Invertebrate Pests-90
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Project Leader and Principal UC Investigators A.A. Grigarick, Professor of Entomology, UC DavisM.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.
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.
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.
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. |