Potential impact of Mexican rice borer non-crop hosts on sugarcane IPM. (2011)

THE Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), was detected in Louisiana in 2008, having first spread from Mexico into Texas. E. loftini is a severe pest of sugarcane, but it also feeds on rice and a wide range of other grasses. Research on E. loftini management has focused on studying the pest in its main crop host plants, with the role of non-crop grasses only recently studied. Two four-replication sentinel plant studies were conducted in 2006 and 2007 to assess naturally occurring E. loftini infestations in five selected weed species: Leptochloa panicoides, Sorghum halepense, Paspalum urvillei, Urochloa platyphylla and Echinochloa crus-galli. Results showed that L. panicoides, a common weed in Louisiana rice fields, was a highly suitable host, harbouring the highest E. loftini infestations with as many as 78% of the plants infested with at least one larva. In addition, S. halepense and P. urvillei, two ubiquitous perennial grasses, also supported complete larval development of E. loftini. On the other hand, both U. platyphylla and E. crus-galli, which are two common weeds in and near rice fields in Louisiana, proved to be poor E. loftini host plants. Continuous pheromone trapping in the southeast Texas rice area showed that adult moths are active throughout the year. Our studies showed that non-crop hosts could play a key role in E. loftini population build-up, thus warranting a better characterisation of E. loftini sourcesink interactions in Louisiana sugarcane producing areas. The manipulation of E. loftini non-crop sources has the potential to decrease a significant proportion of area-wide populations, minimising damaging infestations in sugarcane fields. Introduction The Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), is indigenous to Mexico and was first reported in 1980 in south Texas (Johnson, 1984). This borer quickly became the most damaging insect pest of sugarcane (Saccharum spp. hybrids) in the Lower Rio Grande Valley of Texas, where it represents more than 95% of stem borer infestations (Legaspi et al., 1997). After expanding its range in a northeast direction along the Gulf Coast (Reay-Jones et al., 2007b), E. loftini has also become an increasing problem for rice (Oryza sativa) production in southeast Texas. E. loftini was detected in Louisiana for the first time in December 2008 (Hummel et al., 2008), representing a serious threat to the state’s sugarcane and rice industries. The imminent establishment of E. loftini in Louisiana sugarcane producing areas encouraged proactive studies that integrate cultivar resistance, biorational insecticides, and irrigation-based population suppression to develop an effective management program (Reay-Jones et al., 2005). Insecticides and cultivar Beuzelin, J.M. et al. Proc. Int. Soc. Sugar Cane Technol., Vol. 27, 2010 ______________________________________________________________________________________ 2 resistance have also been studied in rice, which is also grown in sugarcane areas of Louisiana. In addition to crop hosts, Van Zwalunwenburg (1926) stated that E. loftini ‘attacks practically all the grasses large enough to afford it shelter within the stalk’. Non-crop grasses may therefore play a role in the overwintering and build-up of E. loftini populations, and should be integrated into the development of new cultural practices for an improved pest management program. This paper reports on initial studies with E. loftini non-crop hosts and discusses their possible importance in future sugarcane integrated pest management (IPM) for Louisiana. Material and methods Sentinel plant experiments Two sentinel plant experiments were designed to compare E. loftini infestation development on selected non-crop grass species under natural infestations. Experiments were conducted in southeast Texas during 2006 and 2007 near Ganado (N 29.0267°, W 96.4394°) and Hankamer (N 29.8554°, W 94.5451°), respectively, where E. loftini populations naturally occur at high densities. Five weed species that are abundant in or near sugarcane and rice fields and have the potential to host E. loftini populations were studied (Table 1). Rice (cultivar Cocodrie) served as a control. Seeds were obtained from Azlin Seed Service (Leland, MS), except for Paspalum urvillei seeds that were collected in Lafayette Parish, LA. Plants were grown in a greenhouse in 7.57 L pots, each containing eight (2006) or six (2007) evenly spaced plants. In mid-August, after growing for two months under greenhouse conditions, the potted plants were placed in a rice field near a levee. For each plant species, six pots constituted a plot, and plots were arranged in a randomised complete block design with four blocks (1 replication per block). Plots were separated by 75 cm (2006) or 2 m (2007) spaces. Plants remained in the pots, but pot bottoms were removed to facilitate better equalisation with field moisture conditions. In 2006, ten plants from each plot were randomly selected and cut at the base both four and nine weeks after transplanting. In 2007, 12 plants were sampled both four and seven weeks after transplanting. Each tiller was measured and the number of leaves counted. Plants were observed for borer feeding signs and dissected for the presence of larvae and pupae. Statistical analyses were performed using Proc GLIMMIX (SAS Institute, 2008). Generalised linear mixed models (GLMMs) with an over-dispersion parameter were used to analyse the proportion of plants infested with E. loftini (binomial distribution) and E. loftini abundance as affected by plant species (Poisson distribution). Because sugarcane borers, Diatraea saccharalis (F.), also infested sentinel plants in 2006, a GLMM with a binomial distribution was used to compare borer species composition as affected by the plant species. The Kenward-Roger adjustment for denominator degrees of freedom was used in all models to correct for inexact F distributions. Adult pheromone trapping Male E. loftini moths were continuously monitored to determine seasonal patterns of flight activity. From April 2007 to April 2009, monitoring was conducted at three sites in southeast Texas. Two standard universal pheromone traps were used at each site according to the method of ReayJones et al. (2007b). Traps were located near the Texas AgriLIFE Research Center at Beaumont (N 30.0672°, W 94.2932°), and near Hankamer and Ganado where the two sentinel plant experiments were conducted. Traps were checked for E. loftini moths every two-to-three weeks, and trap catches were estimated on a daily basis for each sampling period (Reay-Jones et al., 2007b).