The ecological effects of introduced species on native organisms can sometimes, but not always be significant. The risks associated with invasive alien pests are difficult to quantify. This paper concentrates on the ecological effects of invasive insect predators that feed on pest insects, because the former may potentially affect the biological control of the latter. The literature indicates that invasive predatory insects generally are resistant to changes in environmental conditions, long-lived and voracious with a high reproductive rate, high dispersal ability, able to spread very rapidly across landscapes and exhibit phenotypic plasticity. Their colonization of patches of prey may induce native predators to leave, but the evidence that invaders negatively affect the abundance of the native species is scarce and not persuasive. Insect predators do not substantially affect the abundance of their prey, if the ratio of generation time of the predator to that of the prey is large (the generation time ratio hypothesis), therefore the effect of an invasion by long-lived alien predators on systems consisting of long-lived native predators and short-lived prey on the abundance of the prey is hard to detect.