Horntails, Sirex spp.
Order: Hymenoptera
Family: Siricidae
Description
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.
Quick Facts
- 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
Horntail 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 larvae of Sirex spp. 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, making 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 mm (1/8 inch) to 9.5 mm (3/8 inch) in diameter.
One species of particular importance, Sirex noctilio, is considered a secondary pest of trees throughout its native range in Europe, Asia, and North Africa. However, this species has become established in North America where it was first discovered in 2004. Sirex noctilio is currently the only species of horntail known to kill healthy trees, most notably pines (Pinus spp.). This ability to overcome host defenses is due to the combined effects of a phytotoxic mucus and the fungus A. areolatum, which are introduced to host trees during oviposition. The presence of A. areolatum is also necessary for immature wasp development, which can take one to three years. Adults tend to emerge from early summer to early fall, with the exact time depending on geographic location. After emerging from the natal tree, females begin searching for a suitable host tree by probing the sapwood with their ovipositor. When searching for a host tree, females tend to avoid laying eggs in trees infected with blue stain fungi previously introduced by bark beetles. After laying eggs, females tend to remain in the vicinity of the host tree. This invasive species is considered a high priority on the exotic species target list in the United States and Canada due to its ability to cause financial losses in pine plantations and the high risk posed to pine forests. In addition to attacking healthy pine trees, S. noctilio can attack fir (Abies spp.), larch (Larix spp.), spruce (Picea spp.), and Douglas-fir (Pseudotsuga spp.).
Injury
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, with one major exception being S. noctilio. Currently it has only been documented in the northeastern US but could potentially spread to western states. Oviposition produces scattered beads of resin around the mid-bole, or across the entire bole in heavily infested trees. The foliage of trees infested with woodwasps will wilt and become discolored, changing from dark green to light green, then yellow, and finally to red. The tree may die as a result, which can take anywhere from a few weeks to one year. In the following summer, circular emergence holes may be observed on the bole, but alone do not confirm an S. noctilio infestation since other wood-boring species produce similar exit holes. Accurate identification is important for S. noctilio since it resembles two other species of horntail (Sirex juvencus californicus & Sirex longicauda). See the fact sheet below for additional resources on how to distinguish between these species.
Monitoring
Effective monitoring is necessary to minimize the impacts of S. noctilio. Currently, there are no official monitoring or quarantine guidelines in the United States. However, forestry professionals can support monitoring efforts by voluntarily conducting ground surveys for injuries on potential host trees. The ideal time to conduct surveys is in the fall after adult flights have ceased but before injuries become obscured by harsh winter conditions. It is recommended that these surveys be prioritized on visibly stressed trees or those with fading canopies. The use of binoculars can be beneficial when inspecting for resin beads on distant or taller trees.
Traps can also be used for monitoring S. noctilio but are often more costly and less reliable than ground surveys described above.
Prevention
In North America, stands that are well-managed tend to experience fewer S. noctilio injuries. Thinning stands have been clearly shown to be beneficial, and selecting trees that are well-adapted to site conditions is also recommended, as is minimizing stress, and promoting tree vigor. Stands most vulnerable to S. noctilio in North America include those that are overstocked, unmonitored, prone to stressors such as drought, and have poor growing conditions.
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.
Biological control
Natural enemies of horntails include nematodes, parasitic wasps, and woodpeckers. In its native range, these natural enemies prevent S. noctilio from inflicting extensive injury to host trees. The absence of natural enemies for S. noctilio in North America contributes to the high rates of infestation, although heavy woodpecker activity can be an indicator that an infestation is present. In North America, I. leucospoides ensiger is considered a parasitoid of S. noctilio eggs and young larvae. Although it is commonly found in association with S. noctilio, I. leucospoides ensiger is not regarded as a major control agent. Certain Rhyssa spp. and Megarhyssa spp. attack mature larvae in North American populations of S. noctilio. Parasitism by Rhyssa spp. is less common than I. leucospoides ensiger, and it is currently understood that these parasitoids alone do not effectively regulate North American populations of S. noctilio. However, they should not be ignored as important natural enemies S. noctilio since they can cause significant mortality.
The nematode Deladenus siricidicola is believed to have been introduced to new locations with S. noctilio, and since its introduction, it has become established in the United States. North American strains of D. siricidicola do not sterilize the eggs of S. noctilio, however infections can limit the body size, dispersal, and fecundity of the invasive pest. The Kamona strain of D. siricidicola invades the eggs of S. noctilio after pupation, which results in sterilization. The latter strain of D. siricidicola has been successfully used biocontrol of S. noctilio in Australia. This involves creating trap trees 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 (2/5 inch) 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.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Encyclopedia Britannica. (n.d.). Horntail. Encyclopedia Britannica. Available https://www.britannica.com/animal/horntail
Hajek, A., L. Haavik, and F. Stephen. 2021. Biology and Ecology of Sirex noctilio in North America. United States Department of Agriculture – Forest Service. Available https://www.fs.usda.gov/foresthealth/technology/pdfs/FHAAST-2019-01_Biology_Ecology_Sirex_noctilio.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
Slippers, B., P. de Groot, and M. Wingfield. 2011. The Sirex Woodwasp and its Fungal Symbiont: Research and Management of a Worldwide Invasive Pest. Springser Science & Business Media. Available https://books.google.com/books?hl=en&lr=&id=oiSYvkvn4pgC&oi=fnd&pg=PR3&dq=The+sirex+woodwasp+and+its+fungal+symbiont+:+research+and+management+of+a+worldwide+invasive+pest&ots=muIjlf5B8F&sig=mgE5BJSH5jYB9wK9KuWt-nM37mY#v=onepage&q=The%20sirex%20woodwasp%20and%20its%20fungal%20symbiont%20%3A%20research%20and%20management%20of%20a%20worldwide%20invasive%20pest&f=false
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