talk borer insects are increasingly significant pests of Louisiana rice. More than 35 percent of rice acreage in Louisiana is at high risk of stalk borer attack. The sugarcane borer was observed to cause up to 95 percent stalk breakage on some rice farms in 2002. The European corn borer joined the list in 2003, averaging up to 75 percent larval infestation in rice areas of northeast Louisiana. A third borer species, the rice stalk borer, has statewide distribution. Luckily, it remains sporadic and of minor importance. The sugarcane borer and the European corn borer are very aggressive species when it comes to damaging rice. Their destructive behavior can be compared only to that of another stalk borer species, the Mexican rice borer. The dreaded Mexican rice borer is not yet present in Louisiana, but it is approaching from southeastern rice and sugarcane areas in Texas.
Stalk borers have increased numerically and geographically in rice. This expansion is favored by the rising adoption of minimum-tillage in several crops, increasing cereal crop acreage, a high incidence of grassy weeds, and several years of mild and dry winters. These conditions allow high survival rates of hibernating larvae through winter months.
Rice plants become attractive to infestations as soon as stems get thick enough to sustain larval growth. Therefore, rice may be infested anytime from panicle differentiation up to grain filling. However, in central and northeast Louisiana, the first and second borer generations prefer corn and grain sorghum, which are highly attractive for borer oviposition. As these crops mature in July, they drop in attractiveness for additional egg-laying. Being at early-boot and boot stages, rice becomes a lone and highly attractive target for peaking borer populations after corn and sorghum are harvested.
During borer infestations around panicle differentiation, larvae feed on and kill growing tissues, resulting in “deadheart.” Later infestations during early boot allow larvae to chew on the narrow panicle neck. The affected panicle emerges empty and white, creating what are called “whiteheads.” Larvae feeding during late boot may destroy developing kernels which show up as several white areas of chewed, empty kernels, and are thus referred to as “partial whiteheads.”
Stalk borer management in Louisiana requires an integrated, multi-crop approach. We’ve worked closely with corn and sorghum producers so they are able to manage their borer problem. Good stalk borer management on these crops means less borer problems later in rice. Planting rice early helps avoid high borer populations during the late season in areas near corn and sorghum with a history of borer attack. Rice producers also are warned of borer infestations in nearby corn and sorghum so they will prepare for a potential attack in rice.
Rice field scouts must pay attention to creamy-white moths in their rice because it is the first sign of a borer invasion. Moths fly off short distances when disturbed as you walk through a rice field. Current control measures, however, aim to kill young larvae before they enter stalks. Fortunately, the larvae don’t enter the stalk immediately upon hatching. Young larvae move down the leaf to the junction of the leaf blade and sheath. They feed on the inside surface of the leaf sheath for about 10 days before boring inside the stalk. Lesions caused by larvae feeding on leaf sheaths resemble sheath blight. However, peeling off the affected leaf sheaths reveals the presence of a small caterpillar and its frass.
Fresh feeding scars on the leaf sheath are the last warning you will have while you can still control larvae. If you wait too long, the larvae will bore inside the stem and any control options are gone. Once inside the stalk, larvae cannot be reached by foliar insecticides, and there is no rescue treatment. Last year, Karate-Z added stalk borers to its label in rice. However, no threshold levels are yet developed for these emerging pests. We still are researching the most appropriate time to put on foliar sprays for optimal control.
When an infestation is detected before larvae enter stalks, one foliar spray at early boot with pyrethroids currently used in rice provides excellent control in Louisiana.
It is also important to destroy the crop residue after harvest. This applies not only to rice, but also, and perhaps more important, to corn and sorghum, which may harbor high numbers of overwintering larvae. The sugarcane borer and rice stalk borer are distributed statewide in Louisiana. The European corn borer seems to prefer the northern areas of the state. The eastbound migration of the Mexican rice borer in Texas represents an alarming threat to the major rice areas of southwest and south-central Louisiana. There are too many stalk borer species coming to us from the north and south—we are being squeezed on all sides. RJ
Boris A. Castro is an Extension entomologist at the LSU AgCenter.
