Watch out for these silk feeding insects

July 9, 2024 11:02 AM
Blog Post

Corn is beginning to tassel and there are fields with small ears, which means pollination is just around the corner. Corn pollen is highly attractive to many insect pests, so it is a good idea to assess pollination rates once reproductive development starts. This could be especially complicated in areas or fields with staggered planting dates and growth stages of corn. Some of the main silk-feeding insects in Iowa field corn are Japanese beetle, corn rootworm adults, and grasshoppers. All of these insects are already active in fields across Iowa and ready to feed and clip silks. Late-planted and late-maturing cornfields should be of particular concern this year.

Japanese beetle

You may have already noticed Japanese beetles in your area, but they began emerging a few weeks ago and are looking for things to eat. They are attracted to green silks and often mate and feed in clusters on plants. Japanese beetles and their injury are typically more abundant near the field edge, but adults are highly mobile and constantly move around the landscape to feed on various plant species.

Japanese beetles may interfere with pollination, but it is important to remember a few things: feeding is worse at the field edge, and silks must be severely clipped throughout an entire field to result in poor pollination of corn plants. Insecticide treatments are recommended when three criteria are met: there are three or more beetles per ear, silks have been clipped to less than one inch, AND pollination is not yet complete.

Management of adult Japanese beetles with insecticides is complicated. Research from the Hodgson lab suggests that adults can be killed with contact insecticides, but beetles continue to move into the field after a few days. Continue to scout until pollination is complete, and alternate insecticide modes of action if a second application is required. Variable planting dates, hybrid maturities, and sweet corn in the area may complicate management as beetles will move around to find fresh green silks.

Japanese beetles feeding on corn silks
A cluster of Japanese beetles feeding on green silks. Photo by Ashley Dean.

Corn rootworm adults

Another common silk-feeding insect is corn rootworm beetles (western corn rootworm and northern corn rootworm), which have begun emerging across Iowa. Similar to Japanese beetles, corn rootworms are attracted to pollen and silks and will move around the landscape to encounter fresh food.  Like Japanese beetles, silk clipping is often worse near the field edge, beetles are highly mobile across the landscape, and insecticides are recommended if three criteria are met: there are five or more beetles per ear, silks have been clipped to less than one inch, AND pollination is not yet complete. In fields with adequate moisture, the threshold can be increased to 15 beetles per ear.

corn rootworm beetles feeding on corn silks
Corn rootworm beetles feeding on green silks. Photo by Ashley Dean.


Grasshopper adults and nymphs may also clip corn silks, but unlike Japanese beetles and corn rootworm beetles, grasshoppers are equally happy to feed on leaves. Grasshoppers are more likely to impact corn at the field edge and are highly mobile insects as well. There is no established threshold for grasshoppers feeding on silks, and it is difficult to sample for them to estimate densities. Keep an eye on grasshopper populations, silk feeding, and pollination success to determine whether an insecticide might be necessary. Oftentimes, a border treatment is a cost-effective decision.

grasshopper on a corn plant
Grasshopper on a corn plant. Photo by Ashley Dean.

Continue to scout for ear-feeding insects!

As ears continue to develop throughout the summer, keep an eye out for insects feeding on the ears. This new resource will help to identify different insects that might be injuring ears. Although the JIPM article is focused on injury without the pest, we stress the importance of scouting during the season to know for certain who caused injury to corn ears.


Ashley Dean Agriculture and Natural Resources Extension Specialist II

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, the Regional Corn Rootworm Monitoring Network, and the Iowa Pest Alert Network. She also develops educational resources for field crop pests in Iowa and ...

Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...