Rice Farming

After two years of testing a floating aquatic barrier trap to monitor adult water weevils, university researchers are reluctant to go public with their findings because they want to conduct additional research. But they say the trap could make a big splash with growers and crop consultants when it’s marketed.

Although an Arkansas manufacturer has agreed to assemble the weevil traps, no one has committed to marketing them—yet.

“When these traps get manufactured and released, we’ll not only have some documentation to go with them, but some of the county agents will have used them and can distribute that information to growers,” says John Bernhardt, a University of Arkansas entomologist at the Rice Research and Extension Center near Stuttgart.

Bernhardt and others have been conducting experiments and verification trials since the trap was developed in 1998. The researchers’ main interest lies in the trap’s ability to monitor adult weevils instead of larvae.

Furadan, which went off the rice market in 2000, controlled larval infestations. But subsequent products like Fury and Karate (Warrior in California) only are effective controlling adults. Once larvae reach threshold levels, it’s too late to treat.

Dimilin controls some adults but mainly prevents egg laying and hatching.

That is why researchers have been trying to pinpoint when insecticide applications are most appropriate. Scouting for leaf scars is not a precise science, because it does not indicate how many adults are in a field—just that they are feeding, researchers say.

The floating trap, on the other hand, could provide an answer because the number of trapped adults can be used to predict how many damaging larvae might be produced later in the season. If that number indicates a potential problem, it allows growers to get a jump on insecticide applications.

“The trap is very effective in capturing adults, regardless of the state where the rice is grown or the different cultural conditions,” Bernhardt says. “The real research work involves taking the adult data given by the traps and predicting the number of larvae.

“Each state has different thresholds of larvae based on a per-plant number or number per core sample. Researchers must develop the mathematical relationship between the number of adults and the number of larvae—Arkansas has done this and is verifying the recommendation.”

Verifications also are being conducted in other rice states such as Texas, Louisiana and California.

What makes them tick
Adult water weevils emerge from their overwintering habitats in the spring (levees in California and marginal woodlands elsewhere) and head toward flooded rice fields to feed and lay eggs.

After the eggs hatch, the emerging larvae float down and begin feeding on plant roots. It is the feeding that causes, for example, the 10 percent to 30 percent yield losses commonly found on California’s rice acres.

Larval feeding also inflicts heavy damage in other rice-producing states such as Texas, says Mo Way, research and Extension entomologist at Texas A&M University Agricultural Research and Extension Center in Beaumont. The economic injury level for water weevil infestation in that state is three to five larvae per core sample.

A core sample is a 4-inch-long-by-4-inch diameter plug of soil containing at least one plant.

“Based on our data, where we have a standard set of conditions and cultural practices, we say that on average if a farmer has one larva per core throughout the field, then they can expect about an 80 pound yield reduction,” Way says. “And that’s a linear relationship; three larvae is going to be approximately 240 pounds. When you get to that point—three to five larvae—the value of the yield saved pays for the cost of the insecticide.”

If researchers can predict larval densities from adult trap captures, they are at a distinct advantage. The traps capture the weevils as they swim around a field looking for places to feed and lay eggs.

Scars don’t tell all
While some growers still rely on feeding scars to gauge a potential water weevil problem, Way says there is no correlation between the number of feeding scars per plant and the number of damaging larvae later in the season in Texas. The scars only indicate adults are present and feeding on rice plants prior to laying eggs.
However, no real numbers are available from previous or continuing research on the traps.

Researchers did find a direct relationship between the number of trapped adults and the number of larvae found in core samples in two University of California tests in 2000 and 2001. In the 2000 test, three larvae were captured per one adult. One larva per one adult was found in 2001. This was because researchers deployed the traps at around the two-leaf stage in 2000, says UC Davis entomologist Larry Godfrey.

In the 2001, test more than 50 percent of adults were captured by the one-leaf stage and 75 percent during the two-leaf stage.

“This bodes well for the use of the trap as a management tool, as current guidelines suggest insecticide applications be made between the two and four-leaf stage,” authors Richard Lewis and Godfrey wrote in a report on the trials.

Godfrey says counting leaf scars is “ify” because they only appear at the three-leaf stage, and insecticides should be applied at the two-leaf stage or just at the three-leaf stage if they are to be effective. Plus some plants are still submerged at the three-leaf stage.

Where the challenge lies
While the traps look like the best method for timing applications, researchers say, no mathematical model exists that is suitable for every field across the rice-producing states. Researchers have not been able to conclude how many larvae can be predicted for a given number of captured weevils.

Often the traps’ effectiveness depends on individual rice field conditions and differing cultural practices from state to state or region to region. For example, water-seeded rice in Arkansas occupies about 8 percent to 10 percent of total acres, while most rice grown in Texas is dry-seeded. About 50 percent of Louisiana’s rice acreage is water-seeded and nearly all of California is water-seeded.

Water-seeded rice can be more susceptible to weevil infestation because they are attracted to flood areas. Flooded rice fields give them the perfect spot for egg laying and feeding.

While the economic injury threshold for water weevil infestation can be between 10 and 20 larvae per core sample in Arkansas, it’s only three to five per core in Texas.

Researchers agree they must conduct more tests to be confident about how many traps should be deployed per field or if the traps are as accurate as they appear.

“If you have to put a bunch of those traps out in a field and monitor them, they probably won’t be adopted by farmers or consultants because they might be way, way too time consuming,” Way says. “But I don’t know.”

Way recalls seeing as many as 50 to 100 water weevils caught per trap in a single day during an experiment last summer. Still, it’s hard to say if one trap should be used per 40 acres, per 100 acres or per half acre, he says. “As far as I’m concerned, as far as Texas is concerned, this is very preliminary information.”

