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5/28/2012 - 6/3/2012

Insect Activity Update

By Erin Hodgson, Department of Entomology

During the last week of May, I heard about a few insect sightings in Iowa. The first was a report by ISU Extension and Outreach Field Agronomist Brian Lang in northeastern Iowa. He saw a soybean aphid on VC soybean in his small research plot near Decorah on May 28. I wasn’t surprised, given that part of the state is where we usually first see soybean aphid every year. Winged females deposit a few nymphs per day in May and June. You may need a hand lens to see first instars on small plants (Fig. 1A). Often I confirm early-season colonies in soybean by looking for ants and lady beetles (Fig. 1B).


Figure 1. Soybean aphids start migrating to their summer host in May and June. A) Winged females deposit nymphs on expanding trifoliates. B) Colonies often will be tended by ants and can help with early-season detection while scouting. Photos by Brian McCornack, Kansas State University.


Japanese beetle adults are starting to show up in central Iowa. Kelly Gill, ISU entomology graduate student, saw them destroying rose buds near the library in Ames, Iowa (Fig. 2). We don’t typically see adults until mid-June in Iowa, but our mild winter has accelerated insect development. Japanese beetles have also been reported in other states like Illinois. They could be pests in corn and soybean later in the season, but keep an eye on their population densities in June.


Figure 2. Japanese beetles are strongly attracted to roses and other ornamentals. Eventually, they can be pests in corn and soybean. Photo by Erin Hodgson.

 

Black cutworm is still causing damage in some parts of Iowa. There are reports of significant stand loss due to feeding and clipping in young corn (Fig. 3). Mark Carlton, ISU Extension and Outreach field agronomist, reports some late-planted corn fields are being replanted in southeastern Iowa this week due to stand loss by black cutworm. I recommend scouting for black cutworm until corn reaches V5. Read this ICM News article for more scouting information.


Figure 3. Black cutworm can significantly damage young corn plants. A) Larvae often enter young corn plants above ground by making an entry hole. B) Cutworms curl up when disturbed. Photos by Jon Kiel.

 

A few other caterpillars are showing up in corn, too. First generation European corn borer eggs and young larvae can be found on corn leaves (Fig. 4). Older, non-traited corn should be scouted now to estimate densities. A dynamic threshold calculator.xls is available here. Tracy Cameron, an agronomist near Creston, also found a few corn earworm caterpillars in young corn. This is a little early to see corn earworm in Iowa, but most insects are showing up 1-2 weeks earlier than normal.


Figure 4. European corn borer moths have been collected in black light traps and with sweep nets around early-planted corn. A) Females will deposit first generation eggs on corn leaves. B) European corn borer larvae have a high predation rate; here a pirate bug is hunting down a young larva. Photos by Thomas Hillyer.

 

 

Erin Hodgson is an assistant professor of entomology with extension and outreach research responsibilities; contact her at ewh@iastate.edu or phone 515-294-2847.

Stunted, Yellowing or Wilting Corn: Could Nematodes Be the Cause?

By Greg Tylka, Department of Plant Pathology and Microbiology

There continues to be lots of questions about whether plant-parasitic nematodes are causing damage to Iowa’s corn crop. This varied group of microscopic worms has some species that cause damage to corn at very low population densities (numbers) and other species that are not harmful until population densities reach many hundred or more per 100 cm3 (a little less than a half cup) of soil.

It is common for several different species of plant-parasitic nematodes to occur in Iowa cornfields at low numbers.  But if numbers increase to damaging population densities, symptoms of injury will appear. 

What are symptoms of nematode damage to corn?

Nematode damage symptoms on corn include stunting of plants (Figure 1), yellowing of leaves, and mid-day wilting or leaf curling.  Roots may be stunted, fine roots may be lacking (Figure 1), and there may be discrete areas of black, dead tissue, called lesions, on the roots. Also, some nematodes cause roots to swell.

Figure 1. Two young corn plants stunted from nematode feeding (on right) compared to a healthy corn plant (on left). Note stunting of root systems as well as plant tops and also lack of fine roots.

 

When do symptoms of nematode damage appear during the season?

It would be very unusual for symptoms of nematode damage on corn to occur in the first month of the growing season - except in fields with very sandy soil. For fields with medium and fine textured soils, the aboveground symptoms caused by nematode feeding likely will appear more in the middle of the growing season.

When should fields be sampled?

Samples should be collected when symptoms of damage are seen.  Collect soil and root samples from near plants that are showing obvious symptoms of damage, but avoid sampling near plants that are dead or nearly dead.  There is no reason to collect samples from corn that is not showing some symptoms of possible nematode damage.

What type of sample should be collected?

Up until V6 growth stage of corn: collect soil and root samples.

  • Use a soil probe and collect cores that are at least 12 inches long.
  • Collect 10 or more soil cores to represent an area.
  • Collect soil cores from within the root zone of plants showing symptoms of damage. Combine, but do not mix, the soil cores and place them in a sealed plastic bag labeled with permanent marker.
  • Also collect, with a shovel, the root mass from four to six plants with symptoms of damage (Figure 2).  Take care not to strip off the smaller seminal roots.  The tops of the plants can be cut off and discarded.  Place the root samples in a sealed plastic bag labeled with permanent marker.
  • Protect the samples from physical jarring and from high temperatures (above room temperature).

Figure 2. Young corn plant collected to test for plant-parasitic nematodes in root tissue.

 

From V6 through R3 (milk) corn growth stage: collect soil samples.

  • Use a soil probe and collect cores that are at least 12 inches long.
  • Collect 10 or more soil cores to represent an area.
  • Collect soil cores from within the root zone of plants showing symptoms of damage. Combine, but do not mix, the soil cores and place them in a sealed plastic bag labeled with permanent marker.
  • Protect the samples from physical jarring and from high temperatures (above room temperature).

From R4 (dough) corn growth stage to harvest: sampling is not recommended.

There is not a good relationship between crop damage/yield loss and the number of nematodes in soil and roots once the corn crop reaches the R4 growth stage.  Therefore, sampling is not recommended after this point in the growing season.

Where to send samples

Several private laboratories and most land-grant university plant diagnostic laboratories process samples and determine the identities and numbers of plant-parasitic nematodes present. Here is a list of the university laboratories and their contact information.  At Iowa State University, the facility is:

Plant and Insect Diagnostic Clinic
Room 327 Bessey Hall
Iowa State University
Ames, IA 50011

The test for nematodes that feed on corn from the ISU Plant and Insect Diagnostic Clinic is called the complete nematode count. Samples sent to the ISU Clinic should be accompanied by a completed Plant Nematode Sample Submission Form (referred to on the ISU Extension Online Store as PD 0032) and a check for the $35 per sample processing fee.

Management options, if nematode damage is confirmed

If damaging population densities of nematodes are found, there is nothing that can be done to manage the nematodes and lessen the yield loss that will occur in the current growing season. Primary management strategies for future years are use of soil-applied Counter® 20G nematicide and/or seed treatments such as Avicta® and Votivo.

 

Greg Tylka is a professor with extension and research responsibilities in management of plant-parasitic nematode in the Department of Plant Pathology and Microbiology at Iowa State University. He can be reached at gltylka@iastate.edu or 515-294-3021.



This article was published originally on 6/4/2012 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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