Dakota Farmer

Insect is responsible for poor seed production.

July 29, 2016

2 Min Read

A newly discovered insect may explain why it has been so difficult to increase prairie cordgrass seed production.

Prairie cordgrass is an important native grass. The stiff stems, vigorous rhizomes and robust size of the grass makes it useful in wetland restoration and enhancement, streambank stabilization, biofuel production and early-season grazing

But the seed has been expensive because very little is usually produced.

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One reason seed production is low may be due a new gall midge species discovered by Paul J. Johnson, South Dakota State University entomologist, a professor in the plant science department, and graduate student J. Manuel Perilla López. They found it in prairie cordgrass at four locations in eastern South Dakota.

The prairie cordgrass gall midge has a different relationship with its host plant than other gall midge species, which form a pocket called a gall within plant tissues.

“This species feeds on the seeds but doesn’t modify the plant itself. That’s a new discovery in itself,” Johnson says.

The adult gall midge, which is about half the size of a fruit fly, lays its eggs in the plant when it begins to flower in mid to late July. The larvae feed on the developing seed within the plant. Depending on when the eggs hatch, the larvae will even feed on the unfertilized ovule. However, when the adult lays its eggs too late, the larvae don’t develop to adult stage because the developing seed has become too hard. They don’t have mandibles, so must suck the juice of the plant.

Johnson and López also discovered a parasitic wasp that feeds on the gall midge larvae, but doesn’t damage the grass. It could be used as a biological control.

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SDSU researchers plan to determine what can be done to enhance parasitic wasp populations and what population levels will be needed to prevent seed damage. In addition, researchers must address the issue of monoculture versus mixed grass

“Can we put big blue stem in one area, prairie cordgrass in one and switchgrass in another to produce, on average, a good amount of biomass — and how does that affect the insect communities?” Johnson asks.

He expects that complex, mixed communities will be more stable ecologically and have fewer pest problems. That may then help increase production of native grasses for use in biofuel production.

The research was supported primarily by USDA through the North Central Regional Sun Grant Center, which seeks to develop native grasses as a source of biobased transportation fuels.

Defanian is a SDSU writer.

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