Sphecid Wasp of the Genus Pemphedron in Raspberries
There have been some reports of a small black wasp (photos 1& 3 below) associated with hollowed out raspberry laterals and some lateral death this past summer. What confounded us was actually getting a live sample, but we finally had a good one submitted (thanks Jaime!) and could make an identification that these are sphecid wasps of the genus Pemphedron.
According to the High Plains Integrated Management Website, sphecid wasps of the genus Pemphedron are aphid hunters. This is good, but the more interesting part for us on this is that these wasps actually excavate chambers out of the pith of plant branches (see second example photo below). Critically, the wasps gain entrance to the branch by any vertical break - pruning, tipping or branch breakage. Since neither the xylem nor the phloem elements of the plant are concentrated in the pith, one would not expect a lot plant damage coming from this activity. That said, the grower who has been communicating with me on this has indeed reported a correlation with some dead laterals and Pemphedron wasp excavation.
To answer the inevitable question, this is a "C" rated pest, meaning it's not invasive and not going to generate unwanted regulatory attention. I would leave it alone, but things change all the time and so it still it bears watching given the ability to cause plant problems.
Thanks to Jaime Lopez from Reiter Affiliated Companies, Pamela Cassar from the Santa Cruz County Agricultural Commissioner and of course Kevin Williams who did the ID for us at the CDFA. Great teamwork once again to get to the right answer and keep us in the know.