Study on the resistance components in 33 commercial potato cultivars to Colorado potato beetle, Leptinotarsa decemlineata (Col.: Chrysomelidae)

Greenhouse and field experiments were conducted to evaluate the resistance components in 33 potato commercial cultivars to Colorado potato beetle, Leptinotarsa decemlineata (Say) in 2008. In a field choice test, the numbers of attracted beetles to the cultivars was determined as antixenosis index. To evaluate the antibiosis, potato tubers were planted in pots under greenhouse conditions and on each pot, one sleeve cage was set up, in which 15 first instar larvae were released and reared. The Larval weight after 12 days of releasing, mortality percentages of larvae and pupae and the durations of developmental stages were analyzed as the antibiosis indices. To study the level of tolerance of cultivars, the infested and non infested plots were isolated and arranged based on a randomized complete block design in field. 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 the season, defoliation and yield loss among infested plots were determined and compared to non infested plots for each cultivar. Significant differences were observed in all studied traits. The cultivars Cardinal, Carlita and Sinja showed antixenosis effects with the least numbers of attracted beetles, whereas cultivars Carlita, Sinja, Delikat, Aparet and Bridjet showed antibiosis effects in comparison to the others. The cultivars Santana, Satina, Nicola and Bridjet showed tolerance to damage of the pest. Using cluster analysis, UPGMA procedure was based on Euclidean distance and 33 experimental cultivars were grouped in 7 distance groups

Isolation, characterization and toxicity of native Bacillus thuringiensis isolates from different hosts and habitats in Iran

Bacillus thuringiensis is a Gram-positive, aerobic, facultative anaerobic and endosporeforming bacterium. Different strains of this species have the ability to produce parasporal crystalline inclusions which are toxic to larvae of different insect orders and other invertebrates and cause rapid death of the host. To determine the importance of this species in microbial control, we collected native strains and studied their virulence on the diamondback moth, Plutella xylostella. More than 148 samples were collected from Alborz, Guilan and Mazandaran Provinces. Experimental samples, including soil samples from forests, fruit gardens, agricultural fields, diseased and dead larvae, were transferred to a laboratory in sterile plastic containers. For evaluating B. thuringiensis isolates virulence, a cabbage leaf dip method with 106 cell ⋅ ml–1 concentration of various Bt isolates was applied to diamondback moths. Larval mortality was recorded 72 h after treatment. Based on bioassay results, all isolates were classified into three high, medium and low virulence groups. Protein level characterization based on the SDS-PAGE gel analysis showed that two isolates from a high virulence group have proteins of high molecular masses of 121 and 109 kDa. Results revealed that there is a positive correlation between protein masses and virulence of isolates. In addition, this research introduced nine strains that are highly toxic to P. xylostella and would be valuable as insecticidal agents for controlling lepidopteran pests

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 of all entries. Normalized tolerance values were calculated as percent reductions of plant biomass relative to a non-infested control. Normalized indices for antibiosis (X), antixenosis (Y) and tolerance (Z) were used to calculate PRI in the formula: PRI = 1/(XYZ) (Inayatullah et al., 1990). Results and Discussion: Combined analysis of variance showed that interactive effect of cultivar × year was significant for all of the indices at P=0.01. The cultivars Sinja, Bridjet and Cardinal with the least antixenosis index (Y), had the highest antixenosis resistance to the Colorado potato beetle in 2007, also the cultivars Cardinal, Carlita and Elles with the least antixenosis index (Y), had the highest antixenosis resistance to the pest in comparison to other cultivars in 2008. The highest antibiosis (X) resistance was observed in the cultivars Delikat, Carlita, Armada and Sinja with the least antibiosis index, in two studied years. The cultivars Satina, Cardinal, Santana and Nicola with the least tolerance index (Z), had the highest tolrence to the Colorado potato beetle in comparison to the other cultivars during 2007-2008. The cultivars Delikat,Bridjet and Cardinal ere the most resistant cultivars with 52.63, 36.30 and 29.87 PRI, respectively in 2007. In this year, the cultivars Diamont, Aparet and Velox were the most sensitive cultivars with 1.90, 2.06 and 2.26 PRI, respectively in comparison to other cultivars. In 2008, the cultivars Santana, Bridjet and Sinja were the most resistant cultivars with 48.56, 28.36 and 25.80 PRI, respectively and the cultivars Velox, Agria and Bright were the most sensitive cultivars with 1.90, 2.71 and 3.20 PRI, respectively in comparison to the other cultivars in 2008. Conclusion: Significant positive correlation was observed between the plant resistance indices (PRI) in two study years (r= 0.499, P= 0.01). Results of this research revealed that the cultivars Bridjet, Delikat, Cardinal, Carlita, Sinja and Nicola with the highest PRI were the most resistant cultivars to the Colorado potato beetle in comparison to other cultivars during 2007-2008, suggesting that these cultivars can be used in the IPM program of the pest and resistance transfer to high yield varieties through plant breeding programs