I would and did use just this past week the Permethrin, DE and Dawn mix and spray the older plants from top to bottom including all the stems. I would also sprayed my newer plants which don't show any symptoms of spider mites yet but since they are on many of the older plants it is a good precaution and there are probably some on them already.
Older plants that aren't producing are a more complicated issue as to whether they are viable or not. I remove those that look sickly near the top and that don't have any healthy blooms forming. I also give them a good dose of Urban Farms Vegetable food and then a few days later give them a dose of Texas Tomato Food. I also make sure to remove all those low down suckers that form almost daily. It is still important to focus the plants energy on fruit set while maintaining just enough foliage for good production and a bit of shade. If a week or so after that treatment I don't see big improvement out of a plant then I will yank the old one but any that are showing good improvement I will leave a bit longer.
In this heat to get good fruit set you can not let the soil dry out too much and a good heavy mulch is a must to keep the soil cooler and more evenly moist. I also try to give them TTF every week or so to improve the fruit set which is always low in this high heat but it can be improved. I am getting some decent fruit set from some of my oldest plants and good fruit set on my newer ones including the ones set out in mid June.
Bill
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