Invertebrate Pest Control-88
 

 

 

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Project Leader and Principal UC Investigators

Al Grigarick, professor, Dept. of Entomology, UC Davis

 

Patterns of water weevil infestation flights (April July) and conditions that control fight behavior were targets of research by entomologists.

The study correlated daytime temperatures and degree-days with weevil flights. All peak flights occurred following daytime temperatures above 82 degrees that had been preceded by flightless periods when daytime maximums were in the 60s and 70s. Ninety percent flight completion and 300 degree-days took place on May 20, two to three weeks earlier than in the previous year.

Comparisons: Organic and conventional

THE PERENNIAL PEST-Rice water weevil. Most damage occurs as larvae feed on roots of plants, which reduces growth, tillering, vigor and, consequently, yield. Scientists are continuing their search for varieties with tolerance against this invertebrate.

Three pairs of organic and conventional rice fields in Sutter, Butte and Glenn counties were monitored for both pests and predators. Results indicated that:

  • Tadpole shrimp were more numerous in organic fields. (There was little shrimp damage to seedlings in any field.)
  • There were fewer crayfish in organic fields, probably because of land fallowing.
  • Based on scar counts, there were fewer rice water weevils in organic fields, especially in one which was drill-seeded.
  • Predators were more abundant in organic fields.

Chemical control of weevils

Three experimental pyrethroid-like chemicals, considered relatively safe for wildlife, showed encouraging control of rice water weevils in small-scale tests.

There was little difference among the three (F7869, F6308 and Trebon), and they all reduced weevil damage compared to the controls. Of seven plant growth characteristics measured, only plant height was significantly better in the treated plots. However, growth in general, as well as yield, tended to improve when treated and with the higher rates of treatment.

The pyrethroid compounds show good potential for water weevil control, but more tests are needed. Because the chemicals are used on adult weevils to prevent or reduce egg-laying, time of application is a critical factor.

Genetic weevil control

During 1988 continuing research to develop weevil-tolerant rice varieties was complicated by growing conditions that minimized weevil damage to all the field test plantings-tolerant and susceptible alike. .

In standard field tests, all varieties recovered from early-season weevil damage as a result of good weed control and a somewhat later weevil infestation than in 1987. There was little yield difference between treated and untreated susceptible varieties. (In 1987, yields of untreated susceptibles were reduced by half.) Of seven cultivars carried over from previous years for testing, four maintained high levels of tolerance and three were replaced by new lines with better agronomic qualities.

A similar field test measured plant growth in the search for mechanisms of weevil tolerance. Again, few if any differences in growth were noted. However, one experimental cultivar (87-W-09) showed significantly fewer weevil larvae than the standard susceptible variety. Something about that line, the researchers noted, appears to hamper water weevil development. The same effect did not occur in previous years.

In greenhouse tests, rice plants of different ages (14, 21 and 28 days) were infested with various numbers of weevil larvae. The results demonstrated that younger plants, tolerant or not, are clearly more susceptible than older ones; and that one experimental cultivar (87W-09) carries what the researchers termed "a rather weak varietal tolerance under greenhouse growing conditions."

Cultural weevil control

Could weevil damage be reduced and yield increased by draining rice fields for a week or two shortly after the seedlings emerge from the water? One experiment at the Biggs station produced encouraging results.

After seeding on May 11, half of the paddies were drained for 10 days in early June-long enough so the rice plants showed signs of wilting. Counts of immature weevils at the end of June were 89 percent lower in the drained fields. At harvest the drained treatment produced 10,557 kilograms/hectare, compared to 9,824 kg in the continuously flooded fields. It appears, the researchers concluded, that the lower weevil population offset the stress caused by drying out the field. Small-scale greenhouse trials gave similar results, with 76 percent fewer weevil eggs found in soil that had been drained.

Since weevil flight depends on accumulation of degree-days and on evening temperatures, another experiment was designed to see what effect delayed planting dates might have. The experiment ran through maximum weevil flight. Fewer larvae and less feeding activity were found in the two plantings following the maximum flight period, but the delays in planting also reduced yield.

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