Luis Espino

Increased fitness of rice plants to abiotic stress via habitat adapted symbiosis: A strategy for mitigating impacts of climate change

Climate change and catastrophic events have contributed to rice shortages in several regions due to decreased water availability and soil salinization. Although not adapted to salt or drought stress, two commercial rice varieties achieved tolerance to these stresses by colonizing them with Class 2 fungal endophytes isolated from plants growing across moisture and salinity gradients. Plant growth and development, water usage, ROS sensitivity and osmolytes were measured with and without stress under controlled conditions. The endophytes conferred salt, drought and cold tolerance to growth chamber and greenhouse grown plants. Endophytes reduced water consumption by 20–30% and increased growth rate, reproductive yield, and biomass of greenhouse grown plants. In the absence of stress, there was no apparent cost of the endophytes to plants, however, endophyte colonization decreased from 100% at planting to 65% compared to greenhouse plants grown under continual stress (maintained 100% colonization). These findings indicate that rice plants can exhibit enhanced stress tolerance via symbiosis with Class 2 endophytes, and suggest that symbiotic technology may be useful in mitigating impacts of climate change on other crops and expanding agricultural production onto marginal lands.

Effect of Planting Date on Lissorhoptrus oryzophilus (Coleoptera: Curculionidae) Density—Yield Relationship on Rice in Southeastern Texas

ABSTRACT Results of planting date and insecticide efficacy experiments targeting Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae) on rice, Oryza sativa L., in southeastern Texas between 2002 and 2007 were used to determine density—yield relationships as a function of planting date. Soil core samples were collected on two dates during main crop development to estimate immature L. oryzophilus populations followed by main and ratoon crop harvests. Analysis of covariance (ANCOVA) revealed that planting date did not affect the density-main crop yield relationship during most years and that these relationships varied substantially among years. For ratoon crop yield, an effect of main crop L. oryzophilus immature infestation was detected during some years, but the real effect of these populations on ratoon crop yield remains unclear. Using estimates of yield reduction per L. oryzophilus immature, economic injury levels were calculated. Main crop yields from treated plots and first soil core sample L. oryzophilus immature populations from untreated plots were significantly higher in plots planted at recommended dates than in plots planted earlier or later. This suggests that the presence of high populations of reproductive L. oryzophilus coincides with the period when rice fields planted at optimum dates are flooded. Results from this study reinforce the importance of managing L. oryzophilus populations when planting rice at recommended dates in southeastern Texas.

Determination of Oebalus pugnax (Hemiptera: Pentatomidae) Spatial Pattern in Rice and Development of Visual Sampling Methods and Population Sampling Plans

Abstract Commercial rice, Oryza sativa L., fields in southeastern Texas were sampled during 2003 and 2004, and visual samples were compared with sweep net samples. Fields were sampled at different stages of panicle development, times of day, and by different operators. Significant differences were found between perimeter and within field sweep net samples, indicating that samples taken 9 m from the field margin overestimate within field Oebalus pugnax (F.) (Hemiptera: Pentatomidae) populations. Time of day did not significantly affect the number of O. pugnax caught with the sweep net; however, there was a trend to capture more insects during morning than afternoon. For all sampling methods evaluated during this study, O. pugnax was found to have an aggregated spatial pattern at most densities. When comparing sweep net with visual sampling methods, one sweep of the “long stick” and two sweeps of the “sweep stick” correlated well with the sweep net (r2 = 0.639 and r2 = 0.815, respectively). This relationship was not affected by time of day of sampling, stage of panicle development, type of planting or operator. Relative cost-reliability, which incorporates probability of adoption, indicates the visual methods are more cost-reliable than the sweep net for sampling O. pugnax.

Culture-Independent Analysis of Bacterial Communities in the Gut of Rice Water Weevil (Coleoptera: Curculionidae)

ABSTRACT Gut bacteria play important roles in the biology of insects. In this study, gut bacterial communities were investigated in the rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae), which is a serious pest in rice growing-areas of the United States and some Asian countries. Overwintered female adults were collected from rice plants in five geographic locations: Beaumont, TX; Colt, AR; and Biggs, CA; in the United States, and Tanghai of Hebei and Yueqing of Zhejiang in China. Using polymerase chain reaction, bacterial 16S rRNA clone libraries were constructed. From the libraries, we identified 19 bacterial phylotypes (operational taxonomic units, OTUs). Of them, only 3–5 OTUs were identified in each population, except in the Beaumont weevils, which had 10 OTUs. The OTUs were affiliated with the bacteria in six classes of four phyla: &agr;-, &bgr;-, &ggr;-proteobacteria, Bacilli, Sphingobacteria, and Actinobacteria, with the &ggr;-proteobacteria being the richest. Bacteria of the genera Pantoea and Wolbachia were detected in all weevil populations examined. There were also some bacteria that were specifically detected in certain locations but had a high relative abundance, such as the ones matched to Enterobacter oryzae, Lactococcus lactis, and Rickettsia felis. Geographic variations were found among bacterial communities in their richness, diversity, and evenness. This was the first study depicting a detailed profile of bacterial communities in L. oryzophilus, and the results provide important information for future studies on the bacterial functions, mutual interactions, and use for developing a biocontrol technique against this weevil.

