The Parasitic Wood Wasps, family Orussidae, are grouped with the sawflies. The Orussids are rare, there are only nine North American species, and they are not well known. They’re rarely seen in the field, let alone photographed. The larvae appear to be parasites on wood-boring beetle larvae (Family Buprestidae). There are two species in the northwest United States.

Three diagnostic characteristics are visible in close-up photos:

A. the antennae arise below the eyes, under a ridge of exoskeleton

B. the head between the eyes bears a number of pyramidal bumps or spikes

C. the wing has a single submarginal cell, a feature not easily seen in field photos

In these locations, I saw two types, one with a red abdomen and one with a black abdomen with a white marking near the tip of the abdomen.

Occasionally I saw two individuals notice each other from several feet apart, approach, then scuttle or fly off (meeting video, right).

I observed several individuals backing into holes in the log (hiding video, hiding slideshow, right). I thought I was observing females laying eggs, but Lars Vilhelmsen, Natural History Museum of Denmark, University of Copenhagen, (See his web page) informs me by e-mail that this is a male, possibly just retreating into holes (hiding video, right). Note that the individual peeks out twice after entering the hole.

Female Orussus can be idetified by the swollen antenna tips visible in this image.

I also observed many individuals tapping the log surface with their antennae (tapping video, right). Vilhelmsen and colleagues have shown that in females, vibrations in the wood caused by the antenna taps are pecieved by the forelegs. Females use this vibrational sensing to find suitable holes for ovipositing.

You can see more photos in a slideshow format here: Parasitic Wood Wasps. You can read comments on my Bugguide.net pages on Orussus.

I photographed many of these Parasitic Wood Wasps in early May, 2008 on the beach south of Marlyn Nelson County Park in Sequim, WA (See Clallam County Parks web page). The park is an east-facing salt-water beach on Sequim Bay, with a marsh directly inland of the southern part. I only saw these wood wasps on a few driftwood logs above the high-tide line and adjacent to the marsh. I also photographed Parasitic Wood Wasps second location is the mouth of the Elwha River, a freshwater beach.

In both locations, they were on logs with nearly identical, circular holes, perhaps made by common buprestids in our area.

They were quite difficult to photograph, dashing along the log and switching direction suddenly. They looked from a distance like large ants. Every once in a while one would appear to disappear, their takeoff was so quick.