Plant replacement is not needed, and the foliage will regrow (if not this summer, then next spring, though we expect at least some replacement leaves will emerge this year). The insects in question are not caterpillars, which is why Bt did not work. These are sawfly larvae, and the Hibiscus Sawfly is a native species. No management or intervention is needed, though for a future generation of larvae, you could try blasting them off the foliage with a strong jet of plain water from a garden hose, rubbing them off by hand, or spraying a low-toxicity insecticide like horticultural oil or insecticidal soap. (Note that neither should be applied while temperatures are above 85 degrees, or plant tissue damage may result.) Hibiscus with healthy root systems tolerate this type of defoliation very well and rebound on their own without any additional assistance, though do keep an eye on its watering needs as parts of Maryland keep experiencing drought conditions or low rainfall.

Miri