Answer: Most insecticides are pest specific so without knowing what the pest is (beetle, fly, aphid - aphids colonize and can look like one big bug), it's hard to know what to recommend in terms of control. However, here's a website with lots and lots of photos of different kinds of insects. http://bugguide.net/node/view/4422
I hope you can find the one that's on your mandevilla. If so, write again with the name of the insect and we'll make a control recommendation.
Best wishes with your garden!
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