Protection of Rice from Invertebrate Pests, 2019


Ian Grettenberger, UCCE Specialist, Dept. of Entomology and Nematology, UC Davis

This ongoing project focuses on the biology and management of invertebrate pests of California rice. The main pests studied in 2019 were rice water weevil, tadpole shrimp, and armyworms. The goal of this work is to refine and advance cost-effective integrated pest management practices that protect the rice agroecosystem.

Rice water weevil

Rice water weevil (RWW) has been in California for more than 60 years and has been the subject of considerable research. In recent years, RWW populations have been in decline and its status as a pest has been shifting. Natural populations were very low in 2019, creating new challenges for researchers.

Coupled with the fact that rice planting was late, not enough RWW adults could be collected to infest research plots at levels used historically. A lab colony supplied over half the RWW used. Because of the low weevil numbers and stand establishment problems, several of the RWW trials were deficient. Testing of insecticides for RWW control was not successful. Treatments that were promising in 2018 will need to be explored further with higher levels of RWW infestation.

The extent of the spring RWW flight has been studied for decades. Monitoring helps scientists assess population levels and flight timing. A light trap at the Rice Experiment Station monitors the flight. In 2019, trapping began in early May. Only 41 adults were caught through July, which was slightly more than the previous year but still one of the lowest number of RWW adults since 1969. For perspective, more than 15,000 weevils were captured in 1969, 1981, and 1989.

Most growers and pest control advisers no longer consider RWW a key pest. The reasons for its decline are unclear but may have to do with the use of improved varieties that can withstand injury better, changes in agronomic practices such as reduced burning and improved straw management, or changes in pest management.

Variety susceptibility

In 2019, seven varieties were grown in aluminum rings to evaluate the susceptibility of rice varieties to RWW injury. The rings were infested with three RWW adults on June 21 and three RWW adults on June 24. For each variety tested, a Warrior® II treatment was included in one treatment along with an untreated control.

There were no RWW larvae or adult scarring in the treated plots, probably due to both the combination of insecticide treatments and very low outside RWW pressure. There were no differences in number of larvae or percent leaf scarring among varieties. Also, because of the low RWW infestation levels, there were no detectable yield or biomass differences.

Tadpole shrimp

Tadpole shrimp are aquatic crustaceans adapted to live in vernal pools. Conditions in rice fields make great habitat for this arthropod. Tadpole shrimp have been a problem in California rice fields since the 1940s. Today, most pyrethroid applications target this pest during the seedling stage. However, there have been confirmed reports of increased tolerance of tadpole shrimp to pyrethroid insecticides, so new tools for control are necessary.

In 2019, leveed plots were established at the Rice Experiment Station in a basin with high levels of tadpole shrimp (TPS). The rice variety used in this research was M-206. TPS populations in the basins were not uniform, developing in some plots but not others.

Nonetheless, complete control was found one day after treatment with post-flood applications of Warrior®, Dimilin® (at two rates), Belay®, and Sevin®. Reduced label rates of Dimilin® and Belay® also proved effective. A treatment with Ferrox reduced populations in a small cage trial, but only by 66% after three days. It did not reduce TPS in large plots.

A field with a suspected pyrethroid-resistant TPS population was examined. TPS were collected from the field and brought back to the lab for bioassays. All TPS died in the Warrior® treatments, confirming that the population was not resistant. Application problems may have lead to poor levels of control and reports of suspected resistance.


When armyworm defoliation of rice plants becomes noticeable, control with insecticides is more diffucult because the pest is further along in its lifecycle
Historically, armyworms have been considered a secondary pest in California rice. Outbreaks occurred but weren’t severe for more than 25 years. But in late June 2015, an armyworm outbreak caught the rice industry by surprise. Growers and pest control advisers tried unsuccessfully to control infestations with pyrethroids. The reason for this sudden emergence of armyworms as pests is unknown. The rice industry obtained a Section 18 registration for Intrepid® (methoxyfenozide), and field reports have been positive. Use of Dimilin® is limited to early season applications.

One problem with managing armyworms is that scouting is based on defoliation. Small armyworms are difficult to find. When defoliation is noticeable, armyworms are much further along in their lifecycle and are more difficult to control with insecticides. Similarly, infestations during the heading stage are difficult to detect before injury occurs to the panicle.

To improve armyworm monitoring, pheromone traps were established in the Sacramento Valley in seven locations in 2016 and 2017. In 2018, pheromone traps were placed in 16 locations across the valley. Traps were also established near rice fields in the Delta.

In 2019, pheromone traps were used in 15 locations across the Sacramento Valley. At each location, three traps for each species—true armyworm and western yellowstriped armyworm—were used. Moths were counted weekly from May until fields were drained and rice was not at risk of injury. Six sites were monitored for larvae. At three of these sites where significant numbers of larvae were found, larvae were collected and preserved in alcohol.

