Horntails, Sirex spp.
Generally, horntail wasps are moderately large at about 37.5 mm (1.5 inches) and have broad heads with small eyes. Adult coloration is determined by its species and gender, but most have a cylindrical black or brown body containing markings that are rust, orange, or yellow in color. Male and female wasps have a hornlike spine at the tip of their abdomen, but only females have an ovipositor for egg laying.
- Common names are horntails and woodwasps.
- Horntails are stingless wasps. Adult wasps have a hornlike spine at the tip of their abdomen, and females have an ovipositor for laying eggs.
- Horntails are primitive wasps and lack the constricted waist commonly seen in other wasp families.
- There are 10 species of horntail recorded in Colorado. There are 28 species of sirex horntail in North America, though not all are native.
- Horntails are most frequently associated with coniferous trees, especially pines. However, the larvae of some species, such as the pigeon tremex (Tremex columba), develop in maple, ash, and elm.
Sirex horntail (Sirex nigricornis) adult. Horntails are stingless wasps that produce wood boring larvae. While they are not typically considered serious pests, an invasive species from Europe (Sirex noctilio) poses a threat to pine trees in North America. Image credit: Gerald J. Lenhard, Louisiana State University, Bugwood.org
Woodwasp larva in a gallery. Compacted frass is present in the upper right half of the gallery. Image credit: Dennis Haugen, Bugwood.org
Sirex horntail damage. Adult wasps exit the host tree through these holes. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Applying parasitic nematodes into a tree infested with sirex larvae. Biological controls like this are sustainable alternatives to chemical management. Image credit: Dennis Haugen, Bugwood.org
Life history and habits
Horntails undergo complete metamorphosis, and have four life stages: egg, larva, pupa, and adult. Egg laying and larval development occur mainly in trees that are in decline or those badly damaged by fire, injury, or diseases. Female wasps can also lay eggs on cut logs or milled wood. When laying eggs, female wasps also introduce a fungus in the genus Amylostereum that causes white rot decay of wood which makes it more nutritious for larvae as they continue to burrow and develop.
As larvae burrow, they cause fine sawdust to be packed in the tunnels. The larvae are creamy white, legless, and have a dark spine on the tip of their abdomen. Larval development takes 1-3 years, sometimes longer. Pupation occurs just under the bark. After emerging, adults exit the host tree by cutting through the bark which leaves circular exit holes that are 3.2-9.5 mm (1/8-3/8 inches) in diameter.
Horntails are not considered serious pests as they do not usually occur in large numbers. Although the boring larvae do damage trees, they are typically restricted to trees that are unhealthy or in decline. One major exception is the European woodwasp (Sirex noctilio), which is an invasive species in North America that is known to damage pine trees in other parts of the world. Currently it has only been documented in the northeastern US. Accurate identification is important for European woodwasp; it resembles two other species of horntail, Sirex juvencus californicus and Sirex longicauda. See the fact sheet below for additional resources on how to distinguish between these species.
Trees infested with woodwasps will display wilting and color changes of foliage from dark green to light green, then yellow, and finally to red. Infested trees can also have resin beads or drippings present where female wasps penetrated the bark to lay eggs.
Infested timber should not be used for construction as the wood may contain developing horntail pupae. This can result in the emergence of adults inside the building after it is constructed. While they can be noisy, the wasps are harmless.
Transporting infested wood poses a significant risk in spreading invasive wood boring insects. The following materials pose a significant risk of spreading the S. noctilio: packing materials made of pine such as wooden pallets, cut Christmas trees collected in the wild or from landscape plantings, or pine firewood. In regions of the US where S. noctilio is confirmed, procedures are in place to minimize the transport of infested wood.
A parasitic nematode (Deladenus siricidicola) has been successfully used as a biocontrol agent for managing S. noctilio. In regions of the US where S. noctilio is present, trap trees are created by applying an herbicide to stress the tree three months prior to the emergence of adult S. noctilio. After emerging, females are allowed to lay eggs in the trap tree.
Trap trees with confirmed S. noctilio infestation are cut down and the branches trimmed to make the nematode inoculation easier. Holes about 10-mm deep are made along the tree, spaced regularly along the length of the log. The exact pattern of the holes depends on the size of the log. The nematode suspension is then applied to the holes using a squeezable tube with a long, tapered nozzle.
The nematode infects larvae and sterilizes females so they cannot reproduce. When infected with the nematode, female S. noctilio lay eggs containing only juvenile nematodes. Interestingly, these nematodes grow and develop by consuming the same fungus carried by egg-laying S. noctilio females.
Encyclopedia Britannica. (n.d.). Horntail. Encyclopedia Britannica. Available https://www.britannica.com/animal/horntail
University of California. (n.d.). Wood Wasps and Horntails. University of California – Division of Agriculture & Natural Resources. Available http://ipm.ucanr.edu/PMG/PESTNOTES/pn7407.html
US Department of Agriculture. 2011. Wood Wasps (Horntails). US Department of Agriculture – Forest Service. Available https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5343820.pdf
US Department of Agriculture. 2008. Proposed Program for the Control of the Woodwasp Sirex noctilio F (Hymenoptera: Siricidae) in the Northeastern United States. US Department of Agriculture – Animal and Plant Health Inspection Services. Available https://www.aphis.usda.gov/plant_health/ea/downloads/SirexEA-final-northeast.pdf
Mississippi Forestry Commission. (n.d.). Sirex Woodwasp. Mississippi Forestry Commission. Available https://www.mfc.ms.gov/forest-health/forest-pests/sirex-woodwasp/#:~:text=The%20foliage%20of%20infested%20trees,at%20the%20mid%2Dbole%20level.
Morris, E. (n.d.). Parasitic castration of Sirex noctilio by Deladenus siricidicola (Beddingia siricidicola). Cornell University – College of Agricultural and life Sciences. Available https://biocontrol.entomology.cornell.edu/pathogens/Deladenus.php