If you’re thinking you wish your perennial hibiscus (hibiscus moscheutos) leaves looked this good, I know why–this leaf has not been chewed by the most pernicious pest of the hibiscus leaf. This leaf was most likely chewed by a slug, an earwig or some other type of chewing insect.
If, however, your leaf looks as if there’s not much left–or maybe just the veins are left–well, then, my friend, you have the hibiscus sawfly larva. But I probably don’t have to tell you what you have–you’ve probably seen the little caterpillar-like worms. This fact sheet from the Connecticut Agricultural Experiment Station can fill in any more blanks.
They are not caterpillars, however, so conventional bT type insecticides will not work. And they do have more than one generation per season so that is why they are so uncommonly destructive. They are the “lily leaf beetle” of the hibiscus.
What are your choices (presuming, of course, that you will not resort to the systemic insecticides that are more and more being implicated in colony collapse disorder among honeybees?)
Vigilance is of course your best option. Know when the pests are likely to occur and watch for them.
At the first sign, begin treatment with something very benign–my remedy of choice is always insecticidal soap. It will work on caterpillar-like insects, particularly if you have just a few to start. Look for them on the undersides of the leaves. Spray very early in the morning or near dusk to minimize harm to beneficials (and your plants, of course).
Combinations sprays like canola oil with pyrethrins and the like, and neem (which has a dual effect) should also work. However, I have never liked neem as a fast acting remedy–and you really need that here. Neem merely makes an insect less hungry and stops it from breeding. having tried it on the sawfly larva I have not seen good results. Perhaps it did not make them less hungry fast enough!
I’ve not tried the newer spinosad, but it is also recommended.
And finally, you can give up in disgust and remove the plant. Supposedly newer cultivars are coming that are sawfly resistant. You can always plant one of those when it gets to our market.