Evaluation of Rice Water
Weevil Sampling and
Distribution - 2009
 
 

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Project Leader and Principal  Investigators

Luis Espino, UC Cooperative Extension farm advisor, Colusa, Glenn and Yolo counties

 

 

 

In recent years, California rice growers have begun experimenting with alternative stand establishment practices as part of new weed management strategies. Dry-seeding techniques may influence distribution and abundance of California’s most important invertebrate pest, the rice water weevil (RWW). This new research project sought to evaluate sampling techniques, take a fresh look at RWW larval distribution within rice fields, and to assess damage potential and impact to grain yield.

Experiments were conducted in commercial rice fields near the Sacramento Valley communities of Colusa, Maxwell, Oroville, and Princeton. Variety M-401 was grown at the Princeton location. M-206 was grown at the other sites. Agronomic information such as planting date, flooding, dry down, sampling details, and harvest dates was recorded. Infestation levels were high in one field and low to moderate in the others.

RWW adult sampling

RWW adult populations were assessed using feeding scars and an aquatic net. Results indicate that the aquatic net is not an effective tool for decision making because no relationship could be established between the number of adults caught with the net and the larval populations. The aquatic net proved inefficient at capturing adult RWW.

Feeding scars at the three-leaf stage were found to be a good indicator of adult RWW activity and subsequent larval populations. If insecticides with larvicidal activity become available in the future, this may become a useful way to establish threshold treatment levels.

RWW larval distribution

Observations of the number of RWW larvae and adult feeding scars at different field locations indicate more severe infestations near borders and levees. In only one location was the RWW infestation widespread throughout the field. Results confirm that rice field edge and levee treatments are adequate to manage RWW populations.

Grain yield impacts

At only one location, a dry-seeded field, was a significant relationship established between RWW density and yield. Estimated grain yield reduction in this field was 396 pounds/acre per RWW larvae/core. If confirmed, this estimate indicates that RWW control in dry-seeded rice is as important as in flooded fields. Estimates of yield reduction can vary due to factors such as overall RWW infestation levels, varietal differences, and crop establishment practices.  Further research is warranted.

In summary, RWW’s higher prevalence and impact in and around rice field borders and adjacent levees indicate that current management guidelines are appropriate.

 

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