Dusky Birch Sawfly

The dusky birch sawfly (Croesus latitarsus) is one of several species that target birch trees. Though they are harmless to humans, these insects can be a nuisance for gardeners and arborists, as they cause noticeable damage to their host plants.

Host Plants

The primary host for the dusky birch sawfly is, as the name suggests, the birch tree (Betula spp.).

Regions impacted

Predominantly found in North America, these insects are widespread across the eastern and northern regions of the U.S. and extend into Canada. They are particularly prevalent in areas where birch trees are common.

Description

The larvae of the dusky birch sawfly resemble caterpillars but are distinguishable by their multiple prolegs. They have a yellow-green body with black blotches on their side, a dark head, and can grow up to 1 inch (2.5 cm) long. Adult sawflies are wasp-like, with a black body and a wingspan of about half an inch (1 cm).

Life Cycle

The dusky birch sawfly has a relatively straightforward life cycle that progresses through the typical egg, larva, pupa, and adult stages. Here’s an overview of their life cycle:

Egg: Female sawflies deposit their eggs on the underside of birch leaves, usually along the leaf veins. This occurs in the spring, and the number of eggs a single female lays can be substantial.

Larva: Once the eggs hatch, larvae emerge and start feeding on the birch leaves. They tend to consume leaf tissue from the leaf margins inwards, giving the leaves a ragged appearance. As they feed and grow, the larvae undergo several molting stages. The larval stage of the dusky birch sawfly is the most damaging, as this is when the feeding takes place. After several weeks of feeding, the fully grown larvae drop to the soil to pupate.

Pupa: In the soil, the larvae create small cells where they pupate. The pupal stage can last for several weeks. Some species might overwinter in this stage, waiting for favorable conditions in the following spring.

Adult: Once the pupal stage is complete, adult sawflies emerge. They are winged and are active fliers. The adults are relatively short-lived, living only long enough to reproduce. After mating, females search for suitable birch leaves to lay their eggs, completing the cycle.

There are two generations per year with larvae feeding from May-July and again in September-October.

Damage and Detection

The dusky birch sawfly causes distinctive damage patterns. Recognizing the signs of infestation can help gardeners and arborists take timely action to prevent extensive harm.

Damage:

• Leaf Consumption: The larvae are the primary culprits of damage. They feed on the leaf tissue from the margins inwards, giving the leaves a ragged and scalloped appearance.
• Defoliation: Heavy infestations can result in significant defoliation, with entire branches or even whole trees stripped of their leaves. While mature trees can usually tolerate some defoliation, repeated annual infestations or defoliation of young trees can stress the trees and make them more susceptible to other pests, diseases, or unfavorable environmental conditions.
• Aesthetic Impact: The aesthetic appeal of ornamental birch trees can be compromised by the unsightly damage and premature leaf drop.

Detection:

• Visible Larvae: The larvae can be easily spotted against the backdrop of birch leaves, especially when present in large numbers.
• Ragged Leaves: The unique feeding pattern of the larvae, which involves eating from the edges of the leaves inward, is a telltale sign of their presence.
• Presence of Adults: While the adults do not cause damage like the larvae, observing them around birch trees can be a precursor to potential damage, as they lay eggs on the leaves.

For optimal tree health and aesthetics, early detection and prompt intervention are key. Monitoring birch trees regularly, especially during the active months of the sawfly, can help in early detection and management.

Prevention and Control

Preventing and controlling the dusky birch sawfly involves a combination of cultural, biological, and chemical measures. Effective management ensures the health and vitality of birch trees, especially when they’re ornamental or young.

Prevention:

• Regular Monitoring: Regularly inspect birch leaves for signs of the larvae or their distinctive feeding damage. Early detection can allow for more effective, less invasive control measures.
• Promote Tree Health: Healthy trees are more resilient to pests. Ensure your birch trees have suitable soil, adequate water, and proper pruning. Avoid stressing the trees, as stressed trees attract more pests.
• Clean Up: In the fall, rake and dispose of leaves under the tree. This can reduce the number of overwintering larvae and decrease the potential for re-infestation the following year.

Control:

• Hand Picking: For light infestations, larvae can be hand-picked from the leaves and destroyed. This method is especially effective for small trees or individual ornamental specimens.
• Pruning: Prune out and destroy the branches they are feeding on.
• Insecticidal Soap: A solution of insecticidal soap can be sprayed directly on the larvae. This is a less toxic option and is effective when the larvae are small.
• Chemical Control: Consider using a registered insecticide if infestations are heavy and other methods are insufficient. Always follow label recommendations and consider the potential impact on beneficial insects.