Latin: Urocerus gigas & Sirex juvencus
A well-known pest in Denmark is the wood wasps. It is also known as giant wood wasp, banded horntail as well as greater horntail. They belong to the group of Hymenoptera, where you will also find ants, regular wasps, and bees.
Appearance
All these pests are characterized by their transparent wings, which consist of a small network of veins. There are two different species of wood wasps, which are frequently occurring in the Danish forests. The giant wood wasp (Urocerus gigas) can grow up to 3 centimeters long and the small wood wasp (Sirex juvencus), which is slightly smaller.
The female wasp is distinctive due to the long laying brood, and the females will also be larger than the males. For both sexes, they are bluish black with yellow or reddish stripes, as we know it from the classic bee or wasp. The wasps lay eggs, which subsequently develop into larvae that can grow up to 4 centimeters long. The larvae are white yellow with three sets of breast feet, and at the tip of the hind body they have a short and dark thorn.
Biology and behavior
The appearance of this pest can be frightening to many and result in panic. Here, it is important knowing the difference between this wasp species and the common wasp, as the wood wasp does not attack humans and at the same time can neither sting nor bite. And they are not equipped with a sting at the hind body.
Wood wasps typically occur during the summer months, and they are especially active on hot sunny days. The females search for conifers in connection with laying eggs, where they drill the laying sting into the trunk itself and place 1-2 eggs in each hole. During the females’ lives, they can lay up to 400 eggs, and the eggs are typically laid in diseased or newly cut down tree trunks as the tree will be softer.
The eggs will hatch after 3-4 weeks, at this point the larvae will gnaw their way through the wood. The larvae gnaw passages that are characterized as round in cross-section, and they will widen as the larvae develops. The excrement from the larvae, known as drilling dust, is tightly packed, and does not scatter when the wood is cut.
The larvae will mature within 2-3 years under normal conditions. If they are exposed to poor conditions, the development can take up to 10 years. Adult tree wasps reside just below the surface of the tree and during the summer it will gnaw its way out. These holes are characterized as circular and with sharp edges.
Prevention and pest control
The wood wasp is harmless to humans, but they can damage the wood, so these cannot be used as for instance floor beams. Holes in the exterior layer of your house can cause deterioration as well as an increased risk of moisture and rot in the layer. It is rarely necessary to take actions against the wood wasps.
If you experience an attack in rafters etc., it will seldom result in a weakened load-bearing capacity, so it is seldom you should replace the wood. You should instead repair the holes that are particularly exposed to wind and weather. If you have timber that you suspect has been attacked, you can only keep an eye on a possible development. It is not possible to use liquid wood preservatives as these agents have little effect on this species.
You can however use heat treatment for control. The wood must be heated up to 75 degrees for minimum eight hours or dried, so the humidity drops below 14%. Treated wood will never be attacked by wood wasp larvae.
Wood wasps does have a negative impact on forestry, which is why there are import restrictions of conifers in Australia, to avid infested wood. The export of conifers often has to have a certificated, which do guarantee wood wasps fee conifers, those certificates are issued by the Danish Plant Directorate.
"Pests in House and Home"
"Bed Bugs - Bites, Stings and Itches"
"Food Pests"
"Husets dyreliv" (Insects Around the House - Only danish)
"Skadedyr i træ" (Timber Pests - Only danish)
"Stuefluen" (Common Housefly - Only danish)