All photographs by Rodney G. Lym.
Canada thistle is the only thistle in North Dakota that has become a cropland pest. The best approach to Canada thistle control in cropland should include an in-crop herbicide treatment to suppress Canada thistle growth, minimize crop yield losses, and prepare the thistle for a fall post-harvest treatment. Preharvest and fall-applied treatments provide the most effective long-term control. The best herbicide to use will vary depending on crop rotation. However, the control program must be uninterrupted for two to three years if the infestation is to be reduced.
Herbicides that can be used for Canada thistle growing in small grains are 2,4-D, MCPA, dicamba, Curtail (clopyralid plus 2,4-D), Curtail M (clopyralid plus MCPA), Ally (metsulfuron), Amber (triasulfuron), Canvas (metsulfuron plus thifensulfuron plus tribenuron), Express (tribenuron), Finesse (chlorsulfuron plus metsulfuron), and Harmony Extra (thifen-sulfuron plus tribenuron) (Table 1). Of these, Curtail provides the best and most consistent control, but a single application will not provide long-term control.
Products containing clopyralid provide good Canada thistle control and are labeled in flax (Curtail M is labeled yearly through Section 18 registration. Contact N.D. Department of Agriculture for current status. Section 3 registration pending.), sugarbeet (Stinger), and corn (Accent Gold, Hornet, and Stinger) (Table 1). Clopyralid may have a soil residual, and peas, lentils, potatoes and broadleaf crops grown for seed should not be seeded until 18 months after treatment. Most other crops can be seeded the following growing season.
Canada thistle may be suppressed in corn with products containing dicamba (Celebrity Plus, Distinct, Northstar and others) (Table 1). Dicamba gives better control than 2,4-D with less risk of corn injury.
Canada thistle growing in soybean or dry bean can be suppressed with Basagran (bentazon) (Table 1). A second application is required 10 to 14 days after the first for satisfactory suppression. If Canada thistle has grown taller than the soybean crop, glyphosate can be applied through a selective ropewick or roller-type applicator as a salvage treatment.
Glyphosate can be used to control Canada thistle in glyphosate resistant corn, soybean, and canola (Table 1). In-crop applications will not kill established thistle stands. However, when used as part of an overall management program, glyphosate can reduce field infestations. Apply glyphosate at the correct crop stage and at labeled rates for each crop. Application beyond designated timing or using higher than labeled rates may result in crop injury. Some formulated premixes are available for use in each crop. Use caution at application to prevent drift to sensitive crops.
Good Canada thistle control is often achieved when herbicides are applied as a preharvest treatments, especially during wet summers. Glyphosate, 2,4-D, dicamba, or combinations of these herbicides can be used for preharvest Canada thistle control in wheat. Glyphosate is labeled for preharvest application in wheat, corn, soybean, field pea, chick pea, and lentil. Allow a seven day preharvest interval for broadcast applications and a 14 day preharvest interval for spot treatment. Use a 2% solution for spot treatment and apply to Canada thistle at or beyond the bud stage.
Glyphosate, clopyralid, 2,4-D, and dicamba or combinations
of these herbicides provides effective Canada thistle control and good
stand reduction when applied as postharvest treatments to any crop or
in the fall, prior to a killing frost and when soil moisture is good.
Canada thistle should have 8 to 12 inches of new regrowth. Dicamba or
glyphosate provides greater stand reduction than 2,4-D. However, dicamba
at lower rates can be mixed with 2,4-D, which may provide greater control
than 2,4-D alone with less risk of carryover. High rates of dicamba should
not be used in cropland to avoid herbicide carryover into the next cropping
season. More cost-effective control may be possible by combining glyphosate
with 2,4-D rather than applying glyphosate alone at high rates. Glyphosate
is sold commercially in premix form with 2,4-D (Landmaster BW) or dicamba
(Fallow Master) and can be used for Canada thistle suppression. Tillage
should be delayed at least three days after herbicide application.
An option for Canada thistle in row crops and fallow that includes both tillage and herbicides is known as the rosette technique. The objective is to prevent the plants from bolting by using tillage and/or herbicide treatments until the daylength is less than 15 hours, the photoperiod required for most Canada thistle plants to bolt. The thistles will then regrow as rosettes only. Research at NDSU has found herbicide absorption and translocation to the roots of Canada thistle is greater when applied to the rosette growth stage than when applied to bolted plants, making fall treatment of rosettes the most cost-effective method for long-term Canada thistle control.
The rosette technique for Canada thistle control in fallow includes the use of tillage and fall-applied herbicides, while control in row crops includes in-crop herbicide treatments, tillage, and fall-applied herbicides. Periodic tillage in fallow is used to control Canada thistle shoots and other weeds until late July when the daylength is less than 15 hours. Herbicides used for Canada thistle control, such as glyphosate, Curtail, or Stinger, are then applied to rosettes in late September or early October (Table 1).
In row crops, herbicides and/or tillage can be used to control Canada thistle in the crop to prevent bolting. Cultivation should be continued until canopy closure in soybean and until early July in corn. Research at NDSU has found that cultivation until late-June prevented more than 90% of Canada thistle from bolting in corn and soybean. A second option in soybean is to apply a split application of bentazon in lieu of tillage (Table 1). Herbicides are then applied in the fall following harvest for Canada thistle control. The rosette technique controls Canada thistle in both fallow and row-crops during the season and maximizes the number of rosettes for better herbicide absorption and translocation in the fall.
