How do I Investigate and test the hypothesis on the effect of street-light regime on insects?
The hypothesis can be examined by t-test. You can group the data (number of catches) into  dimmed light, and fully lit street lights and proceed with t-test to assess whether dimmed light results  in less en-catchment or higher en-catchment or equivalent encatchment.  Alternatively, a linear model can be applied subject to certain conditions  here. PCA is not  appropriate here.