“What is happening to my snowbush hedge ... it seems to be fading away?” came the question.
When I answered, the caller at first thought I had mentioned a new computer virus. But no, this is the snowbush spanworm, not a computer-attacking “spam worm”! Depending on your point of view, your snowbush shrubs (Breynia nivosa and compact B. disticha cultivars) may be getting a free pruning. The snowbush caterpillar is more abundant this year than in previous years and is stripping the foliage from these attractive shrubs.
It seems the caterpillar became abundant in the summer of 2005 and settled in since then and can be found almost year-round. Usually snowbush hedges will re-sprout in a short time, but to make matters worse, some properties may have so many caterpillars that once the foliage is eaten, the spanworms start chewing on the twigs and bark. Caterpillars will defoliate the shrubs and start chewing the bark off of plants so severely that it resembles rabbit feeding.
Description and biology: The yellow and black larva (caterpillar) is the immature stage of a pretty moth called the white-tipped black moth. Only the caterpillars cause the injury. This feeding machine’s metered, looping crawling style gives it away as a member of the inchworm family of moths, also known as spanworms. Inchworms have fewer abdominal legs than many other caterpillars, hence they stretch out the length of their bodies and pull their rear section up quickly, which forms a loop; much like a Slinky toy going down steps.
At about only an inch long, the larva is full grown and ready to change into a pupa. According to local lepidopterist Mike Malloy, it enters the ground and goes through pupation there, and emerges as a moth in about seven days. Pupae also are found in loose silken webbing between leaves in the canopy. This is a day-flying moth, which is unusual, as the typical modus operandi for most moths is to be covert, nighttime navigators. The moths have a wing span that is a little over 1 inch and a striking velvety appearance, with navy-blue-and-black wings and with white margined tips on each of the four wings plus an orange thorax. Swarms of moths have been observed in the heat of the day on tree branches overhanging defoliated snowbush.
Hedges typically have mixed populations of mature caterpillars (only 1 inch long) as well as newly hatched larvae, under one-eighth inch in length. It’s not certain how many generations there are, but one needs to look closely for the tiny, newly hatched caterpillars, which signal the emergence of a new generation. The salmon-pink eggs are about 0.7 mm long and are placed individually on the stems.
When this caterpillar cannot find its favorite snowbush meal plant, it will feed on Otaheite or Malay gooseberry (Phyllanthus acidus), and supposedly white sapote (Casimiroa edulis), and snow-on-the-mountain (Euphorbia marginata).
What to do: If you don’t enjoy the colorful, slinky inchworms and pretty moths, spray solutions may be purchaed containing B.t. (Bacillus thuringiensis) or a spinosad-containing insecticide product (ferti-lome’s Borer, Bagworm amd Leafminer & Tent Caterpillar Spray) both are a narrow spectrum insecticide choice. This means only the bad bugs (the caterpillars) will be affected and the attack bugs, the predators and parasites will be unharmed. A labeled, horticultural, insecticidal soap spray may eliminate the smaller larvae. Management suggestions include pruning the new growth when the chewing has started as the small larvae do not seem to climb back readily. And when it come down to replacement, plants aren’t that expensive, so you may want to add some new ones to fill in the gaps. (I like having the moths around.) It isn’t like replacing a royal poinciana because of the royal poinciana caterpillar attacks, which Marco Island and Goodland are struggling with now. For more details on this caterpillar and the royal poinciana caterpillars, see this publication at:/tinyurl.com/2afv46m
Doug Caldwell, Ph.D., is commercial landscape horticulture extension educator with the University of Florida Collier County Extension. E-Mail firstname.lastname@example.org; call (239) 353-4244 ext. 203.