Entomologist keeps eye on borers, stink bugs

Frances Gould, Schultz, Bruce

Mike Stout checking on stinkbug nymphs.jpg thumbnail

LSU AgCenter entomologist Mike Stout examines a mesh sleeve containing stink bug eggs in an experiment to determine the extent of damage to rice kernels caused by stink bug nymphs. From this project, Stout will determine if thresholds for spraying stink bugs should include nymphs.

The range of the Mexican rice borer continues to move eastward in the rice-growing region of southwest Louisiana.

“I’m fairly certain that they are found in every rice-growing parish in southwest Louisiana,” said Mike Stout, LSU AgCenter entomologist and head of the Department of Entomology.

The pest was found in 2016 for the first time at the H. Rouse Caffey Rice Research Station, he said. In June, 10 moths were found in two pheromone traps at the station, and that number doubled in July. By September, more than 60 were found in the traps.

But Stout said the numbers are unlikely to cause yield losses yet. “The data indicates that the longer they have been established in a parish, the higher the populations are,” he said.

Data are showing that some varieties are more resistant to the borers than others. “For now, we can say that Jupiter and other medium grains have consistently shown susceptibility in experiments,” Stout said.

Dermacor seed treatment currently offers the best protection, he said.

No scouting procedures have been established for spraying after the borers are in a field, although monitoring pheromone traps may prove to be the best way to scout for the insect. The larvae that damage rice plants are hidden inside the plant, and the eggs laid by adults are tiny, he said.

Stout said 2016 was an unusual year overall for insects.

The South American rice miner, only seen sporadically in Louisiana rice in the past 12 years, was widespread.

“I think virtually every plot on the station had some South American rice miner,” Stout said. “But I mostly think the damage is cosmetic. It just causes a leaf here or there to drop off and die. I don’t think it caused any yield losses.”

It’s possible that the rice miner will not be as numerous next year. “There was something about this year, I have no idea why. It was a weird year,” he said.

Stout also conducted a series of tests last year to see if stink bug nymphs are causing reductions in yield or grain quality.

Stink bug eggs were placed inside mesh sleeves that were wrapped around immature rice panicles. The sleeves were left in place while the stink bugs hatched and became adults. When the grain matured, the rice was harvested and inspected for blanks and pecky rice.

The study could help answer the question of whether stink bug nymphs should be included in the count when sweeping for insects, and whether the nymphs could cause significant damage that would prompt spraying for the insects earlier than for adults, Stout said.

“Do the stink bugs develop quickly enough to actually cause damage to the grains? And it looks like they do,” he said. “We’ll probably change our recommendation to include nymphs as well as adults.”

Pathogens enter the holes made by stink bugs to feed on the developing grains, resulting in discolored grain called pecky rice. As grain hardens in the dough stage, stink bugs no longer feed on the hardened rice, Stout said.

Another stink bug threshold study is looking at stink bug numbers and rice damage. The current threshold of five stink bugs per 10 sweeps is probably too low. “We’re probably spraying too much insecticide for stink bugs. It looks to me like we could probably raise our thresholds,” he said.

Stout continues to study the biggest problem insect for rice: the rice water weevil. “We are now analyzing results of five separate experiments conducted during 2014 through 2016 in which we compared the susceptibility of the 8 most commonly grown varieties of rice to the rice water weevil,” Stout said. “We are in the process of doing a statistical analysis of the data and will prepare recommendations and ratings on the resistance of rice varieties to the rice water weevil based on these data.”

None of the varieties showed high levels of resistance, but there are differences, he said.

1/24/2017 8:39:45 PM
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