Managing for bean leaf beetles is now a switch-hit proposition. Should you control for beetle damage, or for the bean pod mottle virus the beetles may be carrying? The correct answer for you lies in your field. But be warned: The control measures for these two yield-robbing pests are timed very differently.
“You cannot, in the strict sense, control the virus,” says Marlin Rice, entomologist at Iowa State University. “You have to target beetles to slow down or prevent the spread of the virus, because they're the ones moving it from plant to plant.”
While controlling bean leaf beetles for damage is a well-established practice, controlling them to slow down the virus is a recent strategy.
The bean pod mottle virus causes a 10-20 bu/acre loss in yield, says Rice. His research the past three years has shown that spraying insecticides on beetles twice — after emergence and again in early July — lowers virus damage.
But whether or not a producer should change control measures depends on how much virus is in the field. Rice says four criteria should be met before producers decide whether they need to manage not only the beetle, but also the virus:
A significant yield drop, for no obvious reason.
A lot of green stem in September and early October.
Harvested seed with bleeding hila.
High bean leaf beetle populations in early September.
“Farmers can determine the first three — poor yields, green stem and bleeding hila on the seeds — when they're riding in the combine,” Rice says. “They might not know the fourth, but if you've got the first three things in abundance, then you've got the virus problem.”
You can't assume that you have the virus on appearance of green stem alone, cautions John Hill, plant pathologist at Iowa State University. “Recent studies have shown that multiple other stressors can also cause green stem.”
If you've determined that you have the virus problem, you need to make some changes. The first thing you need to do is think about planting later, says Rice.
“Not late. We don't want to plant outside that window of opportunity where you can get your maximum yields. But if you're planting the last week of April, you're just going to make your problem worse.” Early planting allows more time for beetles to feed and lay eggs before they die off, he says.
“Recent data shows that later planting can reduce the virus or have no impact,” adds Hill. “That means growers can't depend on early planting alone to solve the problem. It may help, and under conditions of high disease pressure it certainly won't hurt.” But Hill says early planting must be used in conjunction with other control measures such as an insecticide application.
Rice agrees and recommends spraying affected beans with insecticide right after they emerge as second course of action. “The most damaging infections to yield and quality occur at the early vegetative stage,” says Craig Grau, plant pathologist, University of Wisconsin. “From a pathology point of view, we need to control those beetles early.”
Rice's research suggests applying a second insecticide treatment in early July, after first-generation beetles emerge.
David and Phred Linn, Correctionville, IA, have been spraying at emergence and again by mid-July for the past two years. “We've done replicated plots two different years, and it pays big time,” says David, who farms more than 700 acres of seed beans. “The first year we had about an 8-bu yield advantage for controlling the beetles.”
Linn spends $10-12/acre in insecticide, which he figures is worth a little more than two bu/acre of beans, giving him a 6-bu/acre profit. When he adds a seed-quality advantage it's a “no-brainer” for his farm, he says.
Deciding to go with the two-pass program should depend on how much virus a field holds, Linn suggests. While it works for him, he says he knows of places 35 miles from his farm that don't have enough of the virus for the program to pencil out.
“If that's the case,” Rice agrees, “then all you want to do is focus on late summer bean leaf beetles starting to feed on the pods.”
The next step for treating bean leaf beetle may be a systemic seed treatment. Rice says seed treatments have great potential because the insecticide is active as soon as the plant germinates. He also does not expect them to kill beneficial insects, such as lady beetles.
“The potential is there for seed treatment, and that may cut down on the amount of chemicals we have to use. But long term we've got to look for tolerance or resistance,” says Hill. “Long-term insecticide application over the landscape is simply not good IPM (Integrated Pest Management) philosophy. The real solution will be companies breeding soybeans for resistance and tolerance.”
Both Hill and Rice say there is anecdotal information that some existing varieties have resistance or tolerance now, although there isn't enough research to confirm it. And that's what farmers like Linn are waiting for.
“As soon as they come up with resistant varieties, I'll plant them,” Linn says. “In the end, that will probably be cheaper than spending all this time scouting, spraying, sweeping and running around doing it my way. It's more environmentally sound to use a biotech product than to go out and spray an insecticide. But, until then, I'm going to keep spraying.”
It's All In The Timing
Controlling bean leaf beetles in an attempt to control bean pod mottle virus significantly changes the timing of insecticide applications. For virus control, spray for beetles after soybean emergence and again after the first-generation beetles emerge. To control beetles, sample to find if the first generation hits threshold. Then spray when the second generation emerges.