PLANT RESISTANCE Inheritance and Categories of Resistance in Wheat to Russian Wheat Aphid (Hemiptera: Aphididae) Biotype 1 and Biotype 2 S. A. KHAN, 1,2 M. MURUGAN, 2,3 S. STARKEY, 2 A. MANLEY, 4 AND C. M. SMITH 2,5 J. Econ. Entomol. 102(4): 1654Ð1662 (2009) ABSTRACT The Russian wheat aphid, Diruaphis noxia (Kudjumov) (Hemiptera: Aphididae), is globally one of the most devastating pests of wheat, Tritium aestivum L., and barley, Hordeum vulgare L. Host plant resistance is the foundation of cereal insect pest management programs, and several D. noxia resistance (Dn) genes from wheat have been introduced in commercial cultivars of wheat to manage Russian wheat aphid (RWA). Emergence of D. noxia biotype 2 (RWA2) in Colorado has made all known Dn genes, except the Dn7 gene from rye, Secale cereale L., vulnerable and has warranted exploration for sources of resistance to RWA1 and RWA2. Here, we report antibiosis resistance to RWA1 and RWA2 identiÞed in the wheat breeding line KS94H871. Additional experiments indicated that tolerance and antixenosis are not operating in KS94H871. Segregation studies involving F 2 -derived F 3 families indicated that KS94H871 resistance to RWA1 is controlled by one dominant gene and one recessive gene, whereas resistance to RWA2 is controlled by only one dominant gene. This new genetic resource may serve as a good source of resistance in future breeding programs with proper under- standing of the genetics of resistance. KEY WORDS wheat, Diuraphis noxia, resistance, inheritance, antibiosis The evolution of agriculture has increased humansÕ dependence on four major cereal crops for energy needs. Among these is wheat, Triticum aestivum (L.), one of the worldÕs most important food crops (Cordain 1999). Wheat is the second most important staple crop in the developing world and serves as a staple food for 35% of the worldÕs population (CIMMYT 2009). The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), is one of the most devastat- ing pests of wheat and barley. D. noxia is considered indigenous to Afghanistan, Iran, southern Russia, and countries bordering the Mediterranean Sea (Hewitt et al. 1984) and was Þrst reported in North America in 1986 (Stoetzel 1987). Yield losses due to D. noxia are severe, and individual plant losses as high as 90% are possible (Du Toit and Walters 1984). Under heavy infestation, grain weights can be reduced by as much as 80% of normal yield (Hein 1992). Cumulative losses to all U.S. small grain production due to D. noxia control, grain losses, and lost community economic activity from 1986 to 1993 were valued at $1 billion (Morrison and Peairs 1998). D. noxia feeds mainly on the adaxial surface of the newest growth in the axils of leaves or within rolled leaves. During feeding, aphids inject saliva into plant leaves and remove phloem sap from sieve elements. D. noxia salivary digestive enzymes break down leaf chlo- roplasts, leading to white, yellow, purple, or redÐpur- ple longitudinal streaks on leaves of infested plants (Fouche et al. 1984, Pike et al. 1991). Symptomatic leaves have reduced photosynthetic ef Þciency, which leads to reduced vigor and increased sensitivity to environmental stresses. Additional damage symptoms include leaf rolling and leaf folding. In young plants, heavy infestations cause tillers to become prostrate; in mature plants, infestations cause tillers to become trapped in the rolled ßag leaf; and severe infestations result in plant death (Walters et al. 1980). Management options for reducing D. noxia infesta- tion include insecticides (Webster et al. 1987, Nel et al. 2002), cultural techniques (Peairs 1998), and bio- logical control agents (Wraight et al. 1993, Prinsloo 2000). However, D. noxiaÕs feeding habit of hiding in rolled leaves of damaged plants often complicates ef- fective management with insecticides or biological control agents. Insecticidal applications have their own limitations, such as environmental and health hazards. Host plant resistance, however, is an eco- nomically sound and ecologically safe method for managing insect pests (Smith 2005). Researchers in the United States estimated that the return on invest- ment was 13:1 for the development of the resistant ÔHaltÕ wheat against D. noxia (Webster and Kenkel 1999). 1 Department of Plant Protection, NWFP Agricultural University, Peshawar 25000, Pakistan. 2 Department of Entomology, Kansas State University, Manhattan, KS, 66506-4004. 3 Department of Plant Molecular Biology and Biotechnology, Cen- tre for Plant Molecular Biology, Tamil Nadu Agricultural University, Coimbatore 641003, Tamil Nadu, India. 4 921 E. Central Ave., Ponca City, OK 74601. 5 Corresponding author, e-mail: cmsmith@ksu.edu. 0022-0493/09/1654Ð1662$04.00/0  2009 Entomological Society of America