California Horntail Wasp

Storey brought in this excellent specimen of a California Horntail. It was one and half inches long, dark blue to black, with a spot behind each eye. California horntails are the most commonly collected horntails in the state.

This species, Urocerus californicus, appears to prefer to lay its eggs in fire or wind-damaged fir, spruce, or, in California in particular, pine trees. The female inserts her long ovipositor into a crevice in the bark and when she finds the right spot, will actually drill into the heartwood, where she deposits her eggs.

The larvae are commonly said to feed on the heartwood, which struck me as peculiar, as I don’t see how they could digest it. So I dug deeper. Turns out that a researcher discovered that a species of horntail, Tremex fuscicornis,  in the Czech Republic, actually places spores of a symbiotic fungus, in with her eggs. The fungus is necessary for survival of the woodwasp larvae, and it’s clear that the larvae are feeding on the fungus that is feeding on the wood. So all that talk that woodwasp larvae eating the wood is wrong; they are eating the fungus and, presumably, its digestive byproducts. It’s a potentially slow process, as it can take 1-3 years for the larvae to reach the stage to pupate. By the way, that fungus is one you have probably seen growing out of dead trees. The woodwasps and the fungus have a nice symbiotic relationship.

Since that 2007 Czech publication, more work has come out to show that woodwasps live in an obligatory relationship with fungi.  A 2010 paper coauthored by Miroslav Kolarik (he is on this site in relation to foamy bark canker), specifically states “Siricid woodwasps live in obligatory nutritional symbiosis with fungi”.

15 October 2020. Shell Road area, Jamestown Ca.

Here is a link to the Bohart Museum page on horntail wasps.