Understanding warming impact on herbivores facilitates predicting plant/crop dynamics in natural/agricultural systems. However, it remains unclear how warming will affect herbivore population size and population composition, consequently altering herbivore colonization in a tri-trophic system (plant-herbivore-predator or crop-pest-biocontrol agent). We studied a soybean-aphid-lady beetle system, by conducting (1) a laboratory warming experiment to examine warming impact (+2 °C or +4 °C) on the aphid population size and composition (alate proportion), and (2) a field colonization experiment to examine whether the warming-induced effect subsequently interacts with predators (lady beetles) in affecting aphid colonization. The results showed that warming affected the initial aphid population composition (reduced alate proportion) but not population size; this warming-induced effect strengthened the top-down control by lady beetles and slowing aphid colonization. In other words, biocontrol on crop pests by predators could improve under 2-4 °C warming. Furthermore, aphid colonization was affected by an interaction between the alate proportion and predator (lady beetle) presence. This study suggests that warming affects herbivore population composition and likely mediates top-down control on herbivore colonization by predators. This mechanism may be crucial but underappreciated in climate change ecology because population composition (wing form, sex ratio, age/body size structure) shifts in many species under environmental change.