With a mild winter and a warm start to summer, insects are getting a head start this year. Farmers are already finding Japanese beetles in scattered soybean fields in southern and eastern Iowa. Some growers are adding insecticide if they are planning a second herbicide pass across bean fields that have beetle infestations.

Iowa State University Extension field agronomist Virgil Schmitt, located at Muscatine, found Japanese beetles along Interstate 80 in eastern Iowa in late June. He began getting calls about Japanese beetles 10 days earlier in counties to the south. “In soybeans, you need to evaluate the entire soybean plant for defoliation,” he advises. “Most defoliation occurs on the top leaves, so it’s easy to overestimate damage.”

In corn, Japanese beetles can damage silks and interfere with pollination. For that reason, this insect is more likely to be of economic concern in corn than soybeans, says Erin Hodgson, ISU Extension entomologist. Her advice is to consider using a foliar insecticide during tasseling and silking if you have three or more beetles per ear, if silks have been clipped to less than a half-inch and if pollination is less than 50% complete.

Insects to scout for in corn, soybeans
Soybean aphids have also made an early appearance this year, says Hodgson. “Aphid numbers are low so far, but some have been found in fields two to three weeks earlier than usual. If aphids get a head start, they could reach economic threshold populations in some fields. This is definitely a year to be scouting.”

Corn rootworm remains the No. 1 pest that steals corn yield in Iowa. Scouting corn for silk clipping by adult corn rootworm beetles is recommended, especially in fields of continuous corn. Later on in July, scouting corn for root injury by the larva is recommended. Even if you’ve planted corn hybrids with a Bt trait to protect against rootworm, of if you applied a soil insecticide at planting, you should still check corn roots to see if there are problems and to evaluate control.

On soybeans, bean leaf beetles are another insect to watch for, says Brian Lang, ISU Extension field agronomist in northeast Iowa. This insect has two generations per year in Iowa — first generation peaking in early July, and the second generation peaking late in the summer. Lang provides the following crop insect scouting observations and links to more information:

• Bean leaf beetle. If first-generation bean leaf beetle (BLB) were to be a problem, they would be noticed by now, as their first generation population should be peaking. A sweep net can be used in soybeans to scout this pest as described in the ISU article A New Bean Leaf Beetle Threshold Calculator is Created. Late August to early September is the peak population window for second-generation BLB, but no activity for first-generation means little activity for second-generation.

• Corn rootworm lava. Larval feeding on corn roots continues in July. It generally concludes around late July, which is when ISU evaluates its rootworm product trials. As you troubleshoot fields in July for other problems, keep this pest in mind. “I always travel with a bucket and water jug this time of season to soak root digs,” says Lang. Conducting Root Digs for Corn Rootworm in Corn offers

photos of larva injury and a description of the university’s zero to 3 node injury scale.

• European corn borer. For fields planted to corn hybrids that don’t have Bt traits for European corn borer (ECB) protection, now is the time to scout the later-planted corn for this pest. All corn is protected up to about 17- to 21-inch extended leaf height by a naturally occurring compound in corn called DIMBOA. Once corn grows beyond the 17- to 21-inch extended leaf height, fields that are not protected by Bt traits are scouted for this insect, identified in the Handy Bt Trait Table.

Once newly hatched corn borer larvae enter the stalk, no insecticide application can help. Scouting tips and a link to and economic threshold spreadsheet is at ISU online. Photos of initial feeding by ECB, and many other facts, are available at the ISU Extension Store.

• Leaf defoliators in soybeans. The most common are bean leaf beetle, caterpillars (green cloverworm, loopers, thistle caterpillar), grasshopper and Japanese beetle. Regardless of which ones, a threat is defined as greater than 20% defoliation with soybeans in the reproductive stages. Estimating Soybean Defoliation provides an illustration to help you define percent defoliation. Also, the Soybean Insect Guide provides a list of common insects, photos, identification and management tips, including the pests listed above.

• Soybean aphid. Populations that Lang is finding now continue to be similar to activity found the last few growing seasons — still low levels at this time. Just as in the past few seasons, mid-July is the time to start scouting weekly. “Be aware that while soybean aphid activity tends to remain low until August, we have had some hot-spot flare ups in late July in various regions of northeast Iowa,” he says. “In recent seasons, I found high-population fields in July north of Cresco, south of Decorah and west of Strawberry Point. No rhyme or reason that I can figure out. In fact, one field that I’m currently scouting has a small hot spot (less than 100 square feet) within the field with an over-threshold population. But the rest of the field is about 50% infestation, with an average of 11 aphids per plant.”

The threshold is still greater than 80% of plants with aphids and an average population of 250 per plant, he says. The Speed Scouting method is preferred, as it is easier and faster. With Speed Scouting, you do not count a plant as infested until it has 40 or more aphids on a plant. You get to 40 aphids on a plant, you quit counting and move to the next plant. If you walk into a field and find less than 7 out of 11 plants with 40 or more aphids, you’re done scouting that field for a week.

• Potato leafhopper. “This warm, drier weather we’re having in July is very favorable for potato leaf hopper populations — a pest of alfalfa,” notes Lang. Scout for PLH in alfalfa through August. Scouting and management tips are available at ISU Extension. The only way to scout PLH is with a sweep net. The net should be 15 inches in diameter. A couple of resources that offer a quality sweep net are BioQuip and Forestry Suppliers Inc.