I already have a good veg garden and am considering a polytunnel for tomatoes, melons, pepers, strawberries etc I was going to construct 9 inch raised beds within the tunnel, will the soil need changing every so often due to infestation or would I be better off to have a gravel area and do some stuff in pots. I have read about this infestation, is it really a problem or does it only effect some plants.
In the home garden situation, there is hardly ever any need to take out or change soil.
If you set up a polytunnel, simply dig in plenty of organic material to the exisiting top-soil, Lay a path of paving slabs for access. There is no need to raise beds, which tend to dry out, and growing in pots is even more onerous in terms of watering.