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Rui, can you tell us what you are trying to achieve. Do your objectives really require quantitative survey? There is a huge difference between quantitative and qualitative surveys, the former require considerable care and rigour, lots of duplicate samples, ideally over several years to overcome all the variables out there in the "wild". Really, to do it properly, it is a PhD project.

Even if you do all this, and get a result, how will you interpret it? Any threshold diversity/ abundance indices there might be would not necessarily be valid in Portugal.

If all you want is a quality assessment of a site then a qualitative survey will do, but even here you need other sites in Iberia that have been assessed using the same techniques with which to compare.

In GB such qualitative assessments are done by producing a species list (min 100 species, 200-300 better) then applying species quality grades to all species. One can then come up with an assessment based on a comparison with known high quality sites. In Portugal I doubt if such species grading or previously assessed sites exist.

The best indicators depend on habitat, basically parasitoides and predators and dead-wood specialists are most sensitive to habitat disruption so (in GB at least) a good list of these will indicate a high quality site. Least good are phytophages and highly mobile species (so butterflies not very good, but are used because the ease of identification together with an army of amateur recorders means you can get large datasets and good ecological knowledge of them allows one to understand results better). I am biased but I really think Diptera plus aculeate Hymenoptera provide the best ecological coverage, there are almost no ways an insect can live which is not found in these groups. Also both can be sampled with a sweep-net and in the case of Diptera, large samples can be obtained in a short time. Beetles are very useful but it is impossible to get such a large sample in the same time without using diverse trapping techniques, which makes a survey so much more complex.

In conclusion, unless you have tens of thousands of euros to play with, do a minimum of three field trip days, May, June and August (but in Portugal I suspect April, early June and September would be better), sample selected Diptera (depending on habitat and identifiability) and aculeate hymenoptera.

Hope this helps.