I sort of cringe when people mention international trading of plants, but if you must, please be very, very careful to make sure that the plants are disease and insect-free. My two worst insect pests, Argentine ants and giant whitefly, are not native to the U.S. and got here accidentally. Giant whitefly just appeared in my yard in the past few years. They are very hard to control and have killed my dahlias and disfigure at least a dozen other species. They don't seem to have any preditors in this country. The pictures at this link will show you how gross giant whiteflies are.
I agree, very good points to remember! Soil can also carry a lot of nasties too. I don't trade plants outside the EU, I'd sometimes like to send a friend a bit of something but always worry about getting them into trouble, either with customs or pests.
I trade plants and seeds internationally OFTEN. I have the most pleasant experiences. I am never required to fill out any type of custom forms unless my package exceeds a certain size limit. Recently I sent a very very large box of plants to the UK. I had to fill custom forms because of the SIZE of the package. I wrote exactly what kind of plants were in the box. My receiver got them unopened in great shape. He said they looked like they had just been dug. I hear lots of stories and most aren't good. So I had to write, incase someone out there is thinking about it. I have never had any trouble. I also don't have many bugs in my gardens. Mostly the Rabbits are the ones that eat everything. So maybe it depends where you live? What type of bugs and what they like to feed on? I don't use insecticides and the foliage on my plants are great! My Dahlias are lovely and every leaf is gorgeous. Thanks everyone