Investigating the Role of Plant Tolerance as Defense Against Rice Water Weevil in Irrigated Drill-seeded Rice in Louisiana

Objective 1: Compare yield losses due to rice water weevil injury in commonly grown rice cultivars in Louisiana under varying field conditions.

Field experiments were conducted from 2017–2019 at the LSU AgCenter, H. Rouse Caffey Rice Research Station in Crowley, LA. Rice cultivars used in this study were selected from commonly grown inbred rice cultivars in Louisiana (Catahoula, Cheniere, CL111, CL151, Cocodrie, Jazzman-2, Jupiter, and Mermentau). Seeds were drill-planted in plots measuring 5.5 m in length with seven rows spaced 18 cm apart at a seeding rate of 67 kg ha^-1. Pest infestation levels were manipulated by treating seeds with chlorantraniliprole (Dermacor X-100, Corteva Agriscience). Three rates of seed treatment were employed – 0 (nontreated), 7.9 (low rate), and 78.5 (label rate) g a.i. ha^-1 –to achieve high, moderate, and low pest infestation levels, respectively. Plots were laid out according to a split-plot randomized block design with four blocks. Each block contained a cultivar (whole-plot) × insecticide rate (split-plot) factorial arrangement. Fields were surface irrigated as needed to facilitate plant emergence and stand establishment. Permanent flood was applied 4-6 weeks after planting, depending on the year. Field plots were maintained by following the water management, weed control, fertilization, and other production practices recommended for Louisiana drill-seeded rice.

Numbers of immature rice water weevil (larvae and pupae) were evaluated 3-4 weeks after the permanent flood. Root-soil core samples, with each sample containing a minimum of one rice plant with intact roots, were taken from each plot using a metal corer (9.2 cm diameter, 7.6 cm deep). Core sampling was conducted twice in 2017 (four cores per plot, 768 samples total), twice in 2018 (three cores per plot, 576 samples total), and once in 2019 (three cores per plot, 288 samples total). Individual core samples were processed by washing the soil from the roots in 40-mesh screen sieve buckets that were then placed in basins of a saturated salt solution. Rice water weevil larvae were counted as they floated to the surface and pupae were counted by inspecting bottoms of sieve buckets. The numbers of larvae and pupae in the 3–4 core samples from each plot were averaged prior to analysis.

At 100% heading, whiteheads resulting from stemborer infestations were visually assessed in each plot. The total number of whiteheads in each plot was recorded and converted to whiteheads per m². Whiteheads were collected and were brought back to the laboratory to retrieve and identify the stemborer species.

Entire plots were harvested using a small-plot combine at grain maturity. Grain weights were adjusted to 12% moisture and converted to kg ha^-1. Yield losses (%) were calculated for each cultivar in each block, using the formula: % yield losses = [(yield_protected – yield_infested)/yield_protected] × 100, where ‘protected yields’ were yields from plots treated with the label rate (78.5 g a.i. ha^-1) and ‘infested yields’ were yields from nontreated plots or plots treated with the low rate (7.9 g a.i. ha^-1) of chlorantraniliprole for a given block.

Rice water weevil density, whitehead incidence, and yield loss data were analyzed using generalized linear mixed models (PROC GLIMMIX). All analyses were carried out in SAS version 9.4.

Objective 2: Assess the independent and interactive effects of rice cultivars and cultural control practices against rice water weevil.

