Hypothesis: The evolution of herbivory in insects opens up many new ecological niches to those insects which evolve it, allowing adaptive radiations to occur. To test this, look at other groups of insects which have evolved herbivory, and look for similar increases in diversity/species number. This would indicate adaptive radiation.
Work Step by Step
Since this question is asking about a large group (insects) which have been adapting for a long time, rather than a controlled experiment you will need to suggest a comparative experiment using organisms which already exist. The hint for generating the hypothesis is the mention of species number in the question-- see that species number seems to be related to herbivory/non-herbivory, and then work from there.