Rice Water Weevil continues to be the most important insect pest in California rice, although armyworms are of increasing concern. Additionally, the recent appearance of and interest in West Nile Virus, prompted a study to evaluate the effects of commonly used rice insecticides on non-target invertebrates. The two main thrusts of 2003 research were to study the biology of invertebrate pests and to refine use of post-flood insecticides such as Dimilin® 2L, Warrior® and Mustang®, with a special emphasis on efficacy and alternative application methods. Significant progress is reported on all research objectives.

RWW Flight

The timing of the RWW adult flight in the spring has been monitored for nearly 45 years at the Rice Experiment Station. The rice industry's switch to the use of post-flood insecticides to control RWW in the adult stage of its lifecycle has placed even greater importance on understanding timing, levels and intervals of peak flights. In 2002 the RWW flight was the lowest seen in recent years at 655, while 2001 saw one of the heaviest at more than 8,000. No RWW flight occurred in 2003 until mid-May because of unfavorable weather conditions. May 21 saw the peak of the flight with more than 800 RWW captured. Total for the season was 2,000 RWW, a significant increase over the previous year.

Chemical Controls

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

In ring plots Dimilin, Warrior and F0570 (a different formulation using the active ingredient in Mustang) all provided excellent RWW control. Other experimentals evaluated last year included Proaxis, Platinum, Cruiser and Icon.

Proaxis is in the same chemical class as Warrior and Mustang. At the three-leaf stage it provided 99 percent RWW control.

Icon, registered in the South, gave excellent control with a soil, pre-flood application.

Thiomethoxam was tested as a soil application (Platinum) and as a seed treatment (Cruiser). The seed treatment was further divided into a treatment on pre-germinated seed and one on dry seed. The soil application ranked most effective against RWW larvae.

Studies also continued on Warrior applied pre-flood to the soil. Applied five days prior to flooding, Warrior provided poor control. A similar treatment in 2002 was very effective. Closer to flooding, Warrior proved as effective as during the standard three-leaf application.

Proaxis and Mustang were also tested pre-flood. At one day before flooding both were extremely effective for RWW control.

Pre-flood experiments were then extended to grower fields. One of these locations had a high RWW infestation and provided a strong test for Warrior. A treatment in June more effectively reduced adult RWW feeding activity than a standard three-leaf application. More importantly, however, it also provided 81 percent RWW larval control, compared to 88 percent for the standard Warrior application. Control with the preflood application seemed to "break" somewhat in July, leading researchers to suspect that the active ingredient had dissipated by that time.

The efficacy of insecticidal applications can make a big difference come harvest. The pre-flood Warrior treatment provided a 1,500 pounds/acre grain yield advantage over the untreated. The standard three-leaf application showed a 2,200 pounds/acre advantage.

Registered products, along with Icon, Platinum and Warrior pre-flood were also evaluated in large field plots for RWW control, as well as for their effects on non-target invertebrates. All treatments provided excellent control against a low-to-moderate RWW infestation. Data on non-target invertebrates are still being evaluated.

RWW Biology

In an effort to identify sources of RWW resistance, nine California rice varieties were evaluated and compared for susceptibility. The nine varieties were L-205, M-104, M-205, Calmati-201, Calhikari-201, M-401, M-202, M-206 and PI506230CK, an experimental line with moderate resistance. Researchers sought to quantify yield effects when these varieties were exposed to an equal infestation of RWW. Some varieties may be more attractive to RWW adults for feeding or egg laying, while others may regrow roots damaged from larval feeding more vigorously. RWW adults preferred M-206, followed by M-401 and M-202 in like amounts. RWW larvae enjoyed munching on M-202 and M-205, all other varieties were considerably lower. Interestingly, M-206 had very few larvae while there was substantial adult feeding on this variety. Conversely, M-205 had a relatively high larval population with little adult feeding.

Research continued in grower fields on the applicability of a floating barrier trap to monitor adult RWW populations. Results from 2003 confirmed the previous two years' observations. Researchers determined that a threshold of 1.5 adults per trap per day during the first 10 days of flooding could be used to trigger control measures.

Another group of studies focused on how long during the season RWW control is needed to protect yield. What is the cost/benefit relationship of applying one or more insecticide applications for RWW. Plant damage and other observations were collected from plots infested at moderate and severe levels. At the highest infestation levels, 100 percent of the plants were scarred. RWW larvae responded well to infestation treatments. Rice grain yields were significantly reduced by severe, early RWW feeding. At the early two-leaf stage, yields were cut in half. A week later yield losses were about 15 percent. However, later infestation timings had no consistent effects on yield.

Although the floating barrier traps have proven themselves a useful tool to monitor RWW infestation levels, there is no commercial source currently available. Entomologists are offering them to growers.

Armyworms have developed into a significant pest of rice during the last five years. In some areas a mid-season insecticidal treatment for them has become common.

Studies and observations were started on armyworms in 2003. Pheromone traps were placed in several Butte County fields starting in late July. Western yellow-striped armyworm moths were captured in August and September with a peak in early to mid-August.

This pest only deposits eggs on broadleaf weeds and will feed on them as well. Researchers suspect recent changes in weed management could be influencing armyworm levels. Scientist will be investigating whether there is a more definable relationship between armyworm numbers and previous insecticide regimes and/or weed populations.