It’s all in the timing New weevil trap map help you synchronize insecticide applications
By Lindsay Jones
After two years of testing a floating aquatic barrier trap to monitor adult water weevils, university researchers are reluctant to go public with their findings because they want to conduct additional research. But they say the trap could make a big splash with growers and crop consultants when it’s marketed. Although an Arkansas manufacturer has agreed to assemble the weevil traps, no one has committed to marketing them—yet. “When these traps get manufactured and released, we’ll not only have some documentation to go with them, but some of the county agents will have used them and can distribute that information to growers,” says John Bernhardt, a University of Arkansas entomologist at the Rice Research and Extension Center near Stuttgart. Bernhardt and others have been conducting experiments and verification trials since the trap was developed in 1998. The researchers’ main interest lies in the trap’s ability to monitor adult weevils instead of larvae. Furadan, which went off the rice market in 2000, controlled larval infestations. But subsequent products like Fury and Karate (Warrior in California) only are effective controlling adults. Once larvae reach threshold levels, it’s too late to treat. Dimilin controls some adults but mainly prevents egg laying and hatching. That is why researchers have been trying to pinpoint when insecticide applications are most appropriate. Scouting for leaf scars is not a precise science, because it does not indicate how many adults are in a field—just that they are feeding, researchers say. The floating trap, on the other hand, could provide an answer because the number of trapped adults can be used to predict how many damaging larvae might be produced later in the season. If that number indicates a potential problem, it allows growers to get a jump on insecticide applications. “The trap is very effective in capturing adults, regardless of the state where the rice is grown or the different cultural conditions,” Bernhardt says. “The real research work involves taking the adult data given by the traps and predicting the number of larvae. “Each state has different thresholds of larvae based on a per-plant number or number per core sample. Researchers must develop the mathematical relationship between the number of adults and the number of larvae—Arkansas has done this and is verifying the recommendation.” Verifications also are being conducted in other rice states such as Texas, Louisiana and California. What makes them tick Adult water weevils emerge from their overwintering habitats in the spring (levees in California and marginal woodlands elsewhere) and head toward flooded rice fields to feed and lay eggs. After the eggs hatch, the emerging larvae float down and begin feeding on plant roots. It is the feeding that causes, for example, the 10 percent to 30 percent yield losses commonly found on California’s rice acres. Larval feeding also inflicts heavy damage in other rice-producing states such as Texas, says Mo Way, research and Extension entomologist at Texas A&M University Agricultural Research and Extension Center in Beaumont. The economic injury level for water weevil infestation in that state is three to five larvae per core sample. A core sample is a 4-inch-long-by-4-inch diameter plug of soil containing at least one plant. “Based on our data, where we have a standard set of conditions and cultural practices, we say that on average if a farmer has one larva per core throughout the field, then they can expect about an 80 pound yield reduction,” Way says. “And that’s a linear relationship; three larvae is going to be approximately 240 pounds. When you get to that point—three to five larvae—the value of the yield saved pays for the cost of the insecticide.” If researchers can predict larval densities from adult trap captures, they are at a distinct advantage. The traps capture the weevils as they swim around a field looking for places to feed and lay eggs. Scars don’t tell all While some growers still rely on feeding scars to gauge a potential water weevil problem, Way says there is no correlation between the number of feeding scars per plant and the number of damaging larvae later in the season in Texas. The scars only indicate adults are present and feeding on rice plants prior to laying eggs. However, no real numbers are available from previous or continuing research on the traps. Researchers did find a direct relationship between the number of trapped adults and the number of larvae found in core samples in two University of California tests in 2000 and 2001. In the 2000 test, three larvae were captured per one adult. One larva per one adult was found in 2001. This was because researchers deployed the traps at around the two-leaf stage in 2000, says UC Davis entomologist Larry Godfrey. In the 2001, test more than 50 percent of adults were captured by the one-leaf stage and 75 percent during the two-leaf stage. “This bodes well for the use of the trap as a management tool, as current guidelines suggest insecticide applications be made between the two and four-leaf stage,” authors Richard Lewis and Godfrey wrote in a report on the trials. Godfrey says counting leaf scars is “ify” because they only appear at the three-leaf stage, and insecticides should be applied at the two-leaf stage or just at the three-leaf stage if they are to be effective. Plus some plants are still submerged at the three-leaf stage. Where the challenge lies While the traps look like the best method for timing applications, researchers say, no mathematical model exists that is suitable for every field across the rice-producing states. Researchers have not been able to conclude how many larvae can be predicted for a given number of captured weevils. Often the traps’ effectiveness depends on individual rice field conditions and differing cultural practices from state to state or region to region. For example, water-seeded rice in Arkansas occupies about 8 percent to 10 percent of total acres, while most rice grown in Texas is dry-seeded. About 50 percent of Louisiana’s rice acreage is water-seeded and nearly all of California is water-seeded. Water-seeded rice can be more susceptible to weevil infestation because they are attracted to flood areas. Flooded rice fields give them the perfect spot for egg laying and feeding. While the economic injury threshold for water weevil infestation can be between 10 and 20 larvae per core sample in Arkansas, it’s only three to five per core in Texas. Researchers agree they must conduct more tests to be confident about how many traps should be deployed per field or if the traps are as accurate as they appear. “If you have to put a bunch of those traps out in a field and monitor them, they probably won’t be adopted by farmers or consultants because they might be way, way too time consuming,” Way says. “But I don’t know.” Way recalls seeing as many as 50 to 100 water weevils caught per trap in a single day during an experiment last summer. Still, it’s hard to say if one trap should be used per 40 acres, per 100 acres or per half acre, he says. “As far as I’m concerned, as far as Texas is concerned, this is very preliminary information.” For questions or comments about this article, contact Rice Farming editor Vicky Boyd at (209) 571-0414 or vlboyd@att.net.
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