Blake E. Wilson, James Villegas and Emily Kraus

The use of insecticidal seed treatments in rice is not a new pest management strategy, but the products have changed over the years. Insecticidal seed treatments are effective in managing early-season pests of rice including the rice water weevil (Lissorhoptrus oryzophilus), a damaging and ubiquitous pest that feeds on the roots of rice and substantially reduces yields. Fipronil seed treatments were removed from the market in 2004, leaving growers with limited options to manage weevil infestations. Foliar applications of pyrethroids were used, but these products are highly toxic to crawfish, and many farmers were concerned about unintentional drift into crawfish ponds. When three new seed treatments were introduced in 2010, growers quickly adopted the new technology. Insecticidal seed treatments (Figure 1) are now used on more than 80 percent of rice acreage in Louisiana. This high adoption rate is largely justified because the treatments provide economic returns in the vast majority of fields to which they are applied. The products effectively control the weevils as well as other insect pests, but the spectrum of pests controlled varies among products with differing modes of action.

While all rice farmers across the state are fighting the rice water weevil, secondary pests have the greatest influence on which insecticides are used.

Chlorantraniliprole seed treatments (Dermacor X-100 from DuPont) control caterpillar pests, including stem borers and fall armyworms. Two neonicotinoid seed treatments, thiamethoxam (CruiserMaxx from Syngenta) and clothianidin (NipsIt Inside from Valent) control grape colaspis, chinch bugs and thrips. The decision on which seed treatment to use must be made before planting. This means growers select the treatment based on anticipated pest problems from previous experiences. Recent surveys of rice growers and crop consultants across Louisiana indicate significant regional differences in insecticidal seed treatment use. Growers in the northeastern production region, where about 20 percent of Louisiana rice is grown, opt to use one of the two neonicotinoid seed treatments. Southwestern growers overwhelmingly use chlorantraniliprole (Figure 2). These differences are not just individual preferences but are driven by variations in pest complexes and production practices.

Stem borers are a much greater threat to rice production in southwest Louisiana than they are farther north. Three species of stem borers attack rice in Louisiana (Figure 3): the rice stalk borer (Chilo plejadellus), the sugarcane borer (Diatraea saccharalis) and the Mexican rice borer (Eoreuma loftini). Stem borers feed internally in rice stems, preventing the flow of nutrients in the plant and resulting in empty panicles or “whitehead.” Stem borer populations have increased in recent years as the range of the invasive Mexican rice borer moved farther eastward into the state. Chlorantraniliprole effectively controls the rice water weevil as well as the three stem borer species.

Grape colaspis infestations in rice occur when rice follows soybean production the previous year, a common practice in northeast Louisiana. It is not a major pest in southwest Louisiana because colaspis larvae are soil insects that cannot survive the flooded conditions of rice and crawfish production. Damaging colaspis infestations are common in the northeast, and neonicotinoid seed treatments are the only chemical controls available to farmers.

As with other agricultural systems, rice pest complexes and available insecticides frequently change from year to year. The availability of multiple insecticidal seed treatment options gives Louisiana rice farmers the flexibility they need to manage whatever insect pests are problems in their unique situations.

Blake E. Wilson is an assistant professor at the Sugar Research Station in St. Gabriel, Louisiana.James Villegas is a research associate, and Emily Kraus is a graduate assistant in the Department of Entomology.

(This article appears in the winter 2019 issue of Louisiana Agriculture.)

Figure 1: Rice seed treated with insecticides: chlorantraniliprole (left), thiamethoxam (center), and untreated (right). Dye is added to treatments to distinguish between treated and untreated seed.

Figure 2: Geographic variation in insecticidal seed treatment use. Survey data reflect percentages of treated acreage. Untreated acreage is less than 20 percent across the study area.

Figure 3: Stem borer complex attacks rice in southwestern Louisiana. Rice stalk borer (left), Mexican rice borer (center) and sugarcane borer (right).