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5/24/2010 - 5/30/2010

Predicted 2010 Corn Rootworm Hatch

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

There were several reports of lightning bugs (fireflies) throughout the state last week. Some people correlate fireflies with corn rootworm larval hatch in the Midwest. Based on conversations with Marlin Rice, former ISU entomologist, and Mike Gray, entomologist from University of Illinois, they believe these events are unrelated. Instead, corn rootworm hatch predictions are more accurately based on temperature accumulations.

Research shows about fifty percent of corn rootworm larvae will hatch from 684 to 767 accumulated growing degree days (base 52 F). The map below displays the number of accumulated degree days from Jan. 1 to May 25 for Iowa. Corn rootworm hatch should be happening in the southeast part of the state. Other parts of the state should experience larval hatch in the next 7 days. 
growing degree days

Accumulated growing degree days (base 52 F) in Iowa from Jan. 1 – May 25, 2010. Fifty percent corn rootworm hatch is expected to occur between 684 and 767 degree days. This map was created by Iowa Environmental Mesonet, Department of Agronomy at Iowa State University.


Ideally, every corn field should be inspected for corn rootworm larvae after reaching 50 percent hatch. Non-Bt fields are most susceptible to larval damage and should be considered a priority. Continuous Bt corn fields with previous damage should also be scouted. Sample for larvae by digging up corn plants and washing the roots in a bucket; larvae should float to the top of the water. Sample corn plants in different areas of the field to estimate infestation levels. 
corn damage

Corn rootworm damage on the roots of a corn plant 


corn rootworm larvae

Larvae of the corn rootworm (Photos by Marlin Rice)



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.

Sampling Strip Trials for Corn Nematodes

By Greg Tylka, Department of Plant Pathology

Many people currently are asking how to collect nematode samples from strip trials of corn treated with new seed-treatment nematicide products. Avicta® Complete Corn and Votivo™ are the two products commonly being compared.

Avicta® Complete Corn, which was widely available to corn producers for the first time in 2010, is a combination of the Avicta seed treatment nematicide (active ingredient abamectin), a seed treatment insecticide and three seed treatment fungicides. Votivo™ is a biological seed treatment containing the bacterium Bacillus firmus that will be available in 2011.

There are no clear guidelines on how to collect samples for nematodes to compare treatments in strip trials. But here are some points to consider.

• The new seed-treatment nematicides provide early season, not season-long, nematode protection; duration of protection is not specified or known. Differences in nematode numbers due to the nematicides may occur early in the season (four to six weeks after planting), when the nematicides are having an effect, and not later in the growing season.

• There are many different species of corn nematodes and they vary in their generation times. For example, the root-lesion nematode can complete a generation in 30 to 50 days; dagger nematode will have only one generation in an entire growing season. The likelihood of detecting a difference in nematode numbers among treatments depends on what nematodes are present in the field and when the nematode samples are taken. It is unlikely that there will be any difference in nematode numbers detected among treatments in samples collected four to six weeks after planting if the predominant plant-parasitic nematodes in the field are those with long generation times like the dagger nematode.

• Populations of plant-parasitic nematodes are extremely variable in fields. Even when consistent sampling methods are used across strips, nematode numbers from the samples will be highly variable. To determine if a treatment is having any consistent effect on nematode numbers, it is critically important to collect numerous sets of samples across strips of treatments and to compare numbers from all samples. One set of numbers from one set of samples collected across strips will not provide any useful information.

• Soil and root samples should be collected to assess nematode numbers. Roots are needed because some plant-parasitic nematodes feed almost entirely inside corn roots during the growing season; their numbers can be deceivingly low in the soil. Whole plants can be submitted if plants are relatively small (see image); otherwise, root balls should be submitted.

Collect several plants or root balls from each sampling area or “plot” so nematodes can be extracted from root tissue.


• It is difficult to determine where to collect samples from strips. One suggested method is to collect 20 12-inch-deep soil cores and a few plants or root-balls from a four- or eight-row wide by 25- to 50-foot-long area of each strip. The sampled area will serve somewhat like a plot for each treatment. Collect sets of samples from “plots” in a straight line (or transect) across the various treatments in the strips. Ideal places to locate the transects in the field would include: 1) areas suspected of having damaging nematode populations, 2) areas where there is poor corn growth and 3) areas with uniform soil conditions (and hopefully uniform nematode populations).

• Soils with higher than 70 percent sand content can be infested with needle and sting nematodes, and these nematodes are difficult to recover in samples collected in the middle of the growing season. If needle and sting nematode infestations are suspected, collect soil samples from strip trials within the first four to eight weeks of the season. There’s no need to collect root samples for these nematodes as both species exist exclusively in the soil.


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

New Invasive Insect Confirmed in Iowa

By Erin Hodgson, Department of Entomology

The Iowa Emerald Ash Borer Team confirmed the presence of emerald ash borer (EAB) in Iowa on May 14. Four EAB larvae were found in one ash tree along the Mississippi River just two miles south of the Minnesota border in Allamakee County. The infested area is owned and managed by the U.S. Army Corps of Engineers and the U.S. Fish and Wildlife Service. The Iowa EAB Team is made up of members from the Iowa Department of Agriculture and Land Stewardship, USDA Animal Plant Health Inspections, USDA Forest Service, and Iowa State University Extension. The team has been scouting for EAB since 2003 using several detection methods.