Field experiments were conducted from 2019–2020 at the LSU AgCenter H. Rouse Caffey Rice Research Station in Crowley, LA. In each year, two separate trials were established – one planted early and the other late relative to recommended planting dates in southwest Louisiana. For each trial, field plots were laid out according to a split-split-plot randomized block design with four blocks. Each block contained a flood timing (whole-plot) × cultivar (sub-plot) × insecticidal seed treatment (sub-sub plot) field plot arrangement. Rice cultivars used in this study are commonly grown in Louisiana (Cocodrie, Mermentau, Jupiter, and Jazzman-2). Seeds were drill-planted at a seeding rate of 67 kg ha^-1 in plots measuring 5.5 m in length with seven rows spaced 18 cm apart. Before planting, each cultivar was either treated or not treated with chlorantraniliprole seed treatment (Dermacor^® X-100, Corteva Agriscience) at a rate of 78.5 g a.i. ha^-1. After planting, fields were surface irrigated as needed to facilitate plant emergence and stand establishment. When the rice plants reached the five-leaf to early tillering developmental stage, the permanent flood was applied to plots assigned to the normal flood timing treatment, whereas permanent flood was applied two weeks later to plots assigned to delayed flood timing. Field plots were maintained following the standard recommendations for weed control and fertilization for drill-planted rice in Louisiana. Weevil density, whitehead incidence, and yield loss data were collected similar to Objective 1.           

Rice water weevil density, whitehead incidence, and yield loss data were analyzed using generalized linear mixed models (PROC GLIMMIX). All analyses were carried out in SAS

Objective 3: Elucidate tolerance-related traits to root injury in rice.

Field experiments were conducted at the LSU AgCenter H. Rouse Caffey Rice Research Station in Crowley, Louisiana in 2020. Two separate trials were established – one planted early and the other late relative to recommended planting dates in southwest Louisiana. For each trial, field plots were laid out following a randomized block design with six blocks. Each block contained a cultivar (hybrid and inbred) × insecticidal seed treatment (treated and nontreated) factorial arrangement. Seeds of the hybrid cultivar ‘RT7521’ (RiceTec Inc.) and inbred ‘CL151’ were drill-planted at a seeding rate of 28 kg ha^-1 and 67 kg ha^-1, respectively, in plots measuring 5.5 m in length with seven rows spaced 18 cm apart. Prior to planting, seeds of each cultivar were either treated or not with a standard rate (78.5 g a.i. ha^-1) of chlorantraniliprole seed treatment (Dermacor X-100, Corteva Agriscience). Treated and nontreated plots of the same cultivar were placed side-by-side to reduce variation in weevil infestations and yields. After planting, fields were surface irrigated as necessary to facilitate plant emergence and growth. Permanent flood was established at 4–5 weeks after planting when the rice plants reached the early-tillering developmental stage. Field plots were maintained following the standard recommendations for fertilization and weed control for drill-seeded rice in Louisiana.

Immature rice water weevil (larvae and pupae) densities were evaluated similarly to Objectives 1 and 2. Core sampling was conducted five times (14, 22, 30, 36, and 42 days after permanent flood in the early-planted trial and 19, 26, 33, 40, and 48 days after permanent flood in the late-planted trial), and two core samples were taken from each plot per sampling point. To determine plant biomass, roots and shoots were separated from each sample and dried in an oven (60°C for one week). Dry weights of roots and shoots were recorded using a digital weighing scale.

Concentrations of nutrients in plant shoots were evaluated in both early- and late-planted trials. Dried plant materials from core samples described previously were sent to LSU AgCenter’s Soil Testing and Plant Analysis Laboratory (Louisiana State University, Baton Rouge, LA). Moreover, rice shoots were collected at 10, 20, and 30 days after the permanent flood in the early-planted trial for the quantification of plant hormones. For each time point, two plant samples (combination of leaves and stems) were taken from each plot and immediately flash-frozen in liquid nitrogen. Samples were sent to Creative Proteomics (Shirley, NY) for quantitative measurement of abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), jasmonic acid-isoleucine (JA-Ile), 12-oxo-phytodienoic acid (OPDA), and indoleacetic acid (IAA).

At grain maturity, entire plots were harvested using a small-plot combine. Grain weights were adjusted to 12% moisture. Percentage yield loss was calculated for each cultivar using a formula similar to that in Objectives 1 and 2.

Data were analyzed separately by planting date. All analyses were performed in SAS version 9.4. Rice water weevil density, plant biomass (roots and shoots), nutrient concentration, plant hormone, and yield loss data were analyzed using generalized linear mixed models (PROC GLIMMIX).