The tadpole shrimp [Triops longicaudatus (Leconte)] has emerged as a significant pest of rice grown in California in recent decades. The change in T. longicaudatus’ pest status has coincided with changes in cultural management of residual rice straw postharvest. Policy changes have reduced the postharvest burning of fields from nearly 95% to less than 10%, promoting increased use of winter flooding as a means of accelerating straw decomposition. Field and laboratory trials were conducted from 2015 to 2017 at the Rice Experiment Station in Biggs, CA and in greenhouses at the University of California (UC) Davis to evaluate the effects of burning, flooding, and a fallow control on T. longicaudatus population dynamics. Experiments demonstrated that burning of rice straw failed to suppress densities of hatching T. longicaudatus and actually had the reverse effect, causing a 51% increase in numbers hatching, perhaps as a result of burning triggering termination of multiyear T. longicaudatus egg dormancy. Winter flooding had no measurable effect on T. longicaudatus hatch. Thus, these changes in winter cultural practices do not appear to be responsible for the emergence of T. longicaudatus as a major rice pest.
Rice water weevil (Lissorhoptrus oryzophilus Kuschel) is a common pest of rice production in the United States whose larvae cause yield loss by feeding on the roots. We conducted studies from 2011–2013 on M-202 and M-206, two commonly grown California medium grain rice varieties, to determine if M-206 demonstrated tolerance to rice water weevil damage. Observations from field studies suggested the possibility of a level of tolerance in M-206 that was more prevalent at high seeding rates. We did this study using two different experimental units, open and ring plots. In both units, we quantified grain yields across four levels, 56, 112, 168, and 224 kg/ha, of seeding rates to detect potential yield recovery by M-206. In the open plots, we used naturally occurring weevil populations compared with controls that reduced the populations with insecticides. In the ring plots, we tested three levels of weevil infestation, none, low, and high, to look at the weevil density effects on yield and scarred plants. Our studies showed that M-206 and M-202 had generally similar densities of immature weevils and yield. Compensation for yield loss did not occur at higher seeding rates. These results suggest that M-206 does not have the ability to tolerate rice water weevil damage better than M-202. There was weak evidence that the number of scarred plants increased as plant density was reduced. The results are discussed in relation to the utility of this study to grower choices of varieties for long-term rice water weevil management.
The tadpole shrimp (Triops longicaudatus (Leconte); Notostraca; Triopsidae) is a vernal pool crustacean that has emerged as a significant pest for many rice farmers in California. Currently, lambda-cyhalothrin is the commercial standard and sole management practice for TPS, but resistance appears to be emerging. Field and laboratory trials were conducted from 2015 to 2018 at the Rice Experiment Station in Biggs, California, and in greenhouses at the University of California (UC) Davis to evaluate the efficacy of several alternative chemical controls and to explore early indicators of TPS activity and damage as monitoring tools. TPS mortality in plots treated with azadirachtin was not significantly different from those in the untreated control across all trials. Copper sulfate, lambda-cyhalothrin, and clothianidin were consistently effective across both lab and field trials. Chlorantraniliprole generated effective control in four of the five trials, whereas two diflubenzuron rates generated adequate control in the field, but not in laboratory bioassays. Thus, there appear to be several effective alternative control options available for farmers, should resistance to lambda-cyhalothrin continue to spread. Water turbidity (measured using a scale of 1–4) and the number of dislodged seedlings were both significantly correlated with TPS counts in field studies. Turbidity exhibited the highest correlations with TPS counts, suggesting that with further refinement it could play a valuable role in monitoring TPS populations.
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