Rice Farming

A new pest, unofficially dubbed the rice whorl maggot, is being found increasingly in gulf state rice fields and is beginning to cause patchy and occasionally severe damage.

Although the discovery is by no means cause for panic, entomologists say consultants and growers should start scouting for the pests this season, especially in late-planted fields.

“I wouldn’t push the panic button, but if I had late-planted seedling rice before the flood, then I definitely would be watching for it,” says Mo Way, Texas A&M associate entomology professor in Beaumont. “I think it might be becoming an increasing problem. I haven’t recommended anybody treat for it over here [in Texas], but it’s something that the consultants and growers should be aware of.”

Because the pest and damage have only been recently observed and identified in Texas and Louisiana, insect experts say they still have numerous questions to answer, such as cultural and chemical control methods and treatment thresholds.

Suspicions lead to new discovery
For the past 10 years or so, Louisiana State University Cooperative Extension agent Ronnie Levy has seen occasional fields with severe infestations of what he thought were leaf miners. In some cases, the larvae had moved into the whorl.

Levy, who works in Acadia Parish, just thought that once the tender young leaves had died, the leaf miner larvae moved deeper into the plant in search of food.

When draining didn’t seem to help the problem, the grower would treat with methyl parathion, which is labeled for leaf miners, and the field would typically respond.

In hindsight, Levy says some of those fields may have been infested with rice whorl maggots, but experts will never know for sure.

Louisiana State University Extension entomologist Boris Castro says he had seen the pest every season since he started with the LSU AgCenter in 2002. But it wasn’t until 2004 when it was causing serious problems in scattered fields that Castro sent samples into the Smithsonian Institute to have it identified.
Although final identification is still pending, Smithsonian entomologists have confirmed the new pest is not a leaf miner, but is a species of whorl maggot.

Way has seen a similar pest in Texas fields for the past two seasons. Originally he thought a beetle similar to a rice water weevil was causing the parallel feeding scars and ragged, coalescing leaf lesions he’d seen in fields.

But when he collected pupae from infested fields and allowed them to hatch, he found a small fly. He, too, has sent samples for identification.

How can I tell a rice whorl maggot?
As with leaf miners, the adult rice whorl maggot fly is not the problem. It’s the larval feeding on the plants that causes the damage.

Leaf miner larvae tunnel and feed between the layers of the leaves, attacking and killing leaves lying on the water or closest to the water. By draining the field, you eliminate the moisture around the leaves and significantly reduce larval leaf miner populations. The rice leaf miner is found statewide and is rarely a concern.

Rice whorl maggots, on the other hand, are much more aggressive in their feeding habits, Castro says. Whorl maggots begin feeding on leaves but eventually move into the plant whorl, or heart, where they spend the remainder of their larval life feeding.

It is here that they do the most damage and can kill seedling plants. Because they are protected in the whorl, insecticides applied at this stage may be ineffective, Castro says.

And draining may actually make a rice whorl maggot infestation worse, Castro says. Lowering the water level allows the maggot to move lower in the plant without drowning.

Unlike the rice water weevil, which makes short feeding scars on the middle of the leaf, rice whorl maggots make very long scars along the leaf margin down to the base of the leaf.

Rice water weevil damage typically is scattered throughout a field, whereas rice whorl maggot damage tends to be in patches. Fields infested with rice whorl maggot may have bald spots where the pest has killed plants. Plants that survive may have delayed maturity.

Levy, Castro and Way visited one severely infested field near the Crowley Rice Research Station last summer where 70 percent to 80 percent of the plants were showing signs of rice whorl maggot damage.

Catching infestations early
The key to any type of eventual chemical control will be catching the pest early before it moves into the plant whorls, Castro says. Once the rice begins looking ragged, it’s too late.

The most critical time for infestation is from plant emergence until about four to five weeks after emergence.
Late-planted rice appears to be more susceptible to rice whorl maggot infestation than fields planted within the recommended planting window, Castro says.

“Most of the late-planted fields I visited last season had it, and in some fields, the damage was very impressive,” Castro says. “It was in patchy areas, but there might be huge spots in the middle of the field.”

What 2005 holds
During 2005, Castro says he hopes to conduct field trials with products already labeled for other rice pests to see if they’re effective against the rice whorl maggot. Currently, no products are labeled for rice whorl maggot.

Conducting the trials in small plots at the Crowley field station won’t be feasible, so Castro will be seeking growers with infested fields willing to cooperate.

He will be working with Extension agents to find infested fields and coordinate the trials.

With the help of Extension agents, Castro also plans to monitor area fields planted during the recommended window to determine the pest’s distribution.

“The best way to identify it is the early feeding marks, and to me, that’s the most logical approach,” Castro says. “Now we have to see how easy it is to find.”

Should you suspect you have a rice whorl maggot infestation this season, contact your local Extension agent.

Contact Vicky Boyd at (209) 571-0414 or vlboyd@sbcglobal.net.

Know thy enemy

Louisiana State University Extension entomologist Boris Castro provides these descriptions of the rice whorl maggot and its life stages:

The rice whorl maggot is legless, transparent to light yellowish in color and about 5 millimeters long. The maggot, or fly larva, feeds along the margins of the leaves and enters the whorl tissue where it kills the "heart" or growing tissue of the plant. The resulting damage is "deadheart," which often leads to plant death. The maggot pupates in the leaf sheath at the collar level of each leaf. We have observed one to four pupae in a single leaf sheath.

Very often, the adult fly can be observed posing on affected plants. The adult is a tiny dark fly about 3 millimeters long.