Biennials. Fall is the preferred time for applying herbicides for biennial thistle control. Fall applications allow for more time to apply herbicides than in the spring and correspond to the most effective time for thistle control. Seedlings that emerge in summer after tillage or previous herbicide applications will not bolt but remain in the rosette stage. Biennial thistles are most susceptible to herbicides in the rosette form.
Herbicides should be applied as late as possible in the fall but prior to a killing frost to allow for maximum seedling emergence and rosette size. Seedlings that emerge after spraying will remain vegetative until the following spring and can be treated then. Long-term eradication of biennial thistles is difficult because of the large number of seeds each plant can produce.
Biennial thistles can be effectively controlled with Stinger (clopyralid), Tordon (picloram), or dicamba (Table 2). Stinger and Tordon are the most effective of these herbicides and may be applied in the spring or fall. Tordon and clopyralid (Curtail) are often mixed with 2,4-D for broad spectrum weed control. Weedmaster (dicamba plus 2,4-D) is an effective treatment and is best applied when the thistles are in the rosette growth stage. Redeem (triclopyr plus clopyralid) is labeled for thistle control in non-cropland and CRP. Escort (metsulfuron) will control biennial thistles in the spring and will eliminate seed production when applied in the bolting to bud growth stages.
Perennials. Curtail (clopyralid plus 2,4-D), Tordon (picloram), Tordon plus 2,4-D amine, dicamba, Redeem (triclopyr plus clopyralid), or 2,4-D will suppress or control perennial thistles (Table 2). Control is greatest when applied to thistle at the early-bud growth stage (early summer) or in the fall to plants in the rosette form. These herbicides applied at low rates may be the most cost-effective method for controlling dense infestations that require broadcast application. Annual retreatment will be necessary for several years to obtain long-term control.
2,4-D is used for suppression only and is most effective when applied in the spring to thistles in the vegetative growth stage (Table 2). Tordon at 1 to 2 quarts per acre (0.5 to 1 pound per acre), dicamba at 3 to 4 quarts per acre (3 to 4 pounds per acre), Stinger (clopyralid) at 1.3 pints per acre (0.5 pound per acre), or Curtail at 3 quarts per acre (0.28 plus 1.5 pounds per acre) will provide near complete control for several years, but are expensive treatments. Redeem should be applied at 2.5 to 4 pints per acre (0.7 plus 0.25 to 1.1 plus 0.4 pounds per acre) Redeem herbicide contains 0.75 pounds per gallon of clopyralid and may be a more cost effective treatment than Curtail which contains 0.4 pounds per gallon clopyralid.
GRAZING RESTRICTIONS VARY WITH HERBICIDE AND APPLICATION RATE SO READ THE LABEL CAREFULLY BEFORE USING.
For perennial thistle control in non-cropland, in addition to herbicides listed above, glyphosate may be applied in the summer or fall when thistle is at or beyond the bud stage of growth (Table 2). Perennial thistle control is usually greater when glyphosate is applied in fall rather than spring.
Control of Thistle
Insect biocontrol agents have been released on both musk thistle and Canada thistle with limited success. The seed weevil Rhinocyllus conicus was introduced from Eurasia to control musk thistle by reducing seed production. Larvae develop in the flower head and consume the seed as it develops. The weevils can reduce seed production by nearly 80%, but they are attracted more to earlier blooming rather than to later blooming flowers. The late season flowers produce seeds with little damage from the weevil, which sustains the musk thistle population. It takes five to 10 years to build a high enough population of insects to greatly reduce seed production.
R. conicus will also attack seed heads of Canada thistle and many other thistle species, both native and introduced. However, the resulting damage to various thistle populations has been minimal to date.
Another weevil introduced for musk thistle control is Trichosirocalus horridus which feeds on the apical meristem of the thistle rosette and developing stems. The feeding causes multiple stems to be formed when the plant bolts instead of a single stem. The multiple stems produce small flowers with few seeds, which is beneficial to the Rhinocyllus population. However, even with the two biological agents working together musk thistle is only partially controlled. A second control method such as chemical is needed to stop the spread of the weed.
Two biological control agents have been introduced for Canada thistle control, and a third was accidentally introduced. To date, none have been effective at reducing the weed on a large scale. Larvae of the Ceutorhynchus litura weevil feed on the underground parts of Canada thistle which in turn are winter-killed. The effects of the weevil must be supplemented by another biocontrol agent or chemical control for effective control. A gall-producing fly, Urophora cardui, causes meristematic galls but does little long-term damage to the perennial thistle. The Canada thistle bud weevil Larinus planus was an accidental introduction into North America. The insect feeds on developing flowers to prevent seed production. Although L. planus can survive under a wide range of climates, it has not reduced established Canada thistle stands.
The painted lady butterfly (Vanessa cardui) can be a very effective biological control agent but only on an intermittent basis. Larvae of the butterfly feed on Canada thistle plants and can eliminate an infestation. However, the insect generally is only found in southern states such as Arizona and New Mexico and will build up populations large enough to migrate north only once every eight to 11 years. The insect will migrate north as far as Canada and those fortunate enough to reside within the migratory pathway will see a dramatic decrease in the Canada thistle population. Top