April’s Ag Advice

April's Ag Advice by April Borders Soybean producers are encouraged to monitor their fields for the presence of the soybean aphid. Plant injury is caused by the insects feeding on the plant sap which can cause yellowing, cupping and wilting of soybean leaves. Plant growth may also be stunted. Soybean aphids are also capable of transmitting mosaic viruses that can cause soybean diseases, such as soybean mosaic virus, bean yellow mosaic virus and alfalfa mosaic virus.

Scouting for soybean aphids should start as soon as the plants produce two trifoliate leaves (V2) and continue until the beginning seed stage (R5) of soybeans development. The insects can be found mainly on the young leaves and stems of the soybean plant early in the season. Count soybean aphids from at least 10 plants at 10 representative locations in the field. The entire plant needs to be examined as the aphids could be in the stems, growing points, flowers, pods, or underneath the leaves.

SDSU Research in 2001 has shown that the soybean plant is quite sensitive to aphids during the reproductive (R) stages of soybean development. Research from 2002 revealed that spraying at beginning seed stage (R5) may still result in significant yield advantages if economic numbers of soybean aphids are present in the fields.

During the 2003 growing season, SDSU research indicated that soybean aphids can increase very, very fast with peak numbers occurring just before R5 (beginning seed stage). For example, it took only 15 days for aphids to multiply from 10 aphids to 1,214 aphids per plant.

With this in mind, a soybean aphid population of 250 aphids per plant close to R1 (beginning bloom) could reach more than 1,000 aphids per plant within seven days. Soybean aphid numbers, however, do decline precipitously after R5 (beginning seed stage).

Based on these findings, SDSU Extension Service recommends that soybean aphids be controlled at R2 (full bloom stage). This stage also marks the beginning of the most sensitive stage in soybean growth and development. Remember the old saying: "Soybean yield is made in August." Dr Mike Catangui, SDSU Extension entomologist would like to add to the saying: "Protect thy bean in early August."

In the past the economic threshold was listed as 200 or more soybean aphids per plant. Based on findings and research done in 2003, the economic threshold has been lowered. SDSU Extension Service now recommends that the economic threshold be determined using the following guidelines:

1. Calculate the gain threshold (GT)

GT = chemicals plus application cost � soybean market value.

2. Express the gain threshold as a percentage of the soybean yield potential of the field.

GTP = (GT x 100) � yield potential

3. Calculate the EIL ( Economic Injury Level) as follows:

EIL = (GTP + 8.01) / 9.6

The unit of the EIL is in aphids per plant.

Here is an example: If the coop is charging $8/acre (chemical plus application) to spray the field. The predicted market value of soybeans is $7/ bushel. The field has been yielding 50 bushels per acre over the past years. The soybeans were planted in 30-inch rows. It is now July 14 and the soybean is nearing full bloom. What is the economic injury level of the soybean aphid if you want to spray your field at full bloom (R2) stage?

GT = $8/acre � $7/bushel = 1.14 bushel/acre

GTP = (1.14bushel/acre x 100) � 50 bushel/acre = 2.29%

EIL = (2.29 + 8.01) / 9.6 = 2.92 aphids per plant or 29.2 aphids per linear foot (assuming 10 plants per foot) or 292 aphids per 10 linear feet.

Insecticides recommended for control of the soybean beetle include Asana, Dimate, Furadan 4F, Lorsban 4E, Mustang, Mustang MAX, Penn- cap-M, Pounce 3.2EC and Warrior.

More information is available in the Soybean Aphid in South Dakota Fact Sheet 914 available at your local Extension Office or on Dr. Mike Catangui's Web page at http://plantsci.sdstate. edu/ent. The agronomy Extension Educators in the S2 Field Education Unit can also assist you with your questions. You can contact April Borders (Clay County) at 677-7111 or Steve Sutera (Bon Homme County) at 589-3531.

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