Invertebrate pest research continues its primary focus on Rice Water Weevil (RWW), although armyworms are of increasing concern. The post-flood insecticides Dimilin® 2L, Warrior® and Mustang® are the pesticides used to control these pests. Investigations into insecticidal management are building cost-effective, environmentally compatible management schemes.

A study continued to evaluate the effects of commonly used and experimental rice insecticides on non-target invertebrates, organisms that could play an important role in mosquito management.

Significant progress is reported on all research objectives.

RWW Flight and biology

The 2004 RWW flight was comparatively light. Only 703 adult RWW were trapped at the Rice Experiment Station, where the spring flight has been monitored for more than 45 years. There was a small peak between April 7 and 9, with the remainder of the flight concentrated between April 23 and its completion on May 3.

Monitoring weevil flights is important to determine the levels and intervals of peak flight periods and to compare RWW trends through the years.  Switching to post-flood insecticides (i.e. an adult RWW control strategy) places even greater importance on understanding flight timing.

Research has been done in grower fields the last three years to evaluate the effectiveness of a floating barrier trap to help assess RWW populations and to make better treatment decisions.  The trap most effectively samples adults during the first week after flooding.  This was confirmed in small plot studies when 8 percent of the introduced adults were captured at the one-leaf stage compared with none at the five-leaf stage.

Ten varieties were compared for their susceptibility to RWW infestation and yield loss.  M-202, Calmati-201 and M-401 had the most leaf scarring from RWW adults.  Larval levels were highest in M-202, Calhikari-201 and M-402. Larval populations varied by a factor of five across varieties.  Yield losses, at the larval numbers recorded, were minimal.

Refined seedling establishment techniques, driven largely by weed management concerns, are also being investigated for their influence on insect pests.  Researchers monitored RWW adult and larval populations but infestations were low.

Chemical Controls

Studies continued in ring plots and large field plots to evaluate experimental materials and registered standards for RWW control.

Three new experimental insecticide active ingredients were tested in 2004 — Etofenprox, Dinotefuron and Steward.  Agrichemical companies and regulatory agencies are showing a renewed interest in this area because of the revoked registration for Icon® in Southern rice and the additional scrutiny placed on rice insecticides due to West Nile virus.

Etofenprox and Steward, both applied at the three-leaf stage of rice, were very effective for RWW control.  Etofenprox is used for RWW control in Japan and Steward is registered in the U.S. for control of a related species, the alfalfa weevil.  Dinotefuron was ineffective against RWW in 2004 testing, a problem attributed to product formulation.  This chemical belongs to a class of compounds known to be good soil insecticides and thus warrants continued testing.

Platinum® (soil applied), Proaxis® (post-flood) and F0570 (post-flood) all provided very good RWW control.

Studies were conducted to evaluate possible changes to Warrior use patterns to improve its efficiency and ease of use.  Warrior applied as a soil treatment would provide growers greater flexibility. In 2004 experiments Warrior applied three, six, even 10 days before flooding gave universal RWW control.  Proaxis and F0570 were also included in these experiments and, applied a day before flooding, also provided excellent RWW control.

Finally, a compound derived from seeds of the neem tree was tested.  In a greenhouse test, this azadirachtin product, Neemazal™0.1%G proved 100 percent effective when the material was applied at the time of seeding or 19 days after seeding (approximate time of RWW egg hatch). Slightly less control (81.5 percent) was achieved from an application made 12 days after seeding.  Additional testing of this product is needed, but these results show considerable promise.

Non-target study

In large field plots, registered products, along with Etofenprox, Dinotefuron, Platinum and Warrior pre-flood, were evaluated for their effects on non-target invertebrates.  Based on preliminary data analysis, insecticides used in the preflood treatments had minimal effects on the total number of invertebrates in 2004.  Post-flood applications, however, were more detrimental to numbers of invertebrates, with all five treatments reducing numbers for the first two weeks after application.

The effects of the insecticides were more detrimental in 2003 studies.  Data from 2003 showed that the total number of invertebrates was suppressed by the preflood treatments until early July.  The post-flood treatments were generally detrimental to invertebrates until mid-August.  Dimilin had fewer effects than the other treatments.  The Warrior application made in July (armyworm timing) was particularly harmful to invertebrate populations.  The effects on beetles were significant.  On segmented worms, none of the treatments had any consistent detrimental effect.  It appears that the effect of these materials depends on the species in question and on the season.

Armyworm biology and infestation

Armyworms have developed into a significant rice pest in the last five years.  In some areas a mid-season insecticide treatment is common.  Studies and observations on armyworms in California started in 2003. 

This pest has many host plants.  Interestingly, the western yellow-striped armyworm is reported to lay its eggs only on broadleaf weeds and prefers to feed on these plants rather than rice.  Therefore, weed populations may influence armyworm populations.

Researchers investigated this relationship in 2004.  No armyworms were sampled until late July.  Populations peaked in mid-August.  Fewer armyworms were detected in plots with weeds controlled versus plots with high levels of weeds.

 Pheromone traps were used to gain insight on the timing of armyworm moth flights.  In addition, larval populations were monitored in rice fields in Colusa and Butte counties.

Armyworm moth captures peaked in early to mid-August.  Western yellow-striped armyworm moth captures peaked in early August and true armyworms were trapped about two weeks later.

Armyworm larval populations in rice fields were near zero until August 10 and increased rapidly by August 17 to 7.6 and 9.8 worms per five-minute search in Butte and Colusa counties. Peak larval populations occurred on August 24.

Significant numbers of this pest’s larvae were successfully controlled with a parasitic wasp, an area that warrants additional research. 

The use of pheromone traps could provide a forewarning of the time sampling needs to be intensified for armyworms in rice fields.