Birch sawfly

The birch sawfly (Cimbex lutea) is a member of the Cimbicidae family within the Hymenoptera order. Adult insects are large, reaching 20–25 mm in length, and mimic hornets with their yellow-brown, robust bodies.

The larvae, known as false caterpillars, can grow up to 45 mm long. They are typically yellowish-green with a distinct dark stripe running down the back and possess the habit of curling into a spiral when threatened.

Adult sawflies have clear wings with dark venation and characteristic club-shaped antennae. Females use a saw-like ovipositor to make incisions in young shoots, where they deposit their eggs individually.

The species overwinters as a prepupa inside a tough, leathery cocoon located in the soil or leaf litter beneath the host tree. Pupation occurs in early spring.

This pest has a univoltine lifecycle, meaning it completes only one generation per year. Emergence of the adults usually coincides with the budding of birch trees in the spring.

The primary host plants for Cimbex lutea are birch trees (Betula species). While they prefer birch, they may occasionally be found on alder and willow trees.

Damage is caused exclusively by the larvae. Young larvae skeletonize the foliage, consuming the leaf tissue between the veins while leaving the harder structures intact.

As larvae mature, they consume entire leaf blades, leaving only the midribs and petioles behind. This results in severe defoliation of the infested branches.

Large infestations can lead to significant canopy loss, which stresses the tree, reduces its growth rate, and renders it susceptible to secondary infestations by wood-boring beetles.

In forest stands or ornamental landscapes, widespread defoliation ruins the aesthetic value of the trees and can lead to branch dieback in weakened individuals.

Early signs of infestation include small, brown, scar-like incisions on young twigs, resulting from the female sawfly's egg-laying activity.

The presence of larvae feeding on the edges of leaves, often found in clusters, is the most obvious sign of an active infestation during late spring and early summer.

Noticeable "lacy" or skeletonized leaves on the periphery of the tree canopy are indicators that larvae are feeding in the upper branches.

Frass (larval droppings) accumulating on the ground or on lower foliage is a strong indicator of a high population density of larvae feeding overhead.

A quick inspection of the branches showing signs of defoliation will usually reveal the larvae, which are easily identifiable due to their size and characteristic coiled posture.

Manual removal of larvae from small trees is a highly effective control method. Shaking branches over a cloth can dislodge the larvae for collection and disposal.

Cultivating the soil beneath the canopy in the autumn helps to disturb the cocoons, increasing the mortality rate of the overwintering prepupae.

Biological control using microbial insecticides, such as those containing Bacillus thuringiensis, is recommended for young larvae without harming beneficial pollinators.

In cases of severe outbreaks, systemic or contact insecticides registered for forestry or ornamental use may be applied to the canopy.

• Encouraging natural predators such as birds and parasitic wasps.
• Monitoring the tree canopy early in the season for signs of egg-laying.
• Maintaining tree vigor through proper watering and soil nutrition.