The relative susceptibility and monitoring of nine widely spread citrus/varieties: Navel orange, Balady orange, Valencia orange, grape-fruit, Balady Mandarin, lime, Lemon, Sour orange and Volkamer lemon to Ptosima undecimmaculata (Coleoptera: Buprestidae) borer infestation as well as the population fluctuation were studied at Sharkia governorate, during the three successive years 2015, 2016 and 2017. The mean rate of susceptibility of citrus species and varieties to P. undecimmaculata infestation reached 16.3% (range, 2 – 46 %). The grand mean degree of susceptibility of citrus species / varieties to P. undecimmaculata infestation approximated 0.377 beetles / tree / year. Infestation increased from 0.341 beetles / tree at the end of 2015 (one year), to 0.720 beetles / tree at the end of 2016 (two years) to 1.331 beetles / tree at the end of 2017 (three years). Summer months recorded the maximum beetle flight (0.242 beetles), followed by spring (0.077 beetle) or autumn months (0.058 beetles) /tree, and stopped during winter. The grand mean degree of susceptibility of citrus species/varieties to P. undecimmaculata infestation approximated 0.094 beetles / tree / season. Generally, Navel orange (19.1%) and Valencia orange (17.2%) were the most susceptible species followed descending by Balady orang (14.3%), Grapefruit, (13.8%) Mandarin (13.8%), Volkamer Lemon (10.3%), while the least was Sour orange (6.4%), Lime (4.5%) and Lemon (2.4%).