Typically, a high humidity situation is enough to discourage spider mites - they dislike wet - so spraying down your little seedling with just the hose on a routine basis may help. Also, mites are not technically 'insects' so many insecticidal sprays are ineffective against them. Neem oil has the advantage of being both an insecticide and a miticide and is very effective with spider mite control. It is advised not to spray above 85- 90F (29-32C) so you have some leeway.

Primarily, you want to avoid spraying in hot sunny weather, as that can often create reactions with the plant foliage. Keep the plant in the shade and spray in early morning when the weather forecast is somewhat mild and you should be OK.

On rereading your post, I get the impression this plant was kept indoors for a time? If so, that's why you got the mites :-) Indoors presents a very dry situation that the mites love. And it also stresses plants like maples to the point where they become more vulnerable to pest infestations. Keep it outdoors - that's what it wants and needs.

As usual, Gardengal is right on.

I wouldn't even think of putting a maple indoors at any time of the year.

Where are you?

I have a partially shaded unheated sunroom where I winter a lot of house plants that can't handle freezing temps. Occasionally a few will get spider mites. I put them outside when it's not freezing and within a week or two the mites are gone. No spray, except water. Outdoors, those plants never get spider mites to the point they are noticeable.

Placing the maple outdoors in the shade and watering the leaves is the best solution for your problem.

Mike

I'm in Canada, near Toronto. I had two box elders and the maple inside a glassed in sun room(unheated/no AC) where they would receive partial sunlight.