Southern summers are seeing soybean loopers make a buffet of some fields where traditional insecticides aren't keeping the dinner bell from ringing for the leaf-chomping critters. And late-maturing beans are bringing more to the table.
“We always see loopers in soybeans,” says Angus Catchot, Mississippi State University Extension entomologist. “Now we have a wider window for them.”
Soybean loopers are a tropical pest that migrates to southern states from the Caribbean and extreme-southern Louisiana. They can overwinter in Florida and Texas. Spring winds from the Gulf of Mexico carry loopers north.
Plants are relatively tolerant of early season defoliation, and soybean loopers usually aren't as big a problem in the Midwest. But southern growers can see major defoliation in mid- to late summer if bugs aren't handled.
Phillip Roberts, University of Georgia Extension entomologist, says larval feeding on foliage is initiated on the interior lower portion of plants. As defoliation proceeds, larvae move toward the tops of plants. Early instars feed on the lower leaf surfaces.
“Older larvae consume irregular areas of leaves, characteristically leaving the larger leaf veins,” he says. “About 90% of defoliation occurs during the last two larval instars.”
ECONOMIC THRESHOLD LEVELS to warrant insecticide treatments are 25% defoliation, or around four to six loopers per foot of row. However, Roberts and Catchot stress that the soybean looper is difficult to control with insecticides, noting that resistance to pyrethroid insecticides by soybean looper has been documented in several states.
“We have to go with something other than pyrethroids for control,” says Catchot, adding that since pyrethroids are effective against many other insects, it's an added expense for growers.
The preferred chemicals for soybean looper control are Intrepid, Larvin and Steward.
Since the soybean looper doesn't hit most southern regions until mid-summer, a treatment isn't needed until nearly August. More than one application may be needed.
A low-end rate of Intrepid is about 5 oz./acre. Using ⅛ to 1/7 gal./acre of Larvin provides control on moderate to heavy populations of larger loopers; Steward — 1/18 to 1/16 gal./acre — is another treatment option.
Catchot notes that Intrepid doesn't disrupt beneficial insects, mites and pollinators, meaning it should work in an integrated pest management program.
He points out the added problems seen by growers in 2008. It mainly involved late-season buildup of soybean looper after growers planted varieties beyond the regular Group IV to low Group V range.
“We also had a large number of acres planted in double-crop wheat beans,” he says. “The later-maturing varieties exposed soybeans to a later window for the loopers.
SOYBEAN LOOPERS ARE also hosted by cotton, peanuts and other southern plants. Roberts says the insect larvae are light to dark green with light longitudinal stripes and two pair of abdominal prolegs.
The body is thick and tapers toward the head. Frequently, soybean looper larvae will have black legs and/or markings on the head and body. However, some soybean looper larvae may have no black markings and may reach a length of 35 mm.
Roberts says soybean looper moths have forewings that are mottled with a bronze to golden sheen and prominent silver markings near the center. Hind wings are dusky-brown.
If left uncontrolled, the looper will enjoy the leaf buffet that most growers can't stomach.