Concern, yes; panic, no Be on the look-out for new pest— rice whorl maggot—to catch it early
By Vicky Boyd Editor
A new pest, unofficially dubbed the rice whorl maggot, is being found increasingly in gulf state rice fields and is beginning to cause patchy and occasionally severe damage. Although the discovery is by no means cause for panic, entomologists say consultants and growers should start scouting for the pests this season, especially in late-planted fields. “I wouldn’t push the panic button, but if I had late-planted seedling rice before the flood, then I definitely would be watching for it,” says Mo Way, Texas A&M associate entomology professor in Beaumont. “I think it might be becoming an increasing problem. I haven’t recommended anybody treat for it over here [in Texas], but it’s something that the consultants and growers should be aware of.” Because the pest and damage have only been recently observed and identified in Texas and Louisiana, insect experts say they still have numerous questions to answer, such as cultural and chemical control methods and treatment thresholds. Suspicions lead to new discovery For the past 10 years or so, Louisiana State University Cooperative Extension agent Ronnie Levy has seen occasional fields with severe infestations of what he thought were leaf miners. In some cases, the larvae had moved into the whorl. Levy, who works in Acadia Parish, just thought that once the tender young leaves had died, the leaf miner larvae moved deeper into the plant in search of food. When draining didn’t seem to help the problem, the grower would treat with methyl parathion, which is labeled for leaf miners, and the field would typically respond. In hindsight, Levy says some of those fields may have been infested with rice whorl maggots, but experts will never know for sure. Louisiana State University Extension entomologist Boris Castro says he had seen the pest every season since he started with the LSU AgCenter in 2002. But it wasn’t until 2004 when it was causing serious problems in scattered fields that Castro sent samples into the Smithsonian Institute to have it identified. Although final identification is still pending, Smithsonian entomologists have confirmed the new pest is not a leaf miner, but is a species of whorl maggot. Way has seen a similar pest in Texas fields for the past two seasons. Originally he thought a beetle similar to a rice water weevil was causing the parallel feeding scars and ragged, coalescing leaf lesions he’d seen in fields. But when he collected pupae from infested fields and allowed them to hatch, he found a small fly. He, too, has sent samples for identification. How can I tell a rice whorl maggot? As with leaf miners, the adult rice whorl maggot fly is not the problem. It’s the larval feeding on the plants that causes the damage. Leaf miner larvae tunnel and feed between the layers of the leaves, attacking and killing leaves lying on the water or closest to the water. By draining the field, you eliminate the moisture around the leaves and significantly reduce larval leaf miner populations. The rice leaf miner is found statewide and is rarely a concern. Rice whorl maggots, on the other hand, are much more aggressive in their feeding habits, Castro says. Whorl maggots begin feeding on leaves but eventually move into the plant whorl, or heart, where they spend the remainder of their larval life feeding. It is here that they do the most damage and can kill seedling plants. Because they are protected in the whorl, insecticides applied at this stage may be ineffective, Castro says. And draining may actually make a rice whorl maggot infestation worse, Castro says. Lowering the water level allows the maggot to move lower in the plant without drowning. Unlike the rice water weevil, which makes short feeding scars on the middle of the leaf, rice whorl maggots make very long scars along the leaf margin down to the base of the leaf. Rice water weevil damage typically is scattered throughout a field, whereas rice whorl maggot damage tends to be in patches. Fields infested with rice whorl maggot may have bald spots where the pest has killed plants. Plants that survive may have delayed maturity. Levy, Castro and Way visited one severely infested field near the Crowley Rice Research Station last summer where 70 percent to 80 percent of the plants were showing signs of rice whorl maggot damage. Catching infestations early The key to any type of eventual chemical control will be catching the pest early before it moves into the plant whorls, Castro says. Once the rice begins looking ragged, it’s too late. The most critical time for infestation is from plant emergence until about four to five weeks after emergence. Late-planted rice appears to be more susceptible to rice whorl maggot infestation than fields planted within the recommended planting window, Castro says. “Most of the late-planted fields I visited last season had it, and in some fields, the damage was very impressive,” Castro says. “It was in patchy areas, but there might be huge spots in the middle of the field.” What 2005 holds During 2005, Castro says he hopes to conduct field trials with products already labeled for other rice pests to see if they’re effective against the rice whorl maggot. Currently, no products are labeled for rice whorl maggot. Conducting the trials in small plots at the Crowley field station won’t be feasible, so Castro will be seeking growers with infested fields willing to cooperate. He will be working with Extension agents to find infested fields and coordinate the trials. With the help of Extension agents, Castro also plans to monitor area fields planted during the recommended window to determine the pest’s distribution. “The best way to identify it is the early feeding marks, and to me, that’s the most logical approach,” Castro says. “Now we have to see how easy it is to find.” Should you suspect you have a rice whorl maggot infestation this season, contact your local Extension agent. Contact Vicky Boyd at (209) 571-0414 or vlboyd@sbcglobal.net. Know thy enemy Louisiana State University Extension entomologist Boris Castro provides these descriptions of the rice whorl maggot and its life stages: The rice whorl maggot is legless, transparent to light yellowish in color and about 5 millimeters long. The maggot, or fly larva, feeds along the margins of the leaves and enters the whorl tissue where it kills the "heart" or growing tissue of the plant. The resulting damage is "deadheart," which often leads to plant death. The maggot pupates in the leaf sheath at the collar level of each leaf. We have observed one to four pupae in a single leaf sheath. Very often, the adult fly can be observed posing on affected plants. The adult is a tiny dark fly about 3 millimeters long.
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