How Adenium obesum plants became known as desert roses is anybody's guess, because they are far as plants get from the ornamental roses (Rosa spp.) found in U.S. Department of Agriculture plant hardiness zones 2 through 11. Desert roses' twisting, grayish-green branches fan out from a bulging base of water-storing humps. Cut into one of them, and it oozes toxic sap favored by African tribal hunters as a poison lethal to big game. In some parts of their growing range of USDA zones 10 through 12, these bizarre plants attract caterpillars nearly as strange as they are.
Long, black bristles sprouting from black, wart-like bumps on their bright-orange bodies give oleander caterpillars (Syntomeida epilais jucundissima) the look of escapees from a Halloween-themed horror movie. After growing from 1/10 to 1 1/4 inches long during a month-long eating binge, they leave desert roses to spin black-shrouded clusters of brown cocoons.
After pupating, they emerge as iridescent, white-spotted blue-green polka-dot wasp moths to mate and lay clusters of creamy-white eggs on the backs of their host plants' leaves.
Groups of the newly hatched caterpillars chew through the undersides of the new leaves, turning them brown. After molting three times, the groups separate, and individual caterpillars begin devouring entire leaves. Without treatment, oleander caterpillars may completely strip a desert rose's sparse foliage.
Desert roses seldom die from oleander caterpillar infestations, but repeated leaf loss may sap their ability to withstand damage from other pests.
Oleander Caterpillar Predators
Their diet of toxic desert roses takes these caterpillars off the menu of caterpillar-preying birds and small mammals. The ominous-sounding, shield-shaped spined soldier bug, however, chows down on them with gusto.
This predatory stink bug injects the caterpillars and polka-dot wasp moth eggs with paralyzing, tissue-dissolving enzymes and sucks out their liquefied contents. It also kills up to 90 other common plant pests.
While releasing one or two of the commercially available soldier bug eggs once or twice a month controls the caterpillars on a single desert rose, they're expensive for that purpose. If you spot the brown bugs or their round, metallic-bronze eggs in your garden, leave them alone; they don't harm the plants and could save them.
To manage the early-stage caterpillars, prune the infested leaves; freeze them in a sealed plastic bag for a day, and dispose of them in the trash. Wear gloves to protect yourself from the oozing sap, and disinfect your tools in rubbing alcohol between cuts and when you finish so they don't spread disease.
Again wearing gloves, hand-pick late-stage caterpillars and drown them in soapy water.
Avoid handling the caterpillars and pruning with ready-to-use, microbial Bacillus thuringiensis var. kurstaki spray. Inspect the plants daily for egg masses on the backs of the leaves. When all the eggs have hatched, spray both sides of the leaves when no sun is shining on the plant. The microbes penetrate the caterpillars' intestinal walls and poison their blood.
Repeat the application after rain or every five to seven days while caterpillars continue feeding. Wear protective clothing, including long pants, a long-sleeved shirt, safety glasses and a respiratory mask when spraying.