It is Thistle Time Again! Now is the Time for Chemical Control of Thistles
Thistles are still reaping havoc on our farms. Musk thistle and bull thistle are biennials causing major problems in perennial grass crops including pastures, roadsides, conservation reserve and fence rows. Biennials require portions of two growing seasons to reproduce. They grow from seed the first season as a rosette (a taproot with a cluster of leaves on the soil surface). The rosette overwinters and cold causes the rosette to send up a flowering stalk the next season and produce seed. Once seeds mature, the plant dies. Destruction of rosettes prior to flowering is an effective means of preventing seed formation and subsequent spread.
Management of pastures infested with biennial thistle requires special consideration. Since biennial thistles reestablish from seed which is dispersed by wind, it is helpful to prevent seed formation adjacent to pastures. Also it may take two or more years of excellent control before seeds are reduced to the point that allows for legume reestablishment for pasture improvement. One year of poor thistle control will result in having to start the control program over.
Most of the herbicides used for control of bull and musk thistle also kill pasture legumes. Spot spraying individual plants or patches rather than broadcast spraying the entire pasture also spares the legumes.
Musk thistle normally initiates flower stalks in early May and reaches full flower in early June. Seed production is prolific and usually completed in mid to late June. Rosettes reestablish nearly any time during the growing season. Some rosettes may be three or four feet in diameter during the growing season. Musk thistle is extremely aggressive.
Bull thistle normally initiates flower shoots in June and reaches full flower in July and August. Seed production is usually completed in late summer. Rosettes reestablish during and early fall.
Chemical Control of Biennial Thistle
The best time to treat biennial thistles with herbicides is in late fall or early spring when the rosettes are present but before flowering stalks emerge. Musk thistle and bull thistle plants with seed stalks are more difficult to kill than the rosettes. Thistle rosettes need to be treated when they are actively growing and not under drought stress. The younger the rosette, the more susceptible it is to herbicide.
FOLIAR TREATMENTS FOR SELECTIVE REMOVAL OF BIENNIAL THISTLE FROM PASTURE GRASSES
One properly timed treatment per year should prevent seed formation. Several herbicides provide good to excellent control of thistle rosettes. Fall treatments should be made late enough to kill all rosettes germinated before winter. Late germinating rosettes that establish after early fall herbicide applications could flower the next growing season. Early spring treatments should kill all overwintering rosettes.
The following are recommended chemical controls in pastures for thistles.
Herbicides Per Acre Per Gallon
2,4-D amine 1-2 Qts. 3-6 Tb.
2,4-D Ester 2 Qts. 3 Tb.
Grazon 1 Qts. 3 Tb.
Now is the time for chemical control of thistles.
Armyworms are on the March in SW Missouri
Extension agronomy specialists have been monitoring true armyworm activity in various crops in
Missouri this year. This comes after reports of significant damage to forages and crops in northern Arkansas.
To date there have been evidence in sections of southwest Missouri of their activity on fescue, bermudagrass,
wheat and corn. Some high numbers of moth counts have been observed in southern Missouri monitoring traps
in the last month.
According to Tim Schnakenberg, agronomy specialist based in Galena, Missouri, “Young corn seedlings
and plants can be at risk. The economic threshold for spraying this pest is when 25 percent or more of the plants
are being damaged. If the outbreak elevates to a high level, armyworms can consume a large amount of foliage
and stunt the crop. Check the seedlings and the whorls for leaf feeding.”
“Fescue seed and wheat producers should also be watching closely because large numbers of them may not
only defoliate the plant, but they can clip heads as well. Treatment is justified in pastures and wheat fields when
there are an average of four or more half-grown or larger larvae per square foot and before more than two or three
percent of the heads are cut.”
Dusk or dawn is the best time to be scouting for them because the young larvae are typically night feeders.
During the heat of the day they will be under plant debris on the ground. As the larvae get bigger they will do an
increased amount of day-time feeding. The larvae have a greenish-brown body, nearly hairless, with two orange
stripes along each side. The head is brown with honeycomb markings.
Schnakenberg reports that once the moths move into an area, they lay their eggs and it may take 2-3 weeks
for the young larvae to start doing damage. It only takes the larvae 5-7 days for them to grow from ½ inch to 1.5
inches in length. After they reach about 1.5 inches they begin to pupate, going into the ground and making a cocoon.
This first generation of true armyworms is generally the most damaging.
Producers can also monitor insect activity in Missouri through the Missouri Pest Monitoring Network at
http://ipm.missouri.edu/. On this site, producers can find insect activity such as armyworm, cutworm, Japanese
beetle, corn borer and corn earworms reported in traps maintained by extension agronomy specialists. Producers can
also sign up for a free email notification if an insect alert occurs around the state.
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