Montana Field Guide

The European Woodwasp is a large insect similar in appearance to a wasp. Although the species is found within the same order as true wasps which they superficially resemble, it is actually a sawfly (suborder Symphyta). As with other species in this group the European Woodwasp is xylophagous, meaning that it feeds on wood. The larvae of the species feeds primarily on pine trees (Pinus), and it is a significant pest for commercial timber in areas outside of its native range. The species is native to Europe, Asia, and northern Africa, but has been introduced to Oceanian, and the Americas. Infestations of this species can damage trees, and in severe infestations kill the host tree. Damage not only comes from egg deposition and larval development but also the inoculation of a symbiotic fungus Amylostereum areolatum which feeds on the wood and in turn is food for the larvae. The species has been successfully managed through use of biological controls including the parasitic nematode Deladenus siricidicola.

The phenology of this species is variable across its native and introduced range. Larval development is primarily controlled by the regional climate but also influenced by temperature within the host tree. Warmer temperatures result in more rapid maturation and a life cycle of about 1 year. In colder temperatures development may take up to 3 years.

As the species spends almost all of its life within the host tree, adults are present within a relatively brief period during the summer and early fall. Emergence periods vary in length and timing across the range of the species. In warmer climates the emergence period may extend for up to 5 months, while at the latitudinal extremes this period may reduced tow 2-3 months.

Adults are large insects, ranging from 2.5 to 4 cm in length. The species possesses traits typical for horn-tail wasps. The abdomen and thorax are not separated by a narrow waste as in true wasps. Both males and females have a spike like projection at the posterior of the abdomen, and females have a prominent ovipositor below this projection. The species can be distinguished from other woodwasps by their black body and black antennae. The wings are transparent. Males have an orange spot in the middle of the abdomen. The rear legs of males are black and females orange (Slippers et al. 2012). Some characteristics are shared between this and native species, see Schiff et al. (2012) for a species identification key.

This species is not known to migrate in its introduced or native range.

The European Woodwasp can disperse up to 30 – 50 Km per year (Haugen et al. 1990) and have rapidly colonized regions where they are introduced. Of great concern is the introduction of this species in wood products. Larvae may undergo diapause during development and it may take years from egg deposition to the emergence of the adult.

The European Woodwasp is a species of temperate conifer forests. Within its native and introduced range it is closely tied to the distribution of its host species, primarily pine trees (Pinus). In its native habitat the species appears to colonize trees weakened by biotic or abiotic factors such as disease and drought. In areas outside its native range, stressed trees are still selected and crowding within commercial plantations may contribute to outbreaks.

Suitable host trees are exclusively conifers and almost all ways pines (Pinus). Within the Pinus genus, the European Woodwasp appears to be a generalist. The species is known to select two of Montana’s pine species, Lodgepole Pine (P. contorta) and Ponderosa Pine (P. ponderosa). As the species has not spread to western North America, the susceptibility of other species is unknown.

The species will attack living pines, and females typically select trees that are stressed by biotic or abiotic factors such as crowding as occurs on tree plantations or drought. Before depositing eggs, the female will drill test holes in potentially suitable hosts, avoiding egg deposition in those with high osmotic pressure. Even if the tree or area of the tree is not selected, phytotoxic mucus and fungus are injected during these tests holes, which damage the tree and over time can weaken or kill the tree increasing its suitability as a host.

The general life-cycle of the European Woodwasp is typical of many forest pests. Eggs are laid in the tree, larvae develop, pupate, and eventually adults emerge and reproduce starting the cycle again. However, aspects of the cycle are more nuanced and interesting than this general description suggests. Among the most fascinating aspects of this species is its symbiotic relationship with a fungus which helps feed the larvae to the detriment of the host tree.

After mating, female woodwasps locate a suitable host tree. Typically, the female begins oviposition lower on the trunk and works her way up. Using her ovipositor she injects a phytotoxic mucus into the tree, lays an egg, and inoculates the tree with the pathogenic fungus Amylostereum areolatum. The mucus and fungus weaken the host tree’s defenses and the fungus provides food for the larvae during the initial stages of development. The fungus is not pathogenic in the absence of the mucus.

Over the next 1-3 years the egg develops into an adult. Rate of development is thought to be controlled by temperature and may also be influenced by the growth of A. areolatum. Emergence is usually in the summer or early fall. Adult woodwasps do not eat and only live for a relatively short time. Females live for up to 5 days, while males live up to 12. Males emerge before females and aggregate in the crowns of trees and are eventually joined by the females. After mating females find a suitable host tree and begin oviposition.

Across its native range, the European Woodwasp is rarely a threat to forests outside of timber plantations. Within the areas where the species has been introduced, the species has disproportionately impacted plantations of exotic pine species. In fact, areas most greatly impacted by this pest species (South America, Australia, New Zealand) all lack native Pinus species. As the pines themselves are introduced, they lack the community of species that have co-evolved. Given this the establishment and impacts of this Woodwasp species in North America which, like Europe has native pine forests, may be quite different. It is unknown how competition from or interaction with native insects and fungi impact S. noctilio or its mutualist fungus.

Three control strategies have been developed to mitigate the impacts of this species in its introduced range. First, implementing silvicultural practices that reduce stress on trees and make them less desirable hosts for the woodwasps. Second, use of the parasitic nematode Deladenus siricidicola as a bio control. Third, use of parasitoid wasps as biological controls. Use of these control techniques have mitigated the impacts of the European Woodwasp in much of the areas they have invaded (as reviewed in Cameron 2012).

For information and resources on plant pests and diseases see the USDA's Animal and Plant Health Inspection Service's Plant Pests and Diseases Profiles