The use of insect-virus insecticides has been very limited and the development of new insecticides has not been pursued actively. This is mainly because the cost of pest control using the insecticides is very high considering its effect compared with the cost versus effect of using synthetic insecticides. One way of diffusing insect-virus insecticides is to strengthen the ability of the viruses in the formulation to kill pests. Strengthening the virus activity will lead to a drastic reduction in the amount applied per unit area, cost of control will be lowered, and the use of such insecticides will spread. Several groups have studied the synergistic effect of an entomopoxvirus protein, fusolin, on the infectivity of insect viruses to strengthen their ability to kill pests. Fusolin is the main component of entomopoxvirus inclusion bodies called spindles, and the spindles of several entomopoxviruses have been found to strongly enhance many nucleopolyhedroviruses. The enhancement occurs through the disruption of the peritrophic membrane, a barrier against virus infection, by the spindles. Two of these fusolin genes were cloned, and two and one of these genes were expressed in foreign gene expression systems and a crop species, respectively. As a result, the gene products enhanced viral infection. Finally, the functional region for the enhancement in this gene has been shown and the detailed mode of action of fusolin was estimated.