What's Up With Crop Insects and Diseases in Early July in Iowa?

Armyworm, Japanese beetles, potato leaf hoppers, slugs on corn and soybeans are some of the insect pest questions ISU Extension specialists are receiving. They are getting disease-related questions, too.

Published on: Jul 6, 2010
What crop insects and diseases are generating questions from farmers in Iowa as we move through the first week of July 2010? Here's an update from ISU Extension field agronomist Brian Lang at Decorah in northeast Iowa.   

* Armyworm damage. "I got a call last week about more Armyworm damage, but we should be at the end of the first generation activity," says Lang. "At the end of the first generation, the larvae pupate, develop into moths and lay eggs back into grasses.  Entomologists have commented that the preferred hosts for the moths' egg laying is on fine leaf grasses such as in pastures, CRP and grassy hay fields. Entomologists do not think the moths would chose corn outright over fine leaf grasses unless there are few other options available (i.e. under drought conditions or some other unique situation).  However, the moths can lay eggs directly into cornfields, and you can't guarantee this won't happen."

Each generation of armyworm lasts about five to six weeks. "We encountered the first larval feeding complaints as much as five weeks ago, and later complaints up through last week," he says. "Based on this, the second generation activity should be quite spread out. In general, we would anticipate most activity from mid-July through mid-August. With large first generation problems, we have the potential for a serious outbreak of 2nd generation activity, and should not be taken by surprise. Hopefully some beneficial parasitism predators can reduce some of the armyworm population."

Assuming egg laying occurs mostly in fine grass areas, most scouting should be accomplished by checking cornfield edges adjacent to pastures, grassy hay fields, etc. A weekly ATV run around the grass field boundaries of your corn fields will accomplish the job of scouting in this case.

* Japanese Beetles. These and related "June bug-type" beetles are common in July. Farmers should be concerned only with the version that are the Japanese beetles, sometimes feeding on soybeans and corn silks. The feeding on soybeans to any significant degree is usually only seen south of Hwy. 3 in northeast Iowa, and generally worst around Hwy. 30 in east-central Iowa.  "However we have found these beetles as far north as Winneshiek County affecting ornamental plants," says Lang.

Homeowners typically have greater concerns of this pest than farmers.  Japanese beetles love to feed on ornamental plants and trees.  A recent ISU Extension ICM news article provides photos, a description and comments about the different types of "June bugs" and whether they are a concern for farmers.  Go to www.extension.iastate.edu/CropNews/2009/0630hodgson.htm

The general threshold for Japanese beetles and other leaf feeding insects is to treat soybeans in the reproductive stages when there is 20% or more defoliation, or on corn silk clipping with three or more bugs per ear.  A photo of different levels of defoliation is available at www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/soydefoliation.html.

* Potato Leafhopper. You should continue to scout alfalfa fields for this insect pest through August. Photos, thresholds and treatment information is available at www.extension.iastate.edu/CropNews/2009/0615hodgson.htm

* Slugs in Corn & Soybeans. In case you were wondering what is causing the scraping-like damage on corn and soybean leaves, it is likely to be from slugs.   This is common in a number of fields, especially fields with higher amounts of crop residue. "I have not seen anything anywhere close to treatable levels of damage," says Lang. "They do well in wet weather, feeding at night and hiding in the soil during the day. As we dry out with less rainfall and warmer weather, the slug damage should subside. The following article goes into detail about the pest, crop injury and suggested treatments:


* Soybean Aphid. Aphids this year showed up in northeast Iowa around the first week of June, as usual. They can now be found in most soybean fields if you look hard enough, says Lang. The rainy weather over the last month greatly held down population development. Last week the annual ISU research plots near Decorah had aphids on one in 20 plants (5% infestation), and averaged only 0.1 aphids per plant (eight aphids on 80 plants scouted). The largest aphid colony on an individual plant was only four. Starting June 5 Lang will e-mail to people who are on his online newsletter mailing list, his weekly findings from the research trial along with the eight-year summary of previous aphid development from this same location.

