Chemical Controls

In small research plots - the metal ring studies at the Rice Experiment Station - DimilinŽ afforded control of RWW larvae for up to four days. Its effectiveness began to diminish after that. Protection of yield was greatest when infestation occurred between two and six days after DimilinŽ application.

In another ring plot study WarriorŽ efficacy began to erode when rates were dropped from 3.8 (the standard rate) to 3.2 ounces active ingredient per acre, an indication that this product is fairly rate sensitive. RWW control with a preplant incorporated application of WarriorŽ was characterized as surprisingly successful since RWW adults were introduced into the plots two weeks after application. In addition, the properties of WarriorŽ suggest the material is not active in this manner. These results need to be verified through more research.

Experimental products studied in 2000 included MustangŽ, IconŽ and NovodorŽ. The first two products were very effective against RWW, the latter only moderately so. Mustang (zeta-cypermethrin) is a pyrethroid product applied post-flood. IconŽ (fipronil) is applied pre-plant or to seed and is registered for use in the rice producing states of the South. NovodorŽ is a biological insecticide of Bacillus thuringiensis tenebrionis.

DimilinŽ and WarriorŽ were also evaluated in grower field tests at seven locations. DimilinŽ was applied alone and in a tank mix combined with WarriorŽ at various plant growth stages and with two rates and placements (full basin versus border only). Timing studies showed the most effective larval control with an application at 50 percent rice emergence and an 8.3 percent yield gain from this treatment. This timing was slightly earlier than studies in 1998 and 1999, indicating some environmental (seasonal) effects. Studies of DimilinŽ rates (eight and 12 ounces/acre) and placements showed no definite trends. The DimilinŽ plus WarriorŽ tank mix (six plus 1.92 ounces/acre) showed some advantages in larval control.

Greenhouse studies were used to further define the activity of key insecticidal products. Studies with DimilinŽ suggest that this material is somewhat effective against newly hatched RWW larvae. NovodorŽ provided 100 percent RWW control with applications one day before egg laying to two days after egg laying.

RWW Flight

The timing of RWW adult flight has been monitored in the spring for nearly 40 years with a black light trap at the Rice Experiment Station. Monitoring weevil flights is important to determine the levels and intervals of peak flight periods and to compare RWW trends over time. (See bar graph above.) The switch to an adult RWW control program with the use of post-flood insecticides has placed even greater importance on understanding flight timing.

Conditions were ripe for RWW flight in mid-May when there was a very high, distinct peak in the flight. Some slight activity was also observed in late April. Compared to the previous two years, the RWW flight was greater in numbers and more concentrated in 2000.

With funding from the California Department of Pesticide Registration, RWW was monitored at 15 other locations also. Somewhat more flight was recorded during the mid- to late April period at these other locations. Flight magnitude was greatest in Butte County, compared with other locations. Therefore, the 2000 RWW flight could be characterized as a slightly later and higher than normal year.

RWW Biology

The short residual of DimilinŽ and WarriorŽ in water could mean multiple applications may be needed to effectively manage RWW in seasons with a prolonged flight. Post-flood materials provide a "window" of control about seven days long, prompting a key question. How long during the season is RWW control needed to protect yield. Season-long control is not practical, but significant residual control could be derived from multiple applications. A better understanding of the cost/benefit ratio of additional applications is essential.

Entomologists examined RWW infestations at the two-leaf stage and every seven days thereafter for five weeks. These were controlled studies in rings so the infestation timing and severity could be manipulated. At two-leaf stage and seven days later, the RWW infestation had a negative impact on rice grain yield. Estimated yield losses were 23.5 percent and 17 percent, respectively. Later timings showed no impact on grain yield. These are preliminary findings that will need further verification. Nonetheless, if additional research bears similar results, this bodes well for a single, well-timed application of a post-flood material providing acceptable yield protection.

The switch to post-flood insecticide treatments in 1999 has rendered most of the existing economic thresholds obsolete. A new sampling tool - a floating trap - developed at the University of Arkansas holds promise for measuring in-field populations of RWW. Additional research is needed to determine the utility of this trap in the California water-seeded rice system.

Cultural Controls

Two aspects of cultural control were studied in 2000. RWW populations were monitored at the long-term straw management study site in Colusa County. Adult scarring was 60 percent and 45 percent in non-flooded and winter-flooded fields, respectively. Larval populations were nearly double in the non-flooded fields compared to winter-flooded fields. This continues a trend observed in previous years.

The relationship between RWW damage and rice variety has also been studied. At present, there are no California rice varieties resistant to RWW. However, there may be differences in the susceptibility of common rice varieties to RWW. In untreated plots larval populations were highest in M-104 and L-205. Grain yields across all treatments were highest in treated M-202 plots. Treatment for RWW, even at relatively low infestation severity, increased grain yield by 13.8, 3.4 and 1.6 percent in M-202, M-205 and the experimental line PI5062130, respectively. For the five other varieties in the study - Calmati 201, Calihikari 201, M-104, M-402 and L-205 - the treated and untreated plots yielded an equal amount or in some cases the untreated plots had a slight yield advantage over treated plots.