Roberta - I believe it was about this time of year that my Grandparents and I would travel down some old, country side roads in search of Horseweeds. The plants would be nearly 5-8ft tall and thick as your fingers or thumb. It always seemed to be quite hot and dry, so there's a pretty good chance that it was around this time of year.
We always looked at the stalks of the plants - either for a telltale "hole" in the stalk, or a section of the stalk that looked swollen (esp on the stalks of "pinkie finger" size or smaller). We would then cut the plant several inches below the lowest "hole" - then cut off the leaves and branches, leaving a long, bare stalk. These were cut in half, if it could be done without cutting thru the swollen area (or the worm). Occasionally we would cut into the stalk, at the hole or bottom end of the swollen portion, and pry a split in the stalk ... looking inside to confirm the worm was there. Then the stalk was dropped into a burlap bag, and after we'd collected a goodly amount, we'd be off to the lake.
We'd split open a stalk and retrieve the worm ... a small immature one would be a reddish tan, and a "goodun" would be a plump, white worm with light golden brown patches on its back. We'd thread the worm on the long shank thin wire hook ... head to tail or tail to head ... and drop them down on deep points - about 15 feet deep (minimum) in 18-20+ft of water. We deadlined (didn't know about "slip floats" in those days) and hand held the pole lightly in one hand. When the rod tip started heading towards the water - we set the hook. I remember catching, what seemed to be, much larger Bluegill than what we normally did (in shallower depths) ... with the occasional Channel Cat or Flathead as a surprise bonus....sometimes even a stray Crappie or Bass would take the offering.
One of the "other" things I remember about these "excursions" into the roadside stands of Horseweed .... is having to wear long sleeved shirts and lots of insect repellant ....LOL!! The long sleeves were to ward off any number of "other" critters, and to keep the weeds from causing our arms to get "itchy". The "bug spray" was to try and keep the "Chiggers" numbers down as much as possible ...LOL!! You never quite seemed to get away "Scott Free" ... but considered it a "successful" event, if you only got a FEW "chigger bites" out of the deal !!
I don't know about "galls" ... I only remember a "swollen" thickening of the plant stalk, and a yellowing of the coloration. The stalk would appear to be swollen about 25-50% larger, in the section where the worm had bored thru. If the worm inside looks like this, then it's a Horseweed Worm - http://web.extension.uiuc.edu/Sangam...talkborer2.jpg
Hope you find some and catch some "bull gills" with them ....luck2ya ..cp
ps - the plant isn't "actually" true Horseweed ... but is, in reality, Giant Ragweed. And the "worm" is actually one of a variety of "Stalkborers". I just call them the same as my "granpappy" did :D !!