Farmers in some parts of Alabama got a reprieve this year from kudzu bugs, but they should expect the insect pest to be statewide in 2014.
“Next year, I think we have the potential for kudzu bugs to be a statewide issue on soybeans,” says Tim Reed, Auburn University Extension entomologist. “This year, we sprayed for it in about a dozen counties, along Interstate 85 and the Georgia state line and in counties like Blount, Talladega and Tallapoosa.”
This year, says Reed, in certain locations, pressure built on early planted soybeans severely and quickly. “It scared some folks, and they went out and sprayed when they were running five per plants on plants that were less than 1 foot tall. If you see that, it’ll scare you, and they will be bad for a long time during the growing season if they start out on seedling beans at such a high density,” he says.
Kudzu bugs were in large enough numbers to be of any use to researchers in central Alabama, says Reed. “The only place where I’ve got kudzu bugs in large enough numbers that I should be able to get data on yields is at the Prattville agricultural unit. They have been really heavy there this year after not having them at all last year. There’s kudzu on either side of that substation, and I suspect that contributed to it. At Crossville, where we had them last year, they haven’t been bad in our plots. They started out like they might be, but they never built up.”
At Prattville, Reed says he initially became concerned when he started picking up about five per sweep. “Research from the University of Georgia indicates that might be the critical point to start spraying, especially if part of that count is in immature insects. But in the unsprayed plots that were planted in mid-April with a late Group IV, they were running 60 per sweep. That’s pretty heavy, and I’m expecting to see some yield loss in those plots,” he says.
At the beginning of the growing season, Reed says his threshold was based on data from counterparts in Georgia and South Carolina who had been fighting the kudzu bug for a couple of years.
“They were saying to wait until you have one adult per sweep and immature present. That’s not really a good threshold to use because you can have a few adults and then have a few immatures, and you’ll get that in a sweep net or just looking on the plant. But I really think you need a pretty respectable number, and five per sweep is a threshold that worked in one study in Georgia where they sprayed every week starting June 28, on early planted beans.
“They sprayed at R1, at R2, at one per sweep, two adults per sweep. When they had about five per sweep with two adults and three immatures together, they got a yield reduction of about five bushels per acre. That’s one of the best tests I’ve seen in terms of thresholds.”
One thing he’s encountering, says Reed, is that just about every field has kudzu bug in it, but you might not pick up but one per sweep, or one per 20 or 25 sweeps.
“In a lot of fields, you’ll pick up enough that you’re a little bit concerned about them, and you’re picking up a stink bug about every 10 sweeps or maybe two stink bugs per every 10 sweeps. You’re also picking up three-cornered alfalfa hoppers, a few loopers, green cloverworms, and you’re thinking about spraying a fungicide.
The critical decision comes when you’ve got disease and insect problems in a field, says Reed.
“You might make a decision to spray a fungicide and bide your time to see what the disease does. That’s when it becomes more difficult to make a decision to apply an insecticide. If they’re R4 or early R5 soybeans, there’s still time for worms to get bad, and if you spray and kill all of your residuals, it may increase the worm problem.”
Some consultants are cautious about spraying soybeans with sub-threshold levels of insects because of podworms, he says. “If podworms get down in the bottom, you can’t get them with a sweep net, and they’ll chew on the beans and hurt yield.”
Check fields closely
Reed advises growers to look at their fields closely, having a good idea about insect densities. “Keep tabs on the disease situation. When disease starts showing up and is spreading to a wider area, you need to spray for soybean rust if you haven’t already. Before soybean rust was detected, fields were going to be sprayed anyway because the yield potential was so good at R4 or R5. Go by a field-by-field situation, and if you’re making good sweep net counts and have a good grasp on the insect populations, then use that to make your determination. If you just look down the plant, you can see them, and if you see a lot of them, then you need to spray.”
Auburn University researchers have found a wasp parasite that is doing a good job on the kudzu bug, says Reed, and that might be part of the answer to this pest.
“In South Carolina, they’ll have a problem one year and not the next, so hopefully we’ll have that same situation. Kudzu bugs feed mainly on the stem, but they’ll feed on any part of the plant. In 24 Georgia studies, the average yield reduction from this pest was five bushels per acre. When I started spraying for them during the last week of June in Prattville, they would build back up in two weeks. You’ll see that until the first week of August. They continue to move around and get on beans that are in the early vegetative stage.”
(A couple other items of interest on this subject include Soybean growers might as well get acquainted with the kudzu bug and Discovery of parasitic wasp could be game changer in kudzu bug battle).