Abundance and Spatial Dispersion of Rice Stem Borer Species in Kahama, Tanzania

The Structure of Rice Stemborer Assemblages: A Review of Species' Distributions, Host Ranges, and Interspecific Interactions

This review describes global rice stemborer assemblages based on published species distributions, apparent host preferences, and reported shifts in assemblage composition in response to environmental factors. At least 56 moth (Lepidoptera: Crambidae, Pyralidae, Noctuidae) and fly (Diptera: Diopsidae, Chloropidae) species have been associated with rice; however, only 21 species are of potential, large-scale economic importance with a further 2 species of localized concern; most of the remaining species' associations with rice are based on dubious records without economic impacts on rice production. A list of stemborer-host associations indicates that rice stemborers are largely oligophagous on grasses (Poaceae), but a few species are polyphagous (also attacking Cyperaceae, Typhaceae, and some Eudicotyledon plants). Total stemborer abundance is determined by rice cropping patterns and management. Assemblage species richness is determined by geographical location, surrounding habitat (particularly as regards secondary and occasional species), and season. Evidence suggests that stemborer assemblage structure is largely determined through conditional interspecific competition. Regional assemblages typically include a single dominant lepidopteran species (primary species) that is largely restricted to rice and for which the climate is optimal; one or more secondary species that vary based on the age of rice attacked, rice anatomy, and the proximity to other habitats (including other crops); and occasional species that probably spill over from adjacent grasslands. The co-occurrence of lepidopteran with dipteran rice stemborers requires further research attention.

Spatial Distribution of Lepidopteran Stem Borers in Louisiana Rice Fields

Louisiana rice is attacked by a complex of lepidopteran stem borers that injure the vegetative and reproductive stages of rice, reducing tillering and grain output. Currently, insecticidal seed treatments are used in Louisiana as a preemptive means to control stem borers in rice. Our objective was to better understand the spatial distribution of stem borers in Louisiana rice fields. Thirteen nontreated commercial rice fields in Louisiana were mapped using GPS software and surveyed for stem borer damage in 2017 and 2018. Damage was recorded by counting the number of blank panicles (whiteheads) within quadrats located at different areas in each field. An ANOVA, spatial interpolation, hotspot analysis, and negative binomial regression were used to determine where stem borer injury was concentrated and whether the distribution of these pests was influenced by rice stand count, field composition, and site location. The results show that stem borers are typically aggregated along field edges, with population density decreasing toward the center of rice fields. Stem borer damage was 4.2- and 3.9-fold greater along field edges than in field centers in 2017 and 2018, respectively. Hotspot spatial analysis revealed 41.7 and 52.1% of low-density clusters occurred at or near field centers, respectively. Negative binomial regression revealed stem borer injury declines with increasing distance from field edges, with regional differences in infestation levels. This suggests stem borer control efforts could be spatially targeted to improve the efficiency of pest management.