Comparison of Resistance Index Some Potato Cultivars to the Colorado Potato Beetle, Leptinotarsa decemlineata (Say) (Col.: Chrysomelidae)

Introduction: Potato, Solanum tuberosum L. is an important crop with 5.24 million tons of production on 210,000 hectares of irrigated land in Iran. Several pests attack the potato crop which among them the Colorado potato beetle, Leptinotarsa decemlineata (Say), is the most important defoliating pest of potato throughout the world and Iran and considerably reduce crop yield. Growers rely on pesticides to control this pest in the field but this insect is well known for its rapid resistance development to pesticides, so that it has developed field resistance to nearly all insecticides used against it and it is now resistant to more than 40 chemical insecticides. The problems of insecticide resistance, combined with continuing environmental concerns associated with chemical pesticide use, have provided a considerable stimulus over the past 50 years for the development of alternative control methods. Host plant resistance is considered to be an important part of integrated pest management (IPM) system of this pest, which is compatible with sustainable control methods and can reduce the use of chemical insecticides .With the aim of identifying the existence of resistance resources, a study was conducted to evaluate and comparison the resistance index (PRI) of 33 potatocultivarsto the Colorado potato beetle. Detected resistant variety (ies) could be used as a resistance source for IPM programs of this pest. Materials and Methods: Greenhouse and field experiments were conducted to compare resistance index (PRI) of 33 potato cultivars to the Colorado potato beetle, Leptinotarsa decemlineata (Say), in Naghadeh region during 2007-2008. In a choice test, the numbers of attracted beetles to each cultivar was determined as antixenosis index in the field. Also, Percentage of larval and pupal mortality were determined and used as the antibiosis index under greenhouse conditions. To evaluate the tolerance index, infested and non infested plots were planted and arranged based on a randomized complete block design under field conditions. In infested plots, each plant was infested by 40 medium larvae (second and early third instars) 15 days prior to the blooming of plants. At the end of season, yield loss in infested plots were determined and compared with non infested plots for each cultivar and used as the tolerance index. Normalized values for each category were computed on a zero-to-one scale by dividing the mean value of each variety evaluated by the maximum mean value