I want to extract nuclear DNA from aphids but the sample size for extraction is difficult to determine as each and every individual in an aphid colony can be a separate species.
Its depends upon your collection procedure. First you should collect wingless aphids (preferably Nymphs for DNA) from different colonies in a same plant parts and store them separately. Individual from same colony will not be different.
Then you can grind upto 5-10 aphids collectively.
You can use ZR Tissue & Insect RNA MicroPrep™. I have used it for 5 Aphids per sample and got very good concentration.