Protection of Rice from
Invertebrate Pests - 2005



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Project Leader and Principal UC Investigators

Larry D. Godfrey, extension entomologist, Dept. of Entomology, UC Davis




Invertebrate pest research in 2005 focused primarily on rice water weevil (RWW) and armyworms, a pest of increasing concern. The broad goal of this work is to refine Integrated Pest Management techniques for both these pests. Best Management Practices have been developed for the rice industry to aid in the mitigation of mosquito populations, an area of great concern with the arrival of West Nile virus in California. Another study continued to examine the effects of registered and experimental rice insecticides on non-target invertebrates in rice fields.

RWW flight and biology

The 2005 RWW flight was again comparatively light, with 978 adults trapped at the Rice Experiment Station. The timing of the adult RWW flight was delayed in 2005, as was rice seeding, because of the cool, wet spring. In 2005, there were minor flights between April 26 and May 2 and from May 11 to May 13. The majority of the flight occurred between May 20 and May 26.

Twelve rice varieties were compared for susceptibility to RWW infestation and rice yield loss. There were significantly more larvae in M-206 and M-205 than in Calhikari-201. Larval populations were similar among other varieties. An 11-fold difference in larval populations was noted between the least and most susceptible varieties. However, the overall RWW population was too low to substantially impact grain yields. In six of the varieties there was a yield advantage averaging 8 percent for the plots where RWW was controlled.

Adult and larval weevil numbers, as well as armyworm populations, were evaluated in a study of rice systems and seedling establishment. Infestation in this plot was low. Adult scarring did not differ significantly among treatments but did range up to 18.8 percent scarred plants in a stale seedbed, no-till, water-seeded treatment. This was about three times more damage than in drill-seeded plots. Larval populations were significantly higher in a delayed spring-till, water-seeded treatment than in the stale seedbed, no-till, drill-seeded treatment.

RWW chemical controls

Studies on RWW in 2005 included ring plots and large field plots that evaluated experimental and registered insecticides for weevil control, as well as efforts to refine application methods. Altogether, 10 different active ingredients were examined in 22 ring plot treatments.

Testing continued on three experimental active ingredients—etofenprox, dinotefuron and indoxacarb. With the loss of Icon® in Southern rice and developing environmental concerns about pyrethroids, agrichemical companies are showing renewed interest in RWW control products.

Excellent activity was shown from some treatments of all three of these active ingredients. Etofenprox applied at the three-leaf stage of rice provided very good weevil control but was ineffective in a preflood application. Indoxacarb applied at three leaf was very effective for RWW control, although the highest rate was needed. At a lower rate and in a preflood application, Indoxacarb was ineffective. Dinotefuron was also effective, although somewhat less so than the other two products.

A new experimental compound, V10170, provided nearly 100 percent weevil control at the rates and application methods tested.

A biological material was also evaluated in ring and field tests in 2005. A granular formulation of Azadirachtin (Neemazal) tested at two rates did not provide weevil control. In a second field trial, Neemazal was ineffective in six tests using different rates and application methods. However, a liquid formulation of this compound showed moderate activity. Greenhouse studies with this material, especially the liquid formulation, demonstrated a definite rate response.

Ring plot tests also examined preflood applications of pyrethroid insecticides for weevil control. Warrior,® Proaxis® and Mustang® applied four days before flooding provided good larval control.

Armyworm studies continue

Armyworms have developed into a significant rice pest in the last five years. In some areas a mid-season insecticide treatment is now common.

True Armyworm

Yellow Striped Armyworm

Two species of armyworms are present in the Sacramento Valley—the western yellow-striped armyworm (Spodoptera praefica) and the “true” armyworm (Pseudaletia unipuncta). Armyworms damage rice through defoliation and by feeding on panicles and rice kernels. The latter damage is much more important.

The two species of armyworms have similarities and differences. Numerous weed species are known to be suitable hosts for both. In many instances, western yellow-striped armyworm develops first on weed or rangeland plants before moving onto crops. It lays its eggs on broadleaf weeds and prefers to feed on these plants more than rice. Therefore, weed populations may influence populations of armyworms.

Entomologists studied this relationship in more depth in 2005 with four different plots—one site with very few weeds; a predominantly grassy weed site; another site with predominantly broadleaf weeds; and lastly a site with both grassy and broadleaf weeds. Armyworm populations were fairly low throughout the test. Numbers peaked on August 5 and were highest in plots with no weed control. These plots also had the most weeds and were primarily infested with ricefield bulrush, bearded sprangletop and some arrowhead.

Pheromone traps were also used to study the timing of armyworm adult flight. Separate traps for each species were placed near rice fields in four locations in Colusa County and at three locations in Butte County. In addition, larval populations were monitored in six rice fields in Colusa County and at seven rice fields in Butte County. Armyworm moth captures in 2003 and 2004 exhibited distinct peaks of high activity. These peaks are reflective of different generations of this pest. In 2005 true armyworm peaked in mid-July and again in late August. Western yellow-striped armyworm moth captures showed no peaks in 2005 but more of a constant, low flight during July and August. This research shows that pheromone traps could provide an effective forewarning of armyworm infestations.

Entomologists also reared armyworm larvae in a laboratory test to evaluate the potential for control with two different parasitic wasps. Additional research in this area is warranted to determine whether a biological control of the armyworm pests is feasible.

Non-target studies

Studies of insecticides on non-target organisms continued in 2005. Preflood applications of Warrior® had minimal effects on the number of aquatic insects and the number of invertebrates in 2004 research. In post-flood applications, some products (i.e. etofenprox and Mustang®) showed some short-term detrimental effects on populations of aquatic insects. Furthermore, most of the five products examined reduced levels of other invertebrates for two to three weeks after application. Some of these reductions were in the range of 50 percent. However, after this initial reduction, populations recovered and were not affected the remainder of the season. Warrior® was also evaluated as a material that could be applied against armyworms in mid-July and proved quite damaging to these non-target populations.


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