Espino: Rice Water Weevil Relative to Field Borders in California 445 OCCURRENCE AND ABUNDANCE OF LISSORHOPTRUS ORYZOPHILUS (COLEOPTERA: CURCULIONIDAE) RELATIVE TO FIELD BORDERS IN CALIFORNIA RICE LUIS ESPINO University of California Cooperative Extension, 100 Sunrise Blvd., Suite E, Colusa, California, USA 95932 E-mail: [email protected] 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 im- mature 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 forL. 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 popula- tions were higher in plots 4.5 or 30 m from the field’s edge than in plots 60 m from the field’s 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. Key Words: Oryza sativa, rice water weevil, field border, border treatments RESUMEN Durante los años 2009 y 2010, cinco experimentos fueron realizados en campos comerciales de arroz en el valle del río Sacramento en California, para confirmar observaciones previas respecto a la distribución de inmaduros del gorgojo acuático del arroz, Lissorhoptrus oryzo- philus Kuschel (Coleoptera: Curculionidae), cerca a los bordes del campo y su impacto en el rendimiento. En cada campo se establecieron un par de parcelas experimentales, una tratada con insecticida y otra sin tratar, a 4.5, 30 ó 60 m del borde del campo. En dos fechas se colectaron muestras de suelo para determinar la densidad poblacional de inmaduros del gorgojo. Para determinar los rendimientos, en el 2009 se consecharon las parcelas expe- rimentales completas (15 m2) y en el 2010 se cosechó una muestra de 1 m2 por parcela. El resultado del análisis de variancia mostró que en la mayoría de los experimentos las poblaciones de inmaduros del gorgojo fueron mayores en las parcelas que estaban a 4.5 ó 30 m del borde del campo que en las parcelas que estaban a 60 m del borde del campo. Los rendimientos de las parcelas tratadas o sin tratar fueron estadísticamente similares. No se encontró una relación lineal inversa entre el rendimiento de las parcelas y el número de in- maduros. Los resultados indican que las aplicaciones de insecticidas en los bordes del campo para controlar L. oryzophilus son apropiadas. Sin embargo, los productores deben monito- rear sus campos para confirmar la presencia deL. oryzophilus en los bordes y su efecto en el rendimiento. Finalmente, se incluye una discusión acerca de la necesidad de desarrollar métodos de muestreo y niveles de daño economico. The rice water weevil, Lissorhoptrus oryzophi- In California, border and levee insecticide ap- lus Kuschel (Coleoptera: Curculionidae), is the plications are recommended for L. oryzophilus most important pest of rice, Oryza sativa L., in control (Godfrey & Espino 2009). The majority (> the United States (Way 2003). The insect was 99%) of applications with insecticides labeled for first found in California rice in 1959 (Lange & this pest are applied by an airplane (DPR 2009). Grigarick 1959) and has since been considered Generally, applications consist of 1 or 2 passes an important pest (Godfrey & Espino 2009). Both along field borders and 1 pass over levees. Average adults and larvae feed on rice, but only the larvae, airplane swath is 15 m, therefore, 1 or 2 airplane which feed on developing roots, are considered passes will cover 15 or 30 m from the field border, damaging. Larval root pruning causes stunting, respectively. This recommendation was developed chlorosis, and reduction of shoot biomass and til- soon after L. oryzophilus was found in California, lers produced (Zou et al. 2004a). and is based on field observations and few pub- 446 Florida Entomologist 95(2) June 2012 lished studies (Lange & Grigarick 1959; Grig- ducted in a no-till, stale seedbed, water-seeded arick 1964, 1970; Cuneo & Godfrey 1998). Since field. The stale seedbed was created by irrigating the publication of these studies, many cultural the field to encourage weed germination before and pest management practices have changed in seeding. After the weeds had become established, California rice culture (Hill et al. 2006); however, they were sprayed with a non-selective herbicide border treatments are still common. Validation of and the field was flooded and seeded. Planting, early observations regarding L. oryzophilus bor- flood application,L. oryzophilus sampling dates der distribution is needed so growers can - with and other cultural information are presented in confidence - continue to limit insecticide applica- Table 1. Seeding, water management, weed con- tions