The most common and noticeable injury found in a soybean field is defoliation. The culprit are insects feeding on leaves.
These leaf-feeding bugs usually strike twice during the growing season — once in spring at the early vegetative stage, after plant emergence — and then in summer, at the reproductive stage.
Jeff Whitworth, Kansas State University Extension entomologist, says there are five defoliators that have presented problems for soybean growers over the last two years. He warns farmers to scout fields to be on the lookout for the following insects again this year.
1. Bean leaf beetle. “Bean leaf beetle is the No. 1 pest throughout the soybean growing region of the United States, year after year,” he says. They have two different color phases — red and tan — and can be confused with other beetles, like the southern corn rootworm and lady beetles.
Whitworth says the bean leaf beetle has six black spots, a black band on the outside of its wing cover and a black triangle behind its head.
“Adults overwinter in alfalfa and CRP [Conservation Reserve Program] fields under the leaf,” he says. “When soybean plants emerge as seedlings, we can find adults there.” Typically, they appear in April.
What worries Whitworth is the females laying eggs around soybean plants. The first-generation adults chew oval holes between leaf veins. The second-generation feed on pods. It is the second generation affecting the marketed product, he adds.
Adding insult to injury, the bean leaf beetle may also transmit the mottle virus.
While soybean plants can come back from defoliation, Whitworth says that if defoliation approaches 20% in the reproductive stage, treatment may be warranted if insects remain actively feeding.
2. Green cloverworm. These insects caused significant damage to soybeans in the Midwest during 2016 and 2017. “Last year, the majority of larvae pupated and emerged as adults,” Whitworth explains.
Adult moths do not overwinter in states like Missouri or Kansas. They fly in. “Just because you didn’t have a problem last year doesn’t mean you won’t this year,” he adds.
Green cloverworm populations have been increasing in soybean fields. Farmers may confuse this caterpillar with a looper, but it differs by having four abdominal prolegs, in contrast to three for loopers. It also has a narrow white stripe on both sides of its body.
Green cloverworms feed on the underside of leaves. Typically, Whitworth says, a fungus attacks these worms, providing natural control. The infected worms appear sick and stop feeding. The green worm will turn white if infected. Death occurs in two days.
Growers may want to accept a little damage to the plants — if not widespread — to allow the fungus to take over, Whitworth adds. Otherwise, the economic threshold is 20% leaf defoliation, and 10 to 15 more half-grown larvae present per foot of row.
3.Thistle caterpillar. The thistle caterpillar was out in force during the 2017 growing season, causing significant defoliation. Whitworth notes that the damage was localized in fields.
This caterpillar is brown-black in color, with yellow striping on both sides. It has spiny hairs on its body that gives it a prickly appearance. It measures 1¼ inches long.
These pests tend to feed along the field edges. Once a thistle caterpillar hits chrysalis stage, the damage is done — so look for a cocoon.
If there is not much damage, treatment is not needed. If the damage reaches 20% defoliation, consider applying an insecticide.
As an adult, the thistle caterpillar morphs into the butterfly known as the painted lady.
4. Japanese beetle. Prevalent in states like Iowa, Indiana and Illinois, over the past few years the Japanese beetle made a name for itself in Missouri and Kansas, too.
Adult beetles are a half-inch long. and they’re hard to miss with their wing covers colored metallic green and bronze. This beetle appears in fields from July through September.
The Japanese beetle makes soybean leaves look more like a skeleton by stripping away the tissue between the larger leaf veins. The damage can be mistaken for Mexican bean beetle feeding; however, the Japanese beetle destroys more of the smaller veins.
The insect can cause economic damage in soybeans but not as much as in corn, This is because in corn, it is feeding on the tassel, which affects ear fill.
5. Grasshoppers. Grasshoppers are mainly problems for soybean growers during drought years. During dry years, they move into row crops for a food source.
They will be seen along field borders first, Whitworth says. Grasshoppers feed on soybean leaves. However, if it is a dry year, they may also feed through the pod directly to the seed.
Treatment of soybean may be necessary at 40% or more defoliation before soybean flowering, 15% from flowering to pod fill, and 25% from pod fill to harvest.
Whitworth says soybean farmers should remain vigilant in scouting for these defoliating pests this growing season in order to protect their yields.