The black soot is a fungus called sooty mold. This fungus is not a plant disease, but grows on the secretions, called honeydew, of sap-feeding insects such as scales, aphids, whiteflies, and mealybugs. Once the pest is controlled, the sooty mold gradually disappears.
The insects you found on your tree’s trunk are the larvae of a species of ladybug getting ready to become pupae. This ladybug is known as the globe-marked lady beetle (Azya orbigera). The Latin word orbigera means “globe-carrying,” a reference to the two large, round, hairless areas on the back of the adult beetles. It feeds on scale insects, especially soft scales such as croton scale.
The globe-marked lady beetle has been found to be a voracious eater of croton scale, a pest first found in southeast Florida in 2008. This soft scale is found on the underside of leaves and small stems of croton, gumbo limbo, firebush, strangler fig, guava, wild coffee, and other plants.
One of the interesting things about ladybugs is that many species are very particular as to their preferred prey. In Florida, adults and larvae of 75 ladybug species feed on scale insects, with only 13 feeding primarily on aphids. Some species of ladybugs feed only on mites, whiteflies, mealybugs, scales, or aphids. Some feed only on mildew! A few species are not quite so picky and will feed on a broad range of pests.
There are even a few species of ladybugs that are plant pests. Florida has two species of ladybugs that are plant pests: the squash beetle and the Mexican bean beetle. Both are found in the north end of our state and are not a concern in South Florida.
To preserve pest-feeding insects and beneficial mites, avoid spraying plants with insecticides unless it’s insecticidal soap, horticultural oil and a few other “soft pesticide” types of product. These products have less of an impact on the natural enemies of pests than some of the more toxic products.