With soybean acreage increasing dramatically in parts of the Southeast this year, growers will want to be aware of some of the insect pests that could cause problems as the season progresses.

One of the most troublesome is the three-cornered alfalfa hopper, said Tim Reed, regional Extension agent, speaking during the recent IPM Web Conference sponsored by the Alabama Cooperative Extension System.

The three-cornered alfalfa hopper is not only a seedling pest of soybeans, but also has been identified as a source of significant yield loss in Mississippi. “This pest uses its needle-like mouth parts to girdle stems. It also can feed on the peduncle,” he says.

Growers generally first notice damage from this pest when they try to sample plants and the plants fall over due to feeding damage that occurred when the plants were less than 10 inches tall, he says.

Reviewing research data from Mississippi, Reed says that in five tests conducted during 2004 to 2007, Research Entomologist Gordon Andrews obtained yield increases in four out of five tests when he sprayed beans in stages R 4.9 to R 6.7

“Yield increases obtained by spraying for mixed populations of three-cornered alfalfa hoppers and stinkbugs were as high as 5.6 bushels per acre, with the average yield increase being 2.6 bushels per acre, which represented a yield loss of 5 percent,” says Reed.

Soybean yields in these five tests averaged 53.4 bushels per acre, he adds.

The conclusion of this study, says Reed, was that yields of early planted Group IV soybeans were improved significantly when insect populations, that were lower than published treatment thresholds, were reduced with insecticide. Three-cornered alfalfa hoppers were more consistently and significantly associated with yield loss.

“As a consequence of this research, Ron Smith and I decided to add the treatment threshold of one three-cornered alfalfa hopper per 15 sweeps to the 2009 Alabama Soybean IPM guide. This threshold is being used by other states as well. Once soybeans begin setting pods, growers need to consider numbers of the pest present along with the numbers of other yield-reducing insect species when making treatment decisions.”

Insecticide seed treatments for soybeans have gained acceptance in Mid-South states after numerous studies showed they provided a positive return over the cost of treatment, says Reed. Mid-South studies conducted from 2003-2007 showed Gaucho-treated soybeans had an average yield increase of 2.8 bushels per acre in 34 trials and Cruiser-treated soybeans had an average yield increase of 3.5 bushels per acre in 66 trials.

“Last year, Dennis Delaney and I conducted 3 small plot, replicated soybean insecticide seed treatment studies,” says Reed. “Seedling soybeans were sampled by shaking plants over a notepad and thrips were the only insects present in meaningful number. Plants did not appear to be damaged by thrips feeding.”

In a test at Crossville, there was a small numerical increase in yield in plots treated with Cruiser insecticide plus fungicide in comparison with fungcide alone, says Reed. Yield increases ranged from 1.2 to 1.8 bushels per acre with the average increase being 1.5 bushel per acre.

Turning to the impact of stink bugs on soybeans, Reed says the green stink bug tends to be the predominant species of this insect pest on soybeans in north Alabama.

“Stink bugs can damage soybeans from the time beans begin to form in pods until beans are fully developed within pods. I conducted a stink bug test in a field of soybeans in Lawrence County last year. The soybeans were in the R5 stage on Aug. 14 with 2.2 stink bugs per row foot (third to fifth instar plus adults) when a pyrethroid was applied to 12 rows either side of a 12-row area that was not sprayed.

“We sampled four days later and found 0.4 stink bugs per foot in unsprayed and .05 stink bugs per foot in the sprayed areas. I do not know why stink bug numbers declined in the unsprayed portion of the field since there was no drift of pyrethroids.”

Samples taken 27 days post-application showed 1.8 stink bugs per foot in the sprayed area and 0.25 stink bugs in unsprayed areas, he says. “We cut just over 54 bushels per acre on either side of the unsprayed area which cut 51 bushels per acre for a difference of 3.1 bushels per acre. I found a field of R7 soybeans in Marion County with a population of almost four stink bugs per foot of row.”

Green stink bugs comprised 87 percent of the stink bug population and brown stink bugs were 13 percent of the total in the Marion County field, says Reed. Test plots were sprayed Sept. 9. Plots four rows wide and 60 feet long were sprayed with different insecticides that left different levels of stink bugs. There were four reps per stink bug level.

Though yields and seed weights were not significantly different, yields were numerically greater for each reduced level of stink bugs and seed weight increased as stink bugs numbers decreased. Sixteen-thousand seed were weighed to get average seed weights. The data indicate that 3.8 stink bugs per row foot at stage R7 decreased seed weight by 8 percent.

Another consideration for soybean producers is making treatment decisions for leaf-feeding pests, says Reed.

“It is important to make an accurate estimate of the level of defoliation present in the field when making a treatment decision. Most people tend to over-estimate defoliation. Make sure you estimate defoliation for the entire canopy — the bottom, top and middle of the canopy.”

For foliage feeders, Reed recommends preventing greater than 35-percent defoliation during pre-bloom and greater than 20-percent defoliation from pod set to maturity. Treat prior to 20-percent leaf loss when five to eight soybean loopers or velvetbean caterpillars one quarter-inch long or longer are present per foot of row.

“You can also include green cloverworms in the count. Treat when you catch an average of 1.5 worms per sweep. Soybean loopers are harder to dislodge with a sweep net and each looper should be counted twice. Remember that loopers tend to start low on the plant and move upward.”

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