Armyworm pressure in 2019 was similar to 2018. Growers and pest control advisers used Intrepid® during peak activity, limiting the impact of armyworm defoliation on the crop.

Early in the season, the number of western yellowstriped armyworm moths caught was at the highest level since 2016, peaking in May last year. Although adult numbers were high, larvae were not found in rice fields.

True armyworm moth numbers peaked July 1, with a second, smaller peak later in August. Both peaks coincided with the most larvae activity. There was no clear relationship between moth captures and larval counts, although high larval counts only occurred with high moth captures. This demonstrates that the link between adult moth captures and damage-causing larvae is not always a direct linear relationship. The sites with the highest number of larvae capture did have high moth captures, but some sites with high moth captures had few larvae. Experience from the last four years of pheromone trapping indicates that fields with low catches (30 or less) do not experience severe defoliation. When moth catches are high, defoliation may or may not be severe. The influence of different natural enemies on armyworms (insects, spiders, birds, and small mammals) is not currently known.

Average number of true armyworm (TA) and western yellowstriped armyworm (WYA) moths caught in pheromone traps
More frequent trap monitoring could improve detection of armyworm peaks. Other trap types such as traps that offer real-time data daily, could also be used to better time scouting and treatment needs. Various companies are working on automated traps and deploying them for crops across the United States.

In 2019, researchers initiated trials using automated traps. While they appear promising, more work needs to be conducted on trap design. Also needed is a better understanding of how moth captures relate to larval populations and larval development. A development model to predict when the damage-causing late instar larvae are present, coupled with better monitoring, could lead to better prediction of infestation risk and better timing of insecticide applications.

Insecticide testing on armyworms

Insecticides are the primary management tool for armyworm control. Several insecticides were tested in a commercial rice field in Butte County. All larvae collected were identified as true armyworm and most of those were at the sixth instar.

Intrepid®, Coragen®, and Prevathon® provided the greatest level of control at seven days after treatment (consistent with prior trials). Dimilin® and DiPel® provided control to a lesser extent. Combining Spear®-Lep—a spider venom-based product—with DiPel® did not enhance the latter’s efficacy. This trial again showed that Warrior® does not provide good armyworm control.

More testing is needed for insecticide efficacy. To expand the ability to do so, true armyworm larvae were collected from the field and put on rice plants in a greenhouse to start a colony. Researchers are working on improved rearing techniques to make this possible.

Defoliation simulation study

Current thresholds for armyworm monitoring were derived from research conducted in the 1970s with varieties no longer in use. The recent armyworm outbreak has once again brought the issue of thresholds to the forefront. A study continued on the effect of artificial defoliation on rice yield to simulate armyworm damage.

A trial at the Rice Experiment Station was set up June 15 in a basin seeded with M-206. Plots were subjected to four defoliation treatments: none, 25%, 50%, and 100% of plant height above the water. When plants reached second tiller July 24, defoliation treatments commenced.

Plant height was significantly affected. Plots that were 100% defoliated, had significantly shorter plants than the rest of the plots. They also had a significantly lower proportion of panicles emerge than the other treatments. Heading also was was delayed by 10 to 12 days compared to the 50% and 25% treatments, respectively.

Yields were significantly affected by defoliation treatments. Plots defoliated to the water line had a significantly lower yield (about 25%) than plots of all other treatments. No yield loss was evident at either the 25% or 50% defoliation levels.

Because of the late planting date, this experiment should be repeated with more normal planting times to determine the effect of defoliation under those conditions.

Nontarget studies

Evaluation of the influence of registered and experimental insecticides on populations of nontarget aquatic invertebrates is a continuing focus of research.

Treatments were applied at different times during the development of the crop when insecticide applications typically occur in rice. Basins were flooded on June 5 and seeded the next day with M-206.

Two sampling methods were used. Animals were collected with an aquarium net weekly from four areas in each plot. Mosquito dip samples were used to estimate populations of mosquito larvae with 25 dips in each of five locations per plot. Data were collected weekly from August to September.

Collecting the samples for this study is completed during the growing season. Separating, counting, and identifying the specimens in the laboratory occurs in the off-season. Quantification often takes 12 months, so data from 2019 are still being processed. For the 2018 data, there were no significant differences in mosquito larvae populations between treatments. For nontarget invertebrate arthropods, there were no extremely large differences between different treatments.

Stink bugs and other invasives

No new arthropod pests were found affecting California rice in 2019. However invasive stink bug species continue to be on the radar of entomologists, specifically the brown marmorated stink bug.

Brown marmorated stink bug has been expanding its range in California and is in some of the rice producing counties. Research conducted by this project in the past has shown that the brown marmorated stink bug can cause peck in rice. Peck is a kernel discoloration that forms as a result of stink bug feeding. This stink bug has not been found feeding on rice.

In addition, the channeled apple snail remains a pest of concern since it has been present in Contra Costa, Riverside, San Diego, Los Angeles, and Kern counties. If its distribution increases, the snail could threaten rice crops in California, as they have in Asia.