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The Grinch who stole Wisconsin’s Christmas trees

For Wisconsin Christmas tree farmer Dan Hanauer, Jr., the
Grinch is very real. It lurks in his soil, greedily munching on the
roots of his trees and stealing his chances for a profitable holiday
season.

“This thing can just devastate a field,” says Hanauer, who grows trees
on 500 acres near Shawano. “One year, we planted a new 50-acre field,
which is about 50,000 trees, and we lost all of them. That kind of
loss tends to get your attention.”

It”s not a green-skinned ogre causing such Grinchly havoc, but a tiny,
cream-colored grub from the beetle family Scarabaeidae. A larval form
of the common May/June beetle, these grubs spend nearly three years
underground, feasting on the roots of trees, grasses and other plants
before emerging as winged adults. A single grub can devour a young
tree”s entire root system, essentially choking it from the bottom up.

That makes the scarab grub a big problem around Wisconsin, which grows
more Christmas trees than all but four states. Some 1,100 Christmas
tree farmers operate in the state, and as many as one-third deal with
persistent damage from grubs, says Chris Williamson, an associate
professor of entomology and an extension specialist on insect control.

But despite the insects” prevalence, few guidelines exist to help
growers like Hanauer deal with them. “If you go into the literature,
there”s really nothing about management practices for this insect in
Christmas trees. It just hasn”t been studied in this system,” says
Williamson.

UW-Madison graduate student P.J. Liesch is working to change that. For
the past two summers, Liesch has studied nearly 1,000 Fraser fir trees
around Hanauer”s farm to experiment with cultural, biological and
chemical methods of dealing with the grubs. The project, which was
sparked when a frustrated Hanauer contacted Williamson to help explain
the crippling losses on his farm, has raised hopes among state tree
farmers that a proven plan of attack will emerge.

“There are a lot of anecdotal stories out there about things that
work, but they really haven”t been tested,” says Liesch. “The goal is
to find some strategies that are effective and environmentally
responsible in this system.”

Part of Liesch”s work examines insecticides used to ward off grubs in
other systems, such as turfgrass. The early data suggest that some of
those products, chiefly ones with the active ingredient imidacloprid,
can be effective in reducing grub populations around trees. In
Liesch”s trials, insecticides reduced tree losses to less than 15
percent in infested fields, compared to losses exceeding 90 percent
without treatment.

But trees present some unique challenges. Unlike turf, few tree farms
are irrigated, meaning that insecticides may not reach the depth where
grubs live unless they are applied directly to roots, a costly and
labor-intensive process. Christmas trees also typically grow for 10
years or more before harvest, and since a tree”s susceptibility to
grubs declines with age, it wouldn”t make sense to treat every tree in
the field the same.

“There”s no one thing that you can do consistently,” says Hanauer,
whose farm has been in the family since 1966. “There”s just no magic
bullet.”

Liesch”s research isn”t likely to provide one, but it will give
growers better data about the need for and timing of treatments, as
well as alternative approaches that could supplant or reduce chemical
applications. Liesch has examined the effectiveness of introducing
grub-eating nematodes to help suppress the insects” population, which
he says shows modest results in certain circumstances. He and
Williamson also stress the importance of understanding the population
dynamics of the grubs, which tend to boom in three-year cycles, making
those years most critical for intervention.

That knowledge helped Hanauer adopt a tiered-strategy that employs
three different insecticides at specific stages during his trees”
growth. And while he says battling the bothersome bug is still “a war
of attrition,” he”s grateful to have some science on his side.

“My hope is that we find some strategies that work for the whole
industry,” he says. “But from my perspective, just having contact with
the researchers has been really valuable. Every time I talk with them
I learn something.”