Larvae are creamy white with a brown head and are legless. They have flattened, bell-shaped body segments (Fig. 1). Adults have metallic green forewings and copper red abdomens, and are approximately three- to five-eighths inch long (Fig. 2). Adults emerge from May to August and leave distinctive "D" shaped exit holes in the outer bark of branches and trunk (Fig. 3). Many other wood-boring beetles can be confused with EAB (e.g., bronze birch borer, two-lined chestnut borer, white-spotted pine sawyer, cottonwood borer). It is important to distinguish native wood boring beetles from EAB. Adults can be sent to ISU for positive identification.

EAB larvae

Figure 1


EAB adult

Figure 2


EAB exit hole

Figure 3


Why care about EAB?
The first EAB confirmed in the U.S. was in southeastern Michigan in 2002. Larvae kill North American ash species, including green, white, black and blue ash. Larvae feed on phloem just below the bark and create serpentine tunnels, or galleries, that eventually kill trees (Fig. 4). Adults will feed on leaves and create notches on leaf edges.
Several symptoms occur in EAB-infested ash trees.
• Vertical fissure on bark
• Serpentine galleries exposed if bark removed
• Galleries are filled with sawdust-like frass (excrement)
• Increased woodpecker activity
• Canopy dieback begins in top third of tree (Fig. 5)
• Shoots form at the base of the tree

Figure 4


Figure 5


Learn more about EAB
ISU Pest Management and the Environment    
EAB main site     
Ash tree identification guide    



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.

Watch for Early-Season Isopods

By Erin Hodgson, Department of Entomology

Throughout the winter I heard several presentations about isopods, a new early-season soybean pest in the Midwest. Isopods are terrestrial crustaceans most closely related to lobsters and crabs. They have many common names, such as woodlice, pillbugs, sowbugs, and roly-polies.

Although isopods have three major body regions (head, thorax and abdomen) like insects, it is very difficult to distinguish the regions because of the armor-like plates on the back. They have two pair of antennae (usually only one pair is obvious), seven pairs of legs and simple eyes (Fig. 1). Immatures look like adults except are smaller in size and proportion. Most isopods are grey or black and some have dark markings on the back. Adults are oval in shape and three-eighths inch long. In general, the back is convex and the underside is flat or concave.


Figure 1. Isopods have become early-season pests in neighboring states.


Isopods are omnivores that scavenge on dead and decaying plant or animal matter. They will also eat live, young plants such as fruit and vegetables. All life stages breathe through gills, so they must live in habitats with high humidity. No-till field crops can be attractive to isopods because they are protected under crop residue. Isopods are most active in the spring, often feeding at night. Isopods are skittish and some species curl up into a ball when disturbed (Fig. 2).

isopod rolled up

Figure 2. Some isopods curl up when threatened. 


Increasing acreage of no-till systems will encourage isopod development because high moisture is needed for survival. Some areas in Kansas and Nebraska have already experienced economically damaging levels of isopods that required replanting in soybean (Figures 3 and 4). When scouting for other early-season soybean pests, look for clipped or missing plants. Isopods can also feed on unifoliates and scrape off leaf tissue. Managing fields for isopods is difficult because seed treatments and foliar insecticides have not proven effective. Heavily infested areas could till every other year to minimize overwintering populations and reduce soil moisture in the spring.

Figure 3. Isopods have the potential to damage emerging soybean, especially in no-till systems.  Photo by Brian McCornack, Kansas State University


isopod damage

Figure 4. Typical isopod damage includes clipping the cotyledon at emergence. Photo by Brian McCornack, Kansas State University



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.

Crop Minute for Week Beginning May 24, 2010

Erin Hodgson, ISU Extension entomologist, talks about current insect activity which is minimal but will increase with the rise in temperatures. Alison Robertson, ISU Extension plant pathologist, discusses whether the early application of fungicides creates any benefit and offers a quick update of current field diseases. 

Listen to the mp3 files of the May 24 Crop Minute.

Timing Restrictions for Postemergence Herbicides in Corn

by Bob Hartzler, Department of Agronomy

Several factors may delay postemergence herbicide applications. When this happens it is important to be aware of restrictions on the label based on corn size or growth stage. The reasons for these restrictions vary among products, but may involve crop tolerance and prevention of residues in soil or grain. When making application to fields that received frost damage, remember to consider the number of leaves lost to frost.

Another consideration with delayed applications is the size of weeds. The likelihood of the herbicide providing control of weeds larger than specified on the label should be considered.

Several products allow applications to larger corn if a directed application is made. In these situations the sprayer needs to be set up so that nozzles direct the spray between the corn rows to minimize the amount of spray interacted by the crop and to improve coverage of weeds.  


Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities.

2010 Western Bean Cutworm Cooperators Needed

By Adam Sisson, Corn and Soybean Initiative and Laura Jesse, Plant and Insect Diagnostic Clinic

Western bean cutworm (WBC) is a corn pest that has been expanding it’s range across Iowa and into more eastern states. Beginning in 2003, Iowa State University set up a network of pheromone traps to monitor WBC range expansion and to provide moth emergence data to enhance scouting efforts. Pheromone trap cooperators included local seed corn dealers, private corn and soybean agronomists and others interested in pest management issues.

Cooperators are needed in 2010 to monitor traps and record data for the Iowa State University Western Bean Cutworm Monitoring Network.  Anyone interested in becoming part of the monitoring network this year should send an email to with their name, address, and email. The trapping program is explained in detail in the ICM article, Western bean cutworm: Pheromone trapping program.   The 2009 trap captures were reported earlier this year in the article Summary of 2009 Western Bean Cutworm Trapping Program.

western bean cutworm

Western bean cutworm adult moth

This article was published originally on 5/31/2010 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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