Corn Borer On Soybean

Corn borers had been detected on soybeans in Hanson County by Dr. Tom Chase
(SDSU Plant Pathologist). About one percent of the stand was infested. Wilting of
the growing points is a sign of infestation. The stem can also be split to see the larvae.
Although corn borers prefer corn, they could actually feed on a wide variety of plants
from proso millet to okra. I think what happened was that suitable corn was unavailable
for the moths when they started laying eggs and were "forced" to choose soybeans. No
control measure is necessary.

Corn Rootworm Damage

Mark Rosenberg (Spink County) sent me some corn roots damaged extensively by
corn rootworm larvae. According to Mark, they were from a field which was also planted
to corn last year. Most rootworm adult beetles will be emerging from the soil soon. Silking corn
should be scouted to determine whether an at-planting insecticide should be used next year.
This is assuming that corn will be planted again on the same field. Crop rotation still works very
well in South Dakota and must be considered in fields experiencing rootworm damage this
season. Ask Mark for more details on the rootworm damage.

Corn Earworms Numerous

Corn ear worms can become a problem in seed, sweet, and field corn this year. Moths are
carried by southerly wind (perhaps the same wind that dumped a lot of potato leafhoppers)
into the state. Once here, moths lay eggs on green silk and larvae enter the ears. However,
larvae also feed on corn leaves like what I saw in Beresford on 7-21-99 (see picture). Control
may be impractical for field corn since multiple applications of a liquid insecticide is necessary to protect the ears. According to a study in Indiana, 4-6 applications of an insecticide
will give 85-100 percent worm-free ears. Seed corn and sweet corn may be treated starting
when ear silks begin to appear until silks have wilted.

Grasshoppers and Potato Leafhoppers

Grasshopper hatch is still ongoing in the state. Some twostriped grasshoppers are now adults
but most redlegged grasshoppers are still young nymphs and will keep on hatching until mid-August. Flowering to pod-fill soybeans must be scouted and treated if necessary. Thresholds and insecticides  are available on the Internet.

Potato leafhoppers will be numerous until the first killing frost in the fall. Newly-planted alfalfa is vulnerable to stunting. Our SDSU research has shown 1-4 point reduction in percent crude protein (5-21% loss) in established alfalfa exposed to above-threshold numbers. PLH are not hard to control and there are numerous insecticides available. Alfalfa may be scouted using an insect net.

| Extension Entomology Home Page | Current Issue | Archives |