Pest impacts soybean harvest

SIKESTON -- Once considered not much of a problem, a soybean pest has become just that for Southeast Missouri growers this year.

Dectes stem borer --a small, gray, long-horned beetle that attacks soybeans -- has popped up in more fields this year than ever before.

"A lot of people have written this off as a low incidence insect. This year it's not," said Jeff House, University of Missouri Extension agronomist for New Madrid and Pemiscot counties. "I've seen it be an economic impact but never to this scale."

Dectes has been spotted as far north as Cape Girardeau and even in neighboring states like Arkansas, Tennessee and Kansas.

"It was something that was a pest in the late '60s to mid '70s and went away for a while," said Dr. Kelly Tindall, research entomologist for Missouri Delta Center in Portageville.

The pest resurfaced after years of growers using reduced-till or no-till practices, Tindall said. Because there's a vacancy of research over the years, experts are in process of relearning about the pest, she said.

House said the pest first showed up in the area around 2001 or 2002 when it was found in a field west of Portageville. It caused enough damage to affect the yields, he said, adding it was an isolated case.

"I never really thought it was a problem -- and that's the general consensus," House said. "About three years ago, it really seemed to take off, and the number of fields we're finding adult borers in has steadily been increasing."

And over the last two years, the pest has become more prevalent, House said. "I dare you to find a field this year that is not infested. The majority will be 85 percent infested," House said.

Dectes usually begins to emerge in late June and peak in July, but this year, it began showing up in May -- and in large numbers, both House and Tindall said.

"A lot of farmers have them (in their fields) and don't realize it because Dectes are very secretive, very hard to catch," House said.

An infested soybean plant will look normal because the pests feed off the soybean stalk from inside the plant, House said. The pests reduce the plant's ability to move water and soil nutrients to developing pods and seeds, which can result in yield losses in heavily infested fields.

To determine if a soybean plant was infested, take a sharp pocket knife and split the stalk to the root, House said.

"The majority of fields I find (this year) have problems with Dectes and over half of those fields have major problems," House said.

For the week ending Sept. 30, Southeast Missouri farmers harvested 22 percent of soybeans planted with 49 percent of soybeans matured. They rated the crop as 26 percent very poor, 30 percent poor, 27 percent fair, 11 percent good and 6 percent excellent, according to National Agricultural Statistics Service.

Forty percent infestation means about a 10 percent yield loss, and some of the fields in the area had 95 percent infestation which means about a 30 percent yield loss, Tindall said, adding these are "guestimations" and may be higher or lower.

The drought and heat in August didn't help either the soybean crop either, House said, adding all contributed one to the other.

"The plant can handle a drought, and it can handle Dectes, but it can't handle both at such extremes as this year," Tindall said.

House and Tindall, along with other agriculture experts, are working to figure out how to manage the pest.

"We're trying to stay ahead of the curve," House said. "We want farmers to be aware of what's going on."

He continued: "They understand it's out there and that we're working on it. There's a lot of impatience and frustration."

Currently no insecticides are specifically labeled to control Dectes, and it is extremely difficult to properly time insecticide applications because Dectes can appear early and late in the growing season, Tindall said.

"There are (seed) varietal differences, and there's something there -- we just don't know what yet," Tindall said.

Also irrigated soybeans don't look nearly as bad as beans that weren't irrigated, Tindall said.

When Dectes are in the stem around harvest time, they girdle the plant, or chew a ring around the bottom of the stem, Tindall said. If the wind blows or it rains, it's comprised and can fall over, she said.

"That's why if farmers know they have Dectes, they should hurry and get their crop out of the field as early as possible," Tindall said.

Farmers should consider after harvest burying the stalks with Dectes larvae at least three inches deep because it gives them a problem for emerging as adults, Tindall said.

"Deep tillage could work, but if your neighbor doesn't deep-till, it's not going to do any good," Tindall said.

The University of Missouri is trying to answer some of the questions concerning Dectes, and hopefully, researchers will have a better idea into the next growing season, Tindall said.

"There's a promising insecticide we're working to have available next year, Tindall said.

House said there's a lot researchers don't know.

"I'm trying not to get a panic started here," House said. "Will they cycle out and be here next year? Ask me that next year."