It sounds like the plot of a cheesy 1950s sci-fi movie: Creatures that don’t need males to procreate. Females birth hundreds of live babies every 5 to 7 days, all of which are exact DNA clones.
Some of those can then mutate, or select for resistance to chemical weapons that control them, and their progeny will be resistant duplicates.
In a short time, there can be billions of creatures that can’t be stopped by the previously effective chemical weapons.
In a nutshell, says Angus Catchot, that’s the sugarcane aphid, a pest has spread like wildfire across southern states in just one year, decimating grain sorghum crops in its path.
There are photos from Texas, where the pest began its spread, of equipment so covered with aphids that it appears to be thick layers of dust.
In uncontrolled situations, grain sorghum yield loss can be total.
“In 2013, we started hearing reports out of the Texas Coastal Bend area and Louisiana about this pest,” Catchot, who is Mississippi State University Extension professor of entomology and plant pathology, said at the annual meeting of the Mississippi Agricultural Consultants Association. “Late that year, they were confirmed in one western Mississippi county.
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“In 2014, they were in every Mississippi county that had grain sorghum. And I’m sure they were in a lot more counties on johnsongrass, which is also a host. In just one year, it had moved from Texas all the way to South Carolina. That’s how rapidly they spread.”
Anyone planning to grow grain sorghum this year “needs to be aware of what can happen with this pest and how it can affect their crop budget.”
The sugarcane aphid, first identified in Hawaii in 1896 and in the continental U.S. in Florida in 1994, was documented in Louisiana in 1999, Catchot says.
“It was never extremely damaging on sugarcane, but for grain sorghum it’s a different story: They can reproduce and blow up very quickly, and they can be extremely damaging and yield-limiting.”
Unlike the cotton aphid, he says, there are no naturally-occurring pathogens that take it out that we have observed to date. “The crashes we’ve seen have, for the most part, been due to predators like lady beetles,” and thus it is important to protect beneficial insects when spraying for other pests, such as midges.
Taxonomists are still working to “try and figure out exactly what this aphid is,” Catchot says. “Molecularly and taxonomically, it aligns with the sugarcane aphid, but they don’t want us to call it the white sugarcane aphid, and we’ve stopped doing that.
A possible species shift
“The thinking is that there has been a species shift and that it’s a sugarcane aphid biotype that prefers grain sorghum and johnsongrass.”
Unlike corn leaf aphids, which can be present in corn in large numbers but “never really blow up and spread across a field,” this aphid can multiply and spread in a week or less across an entire field.
So far, Catchot says, this sugarcane aphid is not a pest of corn. “I’ve personally put millions of them in corn test plots, but couldn’t get them to blow up the way they do in grain sorghum. They don’t seem to be able to establish in corn — right now. But in the future, who knows?”
The chief worry, he says, “is that this pest is going to drive grain sorghum acres way down. And unless some disease we don’t know about suddenly appears to control them, we think they’re here to stay because of the prevalence of johnsongrass. Even at my house five miles west of the MSU campus, johnsongrass was absolutely covered with them last year. Everywhere in the Delta, I didn’t even have to get out of my pickup to find them — you could see their honeydew all over the johnsongrass. So, that will be a continual host for the pest, even if we had no grain sorghum.”
In 2014, Catchot says, it took two to three weeks for the aphids to get to a 20 percent to 30 percent infestation level. “Initially, they just simmer along, but when they get to those levels things go to hell in a hurry. In five to seven days, you can have an absolute disaster. I’ve never seen anything like it.
“I’ve been telling farmers in all of our winter meetings, ‘If you grow grain sorghum, you’re probably going to have to deal with this pest. It’s an absolute must that you have your fields checked twice a week once aphids are first observed. That means you’ll have to budget more for your consultant to do this. If you scout just once a week, your crop can experience loss before you know it.’
“I’m not saying every case will be like that — but the potential is there. I saw it happen in my own plots last year. Do you want to take that chance?”
