The study was conducted throughout two seasons (2013 and 2014) at Xmatkuil farm at Merida, Yucatan, Mexico, to determine the relationships and the interactions between chemical compositions of condensed tannins and mimosine and their effects on the susceptibility of four Leucaena genotypes: Cunningham and K636 (L. leucocephala), and Nativa and KX2 (L. leucocephala 9 L. pallida) to the infestation of the most destructive insect pests; Leucaena Psyllids, Heteropsylla cubana Crawford, 1914 (Homoptera: Psyllidae), and Onion thrips, Thrips tabaci Lindeman, 1889 (Thysanoptera: Thripidae). Mimosine showed insignificant effect on the population of both pests. However, tannins exhibited a significant effect for Psyllid population on Nativa and K636, and highly significant effect on thrips for the same respective genotypes. The insignificant relationships of tannin effect were found for Cunningham and KX2 to the population fluctuations of Psyllid and Thrips