Field trials and laboratory experiments were undertaken for monitoring the relative susceptibility of four potato varieties namely; Atlas, Spunta, Simone and Nicola to potato tuber moth (PTM) Phthorimaea operculella (Zeller) infestations. Experiments were carried out for two summer plantations during 2006 and 2007. Results obtained indicate that Nicola potato variety was the least susceptible one among the four tested varieties. The least infestation (5.5 & 7.17) as average number of larvae/100

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... leaves, percentages of tuber infestation (5% & 18%) and number of larve/100 tubers (5 & 20) was recorded for Nicola variety in the two seasons, respectively. The analysis of variance yielded significant differences among the four tested varieties. Positive and highly significant relationships were recorded between plant age and the corresponding infestation (r = +0.91 for 2006 and +0.93 for 2007). Chemical analysis for tubers showed a positive relationship between lowest sap content of free phenol and the least level of PTM infestations. That will explain why Nicola variety is considered as least susceptible to PTM infestation where it had the least concentration of free phenols (19.507 ppm.). The other verities which demonstrated high concentrations of free phenols harboured the highest levels of infestation with PTM. Positive and highly significant correlation were detected between free phenols content and both tuber infestation & larval content (r=+0.80 and +0.96) respectively.