Napthalene moth balls can be used to avoid insects in permanent displays. It is used as an insecticide and pest repellent in Museums and Insect collections.
Just one suggestion. If you are going to use naphthalene balls be sure about its quality before purchasing from the market. Most of the naphthalene balls available in the market are either of substandard quality or fake.
You can refer following journal and preserve with natural one. http://www.vam.ac.uk/content/journals/conservation-journal/issue-23/traditional-practices-for-the-control-of-insects-in-india/
Phosphine is the best fumigant which can be used to avoid insect growth especially Sitotroga cerelella and Tribolium species in stored grains, however we can also use them in the grains prepared for display. thanks