how to get rid of tiny red spiders

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I have just planted some flowers. A week ago i found there were a lot of tiny white spots cover the sprout and new leaves. I watched carefully and found there are some tiny red spiders. Then I covered the flower with plastic bag and put a lighted cigarette in it for about 5 minutes to get rid of them. But they survived. I sink the flowers to a barrel with water for 2 hours becouse i believe they can't swim. But i found the little creature come out a couple of days later. Can you tell me how to get rid of red spider and save my flowers.