Organic growers with a Canada thistle problem may want to try some alternative control options, besides mowing, tillage, curses, and prayers. Experiments with bacteria, vinegar, and insects suggest that the prickly perennial weed is not as invulnerable as it seems.

Bacteria in a Blender
Recent university studies of the bacterial thistle parasite, Pseudomonas syringae pv. tagetis (Pst), in Wisconsin and Minnesota have shown Pst can weaken thistles enough to prevent reseeding. A June 19 field day at the Cate's Farm near Spring Green, Wisconsin, hosted by the UW Extension, presented the ongoing Pst research of UW graduate student Ryan Tichich and UW weed scientist Jerry Doll. Their basic approach is to gather naturally afflicted thistle plants, liquefy them with water in a blender, pour the liquid through a mesh filter, add a surfactant, and spray the mixture on healthy thistle plants. Naturally afflicted plants are easily identified, with pale yellow stems and leaves--a condition called chlorosis. Tichich and Doll have found that roughly 50% of the plants treated with their spray also develop chlorosis. University of Minnesota/USDA plant science researcher John Gronwald was able to attain infection rates as high as 67% in similar experiments with Pst.

Tichich and Doll are exploring ways to improve that percentage. Application variables include time of day, frequency, and bacterial concentration in the spray. Possible environmental variables include the force of rain driving the bacteria into the thistles' stomata (tiny pores on the leaf surface involved in gas exchange), genetic resistance on the part of individual plants, and the amount of leaf surface available to receive the spray (they have found that infection rates are lower when thistles are mowed before spraying). "It is difficult to explain how an infected plant can be right next to a healthy plant," said Tichich. Throughout his experiment he has tried various concentrations of bacteria. Doll mentioned breeding Pst-vulnerable thistles, and exploring the use of aphids or other insects to transmit Pst.
"Sick thistles are less vigorous, which gives other vegetation somewhat of a competitive advantage," said Doll. "The real hope would be that enough stems connected to the same root system are infected, so that the level of carbohydrate in the roots is low enough that buds on the roots would not have enough food to produce shoots the next year. This will also require a competitive crop or pasture to keep weakened plants from regaining the upper hand."

Organic growers who want to experiment with Pst preparations need to remember to use an approved surfactant. Tichich and Gronwald use a synthetic organosilicone surfactant, Silwet L-77, because of its very low surface tension. "Without including an organosilicone surfactant in the spray mixture, very few Pst are able to enter the leaf and as a result there is no disease incidence," Gronwald wrote, "and I am not aware of any natural surfactants (e.g. soybean oil) that exhibit the low surface tension of the organosilicones." Phil Radspinner, Commercial Sales Manager for Peaceful Valley Farm Supply, an organic materials supplier, wrote, "Our best surfactant would be Therm X70 yucca extract. We use it for foliar feeds to enhance absorption of nutrients through the stomata…I think it will take some experimentation to truly know the comparative differences (between organosilicones and yucca extract)." For his part, John Gronwald also expressed interest in researching the effectiveness of yucca extract.

"Canada thistle shoots were highly susceptible, with 100 percent kill by 5 % vinegar."--USDA research report

The Vinegar Solution
USDA-ARS researchers in Maryland--Jay Radhakrishnan, John Teasdale and Ben Coffman--began a scientific study of vinegar as a non-selective weed killer about two years ago. Results indicate that vinegar can kill Canada thistle, as well as several important grass and broadleaf weed species. Their report states: "vinegar at 10, 15 or 20 % acetic acid concentration provided 80-100 percent kill of selected annual weeds, including giant foxtail up to 3 inches in height, common lambsquarters up to 5 inches, smooth pigweed up to 6 inches, and velvetleaf up to 9 inches. Control of annual weeds with vinegar at the 5 % acetic acid concentration was variable. Canada thistle shoots were highly susceptible with 100 percent kill by 5 % vinegar. However, there was re-growth from Canada thistle roots."

The report also notes that vinegar is environmentally benign: "Acetic acid readily degrades in water, and shows little potential for bioaccumulation." But it cautions farmers: "WARNING: Note that vinegar with acetic acid concentrations greater than 5% may be hazardous and should be handled with appropriate precautions".

