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6/7/2010 - 6/13/2010

Crop Minute Week of June 7

In this Crop Minute, Bob Hartzler, ISU Extension weed scientist, says now that corn has been sprayed with post-emerge herbicide, it is time to turn attention to soybeans. Water hemp and giant ragweed are two species that soybean producers need to keep an eye on as they spread rapidly and have shown resistance to glyphosate.

Stalk Borer Migration Is Beginning

By Erin Hodgson, Department of Entomology and Adam Sisson, Corn and Soybean Initiative

Warm June temperatures have accelerated insect growth and development, including stalk borers. Start looking for migrating stalk borer larvae when 10 percent movement is predicted. The 2010 forecast for 10 percent stalk borer movement in Iowa is starting this week (Fig. 1). 

Fig. 1. Predicted 2010 dates of 10 percent stalk borer migration.


Stalk borers have a wide host range, with larvae feeding on over 175 different plant species. In the spring, young larvae are commonly found on brome grass and giant ragweed. Stalk borer larvae hatch around 500 growing degree days (base 41 F) of accumulated temperatures. Older larvae can quickly outgrow grass and weed stems and begin moving to corn and occasionally soybean around 1,400 growing degree days.

The larvae are not very mobile and typically only move into the first four to six rows of corn. Look for new leaves with irregular feeding holes or for small larvae resting inside the corn whorls. Larvae will excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk. Exposed larvae can be killed with a foliar insecticide treatment, but tunneling larvae are not susceptible. Young corn is particularly vulnerable to severe damage, but plants are unlikely to be killed once reaching V7 (seven true leaves).

Regular weed management within and around corn fields is crucial for reducing stalk borer populations. Stalk borers can cause damage throughout a field if grasses and broadleaf weeds are not controlled in a no-till system. Just killing weeds in a highly infested area will force larvae to feed on corn - this practice could significantly reduce a stand.

To prevent stand loss, scout and determine the percent of infested plants. The use of an economic threshold, first developed by ISU entomologist Dr. Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae. For treatment threshold guidelines and labeled products, see a previous ICM article.



Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847. Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at or by calling (515) 294-5899.

Maintaining Pesticide Applicator Certification During Military Active Duty

By Brent Pringnitz, ISU Agri-business Education Program and Kristine Schaefer, Department of Entomology

Over 3,000 Iowa National Guard troops are scheduled to be deployed later this year, the largest single deployment from the state since World War II. This will likely include individuals who hold either a private or commercial pesticide applicator certification.

Any service member called for active duty should keep a copy of their DD 214 form and contact The Iowa Department of Agriculture and Land Stewardship (IDALS) Pesticide Bureau when they return from service to determine certification status. IDALS will adjust the certification records as needed and will send a letter to the applicator, and in the case of commercial applicators to the company they are employed with, letting them know what continuing instructional courses they need to maintain certification.

IDALS does not need to be contacted prior to deployment. IDALS and ISU’s Pest Management and the Environment program maintain records of recertification and attendance at yearly continuing instructional courses for private and commercial pesticide applicators. Questions regarding applicator certification status should be directed to Tammy Green at the Pesticide Bureau, IDALS, by calling (515) 281-5601 or



Brent Pringnitz is the coordinator of the Iowa State University Extension Agribusiness Education Program. He can be reached at 515/294-9487 or by email at Kristine Schaefer is a program specialist serving on the Pesticide Management and the Environment team. Schaefer can be reached by email at or by phone at (515) 294-4286.

Threecornered Alfalfa Hopper Found in Iowa

Erin Hodgson, Department of Entomology

Last week a consultant near Creston, Iowa identified an interesting insect feeding on soybean. With the aid of a photo, we were able to confirm it as the threecornered alfalfa hopper, Spissistilus festinus (Hemiptera: Membracidae). This insect is native to the southern U.S. and South America and can be an economic pest of soybean in southern growing regions. ISU entomologists cannot recall a previous detection in Iowa.

The threecornered alfalfa hopper has three life stages (egg, nymph, adult) and can have two to four generations a year in the southern U.S. Nymphs are initially white, but turn lime green or brown as they mature. Nymphs have 12 pairs of hairy spines along the back and protruding "tail" at the end of the abdomen (Fig. 1). In addition to the characteristic spines, this hopper has a triangular, wedge-shaped head. Nymphs go through five instars in about 9 to 18 days, and gradually develop wing pads. The Mexican bean beetle larva (Fig. 2) can be confused with the threecornered alfalfa hopper because it has spines and is bright green, but lacks a tail and has an oval body.  

