My hibiscus plant needs help
I have several hibiscus plants. One has a pest problem (see attached photo). I have tried different sprays, the latest was Ferro-lone Triple Action Plus. Then the plant lost most of it's leaves, but it has new growth now. However, the white powder pest remains on the new buds. Please advice what to do.
Multnomah County Oregon insect issues
The insects are aphids. They are green, and the white things you see are their cast skins. Check an image search online for "hibiscus aphid" and some of those photos look very like yours.
The most simple solution is to wash them off every few days. Here is the cultural control listed in the PNW Insect Handbook: "Wash aphids from plants with a strong stream of water or by hand-wiping. Aphid populations tend to be higher in plants that are fertilized liberally with nitrogen and heavily watered, as these practices produce flushes of succulent growth. Avoid excessive watering, and use slow-release or organic sources of nitrogen."
Here is the link to the whole page, which includes some homeowner chemical control options. Always follow insecticide label instructions. https://pnwhandbooks.org/insect/hort/landscape/common/landscape-aphid