Sticky traps proved to be a cost-efficient alternative to light traps for monitoring flight activity. Catches in thirteen sticky traps at four locations coincided closely with light trap catches.

A survey based on locating adult weevils or feeding scars showed that the weevil has not extended its southern range beyond Merced County.

Chemical controls Produce Mixed Results

Post-flood applications of diflubenzuron, an insect growth regulator that affects the hatch of rice water weevil eggs, produced no obvious visual differences between treated and control plots.

The number of larvae recovered in rice plant core samples were not significantly different among the three treatment levels, but they were statistically less than the number found in the control plot samples. Plant growth characteristics and yields did not significantly differ among any of the treatments.

A second field experiment using diflubenzuron as a post-flood border spray, the experimental organophosphate SC-0567 in pre-flood and post-flood treatments, and carbofuran also produced no significant differences in plant growth among treatments or between treatments and control plots. However, the carbofuran treated plots had significantly fewer weevil larvae than the control plots. There were no significant differences in the number of larvae when the various chemical treatments were compared.

Two other experimental chemicals, fortress (an organophosphate) and flufenoxuron (a growth regulator), and carbofuran were compared in greenhouse studies. Carobofuran killed all of the adult weevils. The experimental chemicals killed only five to 15 percent. Carbofuran also did significantly better in controlling water weevil larvae.

Genetic Resistance Search Shows Promise

Five promising lines from previous tests for genetic resistance to weevil damage and 13 promising lines from last year's tests by plant breeders at the Rice Experiment Station, Biggs, were compared with M-202 and M9 in treated (carbofuran) and untreated plots.

Weevil damage reduced yields from 5.8 to 25.8 percent in the seven top ranking lines. Damage to M-202 and M9 was 50.4 percent and 48.9 percent, respectively.

Last year's most promising line, 86-2404, ranked third in overall growth damage caused by weevils and seventh in weevil effect on yield.

Two studies to obtain clues on how plant tolerance works were completed.

One, a greenhouse test comparing the line 86-2404 wtih M9, demonstrated that young plants are most susceptible to weevil injury. Tolerance did not play a role in preventing injury to 14-day-old plants, but it did reduce the extent of injury in 21-day-old plants.

The other, a field test, compared the tolerant lines 85-2385 and 862404 with M9. The tolerant lines were less susceptible to weevil damage up until mid-season. During the latter part of the season all three cultivars showed some recovery, but the recovery was greatest for 86-2404. Yield from untreated 86-2404 was 36 percent higher than yield from untreated M9 and 40 percent higher than yield from untreated 85-2385.

Cultural Practices Influence Weevil Damage

Studies suggest that weevil damage to rice plants can be reduced by waiting to plant until 90 percent of the weevil flight has been completed.

Two planting dates for M-201 - April 26 and May 11 (after 90 percent flight) - were compared. In the first planting, 92.4 percent of the plants had weevil feeding scars. In the second planting, 64.8 percent of the plants were scarred. Plant growth was greatest in the second planting, but there was no significant difference in yield between the two planting dates.

The effect of seeding rate on weevil damage was studied using M201 planted at rates of 100, 200 and 300 pounds per acre. Visual ratings of both treated (carbofuran) and untreated plots for each seeding rate indicated that weevil injury decreased as seeding rate increased. The greatest differences were between the 100 and 200 pound seeding rates. There were no significant differences among yields in the untreated plots, regardless of seeding rate. However, yield from the treated plot that was seeded at the rate of 300 pounds per acre was significantly lower than yield from the treated plots seeded at the lower rates.

 

Water Weevil Reduce Yields Rice water weevil continues to be the focal point of research in the invertebrate pest project. From a rice field in Butte County back in 1968, the pest has spread throughout California's rice-growing regions deep into the San Joaquin Valley. That's the bad news. The good news is that researchers on this project are making significant progress in developing tools that will limit its damage. Pursuing environmentally sound methods of insect pest control is at the heart of the UC Davis entomologist A1 Grigarick's work. "Rice weevil strictly affects yields," explained project leader Albert Grigarick, UC Davis professor of entomology. After emergence from overwintering sites, adult weevils migrate in greatest numbers to the edges of rice fields. They feed on leaves before laying their eggs. Young larvae then attack roots, where most of the damage is caused and is evidenced by plant stunting and reduced numbers of plant tillers. The result can be up to 65 percent yield reductions. Grigarick's project has a dual purpose - analyzing the physical and biological factors that result in fluctuation and movement of pests and finding the most effective and socially acceptable means of control. Information from the first naturally helps with the second.