Should I treat every infestation as I catch them or is some level of aphids "tolerable" and the presence of aphids in the garden unavoidable?
In other words should I only treat the worst cases?
I'm not sure how bad it is for the plants.
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Giles. Not really...there is no black and white but better a light gray. Sooner one treats aphids the better. Hard spray of water, NEEM sprayed at night as soon as you see crowds of aphids on your roses or vegetables. But if you are on your way to a well-deserved vacation and as you go out the door and catch sight of aphids, keep going to your vacation. When you get back you can take care of it or you just might have enough beneficals around that had a feast! Aphids are so normal, gotta stay on top of them but it is not life and death.
The presence of aphids is fairly common for many gardeners.
Aphids are said to be able to potentially transmit certain diseases to plants. Aphids can stress plants and reduce productivity and/or health of the plants. They can make plants sticky and less ornamental.
Some people like to blast aphids off plants with water. I haven't personally found this to be functional with a normal hose end. However, I've found that using an oscillating sprinkler or a nozzle on the shower setting every three days or so to spray the plants thoroughly (and to water where helpful) seems to keep aphids away, whether or not you aim to spray them off, and it definitely keeps spider mite damage at bay when the air is dry. This may not be recommended if you live in a humid area where the plants will stay wet a long time (and be susceptible to fungal infections). If you have hard water, this might additionally cause issues with your leaves. In my semi-arid, northern garden (where days are long and hot) with city water, the plants love it, though, and it keeps them growing and healthy. I don't guarantee that will work with anyone else's city water (you never know what they put in it). Usually, city water is thought to be inferior for gardening. Maybe pests just don't like chlorine/chloramine.
I had aphids on plants before I got a hose nozzle and an oscillating sprinkler, after which they mysteriously disappeared, thankfully. I guess it could be a big coincidence, but it's why I said what I did. Last year, I only watered the very base of each plant; the plants had more issues and didn't grow as fast. Now I water the whole area of ground, and the leaves and they grow fast and healthy; most people will probably say watering the leaves is a terrible idea, though, and it might be, for you; it depends.
Last year, we had a big aphid problem even though I got extra ladybugs. I did find that doing a foliar spray with calcium nitrate fertilizer on my peppers pretty much got rid of the aphids. They didn't like it. I don't seem to need to do that this year.
I haven't noticed aphids spreading diseases much in my garden (possibly on a few plants last year), but I understand it can be a big problem.
Aphids seem to prefer to feast on new growth, but they'll also feed on old growth.
Some kinds of aphids may be more troublesome than others.
In my experience, many pests prefer plants that are growing in the shade, and unhealthy plants/leaves most. Keeping your plants healthy should help to deter pests to a good degree.
Some kinds of plants are more tolerant to aphids than others. Which ones can vary from area to area.
One thing I recommend is saving your own seeds and replanting the freshly saved seeds every year until your plants are used to the local pests and diseases. I use such as a Z4EX to get rid of any pathogens in the seeds (I put them in a jar of water and zap the water), and so far it seems to work. Last year, I grew Aunt Molly's ground cherries, and several watermelons. They were highly afflicted with spider mites and probably some anthracnose-like fungal infection that they might have spread. I saved seeds and zapped them. I grew them this year, along with several new varieties of ground cherries and watermelons. Before I got my sprinkler and nozzle, Aunt Molly's was one of the only ground cherries that didn't show any evidence of the same problems it had last year, and the only watermelons to have issues, I believe, were new ones. These issues all went away after I began showering the plants (except mysteriously on one Goldie ground cherry that receives less light; it's still alive and growing, however). Last year they eventually killed the plants.
Anyway, acclimatization seems to be real, in my experience (and other people's). The watermelons from my saved seeds are bigger this year, too (especially the one whose parent was grown from a rooted cutting last year, which is now the largest in the patch: i.e. Ledmon). If you're going to be saving seeds with acclimatization to aphids and diseases in mind, exposure to aphids and diseases the first few to several years can be helpful (but it's probably best to keep them away anyway, if you want a harvest and a disease-free yard).
Anyway, leaving aphids alone altogether may not be so bad (it probably depends on how fast they breed and if they're spreading diseases). I haven't had major issues leaving the black winged aphids on tomatoes alone last year. The fast-breeding, quicker, armored aphids on the peppers weren't so harmless, however.
Anyway, I recommend working more on aphid prevention. I wouldn't worry too much about a few aphids here and there, unless they cause problems. Here are some suggestions:
I hypothesize that a foliar spray of sea minerals might deter aphids for a while (it deters spider mites, for a while, if you put it in the soil; it seems to give the plants a growth boost, too; it did, however, make the rind of one of my Blacktail watermelons salty, interestingly, but it tasted pretty good). I don't recommend doing this a lot, due to the sodium in it, but once in a while should be fine.
You have to be careful of some suggestions, since they may harm beneficial insects, too (e.g. neem oil, diatomaceous earth, etc.) I'm not sure if oil sprays would harm other insects, except for maybe predatory mites and whatever was on your plant at the time.