Blake Wilson

Sugarcane and rice have been grown in Louisiana for generations, and insect pests have attacked the crops the whole time. Insects native to Louisiana, like the rice water weevil (Lissorhoptrus oryzophilus), were already present in the state attacking wild grasses and jumped to rice once farmers began planting it here. Other insects, like the sugarcane borer (Diatraea saccharalis), were introduced shortly after cultivation of sugarcane arrived, likely because of importation of sugarcane from Central America and Mexico. The number of pests farmers have to contend with is always growing as new invasive insects enter the state.

Mexican rice borer, Eoreuma loftini — This moth is in the same family as the sugarcane borer and similarly attacks many grass species, including sugarcane and rice (Photo 1). It has been migrating northeastward along the Gulf Coast like a slow-moving wave for nearly four decades. LSU AgCenter entomologists began conducting proactive research in 2000 in Texas to identify pest management strategies in anticipation of the insect’s arrival. The Mexican rice borer was first detected in Louisiana in 2008, and is now present in 14 parishes. Fortunately, thanks in part to early research, strategies have been developed to reduce effects on crop yields.

A borer-resistant sugarcane variety, L 01-299, was developed by the AgCenter variety development program. This variety is popular among farmers because of its high yield potential and ratooning ability to produce multiyear crops. It is now planted on nearly 60% of the state’s acreage. Biological control from predatory insects and judicious use of insecticides have further limited the pest’s effect on sugarcane. The borer is controlled in rice fields by insecticidal seed treatments that are already being applied for rice water weevil management.

Sugarcane aphid, Melanaphis sacchari — This aphid is an invader on a global scale. Its exact origin is unknown, but the pest is on every continent except Antarctica. Infestations in Louisiana sugarcane were first reported in 1999, and the aphid quickly became widely distributed across the state.

Sugarcane aphids are sporadic pests, but heavy infestations are seen in some fields every year. Aphid adults and nymphs (Photo 2a) extract the sugary liquid from leaves. Excess sugar, called honeydew, is secreted on leaf surfaces. This allows for development of black sooty mold, a fungus that covers leaves and impedes photosynthesis (Photo 2b). Unfortunately, few management tactics are available. Registered insecticides aren’t effective and can even exacerbate infestations by killing beneficial predators. Research efforts are underway to develop integrated management strategies and obtain additional product registrations. New tools are needed to help Louisiana farmers battle these small, but troublesome, pests.

Rice Delphacid, Tagosodes orizicolus — This small brown plant hopper (Photo 3a) is a serious pest of rice in Central America, where it transmits the damaging hoja blanca (white leaf) disease. The delphacid, as well as hoja blanca, were reported in Louisiana and Texas rice in 1957-1959 and 1962-1964. Fortunately, cold winters prevented the pest’s permanent establishment and neither the delphacid or hoja blanca was subsequently observed in the U.S.

That changed in 2015, when rice growers in Texas observed large patches of “hopper burn,” areas of heavy infestations where plants are brown and severely stunted (Photo 3b). The delphacid has been observed sporadically since then, and evidence suggests it may be overwintering in Texas. Hoja blanca has not been observed recently, but permanent establishment of the delphacid and the disease poses a major threat to U.S rice production. The delphacid is not known to occur in Louisiana at present. Researchers are studying its overwintering ability and monitoring for potential eastward expansion. Entomologists are also looking for collaborators in Central America who can screen Louisiana rice varieties for delphacid resistance.

Beneficial Invaders

It’s not often publicized, but whether or not an invasive insect is considered a pest depends on its context in the environment. Two highly invasive ants from South America, the red imported fire ant (Solenopsis invicta) and the tawny crazy ant (Nylanderia fulva), are major economic and ecological pests in urban and natural environments. In Louisiana’s sugarcane fields, conversely, both species are considered beneficial. Fire ants have long been known to be highly effective foragers and can greatly reduce borer infestations. Crazy ants have only recently become established in the state’s sugarcane regions, but preliminary research suggests they are as good as, or better than, fire ants at hunting down borer larvae.

On the other hand, interactions with some sugarcane pests may not be so beneficial. Both species of ants are known to tend, or farm, aphids. The ants protect aphids from would-be predators and even help move them to better feeding sites. In return, the ants feast on the sugary honeydew aphids produce. On-going research is examining the ants’ ability to exacerbate aphid infestations in sugarcane fields.

Blake Wilson is an assistant professor and field crops entomologist based at the LSU AgCenter Sugar Research Station, St. Gabriel, Louisiana.

(This article appears in the spring 2020 issue of Louisiana Agriculture, which focuses on entomology.)

Photo 1. Mexican rice borer larva tunneling inside a sugarcane stalk. Photo by Blake Wilson

Photo 2a. Sugarcane aphid adults and nymphs. Photo by Blake Wilson

Photo 2b. Sugarcane leaves covered in sooty mold. Photo by Blake Wilson

Photo 3a. Rice delphacid nymphs and adults on rice plants. Photo by Emily Kraus

Photo 3b. Rice field with hopper burn in Texas. Photo by Mo Way