Members of the mallow family, Malvaceae, are some of the most important crops on earth. This includes cotton, okra, and cacao. In addition to providing fibers we wear and the raw material for chocolate, members of the Malvaceae are valued throughout the world for their medicinal properties as herbal remedies for a variety of afflictions. Mallows are some of the most beautiful flowering trees and shrubs worldwide. In Maryland, hollyhocks, rose of Sharon, and various annual and perennial hibiscuses dazzle our gardens. Common marshmallow adds a splash of color to swales and drainage channels along our roadways in summer.
This week while wandering along a swampy trail, I noticed several patches of marshmallow with tattered leaves. On closer inspection I discovered larvae of the mallow sawfly busily removing tissue from the leaf blades. Although caterpillar-like in appearance, the mallow sawfly, a.k.a. hibiscus sawfly, is a rogue member of the bee and wasp clan, the Hymenoptera. This primitive branch of the Hymenoptera contains mostly herbivores, plant feeders that dine on a wide variety of deciduous and evergreen trees and shrubs.
The mallow sawfly begins its assault on the plant by using its saw-like egg laying appendage called an ovipositor to carve small slits in the leaf surface into which she deposits her eggs. From these eggs issue tiny leaf munchers that pass through six larval stages. Their feeding first creates tiny holes in leave called shot holes, but as they and their appetites grow, soft tissue of the leaf is removed until only the tough veins are left behind, giving the leaf a skeletal appearance. Once the feast is complete larvae move to the base of the plant and underlying soil to form pupae. From the pupae emerge the adult wasps that resume the assault on the mallow. In southern states there may be as many as six generations each year, but fewer likely occur in Maryland. With warm summer temperatures, these rascals may complete a generation in about a month.
Fortunately, mallow sawflies never really developed a taste for cotton, okra, or rose of Sharon, but hollyhocks, rose mallow, and marshmallow are definitely on the menu and by midsummer leaves of these beauties may be in tatters. So, what can you do to prevent a plague of these tiny tormentors? Several varieties of hibiscus, including Hibiscus acetosella, H. aculeatus, and H. grandiflora, are noted to be resistant to this pest. Another strategy to foil the sawfly is to inspect your hibiscus on a weekly basis and simply pluck larvae from the plant and toss them in the compost where they will become food for the ravenous horde of predatory invertebrates dwelling there. While insecticidal sprays are available to kill sawfly larvae, high visitation rates of pollinators and other beneficial insects to the flowers of hibiscus make this a sketchy option. Hibiscuses are about to pop, but keep an eye out for sawfly larvae.
Nibble by nibble, a mallow sawfly larva removes pieces of the mallow leaf.
Two excellent articles, “Evaluation of Twelve Genotypes of Hibiscus for Resistance to Hibiscus Sawfly, Atomacera decepta Rohwer (Hymenoptera: Argidae)” by David W. Boyd, Jr. and Christopher L. Cheatham, and “The sawfly Atomacera decepta, a pest of Hibiscus” by H. H. Tippins were used to prepare this episode.