Most Susceptible Stage of Rice Panicle Development to Oebalus pugnax (Hemiptera: Pentatomidae)

Abstract Oebalus pugnax (F.) (Hemiptera: Pentatomidae) damage to rice, Orya savita L., reduces rough and head rice yields, and grain quality. O. pugnax feeds on developing kernels, introducing pathogenic fungi and causing a discoloration of the grain known as “peck.” The objective of this study was to determine the stage of rice panicle development most susceptible to O. pugnax attack. During 2005 and 2006, in greenhouse and field experiments, rice plants were caged at the boot stage and then infested with adult or nymphal O. pugnax. Plants were infested during one of three stages of panicle development: heading, milk, or soft dough. Insects were allowed to feed on the plants for the duration of each stage and then killed. After maturation, panicles were harvested, and grain was hulled and milled. Grain weight, percentage of pecky grain, and percentage of whole grain after milling were recorded. No differences were found in the weight of rough, brown, or milled rice infested with O. pugnax during different stages of panicle development. Number of filled grains per cage was not affected by O. pugnax, and number of empty grains per cage was affected in two of four experiments. Higher percentage of peck was found in grain from panicles infested during dough and milk than in grain from panicles infested during heading. Adult O. pugnax caused higher percentage of peck than nymphs in all stages of panicle development. An inverse relationship was found between percentage of peck and percentage of whole grain weight only in one of the experiments.

Occurrence and Abundance of Lissorhoptrus oryzophilus (Coleoptera: Curculionidae) Relative to Field Borders in California Rice

ABSTRACT Field experiments were conducted during 2009 and 2010 on commercial rice fields in the Sacramento Valley of California to validate observations regarding the prevalence of immature populations of the rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae), near field borders, and to assess their impact on yield. In 5 commercial fields, insecticide-treated and untreated plots were established 4.5, 30 and 60 m from one edge of the field. Soil core samples were collected on 2 dates and inspected for L. oryzophilus immatures. Rice yields were determined by harvesting 15 m2 plots in 2009 or 1 m2 per plot in 2010. Analysis of variance (ANOVA) showed that in most locations, immature populations were higher in plots 4.5 or 30 m from the fields edge than in plots 60 m from the fields edge. Yields from treated and untreated plots did not differ significantly. Linear regression of immature populations and rice grain yield per plot per location did not yield a significant, inverse density-yield relationship. Results indicate that border applications of insecticides for L. oryzophilus management are appropriate; however, growers are advised to inspect their fields to confirm border populations and effects on yield. Research needs regarding sampling and economic thresholds are discussed.

Attractiveness of Stages of Rice Panicle Development to Oebalus pugnax (Hemiptera: Pentatomidae)

Abstract Greenhouse experiments were conducted during 2004 and 2005 with male and female Oebalus pugnax (F.) (Hemiptera: Pentatomidae) caged on rice plants at different stages of panicle development with the objective of determining the most attractive stage to O. pugnax. Field-collected insects were released inside cages containing potted plants and observed during morning and afternoon hours for 5 d. Results showed that attractiveness of male and female O. pugnax to plants with panicles at milk and soft dough stages was greater than plants at preheading and heading stages. Preheading plants were the least attractive to the insects, confirming field observations. Results imply that insecticide applications during the preheading stage are likely ineffective and that monitoring efforts during the milk and soft dough stages of panicle development should be intensified.

Effects of Seeding Rates and Rice Water Weevil (Coleoptera: Curculionidae) Density on Damage in Two Medium Grain Varieties of Rice

Abstract Rice water weevil (Lissorhoptrus oryzophilus Kuschel) is a common pest of rice production in the United States whose larvae cause yield loss by feeding on the roots. We conducted studies from 2011–2013 on M-202 and M-206, two commonly grown California medium grain rice varieties, to determine if M-206 demonstrated tolerance to rice water weevil damage. Observations from field studies suggested the possibility of a level of tolerance in M-206 that was more prevalent at high seeding rates. We did this study using two different experimental units, open and ring plots. In both units, we quantified grain yields across four levels, 56, 112, 168, and 224 kg/ha, of seeding rates to detect potential yield recovery by M-206. In the open plots, we used naturally occurring weevil populations compared with controls that reduced the populations with insecticides. In the ring plots, we tested three levels of weevil infestation, none, low, and high, to look at the weevil density effects on yield and scarred plants. Our studies showed that M-206 and M-202 had generally similar densities of immature weevils and yield. Compensation for yield loss did not occur at higher seeding rates. These results suggest that M-206 does not have the ability to tolerate rice water weevil damage better than M-202. There was weak evidence that the number of scarred plants increased as plant density was reduced. The results are discussed in relation to the utility of this study to grower choices of varieties for long-term rice water weevil management.