Pest Watch
Purple Weed Worries
DECATUR, Ill. (DTN) -- The color purple has invaded many farm fields as winter annual weeds such as purple deadnettle and henbit are thriving on recent rains and leftover nitrogen. The carpet of these, and other winter annual weeds, covering fields due to delayed planting provide the perfect site for black cutworm moths to lay eggs.
Reports of moth flights have already begun to filter in. "We've been picking them up in traps over the past week," says Sean Evans, Monsanto Technology Development Representative for Insect Management. "I'd recommend growers keep a close eye on fields where winter annual growth has occurred and scout those fields carefully. This is an insect that can cause a lot of damage and get away from us when we get into a rushed planting season. This year seems to be shaping up as one of those when farmers will be trying to do a lot in a little time."
The universities of Missouri, Illinois and Kentucky were all reporting black cutworm moth flights this week. Black cutworms do not overwinter in the Midwest. The bulk of the spring population migrates from Southern states beginning as early as March. Factors that favor black cutworm outbreaks include late planting, heavy infestations of winter annual weeds before tillage, reduced tillage/no-till and corn grown after soybean.
Moth flights are generally heaviest in April and May. Females deposit eggs in areas such as low spots in fields and areas overgrown with grasses or weeds. Chickweed, shepherd's purse, peppergrass and mustards are other favored weeds. Winter wheat is also a potential host.
Young larvae hatch in 5 to 10 days after eggs are laid and start feeding on leaves, resulting in small irregular-shaped holes. Cutworm larvae pass through six instars over the next 28 to 35 days. They become most dangerous when they begin cutting plants at the fourth instar stage. One cutworm larvae may cut an average of three to four plants during its lifetime.
Several other species of cutworm can attack corn, but black cutworm is the common culprit. Dingy cutworms can cause leaf feeding damage, but rarely cut corn. Entomologists don't fully understand why cutworms sometimes get a taste for soybean. However, the same wet, weedy conditions also make soybean more vulnerable.
P[L1] D[0x0] M[300x250] OOP[F] ADUNIT[] T[]
Several states use pheromone traps and have black-cutworm-monitoring alerts to keep growers posted on when to begin scouting -- ask your state extension specialist. Here are some active sites: Missouri: http://bit.ly/…; Iowa: http://bit.ly/…; Illinois: http://bit.ly/…; Minnesota: http://bit.ly/….
SOYBEAN CYST THREAT
Those little purple flowered weeds also could cause soybean cyst nematode (SCN) problems. Purple deadnettle and henbit are both strong alternative hosts, according to research done at Purdue University. Field pennycress is a moderate host. Shepherd's purse, small-flowered bittercress and common chickweed are considered weak hosts.
Research by Bill Johnson, Purdue University weed scientist, has shown delayed winter weed removal -- either due to a wet spring or an attempt to control winter and summer annuals with a single application -- provides SCN juveniles the additional time they need to complete a reproductive cycle. Delayed planting conditions increase the importance of using SCN-resistant varieties.
WEED CONTROL OPTIONS
University of Illinois weed scientist Aaron Hager prefers to see control of winter annual weeds at least two weeks prior to planting to prevent yield loss and prevent weeds going to seed. That could be a tough chore to carry out if weather delays continue to push back planting.
"The purple, white and yellow flowers of winter annual weeds are colorful indicators that [weed] seed production is close at hand, which will help perpetuate these challenges for another season," Hager told DTN. "If existing vegetation is to be controlled with herbicides, be sure to tailor herbicide selection to the weeds present in respective fields.
"As winter annual weed species approach maturity, they can become increasingly difficult to control with herbicides. Be sure to adjust the rate of burndown herbicides upward to account for the large and dense vegetation," Hager said.
Hager suggested glyphosate application rates (alone or tankmixed with other herbicides) should be in the 1-1.5 pound acid equivalent per acre (ae/A) range to control the large vegetation. Include AMS (ammonium sulfate) at 8.5-17 pounds per 100 gallons of spray solution, and use enough water volume per acre to ensure good coverage of the dense vegetation. Remember that adequate control of glyphosate-resistant horseweed can only be achieved by not relying exclusively on glyphosate. Tankmix partners or alternative herbicides are needed to provide adequate burndown control of glyphosate-resistant horseweed.
Contact herbicides, such as paraquat or glufosinate, may not be as effective as translocated herbicides (absorbed into the plant) against larger weeds, but can begin to desiccate existing vegetation much quicker than translocated herbicides. Adding a triazine herbicide such as atrazine or metribuzin to paraquat or glufosinate often improves overall burndown performance. When the application of contact herbicides is followed by one or more cloudy days, expect control symptoms to take a bit longer to develop.
Preplant tillage operations made when the soil is too wet frequently do not provide complete control of existing weed vegetation, especially larger weeds. Large weeds that escape the tillage operation ("cultivator avoidance") are often damaged by the physical disturbance and can be very difficult to control with a subsequent herbicide application, Hager noted.
"Regardless of whether you plan to control existing vegetation chemically or with tillage, it is advisable to avoid the temptation to plant first and worry about controlling weeds later," he said.
Pamela Smith can be reached at Pamela.smith@telventdtn.com
(AG/SK)
Copyright 2013 DTN/The Progressive Farmer. All rights reserved.
