Invertebrate Pests-90




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Project Leader and Principal UC Investigators

A.A. Grigarick, Professor of Entomology, UC Davis

M.J. Oraze, Postgraduate researcher

L. Hesler, Research assistant

D. Palrang, Research assistant


The primary invertebrate pest of concern continues to be the rice water weevil. The spring 1990 flight was extremely light, with 604 adult weevils trapped at the Rice Experiment Station in Biggs. That's a 25-fold decrease from the previous year.

UC Davis Entomologist Al Grigarick addresses growers about his studies into the rice water weevil.

The first and major flight peaked between March 30 and April 15. The flight was 98 percent complete by May 4, so fields flooded after that date should have been relatively .weevil-free. RWW infestation in relation to flooding date was evaluated at several Butte County rice fields.

In general, later flooding resulted in lower infestation levels (see illustration). Fields flooded on or after May 13-15 had infestations that were only near or even below control action guidelines.

Researchers also examined the influence of habitat on RWW flight behavior and physiology. In a greenhouse experiment, they measured the frequency that weevils fly out of confinement chambers with differing water-vegetation-soil conditions. The majority of weevils left chambers without a rice and water combination. One unexpected yet consistent trend was for up to one-third of the weevils to depart from conditions that offered good opportunity for both feeding and reproduction.

A related experiment showed 50 percent weevil mortality in four days under dry soil conditions. Also, weevil flight muscle degeneration was greatest in flooded conditions without rice. This suggested that a flooded field with no emergent rice is harmful to the weevil because of the lack of substrate upon which to rest or feed.

"An experimental pyrethoid insecticide showed control of adult weevils "

Results of tests with the experimental pyrethroid insecticide cycloprothrin (United Agricultural Products) showed control of adult weevils comparable to carbofuran. Researchers say the results are promising but timing of the treatment will be critical to its effectiveness.

Evaluation of the nematode, Steinernema carpocapsae, for control of RWW larvae was conducted in the field and the greenhouse. The previously noted light infestation presumably prevented significant differences among the treatments. Even so, the highest nematode rate, the lowest nematode rate and a comparative carbofuran treatment had 51 %, 69% and 39% fewer RWW larvae than the control.

To refine the application technique, the researchers also conducted a greenhouse experiment to test nematode rates and moisture conditions. While there was no significant difference between nematode application rates, it appeared that a longer water holding period (four days compared to two) may have been detrimental to both nematodes and plant growth.


This bar graph illustrates how delayed flooding may help growers control damage from rice water weevil.

In field experiments during previous years, early season drainage has shown promise as a method of cultural control. Although rain may have affected results from last year's field experiments, researchers noted a trend of reduced RWW eggs per plant in drained fields compared to the continuously flooded fields. Removal of field water for durations ranging from four to 18 days showed an adverse impact on RWW reproductive behavior and physiology

"Cultural controls showing promise include late flooding, early drainage and weed management."

Researchers did note, however, that mature larvae did not appear to be susceptible to the effects of drainage. In fact, drainage treatments directed at mature larvae were so severe that they killed the rice plants but failed to significantly reduce larval levels. There were no significant differences among any of the treatments at the light to moderate infestations in 1990.

Levee vegetation management may also be a cultural control option. Researchers found a reduced RWW infestation on fields adjacent to bare levees in four of five paired associations. The reduction may have been caused by removal of overwintering habitat or that bare levees were less attractive to migrating weevils. Or both factors may have come into play.

Researchers examined the relationship between broadleaf weed control and weevil damage on yield in a field test with Londax® and MCPA. The Londax®-treated field showed lower levels of weevil damage than the plots treated with MCPA or the control.

Rice plants were able to recover from weevil damage more readily when there was less weed competition, as was the case in the Londax®-treated field. No significant yield differences were found when weevils were absent during a light weed infestation. This shows that RWW could contribute to a light weed infestation becoming a more important factor.

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