07/04/17 Hoary vervain (Verbena stricta
) is just starting to bloom at the farm where I live; time to start looking for the fly that bores in the spike...the evidence of which I first found in March, on last year's dead stems:
Not sure if the fly is active in the spike while the plant is blooming, or while it's setting seed, or even afterward...but better start searching as soon as the spikes appear. Sure enough, tearing into the elongating spikes, I find two or three each with a tunnel in the central stalk...and a fly larva in the tunnel!
After photographing this larva in its tunnel, I used a pin to make a very small diagonal hole in the central stalk of another (unaffected) flowering spike I'd collected, and introduced the larva into the hole. This allowed me to place the cut end of this plant (which I cut off well below the spike) into water with little threat of drowning the larva. Fortunately, this transplant "took" -- the larva began boring in the central stalk of its new spike. The plant stayed fresh in water for a week or so, at which point I cut it open...and hooray! The larva had completed its development and pupariated. Puparium in stem shown in this series.
But darn it all, when I tried to remove the puparium from the stalk for better photos, my fingers slipped, the plant stem rebounded, and off the puparium flew. I never found it but I did manage to make the air blue with words. Please tell me I'm not the only one who's done that...
07/12/17 More affected spikes found and collected. Adults, including this one
emerged in late July and early August.
Some hoary vervain plants from this location are affected by a fly that girdles the plant shortly after the flowering spike appears but before it elongates appreciably:
It is unclear to me if the stem girdler is the same species as the inflorescence borer, but my current hypothesis is that the two are different species; I haven't examined genitalia, but it does seem like there are two distinct ways of life lived by the larvae.