Leaf drop from the bottom may be a different problem than insects or pests that feed on foliage. Your hibiscus could be suffering from lack of water a few weeks ago and the damage is showing up now, or high temps, too much fertilizer or something toxic that went into the soil could have caused the leaf drop. Are the leaves at the top of the plant new growth? Keep the plant watered and it will most likely survive. If you would like to send a photo I may be better able to solve this mystery of the missing leaves. Keep Growing, Marianne Binetti