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The chestnut bee, also known as the chestnut gall wasp, is a tiny insect measuring 2 to 3 mm in length. It has the unique ability to reproduce without mating, a process known as parthenogenesis. These bees primarily lay their eggs in the buds or leaves of young branches of chestnut trees. The larvae develop inside galls—abnormal growths formed on the tree's new shoots. Each gall can contain between 1 and 13 larvae, though most commonly 2 to 5.
This pest specifically targets the tender shoots, leaves, and branches of chestnut trees, causing the formation of galls on stems, petioles, and leaf veins. As a result, the affected trees not only fail to produce nuts in the current year but also suffer from reduced yields the following season. In severe cases, entire branches may dry up and die, leading to complete loss of production. For this reason, local farmers refer to the chestnut bee as a "cancer" growing on the chestnut tree.
During years when the chestnut bee population is particularly high, some trees can be infested with over 10,000 galls. However, the damage varies depending on environmental conditions and the presence of natural predators. One such predator is the long-tailed wasp, which plays a crucial role in controlling the chestnut bee population. The adult wasps are about 4 mm long, slender, and have a pointed body with sparse yellow-brown hair. They are often mistaken for the larvae of the chestnut bee due to their similar appearance.
After mating, the female wasp searches for the larvae of the chestnut bee within the galls. She then inserts her ovipositor into the tumor and lays her eggs on the walls of the larval chamber. Within one to two days, the wasp larvae hatch and feed on the body fluids of the chestnut bee larvae by biting through their bodies. A single long-tailed wasp larva typically parasitizes only one chestnut bee larva. Therefore, where parasitism rates are high, the damage caused by the chestnut bee is significantly reduced.
Because the long-tailed wasp larvae live inside the galls alongside the chestnut bee larvae for an extended period, they are often confused with them. This misidentification can lead to the removal of beneficial wasp larvae during winter pruning. To prevent this, it’s important to protect and encourage the presence of long-tailed wasps. In areas with high parasitism, the galls cut during winter should be preserved and reintroduced into the chestnut orchard between April and May to allow the wasps to reproduce.
At this time, it’s also essential to avoid spraying pesticides that could harm the wasps. Burning the stem galls can further reduce the population of chestnut bees, and the natural parasitism rate of the long-tailed wasp can reach up to 40% under these conditions.
Adult chestnut bees prefer to lay their eggs in the canopy of weak, thin branches that are sheltered from wind and rain. Therefore, during pruning, all weak and thin branches must be completely removed to eliminate overwintering larvae. In summer, any tender green galls should be cut and burned or buried deeply. When using insecticides, it is crucial to avoid the adult and egg-laying periods of the long-tailed wasp to preserve their population and maintain natural control of the chestnut bee.