Friday, July 12, 2013

Corn silk feeders are worth scouting right now

I saw my first corn tassels of the season and it got me thinking about silk feeders out and about now. But then again, I saw some corn that was only 18 inches tall and looked a long way from tasseling. It's been a crazy summer!

Of course many of you are noticing Japanese beetles flying around (and maybe hitting your windshields) this week. They are definitely attracted to green silks and can interfere with pollination. Japanese beetles mate and feed in groups. It is not uncommon to see them aggregated on plants, especially along the edge rows. They are highly migratory and are constantly moving around the landscape. Adults have a wide host range (>300 plants) and are likely to find something they like to eat.

There has been a lot of marketing about controlling Japanese beetles this year. I don't think it is wise to treat for adults BEFORE tasseling. I have preliminary research that shows foliar insecticides do not have a long residual, regardless of the chemistry. Most broad spectrum insecticides will kill adults if the droplets make contact on the body. That application won't have a 7-day residual for immigrating beetles.

But if beetles are actively feeding when silks are present, determine densities to make smart treatment decisions. Consider an insecticide if: 3 or more beetles per ear AND silks have been clipped to less than an inch AND pollination is not complete. Again, do not expect a long residual with any product, so continue to scout until pollination is over. If a second spray is warranted, alternate chemistries to reduce the chance of developing resistance.

The tricky part of managing this pest is if you have sweet corn that may have staggered planting dates or late-maturing hybrids. This summer is a good example of field corn at various growth stages, too. Be aware beetles will continue to move to green silks throughout the summer.

Japanese beetles at the ISU Johnson Research Farm in 2012. 

The second most common type of silk feeder is adult corn rootworm. Emergence has started in southern Iowa and it won't be too long before we see them all over the state. Like Japanese beetle, they are strongly attracted to green silks and like to feed and mate in masses. Silk feeding can interfere with pollination, so it is important to scout during this period to ensure kernel formation later this summer. Consider an insecticide if: 5 or more beetles per ear AND silks have been clipped to less than an inch AND pollination is not complete. The threshold can be bumped up to 15 per plant if the field is under adequate moisture conditions.

Adult corn rootworm can clip silks. Photo by John Obermeyer, Purdue University.

Grasshopper nymphs and adults can also occasionally eat corn silks. They are usually found around field borders first and then can infest the field interior later in the summer. Since grasshoppers are so mobile, it is very difficult to try and estimate densities. So as with the Japanese beetle and corn rootworm, it is important to watch for silk clipping and take action if they are interfering with pollination. Because grasshoppers tend to move short distances and attack border rows first, a border treatment may be a cost effective decision. 

The differential grasshopper is common in Iowa field crops. Photo by David Cappaert, www.ipmimages.org.

The fourth potential corn silk feeder in Iowa is the corn earworm. The larvae have highly variable body color, ranging from yellow, to green or orange, or brown and even purple. There are alternating dark and light stripes running the length of the body. Larvae have a textured appearance, with many spines coming out of small bumps. This is unlike black cutworm or fall armyworm that have smooth bodies. 

Female moths deposit eggs on green silks during the night. First instars will feed on silks and eventually move down inside the tip of the ear. Older larvae will destroy developing kernels. Since trying to kill the larvae once inside the ear is almost impossible, I recommend monitoring adults. The University of Illinois has a nice website that summarizes how to properly time a foliar treatment for corn earworm, depending on the type of corn grown. 

Corn earworm larvae can feed on silks before they enter the ear to feed on kernels. Photo by Frank Peairs, www.ipmimages.org.

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