Answer: It sounds like the symptoms of feeding by thrips. Thrips are insects that lay their eggs inside the buds of hibiscus, roses, and other species that make big buds before flowering. The thrip is small but visible if you look for it. The easiest way to see thrips is to take an open flower and shake it over a white piece of paper. Thrips will fall out of the flower onto the paper, looking like small, black pencil lines on the paper. They are much longer than they are wide, and their dark color stands out against a piece of white paper.
I use two different insecticides and rotate their use to make sure the thrips do not develop resistance to either. Good products to rotate are Ortho Systemic Insect Killer and Green Light Spinosad. These chemicals need to be sprayed over the tops of the plants, covering the buds and upper leaves of the plants. Use Green Light Spinosad for two treatments, 5-7 days apart. The switch to Ortho, and use it the same way in a third treatment 5-7 days later. Each treatment will dramatically reduce the number of thrips and increase the number of flowers that open normally. Three treatments is usually enough to end the infestation, but you may repeat this treatment series if you need to.
Good luck with your plants.
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