A floating trap using parts from boll weevil traps shows promise as a tool to help growers and consultants monitor rice water weevil numbers and determine when to treat. But researchers working with the device say they need at least one, and possibly two, more years of large-scale grower field trials before they’ll feel confident enough to make recommendations about trap numbers, trap placement and weevil treatment thresholds.

"We’re not quite ready to release the trap," says John Bernhardt, a University of Arkansas research entomologist at the Rice Research and Extension Center in Stuttgart. "We’re not comfortable with it yet. We need to get more people involved with this. The more fields and the more people, the more data we will have and the more comfortable we will feel."

Even when they do establish economic thresholds, Bernhardt says the number will only apply to Arkansas. Because of higher population pressures, Texas and Louisiana Extension have historically established lower economic thresholds for rice water weevils than Arkansas.

Raymond Hix developed the trap design as part of research for his doctorate at the University of Arkansas. In the South, rice water weevils overwinter in wood lots rather than on levees as in California and fly into fields come spring.

Originally, Hix tried to catch weevils in floating cone traps after fields were permanently flooded as they flew into the fields, but the effort was only marginally successful.

After a wind overturned some of the cone traps, Hix noticed that weevils had swum(swam) into the traps. He found that once weevils fly into a field, they swim to get around.

So he focused his attention on the insect’s swimming behavior and developed what is now dubbed the "weevil intercept trap."

The floating trap is constructed of two boll weevil trap tops linked together with a flat piece of aluminum screen that acts as a barrier. Flotation devices on each end keep the traps properly positioned in the water.

Although Bernhardt admits the boll weevil trap tops are slightly larger than needed, they’re readily available and economical. He’s currently looking for a manufacturer that can mass produce the weevil intercept traps.

"With the new insecticides, you have to make a quick decision within the first 10 days of flooding in the Arkansas system," says Hix, now a University of Califorina, Riverside Extension entomologist. "The trick is to make that decision, you have to have a good tool."

Based on the large-scale field trials Hix conducted, he’s confident that the trap is that tool.

When a swimming weevil encounters the screen barrier linking the two boll weevil traps tops, it changes direction, heading either left or right and directly into one of the traps.

Hix’s research examined whether the double-ended aquatic barrier trap could reliably catch adult weevils and if trap catches could be related to subsequent larval infestations.

He found that for every one adult trapped, you could predict about 1.4 larvae per core sample. The 1.4 was the means, and individual core samples for rice water weevil larvae could be expected to vary between 0.8 and 2.

More than 80 percent of the time, water weevils swim in the top 3 inches of water, so the traps needed to be placed 3 to 3.5 inches below the surface to intercept most of the insects.

That could pose a challenge to some growers who maintain a shallow flood except in ditches or along levees.

This coming season, Bernhardt will be looking to modify the traps for shallow water. He’ll also be placing the devices only near ditches to see how the trap catches relate to number of larvae.

The floating barrier trap is part of a continuing effort Bernhardt started in the early 1990s to find new ways to monitor adult rice water weevil populations.

With Furadan, growers and consultants monitored for weevil larvae by taking core samples. When the number of larvae hit economic thresholds, then growers applied Furadan.

The replacements—Dimilin and Karate—need to be applied before adult weevils begin to lay eggs. If growers wait until they begin finding larvae, it’s too late to treat.

With the new products, growers in Arkansas can monitor adult weevils or count feeding leaf scars. Current Extension recommendations call for treatment when growers find at least 60 percent of the plants surveyed with feeding scars. That should equate to the old threshold of 10 or more larvae per core sample.

But some growers and consultants are uncomfortable with using leaf scars, which is why Bernhardt says he’s pursuing the trap.

"If you have a serious infestation of rice water weevil, your leaf scar count will tell you fairly quickly," Bernhardt says. "Those borderline fields or those that are not heavily infested are the ones that people worry about. [Leaf scar monitoring] doesn’t tell you soon enough.

"Traps tell us early that you have a weevil problem and lets you head it off early."

Based on two trials conducted in 2000, Bernhardt says they were able to determine fairly well when to apply Karate for weevils.

Dimilin is a different story, since growers may chose a single or split application. "We still haven’t gotten enough information to use [the trap] with Dimilin since it has to be put out so quickly to hit the right timing," he says.