EAST LANSING, Mich. – A long-horn beetle known more out west and in the south has been documented for the first time as causing yield losses in soybeans in Michigan.

The presence of Dectes stem borer was confirmed recently by Michigan State University from larvae found in soybean fields west of St. Johns, about 20 miles north of Lansing.

According to MSU Professor and Fields Crops Entomologist Dr. Christina DiFonzo, she has encountered just a few of the beetles during sweeps of soybean fields in her 30 years at the university but never spotted damage or larvae from the insect.

“My field crops colleagues at Ohio State and Purdue also have never seen an infestation. It is no surprise, however, that Dectes is spreading northward since the 1970s with millions of acres of soybeans, more no-till production and warmer winters,” she said in a Fast Fonz Facts science-based update from the MSU entomology department.

Dectes stem borer is established in states like Texas, Oklahoma, Missouri and Kansas and as far north as Kentucky.

DiFonzo said the recent damage marked by yellowing and leaves dropping from soybean plants was discovered along the edges of a farmer’s fields.

“The infestation wasn’t just a plant or two on the edge. It was fairly easy to find larvae when we split stems,” she said.

Larvae from the beetle finds its way inside the stems of soybean plants to feed on the tissue.

The stems are hollow once all the tissue inside has been consumed, which can weaken the plant enough to cause it to fall over at the base during windy conditions, making harvesting difficult and increasing losses from combines unable to pick up the soybeans on the ground.

“With stem rot, the pith in the stem is brown but intact. In this case, the pith was gone and the inside of the stem was dark, reddish-brown. Dectes larvae had bored into and ate the pith, leaving dark frass behind,” she said.

According to University of Missouri researchers, the Dectes stem borer, typically, causes anywhere from a 2-to-10 percent yield reduction from eating the tissue inside the stems. Losses can be substantial if plants collapse to the ground or other negative factors played a role in the declining health of the plants.

DiFonzo is advising Michigan farmers to look closely in their fields for any Dectes larvae, which are creamy-white in color and legless with a broad head typical of longhorn beetle larvae.

“Keep an eye out for this pest and damage over the next six weeks as you conduct field calls or troubleshoot poor stands,” she said.

DiFonzo also wants any findings reported to her along with pictures and location of any discoveries at difonzo@msu.edu.

The purpose is to gain a better understanding of the beetle’s presence in the state.

“The bottom line is that Dectes is here and has probably been here for a while,” she said.

According to researchers, adult beetles emerge in mid-summer from the larvae in a plant lived in by the pest the previous year. The larvae survive the winter by making a chamber to stay warm in at the base of a plant until hatching in the spring.

The Dectes stem borer favors sunflower but also infests soybeans, ragwood and other plants like cocklebur.

In tilled ground, DiFonzo said infestations are often limited to the edges of farm fields because adults emerge from plants in ditches and other nearby locations before migrating into the crops.

“Adults aren’t great movers, so they disperse slowly,” she said.