There are a few reports of corn leaf injury from redheaded flea beetle in Iowa. In at least one case, the defoliation was noticeable and the farmer was considering making an application of foliar insecticide. The fields had not tasseled yet and the farmer estimated there were five to six beetles per plant, says Erin Hodgson, Iowa State University Extension entomologist

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Corn rootworm larvae. Photo by Scott Bauer, www.ipmimages.org

Have you seen this redhead?

Hodgson explains, “Redheaded flea beetles are a leaf beetle in the family Chrysomelidae and are distantly related to bean leaf beetle and corn rootworms. The adults are about one-fourth of an inch long with a shiny, dark body and black forewings. As the name suggests, they have a red head (funny how entomologists come up with common names!) and antennae about half the length of the body. Like most flea beetle adults, the hind legs have large femurs that allow this insect to jump long distances.”

This is what the feeding injury looks like from redheaded flea beetles feasting on corn plants. Photo by Emily Wright, Iowa Soybean Association

Hodgson says this beetle feeds on a wide range of plants, including cabbage, soybean, corn and alfalfa. “They also are found on weeds (e.g., pigweed) and can be a nursery crop pest,” she notes. “Adults can feed on flowers and the upper or lower side of leaves, eventually creating skeletonized injury. Larvae develop in the soil, and feed on organic matter and roots.”

If you notice redheaded flea beetle while scouting for insects and disease, try to estimate percent leaf defoliation. The adults are skittish and move easily if disturbed, therefore making density estimates difficult. If you are quick, you could collect the adults in a sweep net while sampling soybeans to confirm the species. There are no treatment guidelines established for this insect, as it is considered a rare pest. However, if leaf defoliation exceeds 20% and plants are in the reproductive stages, it may be worth an insecticide application to protect yield.

4 other problems to look for in your fields this week

As the 2016 growing season enters July, both Iowa corn and soybeans are progressing nicely. At the same time, however, in some areas of the state, both crops are experiencing stress from low levels of soil moisture; and up until June 29, also suffering from excessively hot temperatures. Here’s a report from ISU Extension field agronomist Mark Johnson on what to scout for in your fields this week.

1) Check corn roots now for rootworm injury

Now is a good time to be digging corn roots and checking for corn rootworm feeding, says Mark Johnson, ISU Extension agronomist in central Iowa. Corn rootworm larvae go through 3 instars or growth stages as they develop. They are mostly in the first and second at of the end of June.

CORN ROOTWORM LARVAE: Now is a good time to dig some corn roots and check for corn rootworm. Also, scout soybean fields for aphids, watch for corn leaf blight on leaves, and make a note of which weeds are surviving in your fields.

* Carry a bucket of water with you. Johnson says you should carefully dig around the plant, dig up the root mass and the plant, and then carefully shake off the excess soil from the roots. “Cut off the top of the plant to within about a foot of the soil line to make handling the plant and the roots more manageable. Dunk the roots in the bucket of water and twirl the root ball in the water. Look for tiny corn rootworm larvae floating to the top. They are quite small, white to creamy colored, and brown on each end. These larvae are small enough that they can be easily overlooked.”  

* Which instar stage are the larvae in? Johnson says the first instar larvae make small scars on the roots and the scar site may be brown. The second instar stage is larger and it feeds on the roots and that feeding should be obvious. The third instar larvae is the largest and does extensive damage to the corn roots.

2) Scout fields for soybean aphid, count the little critters

In the summer, all soybean aphids are female and reproduce asexually, explains Johnson: “Eggs hatch inside the females and adults give live birth to nymphs. Nymphs start feeding shortly after being born. Optimal offspring production occurs at 82 degrees F. Under these optimal temperatures, an adult can produce nine nymphs a day. The range of acceptable temperatures for aphid survival is 47 degrees to 95 degrees. During persistently high temperatures soybean aphid reproduction declines in an effort to conserve resources.”

* Scouting and counting aphids. When scouting and counting, take current temperature and forecasted temperature into account, Johnson advises. “Remember at 90 degrees they are barely hanging on and reproduction is very low,” he says. “And above that, they are even dying off. Plus, the beneficial insects are eating them faster than they can reproduce. Even when temperatures are less, beneficial insects may prevent the population from climbing.”

He adds, “When you find 150 aphids per plant, there is no guarantee the level will ever get to 250 per plant and climbing on 80% of the plants. Look at the weather forecast. If the forecast is continued warm, check back in a week in that field. If the forecast is mild, check back in half that time.”

3) Watch for early signs of corn leaf diseases

Northern corn leaf blight (NCLB) is starting to show up in some fields in some areas of Iowa. “This fungus overwinters on corn crop residue,” says Johnson. “Favorable weather includes above normal rainfall, below normal temperatures, and greater than six hours of dew. The ideal temperature for this disease to thrive is around 68 degrees. Scout for this disease when the corn crop is nearing tassel stage (VT) to decide on whether to apply a fungicide at the VT to R1 growth stage period.”

4) Evaluate weed control, how well did it work?

Scout your corn and soybean fields now to see which weeds are surviving. How did your weed management program perform? The information you gather this year will help you prepare next year’s herbicide program. Consider ordering the ISU Weed Identification Field Guide found here at store.extension.iastate.edu/.