Birch gall midge

The birch gall midge (Semudobia betulae) belongs to the order Diptera, family Cecidomyiidae. This tiny insect is specialized in attacking the generative organs of birch trees.

The adult is a miniature gnat about 2–3 mm long with thin legs and transparent wings. It is extremely difficult to spot adults on a tree due to their small size and high mobility during the flight period.

The larvae are elongated, legless, and creamy or yellowish in color. Larval development occurs entirely inside the seeds, which protects them from environmental stressors.

The insect's life cycle is closely synchronized with the phenology of birch development, ensuring the emergence of adults coincides with catkin formation. Pupae overwinter directly inside infested seeds or in fallen galls under the tree.

This species is a specialized phytophage, so its distribution is limited to the range of various birch species. High pest densities are typically observed in mature stands.

The primary food source for the birch gall midge is the seeds of silver and downy birch. The pest does not harm vegetative parts like leaves or wood but significantly reduces the seed yield of trees.

Seed damage occurs during the fruit formation phase within female catkins. The larva consumes the internal tissues of the seed, leading to the total loss of its reproductive capability.

The level of damage can be very high in years with low seed production. During mass outbreaks, the insect can destroy up to 80–90% of the seed crop on an individual tree.

For forestry, this pest is a serious threat because it significantly reduces the quality of seed material collected for nurseries. This hinders reforestation efforts and breeding work.

Indirect damage involves the weakening of natural birch forest regeneration. In regions with high pest concentrations, there is a substantial shortage of viable seeds suitable for sowing.

Adult flight coincides with the beginning of birch flowering, usually occurring in the spring. Depending on the climatic zone, this period varies from late April to late May.

Females lay eggs directly into the scales of young female catkins. The incubation period is short, and soon after oviposition, the larvae begin feeding on the seed tissues.

Larval development inside the catkins continues throughout the summer. During this time, seeds gradually transform into gall-like structures, within which the insect undergoes pupation.

Overwintering occurs at the pupal stage. Some populations remain in catkins on the ground, while others may persist on the tree until the following spring.

Pupation concludes in early spring, after which the cycle repeats. Monitoring for the pest is recommended during the bud burst and active catkin growth period.

The main sign of infestation is deformed scales on female catkins, which take on a swollen or gall-like shape. These patches stand out against the healthy fruits.

When an infested gall is opened, a larva or pupa can be found inside. The damaged seed often lacks a fully formed embryo and endosperm.

Externally, an infested catkin may look looser or asymmetrical. In later stages of development, the galls may change color or dry out prematurely.

During mass infestation, catkins may fall off earlier than expected. Fallen catkins under the tree canopy are often the main reservoir of infection during the winter period.

• Deformation of catkin scales
• Swelling (galls) instead of seeds
• Presence of hollow seeds
• Premature catkin shedding
• Reduced viability of collected seeds

Effective control of the birch gall midge in forest stands is challenging due to the pest's hidden lifestyle. Chemical treatments in large forests are generally not economically viable.

In forest nurseries and seed orchards, sanitary measures are recommended. An important practice is to collect catkins early, before the adults emerge.

Removing and destroying fallen forest litter under trees in autumn can significantly reduce the number of overwintering pupae, preventing re-infestation in the spring.

The use of biological control agents, such as parasitoids that limit gall midge populations, is currently under study. Natural enemies often help keep pest numbers in check.

To preserve seed stocks, it is recommended to select seeds from trees that show resistance to infestation. Genetic selection for resistance remains the most promising long-term protection method.