By Ed Ricciuti
As scientists fight to keep a step ahead of an invasive aphid that can genetically reinvent itself to penetrate the defenses of wheat varieties designed to resist it, they are following advice from the ancient Chinese military theorist Sun Tzu: Know your enemy. Since the 1980s, the Russian wheat aphid (Diuraphis noxia) has caused $850 million in damage to United States wheat and barley crops, largely because it produces distinct strains, or biotypes, with virulence that can overcome resistance genes or otherwise cause increased damage in host plants.
“By defining the extent of the Russian wheat aphid (RWA) problem, better solutions can be developed,” says Dr. Gary Puterka, an entomologist at the U.S. Department of Agriculture’s Plant Science Research Laboratory in Stillwater, Oklahoma. Recently, as part of the effort to understand the Russian aphid, Puterka has demonstrated that it can hybridize a native aphid, potentially increasing the threat to crops.
Puterka, lead scientist on the study of the RWA’s genetics and ecology for the USDA’s Agricultural Research Service, notes that when the presence of the Russian aphid in North America was first discovered, scientists did not realize it could diverge into diverse biotypes, each with its unique genetic identity. Thus, strains of wheat designed to resist one genetic aphid population failed to stop others as the species adapted to new hosts.
Much as bacteria mutate into drug-resistant strains, D. noxia counters resistant varieties of wheat by morphing into new biotypes, which scientists must then counter in turn. One biotype, RWA2, caused an outbreak in Colorado wheat fields during 2003. Unlike the original strain, designated RWA1, RWA2 produced eggs that can overwinter, promoting survival and increasing the threat it posed in northern wheat-growing states. The outbreak sparked long-term USDA research to learn as much as possible about RWA to head off future aphid depredations. “New D. noxia biotypes will continue to challenge the development of … Russian wheat aphid resistant wheat,” says Puterka. His ultimate aim is the real game changer: development of wheat resistant to all biotypes.
Native to the Russian steppes and Central Asia, but spread virtually worldwide, the 1/16-inch-long Russian wheat aphid popped up in Mexico in 1980 and then during 1986 in the western United States and Canada. In 2013, scientists found that during the fall sexual reproductive season in western Colorado it infested the same grass host—crested wheat grass—as the native western wheat aphid (Diuraphis tritici), creating an environment that might well encourage hybridization in nature. Typically, several generations of female aphids reproduce asexually during summer, until males appear in fall and sexual reproduction occurs.
Puterka has performed experiments showing that, indeed, the Russian aphid can hybridize with the native western wheat aphid—at least, in the laboratory—into progeny that could be more adaptive and virulent to wheat and barley than either parent.
“Emphasis on could,” says Puterka, whose creation of the hybrid is described in the Journal of Economic Entomology. However, as he notes in his paper, other aphid species have hybridized and produced viable offspring when living in close proximity.
Puterka kept the aphids under greenhouse conditions. He found that western wheat aphid males “readily mated” with females of both species. Aphids do not undergo larval or pupal stages but begin life as nymphs, closely resembling adults. Puterka observed his captive aphids as they fed and grew. He compared the virulence of the hybrid colony to parental aphids by evaluating damage to 16 wheat and barley varieties resistant to the Russian aphid. The hybrid damaged two varieties of wheat and two of barley more than either parent. Overall, the native western aphid was more virulent than the biotype (RWA8) of Russian aphid tested.
Puterka postulates that virulence of aphid biotypes is linked to recombination of virulence genes when the aphids reproduce sexually. Genetic changes can influence factors such as the potency of aphid saliva, which plays key role in damage caused when the insect feeds on a plant’s phloem sap. Genetically controlled proteins in the saliva seal the walls of the channel through which it feeds and keeps the sap from clotting and healing the wound.
The study by Puterka also suggested that hybrids might not be as fit for survival as their parents. Only one hybrid survived to become a fully reproductive adult compared to eight from western wheat aphid mating. Even so, says Puterka, a single female could produce hundreds of offspring parthenogenetically. “So, only one unusually fit hybrid could reproduce rapidly and firmly establish itself over time,” he adds.
“Close monitoring wheat and barley fields would be an important aspect in detecting new, more virulent aphid biotypes or hybrid species,” advises Puterka.
Journal of Economic Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.