Horntail Wasp

Need to know: 

• Horntail wasps are typically found on trees that are dead, dying, or stressed. 
• Horntail wasps do not have the ability to sting and are otherwise harmless. 
• Occasionally, adults can emerge from lumber after wood is cut and used to build human structures. 
• Horntail wasps will not lay additional eggs in cut lumber. Therefore, they are not a threat to completed structures.

Description of horntail wasps

An unusual pest problem of large wasps chewing holes through plaster board and emerging within recently constructed homes came to our attention in the summer of 1991. These 1 and 1/4 inch long, black or dark blue wasps have been identified as horntails or horntail wasps. The name comes from the spearlike projection on the tail end of both male and female horntails.

Life cycle of horntail wasps

Horntails tunnel as larvae in dead, dying or recently-felled trees. The female wasp lays her eggs in the wood and the larvae create sawdust packed burrows 1/4 inch in diameter by 1 to 2 feet long. The larvae take 2 to 3 years to develop before the new adult wasps chew out of the wood through 1/4 inch holes.

Horntail wasps emerging into houses through plaster board (sheetrock) come from larvae living in the studs behind the plaster board. Studs that were cut from infested logs can contain horntail larvae that survived the processing. The larvae survived, in part, because the studs were not kiln dried or were inadequately dried. The adult wasps chewing their way out of infested studs also chew through almost any building material used to cover the wood (plaster board or paneling).

Damage caused by horntail wasps

Horntail wasps do not bite or sting. Also, they cannot infest lumber after it has been cut so there is no risk of additional generations of horntails in the studs or household furniture. Most horntail wasps will emerge within the first year of construction, though a few may linger and emerge as long as 2 or 3 years after construction.

Management of horntail wasps

Horntail wasps are typically attracted to trees that are dead, dying, or stressed. If found on a landscape tree, consider stresses that may have made the tree attractive to horntail wasps in the first place and address the tree's needs. Swatting or spraying individual wasps as they emerge, and patching and repainting the emergence holes is the only advised management for this problem. There is no practical way to treat lumber within walls to control horntails that have not yet emerged. Wasps and emergence holes are usually few in number and scattered within the house and the chances of structural damage to the walls are very slim. It has been suggested that if 5 or more horntails emerge from a single stud, there will probably be enough damage to the stud to justify replacing it.

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