FROM THE FIELD: Grasshoppers, snails feeding in no-till soybeans

This crop has had problems, but most concerns have been associated with no-till plantings. The following warrant attention:

Watch no-till fields for grasshoppers. Treatable populations surfaced in early May. Several materials are labeled, but Orthene and Dimlin have shown the most consistent results the last couple of years.

Major differences between spraying now versus later in the season are: (1) coverage and (2) possibly dealing with a different species early versus late.

Orthene should be used at 0.3 pound of product per acre. If you put out at least 10 gallons of water to insure good coverage, you probably will see no difference in control at 0.2 pound of product.

Dimilin is labeled for use on soybeans in Mississippi at a rate of 2 ounces per acre. Recently a label was obtained for grasshopper control out West on pastures and rangeland at 1 ounce per acre. We tested that rate over the past two years with excellent control in every test.

That is less than the labeled rate, but our evaluations indicate the 1-ounce rate is sufficient for grasshopper control. We have asked Uniroyal to pursue a supplemental label for the use of 1 ounce of Dimilin for grasshopper control.

One major difference between the two materials is the time needed for control. If grasshoppers are feeding extensively on your crop, choose Orthene. If feeding is minimal and you have a lot of weed or crop foliage, Dimilin would be good.

Orthene will work in about 48 hours. With Dimilin it will be three to four days before a reduction in feeding activity is observed, and seven to nine days for complete control. Dimilin, however, will give three to three and a half weeks of residual activity.

The pyrethroids do a good job on small grasshoppers if they are applied at the correct rates, but most pyrethroids will not be as economical as Orthene and Dimilin.

Snails and slugs have been observed feeding in no-till fields. They prefer decaying vegetation, but as fields have begun to dry, the snails and slugs were feeding on the crop.

Heavy snail populations are being found around Winona, Miss., on both cotton and soybeans. Normally, they are not a problem, but with large populations, control may be needed.

We have little information on the most-effective control. Two factors affect control: coverage and snail movement. If snails are under crop residue and leaves, coverage will be difficult. Rainfall or irrigation may initiate snail movement and help improve control. These factors may be behind the poor control we have encountered over the last few days. Many products are being evaluated to determine the best.

Last year we achieved excellent control with Karate Z at a gallon to 100 acres, but the application was applied late in the afternoon as snails began to emerge. Until we learn more, we think spraying should be done at a high volume of water — at least 15 gallons per acre. Also consider using higher pressures.

Last week we received a call about Ammo applied to cotton at a gallon to 50 acres in 15 gallons of water with high pressure and the boom close to the crop. Two days after application, results were good.

It appears that under dry conditions, snail movement is virtually non-existent. If that is so, it is easy to understand the importance of coverage for this pest.

Several calls have come from farmers about plant populations. Row plantings of 75,000 to 125,000 plants per acre are sufficient for maximum yields. Populations 25 percent above or below these levels will have little impact on yield.

The key is adequate weed control. If plants are healthy, it is usually better to keep a 50-percent stand than to replant.

I have evaluated thin stands on numerous occasions over the last couple of years. Twice we had 1.8 living plants per foot of row. Both fields were irrigated, and both cut in excess of 60 bushels per acre.

In 1993, I was on a farm in south Mississippi that had two plants per foot of row on 40-inch rows. Weed control was not a problem because the grower planned to cultivate, and post-direct spray his entire crop. The beans were non-irrigated and cut 54 bushels per acre.

Soybeans have a capacity to compensate for thin stands. The down side is slower canopy closure, which is offset somewhat by Roundup Ready soybeans and irrigation.

This has not been a textbook year. If you planted early, you probably noticed erratic weed emergence caused by cool conditions and varying moisture levels. In many fields, early weed emergence was so erratic that even where a few weeds were getting large, there were too few to justify spraying early.

Dry conditions (particularly south of Hwy. 82 in Mississippi) helped by delaying the initial flush. It was a case of free weed control.

I was more willing to ride Roundup Ready soybeans than conventional, but weed control has been above average.

If you sprayed early-planted beans seven to 14 days after emergence and had little emerge, you probably sprayed too soon. Were there enough weeds to justify spraying? Timing is more important than the rate, but you have to have something up to spray.

Alan Blaine is the Mississippi Extension soybean specialist.

e-mail: [email protected]

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