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Biocontrols battle giant reed

In the Southwest, an exotic and invasive weed of riparian habitats and irrigation canals has become an absolute monster.

In its native Spain, Arundo donax, or giant reed, is kept under control by a host of insects.

But that’s not the case here in the U.S., where the invader competes for scarce water resources.

“Whole riparian forests have been displaced by Arundo, and you rarely see birds or any wildlife in these dense patches of the weed,” says John Goolsby, an entomologist at the Agricultural Research Service’s Beneficial Insects Research Unit, or BIRU, in Weslaco, Texas.

The Weslaco laboratory is the epicenter of hope for a solution. Goolsby and colleagues have identified four very promising biological controls that could curb the destructive reed.

Candidate insects would complement existing control methods.

Key Points

• The invasive giant reed competes for scarce state water resources.

• Four promising biological controls could curb the harsh giant reed.

• The scale insect biological control is showing the most promise.

New options

Each of the four attacks the reed at a different place.

• A scale insect, Rhizaspidiotus donacis, which may be released this summer, attacks the root.

• The Tetramesa romana wasp attacks the main stem. This weakens the plant, lessens its overall height, and causes it to form galls and pupae outside shoots.

• The Arundo fly, Cryptonevra spp., eats the inside of new shoots.

• The leaf sheath miner, Lasioptera donacis, destroys leaves.

The wasp has been released.

But the scale insect is the biological control that shows the most promise. It feeds on the part of the plant known as the rhizome, where most of the plant biomass originates. Debilitating the rhizome could have a big impact on the plant’s growth and spread.

America’s Arundo adult female scale can produce 100 to 200 young. “This outstanding reproductive capacity demonstrates the insect’s potential to establish large and long-lived populations on giant reed,” Goolsby says.

DNA analysis by coinvestigator Marie Jasieniuk of the University of California, Davis, also shows promise. Jasieniuk’s assays of Arundo’s leaves strongly suggest that in America, the giant reed doesn’t have much genetic diversity.

Arundo’s low level of diversity is bad news for the plant, but good news for those who want to stop its spread.

Flores and Wood write for ARS.

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THE MONSTER: In northern California, technician Greg Ksander (left) and ecologist David Spencer collect a leaf sample from giant reed (Arundo donax).

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ON ATTACK: A Tetramesa romana larva (shown at bottom) chews its way through this cross section of an Arundo stem.

This article published in the January, 2010 edition of CALIFORNIA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.