Project leader Al Grigarick addresses growers during Rice Day with a progress report on research into developing rice varieties with resistance to the Rice Water Weevil.

The spring migratory flights of adult Rice Water Weevil were extremely heavy, exceeded only twice in the 27 years the pest has been monitored. A massive flight of weevils occurred during late April and early May, when flooding and planting was in full swing. Because the weevils arrived earlier than usual, the rice plants were more vulnerable and, consequently, more heavily damaged. Growers who planted later-as well as the researchers at the Rice Experiment Station did not experience as much damage in their fields. Ironically, this was a problem for the entomologists, who must rely on natural infestations to evaluate experimental control methods.

Weather factors

The researchers discovered that daily weather conditions may play an important and predictable role in influencing the occurrence and magnitude of weevil flights. In general, major flights occurred when temperatures at sunset were more than 70 degrees and winds were below 8 miles per hour. Combined with day-degree data, this information should enable researchers to more accurately forecast peak weevil flights.

Biological and chemical controls

A greenhouse experiment to determine whether nematodes could be used as biological insecticides yielded encouraging results. The researchers discovered that two nematode species applied to drained soils (four to eight days) reduced the number of weevil larvae by up to 87 percent. No control was observed when the nematodes were applied directly into water.

"The researchers discovered that two nematode species applied to drained soils reduced the number of weevil larvae by up to 87 percent."

The researchers conducted a large-scale field test of Dimilin® at four sites in the Sacramento Valley. Unfortunately, three of these sites had light to almost nonexistent infestations, presumably because those fields were planted near the end of the weevil flight. As a result, the effectiveness of Dimilin® on large field plots remains undetermined.

Genetic tolerance

Efforts continue to incorporate weevil tolerance into cultivars with suitable agronomic qualities. Researchers ranked the 10 top experimental cultivars from field tests comparing treated and untreated fields under a natural infestation.

A standard greenhouse test to determine the mechanism of tolerance was conducted at UC Davis. As was found last year, the smaller the plant at infestation the more susceptible it was to damage.

Cultural controls

Drainage of flooded fields at about three weeks after planting has been used as a tool to reduce Rice Water Weevil infestations, but researchers caution that early drainage may also depress yields. Growers need to consider the availability of other weevil control measures, fertilizers and weed management to determine whether the tradeoff is a good one. There are also experiments in progress to see if higher seeding rates may reduce yield losses from weevil damage.

The relationship between broadleaf weed control and weevil damage was further explored last year. Researchers noticed in previous experiments that plots treated with Londax® clearly showed less weevil damage than plots treated with other broadleaf herbicides. While Londax® does not appear to be directly toxic to RWW, rice plants recover more quickly from weevil damage presumably because they've been relieved of early season broadleaf weed competition.