A bit of termite info for those interested...
Out of all the current chemicals Bifenthrin works the best for home use - it is cheap, easily available, and relatively safe. It also works awesome on spiders, millipedes, European Earwigs around the house and sheds. In the old days you'd have laced the posts/soil with any of the now banned organochlorines and you'd be fine for 40 years or more (DDT, Lindane, Heptachlor, Dieldrin, Aldrin etc etc).
Bifenthrin will give you 10-15 years protection. Another good one but very costly at around $300/L is Fipronil (Regent 200SC etc). These two are the main ones used by pest controllers. Most anti cholinesterase chemistry will work on termites.
Also don't get sucked in by the new fangled termite "bait" stations. Termites are forage feeders meaning they don't actually "sniff" out timber, they simply bump into it, yell out to their mates, and bingo dinners on. Because they are forage feeders they can actually slip between the gaps in bait stations and shazzam, they're in your house and you don't even know because you think you're protected. If they have a food source (ie: your house) they have no real incentive to cast the net and forage wider, meaning those bait stations will remain untouched while your house gets chewed down. Bait stations certainly have a place, but pest controllers push them because they then lock customers into expensive annual contracts, and it is guaranteed to bring in a set future income for that particular business.
The specific Insect Growth Regulators (IGR's) used in these bait stations are generally kept pretty quiet - even chemical companies that I deal with at work keep things to themselves as they have different divisions in their company. For example Dow have their agricultural sector and their bait stations are sold via another section of the company. The main reason is that the same "exclusive to pest controllers" IGR's are also found in some grain treatments available to farmers for weevils etc. This is why most of the online information related to termite bait stations does not mention the specific active ingredient. Therefore with enough homework you can potentially make your own bait stations if you were that way inclined and smart enough to get the dosage right. For example "Raxil T" from Bayer that is used to treat grain in silos etc contains Triflumuron which is the exact same IGR used in some termite bait stations. The same active that if you asked your local, friendly pest controller about, will be greeted with an answer along the lines of "Nobody else can get it". Heh heh. Fact is that I bet none of them even know what Triflumuron is.
I've had a lot to do with entomology over the last 20 years and the way pests feed and the effect certain insecticides have on pests is something I find very interesting.
The one thing when treating with Bifenthrin (what I recommend as it has relatively low mammalian toxicity when dry) is to make sure you get good coverage, and make sure it injected deep enough (or covered over with dirt) to avoid UV light. UV light is what lowers the half life of basically all insecticides. For example Chlorpyrifos will provide a lethal dose to insects in the open for maybe 10-14 days. Out of UV light it will remain potent for up to 20 years. If you have a good way of injecting or spraying chemical then there is basically nothing a pest controller can do that you can't do yourself. I just treated a pile of around 6 tonne of cut green Mallee this afternoon with Bifenthrin as it will take a year before it is ready to split and burn and my place is termite central. We have 3 species around my place.
Anyway, my head hurts - it could be from the Bifenthrin solvents...
remove old poles,burn,put new poles in ground and pour 20liters of sump oil in the hole before backfilling,paint the rest of the post as well or just use cypress logs
That works too and helps stop rot to some extent.
are steel post an option?
Timber in the ground is never a long term solution. Gypress will be somewhat termite prone due to the natural toxins, but then it rots. Any timber will rot eventually, even ironbark and even gidgy. If your buget permits, make up some small steel post/brackets and concrete them in, you can still use timber posts from here up, its just means the bottoms can be mad to be 4 inches or so above ground level. Helps avoid rot and termites. They hate day light and the prefer some moisture. You can use simple flat plate to make up very good post brackets. I have used these on 12x12 square ironbark and on big round telegraph poles for pole homes. Keep an eye out for decent flat plate in the 8mm to 12mm range 150mm wide or up.
Sadly Serge your idea is the only one that I can guarantee WILL work
Termites are cagey little bastards...