The 1991 spring migratory flight of the rice water weevil began in mid-April, but the majority of the flight occurred relatively late - after May 1. The three monitoring sites - in Butte, Colusa and Sutter counties - showed a major peak in early May. Weather conditions delayed a second major flight period until the third week of May. No weevils were trapped after June 2.

The season's observations support those of previous years, which suggest that weather plays an important and somewhat predictable role in influencing the occurrence and magnitude of weevil flights. In general, major flights seem to occur when the temperature after sunset is greater than 70 degrees and winds are below seven miles per hour, once a minimum number of degree-day units have accumulated to allow flight muscle development..

Researchers continued to investigate RWW infestation patterns in relation to distance from field margins. Researchers found a slight decrease in the "edge effect" (i.e. greater abundance within the paddy than field margins) in early planted fields compared to later planted fields.

The influence of habitat on rice water weevil flight behavior was studied in the greenhouse by observing the tendency of weevils to fly from confinement chambers with differing conditions.

One experiment tested the weevils' ability to fly from different soil-substrate conditions, while another experiment measured two different habitat conditions. Weevils were unable to initiate flight from flooded soil conditions. Under several dry soil conditions, weevils departed at rates ranging from 40 to 70 percent. In a second experiment, weevils displayed a greater tendency to depart a weedy habitat compared with a habitat containing flooded rice seedlings.

Researchers also examined how different soil-water-vegetation conditions - dry soil, moist soil, flooded soil, moist soil with rice and flooded soil with rice - affect the physiological and morphological characteristics of rice water weevil. Dry soil caused heavy RWW mortality in just four days, while a moderate level of flight muscle degeneration occurred in the other four conditions. Reproductive development occurred much sooner in flooded soil with rice compared to the other conditions.

This and other information learned about the pest's biology and seasonal flight behavior may help researchers develop future alternative control tactics.

Previous studies indicated that delayed flooding may help growers control RWW. However, 1991 field experiments in Butte, Colusa and Sutter counties, found that late flooding and planting dates did not consistently or predictably lower infestation weevil levels. The researchers believe this was due to considerable local variability in infestation level in relation to flight within and among counties, as measured by a light trap at a central location in each county.
 

Growers learn that weed-free levees may be one of the more reliable methods of controlling rice water weevil.

Drill seeding, which could offer growers a means of eliminating or reducing damage by tadpole shrimp and other aquatic invertebrates, was studied for its effectiveness in controlling rice water weevil. Researchers found fewer weevil feeding scars on the leaves of drill-seeded rice than on continuously flooded, broadcast seeded rice on the first count. However, the effect seemed to diminish with time. No significant differences in feeding scars were noted between drilled or broadcast rice two weeks following the first count. Further, lower scar counts were not associated with lower infestation levels of immature weevils on the roots. This may, however, be due to the late spring migratory flight.

Researchers studied for the second year how vegetation removal affected field selection by RWW in two locations in Colusa County and one location each in Butte and Suffer counties. The experiment compared pest damage in fields with bare levees with fields that had vegetated levees.

Scar counts showed that lower infestation levels were associated with bare levees compared to the weedy levees at three of the four locations. Unlike some of the other alternative weevil control methods studied during 1991, vegetation; removal was the only promising method in the majority of fields, irrespective of weevil flight timing.