Farm Talk


September 14, 2010

Scout soybeans for soybean podworm

Parson, Kansas — Normally, at this time of year, we assume that we can “coast” through to harvest with the soybean crop. However, for those with late planted soybean or double crop soybean, I strongly encourage you to scout your fields immediately for soybean podworm (same insect as corn earworm). Within the past two weeks, numerous fields throughout west central Missouri have seen levels of this insect well above the economic threshold. Leaves, stems, and flowers can be damaged by the soybean podworm, but pods and seeds are especially vulnerable to attack by the third generation larvae. Podworm will feed on soybean up to the R6 (full seed) growth stage; they normally do not eat R6 soybean, so R1 (flowering) through R5 (beginning seed) are the most critical stages.

If there is more than one podworm per one foot of row, or if more than five percent of pods are damaged, treatment is recommended. When scouting late planted soybean fields, examine the size of the podworms. If the majority of the worms are 1.25-1.5 inches, they will pupate soon and their damage is already done. Damage varies from field to field, so it is very important to scout. If podworms are small and pod damage is near threshold levels, then an insecticide application is recommended. As with any input cost, yield potential of your fields should also be considered.

For those fields in which an insecticide treatment is warranted, both aerial and ground application are options; however, it is important that a high gallonage of water be used to achieve good coverage of the soybean canopy. For ground applications, use 15 GPA and crop oil with the pyrethroid insecticide. For aerial applications, it is important to apply the insecticide with five GPA at least and to fly low to the crop. Time of day has also shown to be influential in the level of control achieved; applications made in the middle of a hot day with low volume (gallonage) were not as effective.

Numerous pyrethroid and combination insecticide products are labeled for soybean podworm (corn earworm) in soybean; examples include Asana XL, Baythroid, Cobalt, Hero, Mustang Max, Pounce, Steward, and Warrior. For a full listing of products labeled for this particular insect, please contact your local MU Extension Center. It is important to treat only if warranted. Unnecessary insecticide applications will do more harm than good, as they reduce beneficial insect populations for approximately 30 days. 

Generally, soybean podworm will feed for at least two weeks; however, if there is quite a range in the size of worms, the feeding time may be extended past the two week point. Once the larva is full grown, it will crawl down the soybean plant and pupate in the soil. The next generation of moths will emerge within 10-25 days.  See MU Guide ‘Corn Earworm in Missouri’: extension.missouri. edu/publications/DisplayPub.aspx?P=G7110

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