When he began working in Louisiana rice fields two years ago, Boris Castro began seeing an unfamiliar insect.
“I was taking many pictures of rice pests I came across,” said the LSU AgCenter entomologist. “That's when I noticed some larvae affecting two plants. Maggots were going inside a rice plant's stem. Taking everything together, this didn't seem to match any larvae I was familiar with.”
Interested to see if others had found anything similar, Castro called Mo Way, an entomologist with Texas A&M University. Way reported he'd seen the same larval behavior and was also unsure what it was.
“So we agreed to keep an eye out for this again in 2004. We both thought it was weird,” said Castro.
Then, last year, the pest hit some Louisiana rice very hard. Fields were severely hurt — denuded, in several cases — in Acadia, St. Landry, Vermilion, Jefferson Davis and Concordia parishes. Way said the larva was also in several Texas counties including Wharton, Jefferson and Calhoun.
“If you look at the range of the pest,” said Castro, “it's obviously already well-distributed across the southern rice belt now.”
Only recently, with the help of the Smithsonian Institution, were Louisiana entomologists able to find the actual name for the mystery pest species: Hydrellia wirthi.
“In March, the Smithsonian was able to compare our specimens from others gathered in South America. They matched. This is a pest that hasn't been reported in the United States previously. It's commonly known as the ‘rice leaf miner’ in South America.”
The pest is so new it doesn't have a common name established in this country. Together, the LSU Agcenter, the Smithsonian and Texas A&M University have proposed “South American rice leaf miner.”
The proposed name has yet to complete a process shepherded by the Entomological Society of America (ESA). The ESA must post the name and allow time for comments. That usually takes 3 or 4 months.
In the meantime, “we're using the ‘South American rice leaf miner’ name among producers,” said Castro. “Doing so will allow us to differentiate from the rice leaf miner (Hydrellia griseola) already in the country. It's very important to differentiate between the two.”
The two species, to the naked eye, are the same. The differences in the morphology of the pests can be seen under the microscope.
But while the two species may look alike, what each is capable of in a rice field show clear differences. That, said Castro, is what's important to know.
The species of leaf miner U.S. rice farmers are already familiar with is considered a secondary, sporadic pest. It causes “mining” damage to the leaf blade — normally on leaves touching water. That's why a common practice to control the pest is to drop water levels so the leaves aren't wet.
“Often, just dropping a flood down can take care of the problem.”
However, the new South American rice leaf miner isn't so easily dealt with. The maggots — or larvae — begin by scratching the leaf surface and then eat their way inside the stem or tiller. This behavior is the reason many began calling the pest “whorl maggots.”
“Last year, for the most part, we saw the maggots already inside the plant tissue. So the damage is much more problematic. This new leaf miner is far, far more aggressive than the one we already know.”
Leaves damaged by the pest tend to dry out and sometimes curl or even break off.
“When inspecting the plant, you'll usually find a maggot inside the stem. Last year, we found one or two maggots per tiller typically. But sometimes, especially in Jefferson Davis Parish, we found up to five maggots per tiller.”
A new pest found
Louisiana rice fields hurt by the South American rice leaf miner were affected mainly well into last growing season. Most of the damaged fields were late-planted from mid-May through June.
“The first report of damage we received was in the second week of June,” said Castro. “That field had been planted in mid-May. The rest of the damage reports came between the end of June through July.”
The pest caused a lot of stand density reduction, according to the entomologist. “One thing we've noticed is it tends to really hurt young plants: from one week post-emergence through 6 weeks post-emergence. The larvae either kill the plant outright or cause growth to be retarded. Floods then drown weakened plants and, if a weed flush occurs, it can reduce density even further.”
Last season, Castro was first alerted to problem fields by an Extension agent in Acadia Parish. “He called me and said, ‘You need to come over because we've got a severe infestation of leaf miner.’ So I went and it was obvious this wasn't our known leaf miner.
“At that time, he said he'd seen similar damage before. He just assumed it was the known leaf miner. He had no reason to think otherwise, you know.”
In speaking with Extension agents and farmers since, Castro has found others who say they've seen such damage in the past few years. For this reason, the entomologist is reluctant to guess how — or how long ago — the South American rice leaf miner found its way into the United States.
“This is a tiny fly (larvae are typically an eighth inch to a quarter inch long), that could be driven by the wind for long distances. It just took a severe outbreak before we found this it. That's why I can't guess when or how this pest got here. It probably entered in the last few years. But, whatever the case, now we must deal with it.”
From the damaged fields, Castro and colleagues collected many larvae and grew them in a lab. He then sent the adults to the Smithsonian Institution.
Castro wonders where the new pest is overwintering and how it has adapted to its new home. Now that he knows it comes from South America, he's looking for literature and studies that will provide good leads.
“As far as habits and biology in this country, we'll be studying that very hard this year,” he said. “The first thing we're going to check is to see if management techniques already proven to help with other pests can help with this one. Will floods or draining make a difference? Hopefully we'll find out the arsenal of cultural practices we already use will help. If so, that will help avoid additional costs for producers.”
If that doesn't work, Castro said a chemical “backstop” will be needed. Currently, there are no pesticides labeled for the pest. And even if there is a pesticide proven to work, Castro worries that application timing will be crucial.
“The way this insect goes inside the plant, it's likely an insecticide must be sprayed at just the right time. If it comes too late, it's unlikely the larva will be killed. That's why we need to know about infestations very early, before they cause plant kills. It'll be hard but we must try. It will be interesting to see if any of the insecticides we already use will be effective on this leaf miner.”
A more vulnerable crop?
Much of the Delta's rice acreage is being planted late — especially compare to last year. Castro said this creates a better opportunity for the South American rice leaf miner to gain a toe-hold.
“With a late-planted crop this year, we're set up for seeing this pest more. The fact that our planting is behind schedule is a concern.”
He said that's one reason the LSU AgCenter developed a state survey protocol to deal with the pest. Originally planned just for Louisiana, the survey is being taken national.
“With the help of some universities, the USDA plans to take the survey to every county where rice is produced: Louisiana, Arkansas, Texas, Mississippi, Missouri and California. Everyone will be watching. We want to know how far this insect has been distributed. And we want to know how intense infestations can be.”
Castro said he has no proof, but wouldn't be surprised if the pest has already moved outside Texas and Louisiana borders. “It's been found in Concordia Parish, right next to Mississippi. I also found two infested plants in Franklin Parish which also borders Mississippi. Hopefully they don't have it there, but it's something to really watch for.”
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