Cleaning up Johnsongrass is the first line of defense against sugarcane aphids (SCA) in grain sorghum, particularly in the lower South and Southwest. The grass, a close cousin to grain sorghum, is one of SCA’s favorite natural hosts.
As a pest in sorghum, the insect came out of nowhere four years ago and has drastically altered how farmers and crop advisors now grow the crop. Once only found on sugarcane, a population has developed that prefers grain sorghum, to the point that it will barely survive on sugarcane.
Initially, the insect turned up on grain sorghum in southeast Louisiana and Texas. By the end of 2015, it had been confirmed as far east as Virginia, as far north as Nebraska and as far west as New Mexico. Its southern range now extends into several Mexican states.
Johnsongrass is a particularly important stepping stone for SCA, especially in areas with milder winters.
“In south Louisiana where we don’t get very hard freezes every year, Johnsongrass may act as a very good overwintering site,” says David Kerns, Entomologist, Louisiana State University. “It becomes a source of aphids and will carry them through the winter. As you go north, Johnsongrass freezes back, so you may not see this early effect.”
Growers in south Louisiana already have been asking Kerns about the relationship between SCA and Johnsongrass, then taking action as needed. One farmer in February sent Kerns photos of patches of Johnsongrass with SCA colonies forming on the leaves. The grower has since killed the grass.
Taking out the grass early doesn’t mean that SCA won’t still build in the field later, but controlling the grass now does deny the insect an early beachhead. If SCA are present in or near a field, they can infest grain sorghum as soon as the crop emerges. The standard recommendation now is to use an insecticidal seed treatment, which can provide up to 40 days of protection.
By cleaning up Johnsongrass ahead of or at planting, SCA lose that jumping off point once the seed treatment plays out, Kerns notes. The insect can still move into the crop later. But the farmer has at least bought extra time. That, in turn, delays infestations long enough that he may reduce the number of insecticide applications or avoid them altogether if he planted early, selected a tolerant hybrid, and other factors line up in his favor.
“In many cases I’ve seen infestations start in grain sorghum where plenty of Johnsongrass was growing along rows, in ditches bordering the field or even in clumps in the field,” Kerns says. “The initial colonies build there. Once the seed treatment wears off, SCA move into the crop.”
Where Johnsongrass is burned back by cold weather, it can rebound quickly from rhizomes and then harbor aphids that do move into an area, Kerns adds. Johnsongrass will likely gain a big head start in 2016, thanks to warmer-than-normal weather through parts of the South and Southwest.
Case in point: on March 6, immature SCA already were being found in Comanche County in central Texas on 2-leaf Johnsongrass.
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At least right now, research doesn’t show that the insect can overwinter on rhizomes, themselves. When infestations take hold farther north, it appears after the aphids have been moved by weather fronts from overwintering grounds to the south.
Again, if Johnsongrass is up and growing at that point, it serves as a safe haven while SCA wait for grain sorghum to emerge and/or become vulnerable as seed treatments weaken.
“Johnsongrass can really come on fast,” Kerns says. “Even if it has been burned back by winter weather, it can make a quick start from rhizomes and be available when the aphids do turn up.”