The fast-growing Central American tree Leucaena leucocephala (Lam.) de Wit (Leguminosae: Mimosoidaeae) has been widely planted in the tropics since the 1970s for agro-forestry, reforestation, and fodder. Since the mid-1980s the tree has suffered serious damage throughout its exotic range from the psyllid Heteropsylla cubana Crawford, which is also native to Central America. This article summarizes field studies on the tree and the psyllid conducted at 2 sites in north Thailand: a cool highland and a warm valley site. In the highlands, mean psyllid densities per shoot were slightly higher, defoliation was greater, and dry matter production losses due to the psyllid were >3 times higher than those at the valley site (62.5 and 18.3% losses, respectively). Using field data, the lower thermal thresholds for tree growth and psyllid development were 11.2 and 9.6°C, respectively. Psyllid abundance was negatively correlated to temperature. When maximum temperatures exceeded 36°C, psyllid abundance fell dramatically, mortality increased, and body size decreased. These data suggest that the tree and the psyllid are climatically mismatched. H. cubana prefers new shoots, and population cycles were related to tree growth cycles. Finally, populations of the introduced parasitoid Psyllaephagus yaseeni Noyes (Hymenoptera: Encyrtidae), native coccinellids, and spiders showed little correlation to the psyllid population’s intrinsic rate of increase. Percentages of parasitism by P. yaseeni were low (mean = 1.2–1.9%, maximum = 6.0%) and no evidence of density dependent regulation was found.