T. Eugene Reagan, Julien M. Beuzelin, Waseem Akbar, Howard Cormier and Jimmy W. Flanagan
Between 30,000 and 40,000 acres of sugarcane production in St. Mary, Iberia and Vermilion parishes were substantially affected by storm surge from Hurricane Rita that came on shore in southwestern Louisiana on September 24, 2005. LSU AgCenter scientists documented soil salt residual averaging five times greater than normal in many areas inundated by the storm surge, with standing saltwater lasting up to eight days. By the spring of 2006, farmers and consultants observed many effects from the hurricane including reduced plant stand potentially affecting crop yield. The surge also affected the integrated pest management system by reducing wellestablished biological control agents such as fire ants and spiders.
To evaluate the storm surge effect in the integrated pest management system, LSU AgCenter scientists designed a study during the summer and fall of 2006 involving 48 sugarcane fields. The fields were divided into four groups, which included two sugarcane fields flooded by the storm surge and two nearby fields that had not been flooded. Within each of these groups, there was one field planted within six weeks before the hurricane and one field not planted that year. Sugarcane is normally planted only once every three or four years.
Sampling included soil analyses of cane fields in the top 3 inches and 4-6 inches (five probes per field). In addition, two traps were placed in each field for six weeks to assess soil surface arthropods (insects and spiders, primarily). Each farmer was contacted to determine the amount and frequency of insecticides used in the sampled fields for sugarcane borer control during the 2006 crop season. Season-long damage from the sugarcane borer was determined by counting bored internodes from 25 randomly sampled sugarcane stalks on the two rows adjacent to and between the traps.
Soil sampling during the summer following Hurricane Rita (2006) indicated significantly increased salt concentrations averaging 806 ppm in storm surge fields versus only 162 ppm in nonstorm surge fields. Figure 1 shows a 42 percent (stubble cane) to 80 percent (plant cane) reduction in fire ants collected in traps in storm surge fields during the sampling period, July 22 to September 9, 2006. There was a strong trend that spiders were reduced in storm surge areas, also nearly a full year after the hurricane. However, no adverse effects were detected on predatory beetles and earwigs or nonpredatory arthropods.
Many of the producers and consultants working in these areas had to make earlier and more frequent insecticide applications to suppress increasing sugarcane borer infestations in storm surge fields. Insecticide applications averaged two per field in storm surge fields compared to only 0.8 per field in non-storm surge fields, a 2.4-fold increase in insecticide use associated with the storm surge. Even with increased insecticide applications, the percent bored internodes caused by higher sugarcane borer infestations was 10.7 percent in storm surge fields versus 5.9 percent in nonstorm surge fields (Figure 2).
Our studies also showed a greater diversity among various groups of ar thropods (species diversity) in the storm surge habitats compared to the nonstorm surge areas, which were more strongly dominated by the red imported fire ants. Additional sampling during April and May of 2007 for sugarcane borer-caused deadhearts showed a continued significant impact from Hurricane Rita storm surge – 390 deadhearts per acre versus 240 per acre in nonstorm surge fields in Vermilion Parish. Therefore, a full 20 months after Hurricane Rita came on shore with a major storm surge into south Louisiana, the integrated pest management system in sugarcane was still being negatively affected.
Importance of beneficial arthropods in sugarcane
The red imported fire ant is so helpful to sugarcane growers that the predation it provides is equal to two insecticide applications to control the sugarcane borer. Spiders are the primary borer egg predators, and predaceous beetles and earwigs also are important beneficial arthropods. Though crickets are neutral insects in sugarcane fields (neither pests nor beneficials), they are food for predaceous ants. Storm surge mortality to ants and the resulting decreased predation enhanced sugarcane borer pest problems and allowed for a greater diversity among both beneficial and neutral arthropods. Though Hurricane Rita enhanced a greater balance among various arthropods, this new balance interfered with the natural sugarcane borer control provided by fire ants and spiders.
Deadhearts in sugarcane result when sugarcane borer larvae kill the growing point of the plant, causing the center leaves to turn brown. Deadheart abundance is related to borer infestations during the spring. Surveys conducted during the spring of 2007 showed there were greater than 150 percent more deadhearts in fields flooded by the Hurricane Rita storm surge.
(This article was published in the winter 2008 issue of Louisiana Agriculture.)