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Extension Entomology

Category: Corn

Western Corn Rootworms

–by Dr. Jeff Whitworth

Western corn rootworm larval feeding seems to be almost finished around the north central part of Kansas, anyway. Root monitoring yielded mostly mature larvae– but also one that had just recently pupated (see fig 2). No western corn rootworm adults have been observed yet.

Figure 2 Two WCRW larvae plus new pupa  (by Cody Wyckoff)

 

CORN ROOTWORMS

–by Dr. Jeff Whitworth

Western corn rootworm (WCR) larvae are voraciously feeding on corn roots (see fig 1) and thus continuing to grow and develop as seen in fig 2.  The WCR larva on the right, in this photo, was collected on 3 June 2020, while the ones on the left were collected from the same field on 17 June 2020.

Figure 1: WCR emerging from root (Cody Wyckoff)

Figure 2: WCR larvae (Cody Wyckoff)

 

CORN—Herbicide Roundup

–by Dr. Jeff Whitworth

Just FYI: Many weed species have become resistant to the herbicide Roundup. However, as can be seen in this photo (fig 1.), non-Roundup Ready corn is still highly susceptible to Roundup while the Roundup Ready corn, which was sprayed at the same time, is still highly resistant!

Figure 1  Non-Roundup Ready corn in the foreground, Roundup Ready corn in the background (picture by Cody Wyckoff)

Corn—corn rootworm larva

–by Dr. Jeff Whitworth

Corn root sampling yielded the first western corn rootworm larva from north central Kansas on 3 June (fig 1). Many areas of continuous susceptible corn were sampled from fields that had significant corn rootworm adults in 2019. This was the only larva found but rootworm larvae will probably be hatching and start root feeding over the next 7-10 days.

 

Figure 1 Corn rootworm larva

 

Sorghum “Ragworms”

–by Dr. Jeff Whitworth and Dr. Holly Davis

Much of the sorghum around north central Kansas is at, or just coming into, the whorl stage.  As the leaves unfurl and grow out of the whorls, a pretty high number of plants are showing large holes that have been chewed in leaves.  These holes are from smaller larvae (most that we have sampled are corn earworms) that feed on the leaves while they are still furled. When leaves grow out of the whorl they have showy, ragged looking feeding that may cause concern.

Larvae sampled this week still have about one additional week of feeding within these whorls, then they will exit and crawl down the plant to pupate in the soil.  Larvae in the whorl are rarely worth spraying for four reasons: 1) by the time the leaves unfurl making feeding damage visible, most larvae have already accomplished most of their development and thus feeding, 2) insecticides usually can’t penetrate far enough down into the whorl to actually impact the larvae, 3) a general insecticide will kill most beneficial insects, and 4) ragged looking leaves during this stage have little to no effect on yield, and no, you cannot eliminate the next generation by spraying this generation.

 

Corn Update – Corn Rootworms and “Ragworms”

–by Dr. Jeff Whitworth and Dr. Holly Davis

Corn rootworm larvae continue to be very active in fields of continuous corn that have been planted to susceptible varieties.

Corn earworms have been feeding in north central Kansas corn for about a week now and signs of this feeding are now becoming visible as the leave start growing out of the whorl.  The small larvae may consist of corn earworms, fall armyworms, and/or armyworms, but all may cause the same type of ragged looking leaves, earning them the name “ragworms”.

 

 

This type of leaf feeding can be highly visible, and many plants can be impacted, but the data has always indicated there is little to no effect on yield.  In addition, the larvae are well sheltered within the whorl and thus insecticides only impact them when they exit the whorls to pupate in the soil.  And, by that time, all the feeding is completed anyway.

Corn Rootworms

— by Dr. Jeff Whitworth and Dr. Holly Davis

Western corn rootworm larvae have been detected for the 1st time this year in north central Kansas.  All were 1st instar larvae and most were still very small (recently hatched).  Thus, if you have any fields of three-plus year continuous corn planted with corn rootworm susceptible varieties, the rootworm feeding may become more and more evident over about the next three weeks.

For more information on corn rootworm management, please see Corn Rootworm Management in Kansas Field Corn: https://www.bookstore.ksre.ksu.edu/pubs/MF845.pdf

For more information relative to all corn pests, please see the KSU Corn Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF810.pdf

 

Revised Extension Publications

–by Jeff Whitworth and Holly Davis

 

Blister Beetles in Kansas, MF959, originally published by Robert Bauernfeind, Randall Higgins, Sue Blodgett, and Lowell Breeden in 1990 has been revised by Holly Davis and Jeff Whitworth. It is now available at: https://www.bookstore.ksre.ksu.edu/Item.aspx?catId=573&pubId=1549

 

Corn Rootworm Management in Kansas Field Corn, MF845, originally published by Randall Higgins, Gerald E. Wilde, and Timothy Gibb in 1995 has been revised by Holly Davis and Jeff Whitworth.  It is now available at: https://www.bookstore.ksre.ksu.edu/Item.aspx?catId=221&pubId=1502

Corn and Sorghum Update

— by Dr. Jeff Whitworth and Dr. Holly Davis

Chinch bug populations continue to increase at a very disconcerting rate.  Most corn throughout north central Kansas is far enough along in its development to tolerate large numbers of chinch bugs.  Smaller sorghum can still be seriously stressed by growing populations of chinch bugs, especially as the hot and dry conditions return.  For more information of chinch bug management in sorghum, please see the KSU 2018 Sorghum Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf742.pdf

 

One relatively mature fall armyworm larva was also detected.  Thus, in approximately 2-3 weeks there will be a new generation of fall armyworm larva ready to start feeding, most likely, on sorghum.

 

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