High propagule pressure is arguably the only consistent predictor of colonization success. More individuals enhance colonization success because they aid in overcoming demographic consequences of small population size (e.g. stochasticity and Allee effects). The number of founders can also have direct genetic effects: with fewer individuals, more inbreeding and thus inbreeding depression will occur, whereas more individuals typically harbour greater genetic variation. Thus, the demographic and genetic components of propagule pressure are interrelated, making it difficult to understand which mechanisms are most important in determining colonization success. We experimentally disentangled the demographic and genetic components of propagule pressure by manipulating the number of founders (fewer or more), and genetic background (inbred or outbred) of individuals released in a series of three complementary experiments. We used Bemisia whiteflies and released them onto either their natal host (benign) or a novel host (challenging). Our experiments revealed that having more founding individuals and those individuals being outbred both increased the number of adults produced, but that only genetic background consistently shaped net reproductive rate of experimental populations. Environment was also important and interacted with propagule size to determine the number of adults produced. Quality of the environment interacted also with genetic background to determine establishment success, with a more pronounced effect of inbreeding depression in harsh environments. This interaction did not hold for the net reproductive rate. These data show that the positive effect of propagule pressure on founding success can be driven as much by underlying genetic processes as by demographics. Genetic effects can be immediate and have sizable effects on fitness.