In an Iowa State University review of the study Dr. Teasdale suggested vinegar works in a manner similar to that of paraquat, in that it causes "the rapid dissolution of cell membrane integrity" in the leaves. The result is that the leaves dry up, and the plant dies. ISU had poor results controlling larger weeds with vinegar at their Nashua experimental farm.

As always, organic growers must check with their certifying agency before trying vinegar on their crops. Non-approved vinegars may be derived from a genetically-modified plant source. They may also be concentrated synthetically, by a catalyzed reaction with methyl alcohol and carbon monoxide, or the oxidation of acetaldehyde or petroleum. By contrast, natural acetic acid concentration of plant-derived vinegar can be increased from 5% to 15% via distillation and to 30% via freeze evaporation. Last May the Organic Materials Review Institute (OMRI) approved an herbicide for the first time: a combination of vinegar, garlic, and yucca extract called "Alldown," produced by SummerSet Products, Inc., of Bloomington, Minnesota.

The USDA research suggests that banding may be a cost-effective application method for vinegar. The scientists found that corn plants were not affected by vinegar sprayed on their base, while 90-100 percent of the weeds were killed. They estimate that broadcast application would cost around $66.00 and $99.00 per acre, for 20% and 30% acetic acid concentrations, respectively, but band application would only cost around $22.00 and $33.00 per acre, respectively. A "hooded sprayer" was displayed at Nashua July 1, for an ISU organic weed control field day. The sprayer is intended to protect crops during a vinegar application. Also visible were some browning thistles that had been spot-sprayed with AllDown earlier that day. Although the thistles still looked viable during the field tour, several days later SummerSet president Bruce Marrs said that they had completely withered within 24 hours.

Beneficial Bugs
A third alternative control option is introducing insects that naturally feed on thistles. These insects can be purchased or gathered, then released in high enough to numbers to establish a permanent control.

The Canada Thistle Stem Weevil (Ceutorhynchus litura) can kill thistles if they attack the rosettes in early spring, or after regrowth from mowing or tillage. All life stages of this insect feed on Canada thistle. Canadian researchers in Ontario found 87% of thistles attacked by this weevil were afflicted with rust.

Montana State University researchers discovered that they can gather good numbers of young adult thistle stem weevils by sweep-netting in alfalfa fields that are near thistle patches. They found the most insects during the time between 675 and 800 degree-day units, generally late August to late October. (You can visit the UW Soil Science website at http://www.soils.wisc.edu/wimnext/asos/SelectDailyGridDD.html and use their computerized degree day calculator. All you need to know is your latitude and longitude coordinates.) Adult weevils can also be sweep-netted from thistle tops. They drop readily when disturbed, so a single upward stroke should be used. Release in groups of 30 to 50 weevils.

The Thistle Defoliating Beetle, Cassida rubiginosa, feeds on the foliage of Canada, musk, and plumeless thistles throughout the growing season. A research review by Agriculture Canada describes thistle biomass reductions as great as 88% in pastures, and 67% in stands of pure thistle, with 25% of the plants surviving to the second year. But in some places beetle populations are severely reduced by parasites.

The developing larvae of the Thistle Stem Gall Fly, Urophora cardui, form a gall around themselves that reduces the vigor and seed production of the host plant. Research in Canada found limited biocontrol because the thistles are 80% developed by the time the flies attack. The researchers suggest that this could be improved by delaying thistle development with mowing, tillage, or a companion infestation of thistle defoliating beetles.

Most releases in Canada were made with 100 to 200 flies obtained by dissecting larvae from their galls, although colonies can be established with 20 flies. It is easier to scatter the galls on the ground in the fall, but the number should be large enough to allow for staggered adult emergence.

The Canada Thistle Flower Weevil, Larinus planus, larvae feed on flower heads, reducing seed production. The adult feed on thistle foliage. Agriculture Canada reports that this weevil can depress seed production by up 95%. Adults may be collected with a sweep net in late May to early June. Store them in a cool place, in a cardboard box, and release within several days.