Rice has been grown in the United States since the mid-1750s. From available literature, it appears rice water weevil was a problem pest in the first crops — and it remains a major pest to this day.
Pyrethroids are still a very important tool for rice water weevil management. However, they control the weevil adults and have only minimal activity on larvae immediately after they hatch and are headed towards the soil to feed on rice plant roots.
That's why Dermacor, a larvacide produced by DuPont, holds such promise and has the attention of growers.
“Since last year, we've had a Section 18 registration — an emergency label — because of Louisiana's extreme problems with weevils,” says Natalie Hummel, LSU AgCenter entomologist. “There aren't very many control options and the weevil is definitely our biggest rice pest. The Section 18 has been renewed by EPA for 2009.”
There is a catch with Dermacor. The use pattern is restricted to drilled rice or where it's broadcast into a dry seedbed and then incorporated into the soil.
Over the last several years, Hummel and Mike Stout, another AgCenter entomologist, have done much Dermacor research. They have confirmed the product has good efficacy.
“Last season, we did a comprehensive demonstration trial in eight locations throughout the different rice-production areas of Louisiana. We wanted to confirm it worked at the commercial field level — and it does. There were no cases of product failure.”
Applied as a seed treatment, Dermacor is taken up by the plant as it grows. It does cause a bit of mortality in weevil adults but works best when weevil larvae begin to feed on the root and die soon after. It causes paralysis through a mode of action that hasn't been used before.
Might the restrictions on Dermacor be lifted in the future?
Last year, Stout did some small-plot research in Crowley, La., looking at broadcasting dry seed that was treated with Dermacor into a flooded paddy. “It had good efficacy — so the product can handle that system,” says Hummel. “However, DuPont has to decide whether or not to pursue any sort of water-seeded labeling. Obviously, the restriction to drilled or dry-broadcast systems will limit the amount of acreage treated with this product.”
To combat the pest, Louisiana rice growers should also adopt cultural practices such as early-planting, if possible. That helps avoid weevil infestation.
“Also be careful with water management since water-seeded rice is more susceptible to weevil damage. Depth of flood also appears to be important. If a grower has the ability to control and maintain a shallow flood (around 5 inches), that works to the crop's benefit. That possibly reduces the oviposition done by weevils — they just don't have as much area to lay eggs underwater.
Hummel says Louisiana has more trouble with the weevil than other states. “The theory is we have a very good habitat, a good year-round climate for them. In the water, they hang out in riparian habitats along the edges of our rice fields. Louisiana has a lot of wetland areas and there are also a lot of fields with trees around the edges of fields. The weevils overwinter in the soil and dead grass in those areas. As opposed to California, Louisiana has more small acreage fields with trees on the edges. There could be two or three generations yearly.”
In the Dermacor study last year, “we took data out of commercial fields. There were untreated checks to compare. We found an untreated field with an average of 14 larvae per core — the method we use to sample soil around the roots. All states use the same method, so it's a nice way to compare weevil populations.”
With the 14 larvae per core, a farmer may lose 7 to 14 percent of yield in an untreated field — “a substantial problem.”
The real challenge with the weevil is that while the adults are visible, the larvae aren't seen feeding on the roots. “You may not know they're there until the plants fall over. In those cases, the field would typically be drained.”
However, draining is also questionable. “Past studies show mixed results from draining. (Stout) and I are also planning to look at what really happens when a field is drained.”
There are other variables to consider. “Another possibility with draining is that, while stressful to plants, with the reapplication of water and a dose of fertilizer, the plants show a big growth response. Well, that may not be due to stopping the weevils but to the re-flooding and inputs.”
Also, when draining, the field needs to be dried down for a certain time. In Louisiana, where frequent showers aren't uncommon, maintaining a dry field isn't always easy.