The Delta's recent switch to earlier-maturing soybean varieties has caused some changes in insect management practices. Most notably for south Louisiana is the red-shouldered stink bug, an insect especially prevalent since 2000.
“It's also known as the red-banded or Cajun stink bug,” said Matt Baur, research entomologist with the LSU AgCenter. “Lately, there are some expletives used to describe the pest too.”
With a distinctive reddish pattern across their backs, red-shouldered nymphs and adults are usually smaller than those of other stink bug species. Once adult, the species often has a yellow, red or even blackish stripe on their “shoulders.”
Red-shouldered stink bug females require some 20 days between the time they emerge as adults and the time they begin to lay eggs. In considering controls, this characteristic “is something we may be able to turn to our advantage,” Baur told those attending the Tri-State Soybean Forum in Delhi, La., on Jan 6. “Another thing of interest: the development time between eggs to adult is about 30 days. When you add to that the 20-day pre (egg-laying) period, it's at least 30 to 40 days before a generation turns over.”
Baur said one tell-tale characteristic used to identify eggs of the species is egg clusters appear in two rows. “If it's a two-row cluster, it's always going to be this species.”
Once it began to warm up in February and March of 2005, “we began to see migration of (red-shouldered) adults out of woodlots into small spring hosts along field margins, ditches and rights-of-way. We saw them in hairy vetch quite a bit.”
It appeared there were two to three generations on the early spring hosts. For south Louisiana — especially in the soybean/sugarcane production area — “we saw populations peak in Group 3s and 4s. We saw a large migration of adults in to the fields.”
At 24 insects per 100 sweeps or six insects per 25 sweeps there's a lower treatment threshold for red-shouldered stink bugs. For other stink bug species, the threshold is nine insects per 25 sweeps or 36 insects per 100 sweeps.
Because of the lower threshold, in 2005 many producers “pulled the trigger early and made an acephate application. At that time there were no pods on the plants as they were in very early reproductive stages. With that 20 day lag between the time the females showed up and egg-laying, the spraying was done and took out that entire generation.”
Producers claimed excellent control for about 40 days. Then a second population peak hit in July. By that time, most of the early beans had pods.
“Once again, producers pulled the spraying trigger early. They took out most of the insects again and had about 20 to 30 days of control.”
By the time August rolled around, the later-maturing soybeans also had pods and fields held a large range of other crop maturities for the red-shouldered stink bugs to develop in. At that point, “we had not only adult stink bugs but eggs, nymphs, the whole nine yards.”
Spraying provided about two weeks of control. “What happened was the early populations had laid eggs and the eggs were hatching out. As they treated to control the adults, the eggs and nymphs kept resurging. That led to a lot of chemical applications.”
Baur estimates Louisianans saw five to seven generations of the pest in 2005.
Distribution and control
The distribution of the red-shouldered stink bug has begun to change.
“It's moving north and with the warmer weather we're having there's a real possibility we'll see a similar scenario develop in north Louisiana. That's especially true if they begin to move into the early varieties like Group 4s.”
For control of the pest, acephate at 0.75 pound did well. Baur said many producers used a combination of a third of a pound of acephate with 2 ounces of Baythroid. Furadan also provided good control.
“In our experimental fields, we tried endosulfan. It's a chlorinated hydrocarbon that isn't currently labeled in Louisiana soybeans. It suffers from the same problem as Furadan: high mammalian toxicity. They're both very dangerous to use. If you inhale Furadan it can cause bad problems. It can kill you. Endosulfan looked good in our plots, but (due to toxicity issues) I don't know if we want to pursue labeling.”
Another product of interest is out of Brazil, a country that has to deal with red-shouldered stink bugs in large numbers. “Engeo is a Syngenta product and contains (some of same chemistry) as Cruiser or Centric. It's combined with a pyrethroid — since it's made by Syngenta, probably Karate — and provided good control. However, it isn't labeled for soybean use. It's projected the label for this product will come in 2007.”
Another practice out of Brazil didn't work. “We heard Brazilian producers (fighting the red-shouldered stink bug) add salt to an insecticide application. This supposedly causes the insects not to move around — they'll stay put and that increases the level of control. We tried it but there was no evidence to support the use of salt.”
Baur also looked at a seed treatment test with Cruiser and Gaucho. “We didn't see a lot of difference. I suspect that for the seed treatments the insecticide isn't around long enough to provide much help.”
Wondering if current thresholds are useful in dealing with the red-shouldered stink bug, Baur conducted further tests. “What we did was place cages over soybeans and then released red-shouldered stink bugs in the cages. We infested at different levels — no stink bugs, one-third of an insect per row foot, two-thirds of an insect per row foot, and at the current threshold level — and left them there for three weeks. We also infested at three plant stages: R3/R4, R5 and R6.”
In the two-thirds test, Baur said if plants were infested at R3/R4 there was a drop in yield. However, “it wasn't statistically different from the check plots.”
If the pest was turned loose at R5, though, there was “a major yield hit.” The same was true for infestation at R6.
“So if you leave the stink bugs there with beans in the pods, yield will suffer. (Forget three weeks), even if you leave it for a short time, this pest can hurt you.”
Bean leaf beetle
Since moving to early soybean systems, bean leaf beetles have become more of a problem. “I want to remind you that this insect comes out in the early season and infests plants. It can do quite a bit of damage.”
The larvae feeding on roots and rhizomes affect nutrient uptake. Adults feed on leaves. While leaf feeding isn't a huge problem, adults also feed on pods. They can also transmit bean pod mottle virus, “a major trouble for everyone in the tri-state area.”
The three-cornered alfalfa hopper seems to prefer narrow-row production practices currently in use. Work done 20 or 30 years ago shows this insect has two population peaks: one at the end of July and the second into August or September.
The first peak normally occurs before the crop is in reproductive stages. “What happens is the pest causes a lot of girdling on the plants. As long as plant populations are around 130,000 per acre, the crop should be fine. Dropping below around 80,000 plants per acre can cause trouble with the beginning population peak.”
The second population peak causes the most trouble. “That's because pods are on the plants. The insect nymphs girdle the petioles leading to the pods. This causes the pods to drop off.”
Research has shown producers should worry about the nymphs in the second generational cycle when pods are on plants.
“That's when you'll get the maximum yield. That's an important fact because as we go to more early maturing varieties, we may begin to see susceptible plants — plants with pods — during the first generational cycle.”
The problem is nymphs are very hard to sample.
“If it's nymphs you're worried about, it's nymphs that need to be sampled. But how many folks sample soybeans with a drop cloth or beat net? Those are the best ways to sample for nymphs but practically no one uses them. Everyone prefers a sweep net. Sweep nets are excellent for adults but not nymphs. So we need to develop a method to sample adults and predict what kind of nymph populations we have and how much damage can occur.”
e-mail: [email protected]