Cornfields are starting to tassle and silk all over Iowa. Many insects like to feed on silks, so it is a critical time to assess pollination rates. This article highlights a few of the most common silk feeders of importance.
Japanese beetle. Some 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 edge rows. They are highly migratory and constantly move around the landscape. Adults have a wide host range (>300 plants) and are likely to suitable hosts.
Adult management in corn can be frustrating; however, prophylactic applications are normally not profitable. If beetles are actively feeding when silks are present, determine densities to make smart treatment decisions. Consider an insecticide if: three or more beetles per ear, AND silks have been clipped to less than an inch, AND pollination is not complete.
Most products registered for Japanese beetle management kill via contact or ingestion. Data from my lab shows good efficacy when adults are contacted directly by the spray. I don’t expect a long residual as additional adults move into the field. Continue to scout until pollination is complete. If a second spray is warranted, alternate chemistries to reduce the chance of developing resistance. The presence of sweet corn with staggered planting dates or late-maturing hybrids complicates Japanese beetle management. Weather conditions this summer have resulted in field corn at various growth stages. Beetles will continue to move to fields with green silks present throughout the summer.
Corn rootworm. Another common silk feeder is adult corn rootworm. Emergence of westerns and northerns is happening throughout Iowa. 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 fertilization of kernels. Consider an insecticide if: five 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 beetles per ear if the field has adequate moisture.
Grasshoppers. Grasshopper nymphs and adults also occasionally eat corn silks. They are usually found around field borders first and then move to the field interior later in the summer. Since grasshoppers are highly mobile, it is difficult to estimate densities. 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.
Corn earworm. The fourth potential corn silk feeder in Iowa is the corn earworm. The larvae are highly variable in 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 inside the tip of the ear. Older larvae will destroy developing kernels. Since killing the larvae once they reach the inside of the ear is almost impossible, I recommend monitoring adults. The University of Illinois has a website that summarizes how to properly time a foliar treatment for corn earworm, depending on the type of corn grown.
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