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Mississippi entomologist updates pest thresholds. Aims to save producers money. Three-cornered alfalfa hoppers, bean leaf beetles, stink bugs.
ANGUS CATCHOT, MISSISSIPPI entomologist, presents recommendation tweaks and new research information at the mid-January Cotton and Rice Conference. The tweaks, he says, “should save you some money.”
Bean leaf beetles
Collectively, bean leaf beetles, grasshoppers, soybean loopers, green cloverworms are known as “foliage feeders.” Most of them don’t cause direct damage, says Catchot, although bean leaf beetles can feed on pods a bit.
“Yield loss is more or less from leaf-feeding. When you lose a lot of leaf surface and lose photosynthesis, you can get a yield loss.
“We had a graduate student working on foliage loss. The critical period to maintain the foliage is between R-3 and early R-5. If something happens during those stages, say you lose 80 to 100 percent of your leaves, you can see upwards of an 80 percent yield loss.”
While producers must manage these pests, Catchot cautions to be “sensible about it. You don’t need to keep every bit of foliage. The crop can tolerate a little foliage loss -- 20 percent is what most of us are working with in the Mid-South.”
Bean leaf beetles exploded in 2013.
“In Mississippi, we have pyrethroid-resistant bean leaf beetles. We had some fields that hit 300 per 25 sweeps. That led to a lot of foliage loss.
“We used to have a threshold of 50 per 25 sweeps, or a 20 percent foliage loss. We can hit the sweeps threshold easily in Mississippi. We’d make a spray and, a week later, another population would move back into the field. No chemistry would hold them more than five to seven days.
“Suddenly, you be at a two-shot spray -- sometimes three -- for bean leaf beetles. However, the field never got close to the 20 percent foliage loss. That means farmers were making a lot of unnecessary sprayings, while blowing up loopers and other lep pests because the beneficials were taken out.”
That has led to another recommendation change for 2014.
“We’ve basically gone to only a defoliation threshold for bean leaf beetles. The sweep number threshold was getting a lot of producers in trouble and causing multiple sprays.
“You can tolerate the beetle numbers, keep in mind, because the damage is only to the leaves.”
How quickly can the defoliation threshold be reached? If you’re making a once-a-week check and running five percent damage, is there a danger of coming back in a week and finding 40 percent damage?
“Five or six years ago, I was doing resistance screenings around the state. I went to a field of Group 4s and Group 5s growing together. The 4s were drying down.
“When I got to the field, which had a big unsprayed section I was working in, the beetles were running about 150 per 25 sweeps – a 3X threshold. Defoliation was about two to three percent when they were coming out of the 4s.”
While anecdotal, “it took 12 days to get from there to 20 percent defoliation. So, you have a bit of time.”
Researchers have made no changes for bollworm thresholds.
“Currently, all of us in the South are running about nine per 25 sweeps. Most people are spraying at about six.
“We’re working very hard to validate the thresholds. One thing we know for sure: bollworms are more of a problem.”
The good news is the arrival of additional materials to combat the pest. “I don’t think any of us would recommend pyrethroids for bollworm control -- particularly in soybeans or grain sorghum. We’ve seen so many failures. I believe seven out of 10 people that try to use pyrethroids on bollworms in soybeans end up in failure.
“Now, we have the diamide class of chemistry: Belt, Prevathon, Besiege. These products are incredible on caterpillar pests.”
For soybean loopers, Catchot routinely sees around 30 days residual with those products. On bollworms, “we have some really good lab data that suggest the same. However, in the field with bollworms, typically we get 14 to 21 days residual.”