Fig. 1. Threecornered alfalfa hoppers nymphs are lime green with obvious spines along the back. Photo by Charles Lewallen. 

Fig. 2. Mexican bean beetle larvae should not be confused with this hopper. Note rounded head, oval body, and lack of a tail. Photo by Eugene L. Nelson.

Adults are about 1/4 inch long and are taller than wide. The first segment in the thorax is greatly expanded and covers the abdomen (Fig. 3), hence the name "threecornered." Adults are also lime green, but the males have a red or orange stripe along the "shoulders" (Fig. 4). Threecornered alfalfa hoppers overwinter as adults under pine trees. 

Figs. 3 and 4. Adult threecornered alfalfa hoppers. Female left (Photo by Clemson University Extension) and male with red shoulders (Photo by Johnny N. Dell).


Threecornered alfalfa hoppers prefer legumes such as alfalfa, soybean and sweet clover. The nymphs and adults are phloem feeders, and puncture stems and petioles. Older nymphs typically girdle plants a few inches above the soil by creating a series of punctures around the stem (Fig. 5). Girdling restricts water and nutrient uptake and photosynthesis. Brittle stems break off weeks after attack, often following a wind event. Young soybean plants (up to V7) can be killed by vigorous hopper feeding, but adjacent plants often compensate by filling vacated row spaces. Threecornered alfalfa hoppers move up the maturing plant, feeding on lateral branches. Small pods may be aborted due to feeding punctures during the reproductive stages. 

Fig. 5. Typical feeding causes girdling around the soybean stem. Photo by David Adams, University of Georgia.


This hopper rarely causes economic damage because soybean plants can compensate greatly. Reduced tillage areas and late planted fields are more likely to get infested. When plants are less than 12 inches tall, visually look for hoppers and initial girdling symptoms. As plants get larger, use a sweep net to capture adults and nymphs. Consider making a foliar insecticide application if 10 percent of plants (less than 12 inches tall) are infested or if 50 percent of plants are girdled. For reproductive soybean, consider treating when an average of one hopper per sweep is reached.



Reference: Handbook of Soybean Insect Pests, L. G. Higley and D. J. Boethel (eds.), Entomological Society of America, 1994.

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847.

Start Checking Soybean Roots for SCN Females

By Greg Tylka, Department of Plant Pathology

Soybean cyst nematode (SCN) is a serious pest concern for soybean every season in Iowa. Current SCN management options include soil-applied nematicides and resistant soybean varieties. All are implemented at the time of planting. However, effective scouting during the growing season is the key to successful, integrated management of SCN.

Many Iowa fields are known to be infested with the SCN and many more are suspected of being infested. Research funded by the soybean checkoff indicates that about 70 percent of Iowa’s fields may be infested with SCN. Symptoms of damage from SCN on soybean could include stunting of the plants and yellowing of leaves. But more than 30 percent yield loss can occur without the appearance of any symptoms.

SCN females can be seen with the unaided eye on roots of soybean plants. So digging roots and looking for SCN females is an effective way to scout fields for the presence of this pest. The SCN life cycle takes about 24 days to complete under ideal conditions (including soil temperatures of about 78 F).  But depending on spring rainfall and soil temperatures, the first SCN females may not appear on soybean roots until five or six weeks after planting.

To check for the presence of SCN females on roots, carefully dig roots from the ground and then gently shake or crumble soil away from the fine roots and look for the small, white dots that are the SCN females. Checking for SCN by looking for adult females on soybean roots was covered in a previous ICM News article.

Digging roots to check for SCN females.

In addition to observing roots for SCN females to determine if a field is infested with the pest, assessing how many SCN females are on the roots of SCN-resistant soybeans gives an indication of how well the resistance is controlling reproduction of the nematode. There should not be more than 10 to 20 white SCN females on the roots of an SCN-resistant soybean variety if it is controlling reproduction of the nematode well. See “Check SCN-resistant Soybean Roots for SCN Females" for more details.



Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

This article was published originally on 6/14/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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