdomen; sawfly larvae have six or more pairs, while most caterpillars have four or fewer. Larvae are often found feeding on the underside of leaves. The adult sawfly is a stout, black, wasp-like insect, approximately a quarter-inch long, that emerges from the soil in spring to lay eggs.
Non-Chemical and Cultural Management Strategies
Mechanical removal is a highly effective initial control method, especially for small infestations, since sawfly larvae feed openly and are relatively large. Regularly inspecting the undersides of leaves in late spring allows for early intervention. Simply handpick the pale green larvae and drop them into a container of soapy water to destroy them.
A strong jet of water from a garden hose can dislodge larvae from the leaves, which is most effective when the larvae are young. Once knocked off, these larvae have a low chance of successfully climbing back onto the plant. Pruning heavily infested canes can also reduce population pressure if damage is concentrated on specific canes.
Cultural Control
The sawfly life cycle provides a window for cultural control by disrupting the overwintering stage. Mature larvae drop from the canes in early summer and spin cocoons in the topsoil or leaf litter to overwinter. Tilling the soil around the base of the canes in late fall or early spring exposes these cocoons to cold temperatures, predators, and desiccation. This significantly reduces the population that emerges the following year. Alternatively, applying a layer of mulch acts as a physical barrier, impeding adult sawflies from emerging from the soil in the spring.
Applying Targeted Control Products
Targeted insecticide application should be used when cultural methods are insufficient and larval feeding threatens plant health. Timing the application to coincide with the presence of young, actively feeding larvae in late spring ensures maximum effectiveness. Ensure thorough coverage by spraying both the upper and lower surfaces of the leaves to contact the larvae.
Spinosad
Spinosad-based products are highly recommended for sawfly control and are acceptable for organic gardening, as they are derived from a naturally occurring soil bacterium. Spinosad is effective against larvae, acting both by contact and ingestion, leading to a cessation of feeding shortly after exposure. The pre-harvest interval (PHI) for Spinosad on caneberries is typically one day, making it a suitable option close to harvest.
Soaps and Oils
Insecticidal soaps and horticultural oils provide another low-impact option, working primarily by smothering or disrupting the insect’s cell membranes through direct contact. These products are most effective against smaller, younger larvae. Since they offer no residual protection, repeat applications may be necessary. Insecticidal soaps often have a pre-harvest interval of zero days, allowing them to be used right up to the day of harvest.
When applying any spray, take measures to protect beneficial insects, especially pollinators. Spinosad is toxic to bees while wet, so applications must be made late in the evening after bees have stopped foraging. This allows the product to dry completely before morning. Always follow the specific application rates and safety guidelines provided on the product label.