On cucumbers this is often not an insect at all. Downy mildew, angular leaf spot and cucumber mosaic virus all yellow or mottle the leaves, and they need a different response from mites. Turn a spotted leaf over before anything else: downy mildew shows a purplish-grey fuzz underneath in humid weather, angular leaf spot shows water-soaked patches bounded by the veins that may ooze when wet, while spider mites show tiny moving specks and, once numbers are high, fine webbing.
The rest of what gets called “yellow spots on cucumber leaves”. Some are pests we have not profiled yet; some are insects doing you a favour; some are not insects at all, and no treatment will help.
Not an insect. Pale green to yellow spots on the upper surface, angular and bounded by the leaf veins, with a purplish-grey fuzz on the underside in humid weather. Older leaves go first, and in wet or very humid conditions it spreads fast and whole leaves turn brown.
Not an insect. Also bounded by the veins, but the spots start water-soaked, turn grey, tan or brown, and dry and drop out to leave ragged holes; in wet conditions a drop of clear to white bacterial ooze can form on them. Bacterial, and spread by working among the plants while they are wet.
Not an insect itself, though aphids carry it. Irregular dark green areas alternating with light green or yellow in a mottled mosaic rather than defined spots, with small, distorted leaves, stunted growth and malformed fruit. There is no cure; destroy suspect plants promptly.
Not an insect. Yellowing between the leaf veins, sometimes with reddish-brown tints, and nothing living on the underside; the older leaves then brown and drop early. Heavy use of high-potassium feed such as tomato feed can bring it on.
Four questions about the damage separate these better than an appearance can, and this page answers some of them for you.