Bean leaf beetles are giving southern soybean growers more and more fits. Traditional insecticide treatments are failing, and there's added cost to beat them back with an alternative chemical.
So goes the continuing battle with resistance issues, which are among the biggest fears for farmers. Whether it's bugs, weeds or diseases, it seems like growers face new resistance challenges every year.
Fortunately, there's an answer to battling bean leaf beetles when typical pyrethroids no longer make the kill. Fred Musser, Mississippi State University research entomologist in Starkville, MS, says an organophosphate insecticide will normally control the bean leaf beetles.
“A treatment with Sevin or Larvin should work,” he says, noting that the pests are also causing problems in Arkansas and other southern states.
Brian Ward, a Leland, MS, crop consultant, was among the first to notice the leaf beetle resistance patterns — in 2005.
“They've been really exceptional the past few years,” says Ward. “We're noticing that pyrethroids cannot control them.
“We can control them, but I think this is going to be our No. 1 pest around here,” he says.
BEAN LEAF BEETLES are oval shaped and red to brown and black in color. They are about ⅓ in. long and usually have three black spots on each wing cover. The bug has a black head as well as a black triangular area on the forward margin of its wings.
They feed on the youngest tissue available, eat the leaves and also invade pods. There will be a lot of defoliation and holes in leaves. “If they eat off all the leaves it could cause heavy damage,” says Ward. “They can also cause a lot more problems.”
In late season they will eat on pods. The bugs can also transmit bean pod mottle virus, which can discolor beans and lower seed quality.
According to Iowa State University data, the economic threshold for treating first-generation bean leaf beetle varies with the price of treatment and soybean prices. Higher market prices enable growers to elevate the threshold numbers based on the cost of treatment.
For example, with $8 soybeans, the threshold is about 30 per 20 sweeps if treatment costs are $15/acre and about 20 per 20 sweeps if treatment cost is $10/acre. If beans are $10/bu., the threshold is 24 per 20 sweeps if treatment cost is $15/acre. It's about 16 per 20 sweeps if the application cost is $10/acre. For more on ISU bean leaf beetle threshold, go to? www.ipm.iastate.edu/ipm/icm/2005/5-2-2005/integrated.html .
Those numbers don't include potential treatment for bean pod mottle virus.
WITH PYRETHROID RESISTANCE, alternative control can normally be accomplished by using a 24-oz./acre application of Sevin or 18-24 oz. of Larvin. There are also premixes that show strong control.
A premix of 4-4.5 oz. of Indigo and 3.8 oz. of Leverage is an option. Another insecticide being reviewed for beetle control is Binfenthrin (formerly Capture).
However, Ward and Musser worry about the added expense, because a pyrethroid will likely still have to be used to control the dreaded stink bug, which uses alternate hosts that include alfalfa and clover, as well as other insects.
Ward says growers are more and more savvy at controlling insects and will take the right steps to handle the situation. It just may cost them more to do so.