Chasing Rootworms

CHAMPAIGN, Ill. (DTN) -- Perched on a yellow scaffolding tower 30 feet in the air, Joe Spencer cuts a striking figure. With a butterfly net in hand and a safari hat on his head, Spencer squints into the hazy, hot August sun high above a cornfield in central Illinois.

Trucks have ground to a halt on a nearby road in the past, as baffled farmers watched the University of Illinois entomologist, his graduate student Sarah Hughson, and student workers lunge to and fro with their giant nets, swiping rootworm beetles out of the air into the late evening hours.

Unbeknownst to these bemused spectators, Spencer and his crew are hard at work helping them defeat the wily, ever-evolving western corn rootworm. Once back safely on the ground in his lab, Spencer and Hughson will get to work "interrogating" their captives.

By dissecting the beetles and then grinding them to analyze their debris, Spencer can tell how recently each beetle has mated and which nearby fields it has visited in the last 24 hours. This year, his work has been funded by Monsanto as part of a project to study the dispersal of adult rootworm beetles from cornfields. The goal is to add this dispersal information to computer models that could predict the spread of Bt-resistance in rootworm populations in large corn states like Illinois and Iowa.

In the meantime, preliminary results from Spencer's research last year have shown that insects caught high in the air above cornfields are alarmingly mobile, which could complicate resistance management efforts.

Monsanto has funded seven projects like this one under a program called the Corn Rootworm Knowledge Research Program. In addition to chasing rootworms down with his trusty net, Spencer is also the lead investigator on another project which is examining the possibility that some rotation-resistant rootworm populations may be developing resistance to the Cry3Bb1 protein found in certain Bt-corn hybrids.

Monsanto hopes studies like these will help the company and growers find better ways to manage corn rootworm, which has already developed resistance to two Bt proteins on the market and continues to cost corn growers up to $1 billion each year in yield loss and control costs.

TO TRACK A BUG

Spencer uses the Bt proteins in corn hybrids as markers to track the movement of the beetles he catches. In the fields surrounding his beetle-catching towers, Spencer has planted corn hybrids expressing two different sets of Bt proteins, and a nearby farmer's fields contain a third Bt protein that isn't found in Spencer's fields.

Crushing the beetles allows Spencer to test them for each of the three Bt proteins, which betrays the insects' feeding sites for the past day. "Since insects can't order carry out food, they have to have been in that field to contain pollen or silks from, say, a YieldGard rootworm plant," he explained.

Identifying the Bt proteins in the guts of thousands of beetles has allowed Spencer to accurately assess what proportion of rootworms tend to move between fields.

Thanks to Purdue entomologist Christian Krupke and his students, Spencer also has intimate knowledge of the rootworm's sex life, which has helped him understand when in their reproductive cycles beetles tend to disperse.

Studies overseen by Krupke have shown that during mating, male beetles deposit a spermatophore, essentially a plug of sperm and nutrients representing up to 9% of the male beetle's body mass.

Afterwards, the depleted male beetle stumbles off to recover, and the female begins slowly absorbing the snack pack he left her. By examining the presence and size of spermatophores inside the female beetles he catches, Spencer can tell how recently they were mated.

Spencer and his team of student workers also make sure to collect beetles at different heights. Some were collected moving within the corn canopy; others were snagged from the towers 30 feet high in the air, which marks the launch of a major road trip for a bug, Spencer noted.

A MOBILE PEST

With these tools in hand, Spencer's dispersal studies from last summer have yielded some interesting conclusions. More than half of the female beetles they collected in the air were young, recently mated females.

Even more importantly, they weren't all local. "In the air, about half the beetles above a field came from somewhere nearby," Spencer noted. "So they're really moving." Beetles caught down in the canopy, on the other hand, are less mobile -- only 4% were found to be from another field, Spencer noted. But once beetles rise up in the air and catch the wind, they appear to become quite mobile.

Spencer hopes future studies will illuminate the specific characteristics of migrating Bt-resistant beetles and maybe even nail down how far the high-flying beetles he has caught are really going. Older lab studies have shown that up to 15% of young, newly mated females are capable of launching truly long distance migrations of 15 miles or more in a single night.

Practically speaking, Spencer and Hughson's preliminary dispersal conclusions hold an uncomfortable truth for farmers. In short, your neighbor's rootworm management tactics could have just as much of an influence on your field as your own tactics. "We probably aren't going to stop resistance by just managing a particular field," Spencer pointed out.

Community efforts might become part of future resistance control, he added. If researchers can ferret out how exactly far resistant beetles can migrate, local farmers could come together to manage "hotspots" of resistance by agreeing to use a variety of different Bt-hybrids and management tactics within a certain region, Spencer speculated.

Until then, you will probably find Spencer and his students high in the air each summer, day after day, chasing rootworms.

Emily Unglesbee can be reached at Emily.Unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee.

(PS/CZ)

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