I want to know the level of humidity for aphids and spider mites to grow, then I can do the reverse approach.
The best defence against pests (and diseases) is to grow healthy & vigorous plants.
A weak plant is an easy target.
A healthy & vigorous plant is a lot better prepared to defend itself from damage, infection.
Provide optimal growing conditions for the plants you are growing eg
Light, water and fertiliser requirements to encourage healthy & vigorous plants.
Plants aren't overcrowded, good air circulation.
Use appropriate mulching material -- for me that means, compost or shredded leaves. Do not use wood chip mulches.
Keep the gardening area clean -- clear weeds...
Inspect your plants regularly, the sooner you identify a "possible" problem, the sooner you can react appropriately.
Aphids prefer hot and dry conditions.
Spider mites prefer hot, dry and dusty conditions.
You should also consider attracting beneficial insects.
If you are not using pesticides which would kill off beneficial insects, I suggest you do some research into which plants attract and help them to thrive.
Ladybugs and praying mantis are good predators for aphids and other smaller pests. From what I know you can purchase ladybugs in bulk online in the US (I'm in Australia, this is is just something I've heard.)
There was a recent post that aske about this which I will link later, but in the meanwhile just search for 'ladybugs' and you should find it.
I would recommend planting in open areas for your plants (away from trees). Pests tend to afflict shaded plants the most, in my experience, probably because they're weaker and don't have as many beneficial insects around them. This may not be true for every kind of aphid and certainly not always for spider mites, however. I observed this mostly with whiteflies and grape leafhoppers, but also with aphids on members of the Solanaceae family and beans. Aphids ravaged cabbage in open areas, though. Spider mites did more damage to tomatillos in our open area, though, than in the shade, but less damage to watermelon in direct sun than the shaded portions.
In addition to causing shade, trees can also harbor spider mites and aphids. Spider mites like apple trees.
Mulching should help to prevent dust problems that might attract spider mites. Keeping plants well watered should also help.
Keeping plants healthy, strong and vigorous is definitely a boon against spider mites and aphids, too, I believe. Make sure you don't have too little or too much potassium and calcium.
Sea minerals seem to strengthen plants a lot against spider mite damage. A foliar spray once in a while should help.
However, before you go defending against spider mites and aphids, you're probably better off just gardening as usual and keeping plants healthy and strong until you notice a problem.
Ladybugs can help against aphids as can lacewings and other things. Predatory mites can help against spider mites.
Aphids don't seem to like foliar sprays of calcium nitrate fertilizer. It's probably toxic to them.
Spider mites cause serious damage when it's hot and arid, with dry soil. In fact, just spraying plants with water can prevent them from doing damage for a day or two. I haven't noticed a relationship between aphids and moisture, yet.