Bug Day (5): Thistlehead Weevil
If you’ve been following the Bug Day series, you already know that bugs can be the focus of controversy; check out, notably, the Spittlebug item. Well, here’s another poster child of controversial bugs: the Thistlehead Weevil (Rhinocyllus conicus). As the common name implies, this bug is into the heads of thistles. Specifically, it lays its eggs on the thistle flower. The eggs hatch and become larvae that burrow into the base of the flower and munch on the material that feeds the seeds. With enough larvae and time, the plant fails to reproduce. Next year, fewer thistles appear. In a few years, they’re reduced to a minor problem. That’s the theory.
This theory made the Thistlehead Weevil highly attractive to North American farmers and ranchers whose land had become invaded by the Musk Thistle, Carduus nutans. This plant typically grows from 3 to 5 feet tall with multiple sharp spiny branches and leaves radiating three feet or more. A single plant in prime condition may generate 120,000 seeds, light enough to float in the wind. Seeds may remain viable in the soil for ten years or more. The plant is a native of Eurasia and North Africa and found its way to North America in the early 1800s. It’s a certified noxious weed in many U.S. states, including California, where it’s on the A list of noxious weeds. Ranchers report that livestock will not graze near this plant due to the sharp spines, reducing available forage. According to research compiled by the California Invasive Plant Council, this plant also may exude biochemicals that inhibit growth of other plants nearby (allelopathy), and may deplete nitrogen in soil and block the action of clover and similar legumes that ordinarily add nitrogen to soil. A bad actor!
The Thistlehead Weevil is also a native of Eurasia, where it has long been known to feed on the Musk Thistle. In the early 1960s, when ecological science was still in its infancy, unnamed parties collected hundreds of adult weevils from France and Italy and dropped them on thick stands of the Musk Thistle in Canada and the U.S. This seemed perfectly logical: import an exotic bug to fight an exotic weed. Biological control. From the viewpoint of ranchers and farmers whose incomes lay under the shadow of this invasive thistle, the weevil biocontrol measure was a success. However, problems soon developed. It turned out that the imported weevil had an appetite not only for the imported weed, but also for native North American thistles, including thistles that were threatened or endangered. It rapidly expanded its infestation of natives where present. In a few areas, its population mushroomed in stands of native thistles even in the absence of the Musk Thistle. A number of native thistles are sources of high-priced thistle honey. Some have verified medicinal value. A number of birds and small mammals depend on native thistles for nectar, pollen, and nesting materials. The introduced weevil also hurt native plants indirectly; it starved and pushed out native insects that fed on native thistles without threatening the plant’s populations.
Scientists looking back over this scenario noted that it had long been known that this weevil also attacked other thistles besides the Musk Thistle in Europe. But the researchers at the time subscribed to a kind of genetic theory that weevils found on the Musk Thistle there would only feed on the same here. In fact, the weevils fed on whatever happened to be blooming at the time. Evidence contradicting the genetic theory was ignored. The native thistles were considered negligible. The economic importance of controlling the Musk Thistle overrode all hesitations.
Decades of controversy followed. Biocontrol has a long history of such controversies. There are great successes, great disasters, and many cases with a mixture of both. The upshot of the controversy regarding this weevil is that it is no longer considered an effective biocontrol agent. This is partly because of its impact on “non-target” (native) plants, and partly because of its variability in restraining the Musk Thistle. It is highly effective in killing the seeds of the terminal flower (the one on the top) but much less so with the flowers on lateral branches. While some studies report effectiveness in the 90 percent range, others fall as low as 10 percent. As a result, in 2000, the US Department of Agriculture outlawed the shipment of this bug across state lines, effectively killing commercial distribution.
How is it with thistles in Chavez Park? Our Plants database shows the Italian Thistle (Carduus pycnocephalus), the Bull Thistle (Cirsium vulgare), and the Milk Thistle (Silybum marianum). We also have the “Sow Thistle” (Sonchus asper) but that’s not a true thistle and this weevil doesn’t care for it. In the photo above, the weevil is on an Italian Thistle (Carduus pycnocephalus); they tend to have cobwebby hairs up in the flowerhead. Bottom line: we don’t have the nasty Musk Thistle. I don’t know why not; we have just about every other noxious invasive exotic plant known in the temperate universe. And we don’t have any of the native thistles that represent non-target hosts of the Thistlehead Weevil, the star of this Bug Day. This bug has a limited menu here.
There is a big literature on this bug. Wikipedia has just a little bit of it. There is a summary in CABI. The USDA lists a number of useful publications. The California Invasive Plant Council just published a Best Practices Manual with extensive and detailed discussion of biological control. Svata M. Louda of the University of Nebraska has published extensively on this topic; see Louda et al. 1997, Louda 1999, Louda & Arnett 1999, Louda et al. 2002, Louda et al. 2003, Rant & Louda 2012, for incisive, thoughtful reflections on biocontrol in this and related cases. Also of interest, a laboratory study of the weevil reproducing on native thistles by Wiggins et al. 2010.
