<p>The use of microbial insecticides in crop fields has been very limited, especially in developed countries, compared with that of synthetic (chemical) insecticides, even though the former are friendly to vertebrates (including humans and livestock), most beneficial insects, plants, and the environment. This lower use rate is attributable mainly to their more expensive commercial production and lower effectiveness compared to synthetic insecticides. The combined use of microbial insecticides and synergistic agents would strengthen the potency of these insecticides and decrease the amounts of the microbial insecticides used. This, in turn, would lead to lower costs and wider adoption. Therefore, it is important to develop an efficient method of the combined use. Natural synergists are generally less harmful to vertebrates and the environment than synthetic synergists. Here, I review recent studies on two major natural synergists derived from insect viruses: the proteins enhancin and fusolin. Enhancin originates from baculoviruses that infect insects, while fusolin is found in the insect virus group entomopoxviruses and in baculoviruses; the fusolin in baculoviruses is also referred to as GP37. In addition, I discuss prospects for the development of technologies for the use of the proteins in the fields, including the improvement of gene expression systems and genetically modified plants, and the engineering of the two proteins.</p>