Protection of Rice from Invertebrate Pests, 2017


Project Leader

Luis Espino, farm advisor, UC Cooperative Extension Colusa, Glenn, and Yolo counties

Larry Godfrey, UC Cooperative Extension entomologist, Dept. of Entomology and Nematology, UC Davis

This project focuses on the biology and management of invertebrate pests of California rice. The main pests studied in 2017 were rice water weevil, tadpole shrimp, armyworms, and stink bugs. The goal 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 nearly 60 years and has been the subject of considerable research. In recent years RWW populations have been in decline. The reason for this decline is unknown, but the use of pyrethroid insecticides, changes in straw management, or the drought may help explain it.

A light trap at the Rice Experiment Station monitors the number of adult RWW adults in flight from mid-March until mid-June. Two peaks occurred in 2017—early May and late June. These peaks coincide with adult flight as RWW leave overwintering sites early in the spring and the flight of the first generation of weevils emerging from rice.

Studies of insecticidal management of rice water weevil typically take place in aluminum ring plots, small field plots, and greenhouses to evaluate experimental insecticides and registered standards. In 2017, eight active ingredients were examined in 16 treatments in ring plots.

These tests showed that Warrior®, Mustang®, Belay®, and Coragen® can provide good control. The best timing for control with pyrethroids is preflood or postflood, while Belay® only gave good control postflood. Coragen® is only registered for preflood applications and worked well at the recommended rate. Belay® also provided good control when applied at the 5-leaf stage as a “rescue treatment” when an infestation had been identified and the optimal application time had already passed.

Variety susceptibility 

Host plant resistance is an important part of integrated pest management programs. As new varieties are developed and production practices improve, rice plants are becoming more vigorous and higher yielding. This may also influence pest interactions and rice plant response.

In 2017, varietal comparisons of the susceptibility of rice to RWW injury showed L-206 to be the most tolerant. L-206 showed less adult feeding and resulted in less yield loss when infested with the same level of rice water weevil as M-206 or M-209. Tillering is the plant characteristic most affected by heavy weevil infestations.

Tadpole shrimp

Tadpole shrimp are a component of aquatic ecosystems worldwide, including freshwater, brackish, and saline waters. This organism is considered a living fossil, as it has not changed morphologically for 250 million years. The genera Triops is a pest of rice because it can feed on germinating seedlings and its burrowing behavior uproots seeds and small seedlings. Most pyrethroid applications during the seedling stage of development target this pest. There have been confirmed reports of increased tolerance of tadpole shrimp to pyrethroid insecticides, so new tools for control are necessary.

In 2017, field studies on tadpole shrimp control were conducted in ring and leveed plots. With the exception of Aza-Direct®, all treatments significantly reduced tadpole shrimp density when compared to the controls. Postflood applications of Dimilin®, Warrior®, Sevin®, Coragen®, or Belay® were effective at controlling naturally occurring tadpole shrimp.

It is worth noting that the size of tadpole shrimp being targeted is very small. After a field is flooded, eggs hydrate and in a day or two the first instar develops. The tadpole shrimp in these plots were small at time of application, which facilitated control. Once the tadpole shrimp got to a shell length of about a quarter inch, control with Dimilin®, an insect growth regulator, was slower and took up to three days.

The predatory capacity of mosquito fish and aquatic beetles against tadpole shrimp was confirmed. The practical use of these predators is uncertain, as the number of mosquito fish needed and the timing of their introduction could prove challenging. Predacious beetles are not easy to collect, and the number needed for effective control may also be impractical. However, these results highlight the importance of protecting other organisms present in rice fields early in the season.


Armyworms were a serious problem for rice growers in 2015 and 2016, especially in Glenn, Butte, and Sutter counties. A survey of growers and pest control advisers found that average yield losses in 2015 ranged from 4% to 12%—even after two pyrethroid applications.

Intrepid® belongs to the diacylhydrazine class of insecticides and has a novel mode of action that mimics a molting hormone. It was used on about 40,000 acres in 2017. Yield losses due to armyworms in fields treated with Intrepid® were 29% lower in 2016 and 64% lower in 2017 than in pyrethroid treated fields.

Pheromone traps were used again last season to monitor armyworm moth activity from planting to harvest. Monitoring took place at eight locations in rice production areas. The traps showed that a peak of armyworm moth activity occurred in mid-June and in early July. During August, another time when armyworms are seen in the field, moth activity did not peak and outbreaks did not occur. This type of information can help growers and pest control advisers monitor and manage their fields more effectively. The traps also showed that the true armyworm was more prevalent than the western yellowstriped armyworm.

Nontarget studies

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

Insecticides included in this work included Belay®, Coragen®, and Warrior® applied preflood at the 3-leaf stage of rice. Warrior® was also applied mid-July for armyworm control.

Two weeks after the application of preflood treatments, the number of aquatic insects in untreated plots was similar to the number of aquatic insects on plots treated with Belay®. Plots treated with Warrior® and Coragen® had significantly fewer insects than the control. At five weeks and later, all treatments were similar to the untreated.

Effects on the density of nontarget organisms were observed after insecticide applications, mostly on invertebrates other than insects. The effect of Warrior®, Belay®, and Coragen® on insects when applied preflood or at the 3-leaf stage of rice was minimal. Density of invertebrates other than insects was significantly reduced for several weeks after preflood applications, but the differences with untreated plots were small.

Stink bugs

Research conducted by this project in the past has shown that several native stink bugs can cause peck in rice. Peck is a kernel discoloration that forms as a result of stink bug feeding.

In 2017, panicles were placed in cages with redshouldered, conchuela, southern green, and consperse stink bugs, resulting in peck levels that ranged from 1.5% to 3.9%. While stink bugs are not considered a pest of rice in California, growers and pest control advisers should remain vigilant and report any unusual stink bug activity.

A message from the invertebrate pests project team

In 2017, the entomology rice project lost its longtime leader, Larry Godfrey. Larry served the industry well during his years working on rice. His work resulted in the registration of almost all of the insecticides currently available for use on rice. Larry was a great researcher and is dearly missed. His staff research associate, Kevin Godin, kept the project going and did all the work presented in this report with the help of Larry’s graduate student Joanna Bloese.