Fall is the time of purple and gold in My Florida Backyard, and one of the brightest patches of yellow comes from the Winter Cassia (Cassia bicapsularis syn. Senna bicapsularis). This shrub provides a bright burst of color from now through the first frost with its unusual flowers, but that's not the only way it provides gold in the garden. You see, cassia species are host plants for sulphur caterpillars of several varieties. Normally, these caterpillars are the same green shade as the leaves they eat. But when the flowering season arrives, the caterpillars immediately begin to dine on this delicacy, and as a result - they turn bright yellow.
The species above is an Orange-Barred Sulphur caterpillar, but Cloudless Sulphur butterflies (shown below) will lay on cassia as well, along with the smaller Sleepy Orange. They generally lay far more eggs than will ever hatch - Winter Cassia is extremely attractive to ants, and while harvesting nectar the ants will also gobble up any sulphur eggs along the way. Fortunately, sulphur butterflies seem to haunt these bushes, laying eggs from spring to first frost, so their continued success is pretty well-assured.
It should be noted that some organizations place Cassia bicapsularis on invasive species lists, but other groups, like the University of Florida Extension, are not concerned and in fact continue to recommend this bush for planting. To add to the mess, C. bicapsularis is sometimes confused with Senna pendula, a similar and possibly more invasive species. You can use your own judgment on whether to include this non-native in your own landscape, but in My Florida Backyard, we feel comfortable enough with this plant to enjoy the incredible benefits it brings to the butterfly garden, including the bright yellow caterpillars of fall.
P.S. Nov. 7: A quick update - here's a good article from FloridaGardener.com with more info about the differences between C. bicapsularis and S. pendula.