The use of cover crops provides many benefits for farmland. Improving soil fertility, water penetration, and improving the amount of organic matter in soil are only three reasons why many producers sing cover crop praises.
However, as some Mid-South producers are currently being reminded, cover crops can also have a downside.
Just south of Marvell, Ark., soybeans were planted into a cover crop of Austrian winter field peas. Shortly thereafter, pea weevils were discovered chewing on the young soybean plants.
“The cover crop situation is not as widespread or shocking as some might believe,” says Gus Lorenz, University of Arkansas entomologist. “It is interesting, though.
“This is a relatively small incident on, perhaps, 1,500 acres. It comes after a similar thing occurred last year. I want to be clear: it is only associated with fields using Austrian winter field peas as a cover crop. The problem acreage is very limited.”
The pea weevils, says Lorenz, “get down into the debris, the cover crop residue and then feed on the plant. They cause a lot of defoliation and there are a lot of them. An application might take out a lot of them on the crop but there are plenty more still down in the residue, in the soil, waiting to move up. And the immature stage of the weevils feed on the nitrogen-fixing nodules of the soybeans.”
Producers are forced to keep chasing after the pea weevils. “You can spray and remove the ones in the crop, then they’ll come out of the residue or ground and you have to spray again. The grower gets into a situation where there’s a continual re-infestation because he can’t get control deep enough into the residue.”
What really concerned Lorenz is a field of corn across the row that was planted with the same cover crop. “We saw the same thing (in 2013) in a location not far away. The brown stinkbug damage was unbelievable. The Austrian winter pea attracts the brown stinkbugs in high populations. Once the corn comes out of the ground, the stinkbugs begin feeding on it.”
In 2013, the stinkbugs actually killed “a bunch of young corn and stunted a bunch more. The corn actually begins to tiller if the stinkbugs begin feeding at the right time. They move right down the row and there’s a progression of less damage, symptomology, to the plants as they grow.”
So far this season, the grower has sprayed that cornfield twice with a pyrethroid to kill the stinkbugs. Lorenz says the pests are still in the field.
Last year, when the corn began making ears “there were still so many stinkbugs in the field they began to damage the ears. They actually caused the corn to ‘cow horn.’ That’s when the pest feeding makes the ears curved and misshapen. There were parts of the field that had 100 percent damage. They really tore that field up.”
The bottom line for Lorenz is “we must not forget that cover crops come with risks. Everyone wants to talk about how good cover crops are, how positive they are agronomically. And that’s true -- I’m not downplaying the benefits. However, any time you bring in a cover crop there may be some pest management issues that can’t be forgotten.”
Proper management of the cover crops is important to avoid these types of situations. An earlier burndown of cover crops should be considered. Lorenz says those plants need to be killed before planting a primary crop into them.
“Many growers go in with a roller before planting, then they plant, then they spray a burndown right behind the planter as a pre-emerge. That doesn’t provide enough time for the cover crop to be dead -- to push the insects out -- before the next crop is planted. The pests just move over onto the emerging stand.”
Fall armyworm outbreak
Currently, Lorenz is especially concerned with a fall armyworm outbreak in the state. The armyworm populations are “a much bigger issue. Because of the rain pattern we’re in, the growers are letting grass get away from them. That means broadleaf signalgrass, crabgrass and all the rest gets a stand, a solid carpet in many places. Fall armyworm moths come in and lay eggs on the grass as the soybeans are growing. Well, the grower sprays for the armyworms and they move into the soybeans, milo or corn. So, we’ve got half-grown fall armyworm larvae in the grass heading straight for the crops.”
Lorenz is keen to point out that in many situations it isn’t the growers’ faults. “It’s too wet to treat and the grass keeps growing. Last week, I found larvae up to an inch long just ready to be pushed into beans or corn.
“Before spraying grass, I highly recommend that you go out and check it for fall armyworms. If it does, not only are you going to put out a herbicide but also an insecticide.”
Don’t plan on just a quick scan. “One thing I’ve noticed with these armyworm populations is they’re spotty. Just because you don’t see them in one area of the field doesn’t mean they aren’t in the rest. If you do find them, the grass is often literally covered up with armyworms. You can’t miss them.
“The feeding damage is significant; you can see the larvae hanging off the grass. Then, 50 yards away, there’s nothing in the grass. Don’t be fooled by that -- they’re too spotty and you need to scout the field properly.”
Armyworm problems are all over the state. “We’re set up, from what has happened in the last few weeks, for major insect problems.”
Lorenz has also seen similar problems with bollworms in henbit. “In a few fields, growers let the henbit grow up, planted into it, the crop came up, they put a burndown out to kill the henbit and then the bollworms moved into the soybeans. Those bollworms are mid-stage, or larger, and they’re eating the bean plants down to the ground.”
Overall, the caterpillar activity is quite high for this early in the year. Lorenz and colleagues “are worried about what that means when we get into the meat of the growing season. We’re already treating blooming soybeans for bollworms in south Arkansas. That tells me the activity and pest pressure is above average and everyone has to keep an eye out.
“Also, in cotton, we’re finally to the point of squaring. There’s a bunch of seven-, eight-, nine-node cotton out there. A lot of cotton fields -- and I’m talking up and down the whole Delta -- have high plant bug numbers, above treatment levels. They’re coming off wild hosts that are being sprayed around the edges of fields.”
Coming into this growing season, was the expectation that insect pressure would be down following such a cold winter?
“That’s true. I was asked about that at every winter meeting. After 20-some years in this job, I don’t make winter predictions because they’re nearly always wrong.
“The fall armyworm is a great example. After that cold winter I doubt we had a resident population here. They can’t make it through even a normal Arkansas winter. But the southerly wind flow out of the Gulf has pushed fall armyworm up here and now they’re causing problems.”