Germplasms from previous study may thwart new aphid biotype: Russian wheat aphids in differen nymphal stages. The Aphids cause leafrolling, as shown on these susceptible wheat plants

Agricultural Research, April, 2004 by Luis Pons

Scientists in ARS's Wheat, Peanut, and Other Field Crops Research Unit have played a major role in controlling Russian wheat aphid before. Now, it seems this group from Stillwater, Oklahoma, may have done it again.

Starting in the late 1980s, entomologist James Webster (now retired) spent a decade sorting through thousands of wheat and barley germplasm samples, seeking accessions that showed resistance to the aphid, Diuraphis noxia. Geneticists Cheryl Baker and Dolores Mornhinweg then used these accessions lo develop germplasm breeding lines that resisted the pest. These breeding lines led to aphid resistant wheats, some of which have already been released, and barleys that are due out over the next few years.

"The initial work against the Russian wheat aphid was landmark research," says the unit's research leader. David Porter. "The best evidence at the time showed that this threat was under control, thanks in large part to ARS research."

Unfortunately, it wasn't. In spring 2003, a new biotype of the pest was found in Colorado and was later identified in several other plains states. This variant was able to overcome many of the resistant wheat and barley lines, as well as all five of the released resistant wheat varieties.

Confronted with seeing their earlier work rendered obsolete, Baker and Mornhinweg turned to an invaluable safety net: the large collection of wheat and barley breeding lines they had developed while fighting the original aphid biotype. They may have hit pay dirt there in the from of advanced germplasm breeding lines also resistant to the new' menace.

"If this hadn't been the case, we'd be back at square one," says Baker, whose expertise lies in protecting wheat crops from insect pests. "Hopefully, our discoveries will save us 4 to 6 years of new research."

Readiness Was Deliberate

"This was not by accident," says Mornhinweg, who focuses on protecting barley from pests. "'We purposely sought genetic diversity in our resistant breeding lines for just such a possibility."

Mornhinweg says the development may save time in another way. "Usually, once a source of resistance is found, it takes about 10 years to incorporate the resistance into agronomic types appropriate for U.S. farmers,'" she says.

The discoveries may help thousands of wheat and barley growers envisioning a repeat of the original aphid's damage. That biotype has cost North American producers billions of dollars since its appearance in Texas in 1986. The Russian wheat aphid is a major pest worldwide of winter wheat barley and other cereal crops. This tiny, green bug--about the size of a sesame seed--was first spotted in this hemisphere during the early 1980s, in Mexico.

It feeds on crop plants, causing leaves to curl while producing distinctive white, yellow, and purple longitudinal lines along them. These curled leaves provide shelter for the aphid and make them hard to detect until their damage is apparent. Aphids will still feed on resistant cultivars, but the plants' resistance prevents leafrolling; this can significantly lower aphid numbers and reduce plant damage.

Prolific--In Certain Environments

Few farmers were as hard-hit by the pest as those in Colorado. That state's 14,000 wheat growers have suffered through more than $130 million in crop losses and insecticidal control efforts since the original aphid arrived, and its barley industry was brought to a standstill.

Entomologist John D. Burd of the Stillwater lab says biological, climatological, and ecological factors combine to make Colorado "the economic center of Russian wheat aphid problems in North America.

"This aphid--including the new biotype--reproduces asexually, year-round." Females give birth to live females, and no eggs are involved. "This allows the pests to reproduce quickly and in large numbers. They will give birth every 4 to 6 hours under optimum conditions."

This aphid is very particular about its environment. Conditions cannot be too hot or too cold, and there has to be an abundance of volunteer wheat--from spillage, wind, or harvesting--for oversummering. "The grasslands of eastern Colorado provide all these," says Burd.

The Stillwater lab was among many research institutions that responded to the original Russian wheat aphid threat. In collecting as many wheat and barley germplasm samples as possible to test for aphid resistance, its scientists found their greatest resource to be the vast National Small Grains Collection (NSGC), managed by the ARS Small Grains and Potato Germplasm Research Unit in Aberdeen, Idaho.

NSGC personnel provided accessions for testing and collected and made available test data to barley and wheat researchers worldwide. Overall, scientists from the two labs tested 30,000 wheat accessions and all the available 24,000 barley accessions.

Baker also contacted germplasm collectors overseas--a move that may prove vital in the struggle against the new aphid biotype.

Years of greenhouse testing against millions of aphids led to identification of more than 300 resistant wheat germplasm lines. Mornhinweg developed 40 barley germplasm lines she terms highly resistant to the pest, as well as some lines with intermediate resistance.