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The curious case of Dimilin

There's no rancor, animosity or fur flying: on this, all sides agree. However, that doesn't mean there isn't a pointed debate going on. Good-natured or not, when it comes to the use of Dimilin as a yield enhancer or fungicide, Extension viewpoints in Mississippi and Arkansas couldn't be much different. And both sides are willing to tell you why.

The Arkansas take (part one)

Dimilin is marketed as an insecticide, a juvenile growth hormone that inhibits chitin (which forms insect exoskeletons) development. The active ingredient in the Crompton/Uniroyal product is diflubenzuron.

“As far as its insecticide properties, it works great,” says Cliff Coker, Arkansas Extension plant pathologist. “We recommend it for grasshoppers and some other insects.”

Any further claims for the product, says the researcher, have yet to be proven on Arkansas soils. “I've looked at Dimilin in conjunction with fungicides, including Quadris. We saw no yield response. I've also looked at Dimilin by itself. In 1997, I looked at Dimilin when Uniroyal was claiming the product was a yield-enhancer. There were tests spread all over the Arkansas Delta — from Mississippi County down to Chicot County. It was put out on high-yielding beans at a 2-ounce rate at R-3/R-4. We saw no yield response to those tests — while using replicated, small plots, big plots, aerial application, weigh wagons and all the other tools — at all.”

Chris Tingle, Arkansas' soybean specialist, says Dimilin has been tested in the state for years and still is. This year, state researchers will once again look at the product in multiple, replicated experiments.

“I've talked to (former Arkansas soybean specialist Lanny) Ashlock long and hard about this,” says Tingle. “Researchers found nothing under his watch nor have they under mine. We've looked at this product over many environments, at multiple rates, at different growth stages and everything else. We've found nothing. We've seen no yield or fungicide enhancement that's touted in other states.

“It's tough. Before making any recommendation, I have to consider that a product has the potential to be applied to 3 million acres. We must be able to back our recommendations up with data. We just don't have that data for Dimilin.”

Ashlock, who now works at Cullum Seed, agrees with Tingle's characterization. Perhaps, he suggests, “both camps are right because the soybean production environments in much of Arkansas are just different enough from those found further into the southeast. It's kind of strange. These are credible scientists on both sides of the river.”

The Mississippi take

Alan Blaine, Mississippi Extension soybean specialist, has two research colleagues who began studying Dimilin in the early 1980s. “We've had extensive testing on this for over 20 years, and this is not a product to use in every situation. However, I want to highlight some situations where we feel it's beneficial.”

Over the years, Mississippi researchers have observed Dimilin doing three things:

  1. Providing control of certain pest species.

  2. In some situations researchers have noted the reduction of frogeye leafspot in conjunction with Dimilin. “We're not saying Dimilin is a broad-spectrum, across-the-board fungicide. But it appears to help on some diseases, and we continue to study it.”

  3. It has yield enhancement abilities.

“At times, in the absence of insect pests, yield responses have been observed that we feel are due to (Dimilin-related) disease suppression…. In around 50 tests, we've seen results from no response to an increase of 8 bushels.”

Blaine says the normal use rate of Dimilin is 2 ounces (costing about $2.50 per acre). It provides a residual of around three weeks.

One area where Dimilin has been inconsistent is in early-planted soybeans — particularly early Group 4s. Mississippi's best results with Dimilin are seen on Group 5s planted after mid-April.

“If you have some Groups 4s planted in late April or early May, they're going to act a lot like a Group 5,” says Blaine. “In that circumstance, you're much more likely to see a response from Dimilin. Much of the efficacy of Dimilin has to do with planting date.”

Typically, timing of application in the state has been at the R-3/R-4 growth stage. In tests over the last couple of years, researchers have piggybacked Dimilin with a fungicide (most often Quadris). In order to do a combination treatment, the fungicide has been placed at a slight disadvantage.

“To reach a happy medium, we've gone with fungicides a tad late. It's worth the slight risk in order to just have just one trip over the field,” Blaine points out.

In the south Delta region of Mississippi, the predominate worm species is soybean loopers. Blaine and colleagues have found that Dimilin applied as a preventative under heavy looper pressure won't work. That's true, he says, even if rates are increased to 6 ounces. It will suppress the worms, though, buying a grower seven to 10 days.

“That's a big deal,” says Blaine.

Why? Because in some areas (until Mississippi received a Section 18 on Intrepid) growers had been in a two-spray situation to control loopers.

“There was no way around it. We had no really good residual material for loopers. Spraying so much was very expensive — $20 per acre. With Dimilin buying us a week to 10 days, many growers could cut field trips with the higher-cost product from two to one. A lot of times, we're just trying to buy time until September's cooler nights and conditions take loopers out.”

