Iowa Insect Information Notes

Horntail Wasps in New Homes

Horntail Wasps in New Homes

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.

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).

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.

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.

Updated 07/14/2005 - 12:45pm