Different sites in the forest of Papua New Guinea have been found to have very similar species compositions of herbivorous insects, even though each site has a large number of species. These herbivorous insects are therefore said to have low diversity.

low beta diversity

Explanation:

Beta diversity is a measure of the differences in the composition of species among different environments. In order words, it shows how different in terms of species composition is among different environments.  

A high beta diversity indicates there is a little similarity in the composition of species among different environmental sites, while a low beta diversity indicates there is high similarity in the composition of species.

Since the species of herbivorous insects among different sites in the forest of Papua New Guinea are very similar in composition, we can say they have low beta diversity.