Blaine insists Dimilin will also “annihilate” green clover worms, velvet bean caterpillars (a pest common in Mississippi's hill country), cabbage loopers and salt marsh caterpillars.

Three years ago, Dimilin's efficacy was again revealed when grasshoppers threatened.

“We were in some dry weather and seeing a bunch more grasshoppers. We started putting Dimilin (at a 1-ounce rate) in with Roundup. We've since received a label in Mississippi for grasshoppers.”

While Dimilin doesn't work terribly fast on grasshoppers (normally taking from five to seven days), its value to no-till producers rose. Blaine says with Roundup Ready soybeans, farmers often don't just spray the fields but also ditches and turnrows. In so doing, the pest is deprived of its preferred grasses and moves into soybeans.

“If you've planted soybeans no-till and the crop is a week from coming up, a little Dimilin put in with burndown will control grasshoppers before the crop ever emerges. Obviously, if you've got high numbers feeding on the crop, acephate is the product of choice.”

From a fungicidal standpoint, Dimilin is also promising, says Blaine.

“Research has shown that there are several organisms with a chitin component. Since Dimilin prevents chitin development, we feel it offers fungicidal benefits — particularly with frogeye leafspot.”

As proof, Blaine points to 1983 when two Mississippi State researchers “checkerboarded” plots in south Mississippi. Later, they noticed that everywhere Dimilin was sprayed there was no frogeye. It was concluded that this was due to Dimilin/chitin interaction.

Intrigued with the 1983 findings, Blaine saw the frogeye outbreak of 2001 as an opportunity to try and repeat the earlier results. In numerous grower fields, he tried and failed.

Then, last year, “just by pure luck, I had 14 early-planted Group 4 dryland fields in the verification program. I wanted to try and protect the yields because we had a heck of a crop. Some didn't want me to do it, but I decided to put Dimilin on those fields. Today, I'll tell you I should have put out a pyrethroid instead.”

Still, some interesting data came about because at the same time, on those same fields, “we had some remote sensing projects and were having flyovers. (A fellow researcher) suggested we leave an untreated strip right in the middle of the field. On three of the fields, growers were making their last herbicide shot. We asked them to put Dimilin in with their Roundup and to leave an untreated strip in the middle.”

Application was just prior to, or right at, bloom. When the researchers later returned they found that where Dimilin had been sprayed there was no frogeye. Where the fields were untreated, “frogeye was everywhere.”

To prevent frogeye using Dimilin, says Blaine, you must spray the field prior to the disease arriving. Once frogeye is in a field, Dimilin has no impact.

“This year, I'm going to set up several field tests to see if we're on to something with Dimilin and frogeye. I'm going to put Dimilin out at bloom. We've already seen similar preliminary results with frogeye in lab conditions. If this has an application here, we could be looking at two entirely different uses for the product.”

At this point, though, Blaine makes it clear that he's not advocating Dimilin as a standalone fungicide, per se.

“We use Dimilin as an insecticide,” he says, “and then it does other things that mystify us.”

The Arkansas take (part two)

Coker, who terms the situation “very, very interesting”, says he knows of no other product with such divergent views in neighboring states.

With so many questions swirling, Tingle wants to make sure all understand how Extension comes up with recommendations. “We have a great group of researchers representing a whole host of disciplines. We remain in close contact with each other and update each other on research. When we find an interesting situation we report it, but we also try to verify it. So before we tell a grower to put out a certain product, plant this many seeds, whatever, we try and base it on numerous research trials over years and locations. Recommendations are a group effort.”

The Arkansas researchers aren't just paying lip service to Dimilin study. Last year, separate from Coker, Tingle conducted large, replicated trials looking at Dimilin. When yield maps were studied, there were no detectable trends toward yield bumps.

“We must maintain our practice of only recommending things with a proven economic return,” says Tingle. “Right now, Dimilin being used as anything beyond an insecticide hasn't shown that. I've spent a long time considering why we aren't seeing the Dimilin benefits our neighbors are. We're dealing with two environments. When you get down to it, there's a slightly different cropping environment. Mississippi plants a majority of their soybeans in April. Arkansas plants only 25 percent that month. Arkansas has much more rice…. Bottom line: we just can't find a way to make the use of (Dimilin as a yield/fungicide enhancer) pay for itself.”

Despite Arkansas' stance, Blaine stands by Dimilin. “In our program, on Group 5s, if we make a Quadris application, then Dimilin is going on, too,” he says. “I tell you what; I'll challenge any Delta soybean producer. If you want to know if Dimilin is doing anything, checkerboard a soybean field with it. Then go back and check the pest and diseases. (In the situations I've outlined) it works.”

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