Slowly but surely, a troublesome, former nemesis for California rice growers is making a comeback.
And the rice water weevil (RWW) is being sneaky about it, at least in trials at the Biggs, Calif. Rice Experiment Station conducted by resident rice weevil expert, University of California (UC), Davis Entomologist Larry Godfrey.
A RWW herd snuck in over a weekend in Godfrey's research plot at the station. For 95 of the 100 days Godfrey trapped for weevils on the station, he caught nothing.
However, on the weekend of May 5-7, the weevil must have heard there was a party going on at the station with all the workers away, and Godfrey said traps caught 75 percent of the rice water weevil count for the season. The other 25 percent arrived April 26-27. He had never seen that before in many years of RWW work.
With only two flight catches, the assumption would be the total numbers were low. You'd be flying off the wrong handle if that was the assumption.
The two-incident flight pattern was admittedly a first for the veteran UC entomologist, but what was not a surprise was the fact that the count this season was 20 percent higher than 2006.
“Rice water weevil counts have been on an upward trend for the past three or four seasons. It has not reached the peak levels of the mid-1990s, but the numbers continue to go up,” said Godfrey.
An ominous sign was made more worrisome by the implied threat of possibly losing, or regulations severely restricting, the use of rice growers' primary pesticide tool, pyrethroid insecticides.
Godfrey told growers, pest control advisors, and others at the annual research center field day that 95 percent of the pesticides used in rice are in the pyrethroid class. Not only is that a cause for concern for resistance, but it is made doubly alarming by the current re-evaluation of pyrethroids by the California Department of Pesticide Regulation (DPR).
Product registrants and DPR are jointly evaluating pyrethroid use because the compound has been found attached to organic matter in water. Traces of pyrethroids have not been found in water, Godfrey pointed out — only attached to the organic matter in the water. It is the first major environmental red flag for a chemistry class used in agriculture and urban settings for more than 20 years.
Godfrey said any possible new constraints placed on pyrethroids are at least a year or more away. However, he said it behooves him and the rice industry to quickly find possible alternatives to pyrethroids.
Fortunately, Godfrey is looking at three that offer good RWW control in California.
One is trade-named Trebon — the chemical name is etofenprox. It has had a Section 18 label in Louisiana and Texas last season for rice water weevil control. It is manufactured by Mitsui chemicals Inc. and distributed by Landis International.
Godfrey said the company is pursuing a California registration, but he could not predict when it may be available.
Another product is Steward from DuPont. It has the same active ingredient as a now widely used insecticide Avaunt, indoxacarb. Is it active on rice water weevil, but it is also kills crayfish.
For that reason, the company is not pursing registration in the U.S., but Godfrey said the state's rice acreage qualifies California to pursue IR-4 registration for Steward.
The third promising compound is clothianidan from Valent. Godfrey said the company is looking at it on rice in the South as a seed treatment.
“I do not see it as a seed treatment, but we are more interested using it pre-flood or in a water application,” he said. “I have used it in pre-flood in lower and lower rates and it is a very active product. It has a lot of sustainability features, and the university is very interested in getting the product registered. Valent is facilitating that.
“In 2006 these three experimental products applied with various rates and application methods provided as high as 95 percent rice water weevil control and excellent protection of grain yields,” he said.
However, all three products showed only moderate RWW control when improper rates or suboptimal application procedures were used. When used properly, Godfrey added, “all three of these have a good fit in California as an alternative to pyrethroids.”
Godfrey also continues to evaluate rice varieties for insect resistance. “There are varietal differences in attracting both adult weevils and larvae.It is not enough to walk away from weevil control, but there are differences.”
As insecticides continue to be the first line of insect pest control, Godfrey also continues to monitor the impact of these products on invertebrate non-target insects.
This information is particularly useful for mosquito control within rice production areas where biological control of mosquito larvae is critical.
Overall, Godfrey said products registered for insect pest control in rice have a short residual, perhaps two weeks.
“There is no long-term impact on non-target organisms. The products growers use are really short-lived,” he concluded.