In some of his plots, Catchot says, “shortly after they bloomed out, the aphids infested the entire field at extremely high levels. The entire plants, which had received one application of Transform, were killed. There was 100 percent yield loss. We could have made additional sprays but wanted to see what their potential recovery could be after one application.”
Seed treatment recommended
A seed treatment, he says, can make a significant difference. “If you grow this crop, a seed treatment, either clothianidin or thiamethoxam, is going to be an absolute necessity. They work really well, and give you about 40 days protection, to about the pre-boot stage.”
The pre-boot stage is a very sensitive period for grain sorghum, he notes.
“Control works, but you’ve got to get it on the crop in a timely fashion. And even with seed treatment, it only buys you so much time before they can infest the plants, but it will minimize early foliar sprays while it is still active.”
In 2014 plots, Catchot says, Transform applied about the time heads were fully out and aphids were at the 25 percent to 30 percent level, provided control, “but we still had a 67 percent yield loss where we did not apply follow up applications. Even for an infestation that started as late as the soft dough stage, we still had a 21 percent loss. This pest can be damaging even very late in the growth process if not properly controlled.”
Two products used in Texas, Lorsban and dimethoate, didn’t work well in Mississippi plots, he says. “I would not recommend either against this pest in Mississippi. Five days after a dimethoate application for a 20 percent infestation, there were so many aphids the plants were almost dead. It’s scary how quickly they blew up after dimethoate.”
Problems aren’t limited to the growing season, Catchot says. “They can be present at harvest as well.. There are reports, in Louisiana, of growers having to pressure wash machinery at the end of turn rows. You can’t run those sticky grain heads through a combine — it’ll clog up every time. Some growers combined their last shot of Transform with their desiccant, and that seemed to work well.”
Catchot says he worries that “unless we’re very careful, the effectiveness of Transform could be very short term. Every sugarcane aphid that has been legally sprayed, from Texas to South Carolina, has been sprayed with Transform. It’s no secret that aphids can very quickly develop resistance to an insecticide.
Additional chemistries needed
From a resistance management standpoint, we need other products to help relieve Transform. Bayer recently got a full registration for Sivanto at 7 ounces to 9 ounces. But the 7 ounce rate would be very costly. They’re going to follow immediately with a 2ee label for a 4 ounce to 7 ounce rate, which would make the cost comparable to 1 ounce of Transform. We’re also submitting a Section 18 request for Centric. If we should lose Transform in the middle of the season, that would leave us only with Sivanto, and we’d like to have Centric as another option.”
Growers may be able to make fewer applications by planting early, Catchot says. “In 2014, with plantings up to about May 5, many growers got by with zero to one spray. We don’t really know if it was because the aphids weren’t in Mississippi in numbers yet, or if it was due to the early planting.
“Long term, I think the solution may be with germplasm. There are some breeder lines that are resistant to this pest, but it may be years before they’re commercially available.”
Thick plantings, at least anecdotally, seem to be less vulnerable, he says. “Spots that were planted too heavily last year, either in twin rows or whether the planter was improperly adjusted, seemed to have less severe infestations. At the other end of the spectrum, weak spots in fields seem to blow up more quickly.”
Grower are cautioned that they can make the problem of a blowup of sugarcane aphids worse by applying pyrethroids for midge control that isn’t needed, Catchot says.
“This practice is really bad about killing beneficials and blowing up aphids. Midges can be devastating, but they’re pretty easy to scout for, so be careful about applying pyrethroids.
“I definitely think this pest can be managed now that we will likely have more chemistry to target it with, but it’s going to require twice-a-week scouting and care with midge sprays and any other disruptive sprays that kill beneficials.
“I would budget at least two aphid sprays," Catchot says. "Hopefully you won’t need both, but I’d prepare for it. Don’t be afraid to plant grain sorghum — but just be aware that you’re going to have to be on top of things and that you need to budget for extra scouting.”