The Parasitic Wood Wasps, family Orussidae, are related to sawflies. Orussids are rare—there are only nine North American species—and not well known. They’re rarely seen in the field, let alone photographed. The larvae appear to be parasitoids on wood-boring beetle larvae (Family Buprestidae). There are two species in the northwest United States. The individuals shown on this page and on Orussus ovipositing (see menu) are probably Orussus occidentalis.

 For several years we have photographed Orussus on a beach south of Port Williams County Park, Sequim, Washington. The park is an east-facing salt-water beach on Sequim Bay, with a marsh directly inland of the southern part. We see these wasps on driftwood logs above the high-tide line and adjacent to the marsh. They seem to prefer logs of trees previously infected by Buprestid beetles, which leave characteristic round holes about 2.5 mm in diameter. Orussus appears to investigate and hide in these holes (slides 9 and 10, video slide 14). Other insects and spiders sometimes use the holes, and many holes seem to be filled with beach sand.

We have also photographed Parasitic Wood Wasps in a second location, the mouth of the Elwha River. We have not seen Orussus on logs in similar locations elsewhere on the Olympic Peninsula

Orussus is difficult to photograph as they dash along a log and switch direction suddenly. When they take flight, they seem to disappear, their takeoff is so quick. The video in slide 17 shows one Orussus approaching another in what looks like a display. The second Orussus takes off so quickly that even with the video slowed down to 5% of normal speed, it still seems to disappear.

Three diagnostic characteristics are visible in close-up photos:
• The wing has a single submarginal cell, a feature not seen in field photos because Orussus folds its wings tighlty over its back (slide 7).

• The head between the eyes bears a number of pyramidal bumps or spikes (slide 8).

• The antennae arise below the eyes, under a ridge of exoskeleton (slide 9).
• Female Orussus can be identified by the swollen antenna tips (slide 2).

As they run along logs, both sexes tap the log surface with their antennae (video slide 15). Lars Vilhelmsen, Natural History Museum of Denmark, University of Copenhagen, 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 locations for ovipositing. (Vilhelmsen web site)

 Bugguide.net pages on Orussus