Keep an eye out for these crop diseases during July

* Brown Spot in Soybeans. "Following the very rainy stretch of weather in June we can find higher levels of Brown Spot (also called Septoria Leaf Spot) than normal," he says. "This is something that should be watched.  As long as this disease remains in the lower canopy it will not significantly affect yield. However, if the disease advances up the canopy during the early reproductive stages of the plant, it may warrant a foliar fungicide treatment, typically timed at R3 stage soybeans." For photos and additional information of these diseases, go to www.ipm.iastate.edu/ipm/icm/1996/7-8-1996/soyfoldis.html

* White Mold Prevention in Soybeans. Most soybean fields are now at R1 stage of growth. Farmers with fields that have a strong history of White Mold may try a preventative measure of either a fungicide specific for White Mold or a herbicide like Cobra which can stimulate the plants own defense mechanism against White Mold. University research suggests that Cobra is a viable option in suppressing white mold (as is Phoenix herbicide which has the same active ingredient as Cobra). The Cobra label calls for rates of 6 to 8 ounces per acre for white mold suppression, although X.B. Yang, ISU Extension plant pathologist, obtained white mold suppression in trials in the mid-1990's with Cobra application rates as low as 2 to 4 ounces per acre.

Several fungicides (Topsin, Domark, and Endura) are labeled for control of white mold on soybeans. Consider the following factors before you decide to spray a fungicide for white mold on beans: 1) proper application timing is essential; applications should be made at flowering to protect senescing flowers from infection, 2) Topsin and Endure suggest an R1 stage application while Domark suggests an R3 application, and 3) spray coverage is also essential; sprays must penetrate the soybean canopy in order to protect the soybean flowers.

White mold is favored in fields that have a closed soybean canopy -- and a known field history of the disease, cool and wet springs and fields with highly susceptible soybean varieties. Fields less favorable for White Mold and less likely to benefit from a preventative fungicide treatment are those with:  more tolerant soybean varieties, higher temperature environments, fields where canopy closure is slow, and those fields that don't have a previous history of white mold.

* Foliar Fungicide Applications for Corn.  "On average, the best time to apply a foliar fungicide to corn if such a treatment is needed, is from tasseling through silking stage (VT-R1).  However, based on ISU research trials, in years when tasseling occurs quite early, as it may in some fields this season, the more profitable responses to fungicide applications were those applied closer to the R1 to R2 growth stage of corn. In other words, in most years the ISU research time of application trials with foliar fungicides found the most profitable responses were with applications made in mid-to-late July.

As to whether or not to make the decision about the need to apply a fungicide, Alison Robertson, ISU Extension Plant Pathologist, suggests the following:

1)  If the corn has good genetic resistance, don't worry, be happy.

2)  If genetic resistance is moderate or poor, then just prior to spraying time consider the following:

    a)  Go to several locations in each hybrid in each field and

    b)  Inspect several plants at each location, checking the ear leaf and the next three leaves down.

    c)  For a hybrid with poor resistance:  if you find any lesions on those leaves, have a fungicide applied.

    d)  For a hybrid with moderate resistance:  also consider if you only found a few vs. many lesions to decide if a fungicide would be applied.

All replicated research trials from both ISU and the Iowa Soybean Association from 2007 to 2009 show profitable responses of foliar fungicide applications to corn 39% of the time. So 61% of the time it was not profitable. It is important to use parameters such as those above to help decide whether or not to apply a fungicide to increase your probability of an economic return. "FYI, on disease scouting, we are finding some early signs of eyespot on corn this year, particularly in fields that are corn following corn," says Lang.

* It's Time to Sample for Corn Nematodes. Most corn nematode populations peak in early July. This is considered the best time to sample for the pest. The method is NOT the same as for sampling for soybean cyst nematode (SCN). That's because with corn nematodes you are collecting the worm-like nematodes, not cysts or eggs as you collect with SCN. The laboratory testing process of the samples is also a bit more difficult for corn nematodes, so the fee for a corn nematode sample ($30) is more than for a SCN sample ($15). 

The procedure for collect a corn nematode sample is:

1)  Use the Sample Submission form from ISU www.extension.iastate.edu/Publications/PD32.pdf.  On the back of this form under "Tests Available and Fees" you select "Complete nematode count $30.00".

2)  Collect 15 to 20 soil cores taken at 12-inches deep and cored at an angle under corn plants.

3)  Also collect and submit two or three root balls with the soil core sample.

The results you get back from ISU's testing lab will list the different nematodes found, their numbers and what types and numbers of nematodes might be of concern for you in the field that was tested.