When Arkansas rice farmers begin harvesting in mid to late August, yields will give them an idea of how bad lespedeza worms hit their crop this year, says Chuck Wilson, rice specialist with the University of Arkansas Cooperative Extension Service.
Several producers already know because they had to replant in May after the worms caused serious stand reductions.
“This is the most widespread problem with these worms I've seen since I've been with Extension,” Wilson said.
Lespedeza worms are more correctly known as grape colaspis. Their preferred food is lespedeza, a legume, and soybeans, but they will feed on rice if their preferred foods are not available.
The worms emerge from overwintering in farm fields and begin feeding on the roots and underground part of the rice stem. There are no insecticide treatments to control the larvae after the rice emerges since the worms are below ground.
“Our normal control method is to fertilize and flush the field. It doesn't kill the larvae, but it encourages the rice plant to start growing so that it outgrows the potential damage caused by the worms.”
However, it was so cool in May that even after farmers flushed fields, “they still couldn't get the rice to grow. It just sat there, and the worms ate until they wiped out large areas of stands.”
The most widespread losses were in the Grand Prairie region of eastern Arkansas, Wilson noted. But there were also substantial losses in other parts of eastern Arkansas where rice is grown on silt-loam soils.
Wilson identified these areas as being in Ashley and Drew counties in southeast Arkansas and west of Crowley's Ridge in counties including Cross, St. Francis, Poinsett and Craighead. All of these areas have lespedeza worm problems year after year, he said, “and they had big problems this year.”
Lespedeza worms develop into beetles that feed on lespedeza and soybeans, but it's the worms that do most of the damage to rice.
“They'll produce several generations. When frost is about to hit, the last generation of larvae go deep into the soil and overwinter. They migrate toward the surface in the spring when it warms up, and if rice is planted in the field they're in, that's what they attack.”
Wilson said the worms can girdle a plant. They cut off the flow of water and nutrients to the upper part of the plant. If there are enough of them in the field, they can wipe out stands of seedlings. The seedlings will look salt-stressed with yellowing, stunted growth and wilting because of the loss of water to the upper plants.
Wilson said there is a preventative treatment for lespedeza worms. A chemical seed treatment called Icon is an effective treatment, but it has had limited use because of its expense (about $15 an acre), Wilson noted.
University research suggests that reduced rates of Icon are as effective and may reduce the cost.
While Icon, which is marketed for control of rice water weevils, is not widely used in Arkansas, Wilson figures more of it will be used next year because “farmers have seen what the worms can do.”
Lamar James is an Extension communications specialist with the University of Arkansas.