to field borders. trol, fertility and cultural practices were typical Rice grain yield losses due to L. oryzophilus in of California rice (Flint 1993; Hill et al. 2006). Af- studies conducted in California have ranged from ter flooding, the flood was maintained in all fields 21% to 86% (Godfrey & Palrang 1996; Hesler et until harvest. al. 2000). In these and earlier studies (Grigarick 1970; Grigarick & Way 1978), artificial infesta- Experimental Plots tion of experimental plots was used to simulate commercial field conditions. Results from these At each experimental site, plots were estab- studies show that the potential for yield reduction lished 4.5, 30 and 60 m from one of the edges of exists; however, the impact of natural infestations the field. These distances were chosen considering on commercial fields has not been documented. coverage of typical aerial insecticide applications Research conducted in other rice producing states for L. oryzophilus. One airplane pass around field has shown that yield reductions due to L. ory- borders covered the 4.5 m plots, and two passes zophilus can be greatly affected by cultivar, cul- covered the 4.5 and 30 m plots. In a typical aerial tural practices and other management factors, insecticide application against L. oryzophilus, and environmental conditions (Stout et al. 2002; plots 60 m from the field edge are not treated. The Tindall & Stout 2003; Zou et al. 2004b; Espino edge of the field refers to the field margin formed et al. 2009). Research is needed to determine the by the width of the basin, or the headland. Plots impact of L. oryzophilus in California commercial were 2.5 × 6 and 2.5 × 4.5 m in 2009 and 2010, rice fields under current management practices. respectively. To create a range of L. oryzophilus The objectives of the studies reported herein densities that would produce a range of yields, were to validate observations regarding the an insecticide treatment was included in the ex- prevalence of L. oryzophilus larval populations periments. Treatments assigned to plots were in- around field borders, and to assess their impact secticide application (treated and untreated) and on yield in commercial rice fields. distance from the edge of the field. Treated and untreated plots were separated by a 2.5 m buf- MATERIALS AND METHODS fer of untreated rice plants. Treated plots were sprayed with λ-cyhalothrin (Warrior II, Syngenta Crop Protection, Greensboro, NC) at 33.6 g a.i./ Experimental Sites ha once in 2009 (at flooding) and twice in 2010 (at Five experiments were conducted in commer- flooding and 2 wk later), using a hand-held, CO2 cial rice fields located near Colusa, Maxwell and pressurized, 4 nozzle spray rig (TT110015 Turbo Princeton in the Sacramento Valley of Califor- TeeJet Wide Angle Spray tips, 15 gpa). In 2010, nia during 2009 and 2010. In 2009, experiments to avoid movement of insecticides with irrigation were conducted in Colusa, Maxwell and Princeton water, treated plots were isolated using barriers fields (Colusa-09, Maxwell-09 and Princeton-09 made of roofing metal flashing 20 cm high held in experiments), and in 2010, in Colusa and Maxwell place with wood stakes. The metal flashing was fields (Colusa-10 and Maxwell-10 experiments). pushed into the soil after applying the flood and Colusa-09 and Colusa-10 were conducted in the was removed before harvest. Plots were managed same field, but in different basins. The basins in the same manner as the rest of the field. used were rectangular in shape and between 3.5 and 7 ha. Planting systems differed among years Insect Data Collection and locations. Princeton-09 and Maxwell-10 ex- periments were conducted in water-seeded fields, Three soil core samples (10 cm diam × 10 cm the traditional and most common rice planting deep) per plot were collected on 2 sampling dates, system in California. Water-seeding consists of about 6 to 8 wk after seeding and 14 d later; cores flying pre-germinated seeds onto a flooded field. taken in the first and second sampling dates are The Maxwell-09 experiment was conducted in herein referred to as first and second core sam- a dry-seeded field. Dry-seeding is performed by ples, respectively, as indicated in Table 1. To take flying dry seed onto a dry field and immediate- cores, plots were visually divided in thirds and a ly applying water to promote seed germination. core was taken from each third, at 1 m from one Colusa-09 and Colusa-10 experiments were con- of the sides of the plot.