Garden Q&A: Hibiscus sawfly larvae dines only on hibiscus varieties
Question: I have three perennial hibiscus plants, two red and one white. The past two years, the leaves on the red ones have little holes in them and eventually they turn brown. This year they did the same thing, but the leaves fell off and the bloom was poor. What can I do to fix this? Also, I would like to move them. When can I move them? And how far out do I have to go when digging them up?
Answer: It sounds like your perennial hibiscus plants are being attacked by hibiscus sawfly larvae. These small insects are caterpillar-like in appearance, but they are not true caterpillars. Though many species of plants are attacked by various species of sawfly, the one attacking your plants is specific to relatives of hibiscus.
With tiny green bodies that measure a mere 1⁄8 to 3⁄4 of an inch and light brown heads, hibiscus sawfly larvae are difficult to spot.
They are most frequently found on the undersides of leaves, so flipping over the leaves and examining their backs is essential for proper identification of this pest. Though they are no longer actively feeding this time of year, start searching for the larvae next June and try to get a handle on their population before they skeletonize the leaves and cause them to drop.
Hand-squashing the insects in earnest is effective, but it takes a lot of time. Inter-planting your hibiscus with plants such as sweet alyssum, wallflower, lemon balm, cilantro and other flowering herbs helps to lure in the predatory insects that prey on hibiscus sawfly and help keep their numbers in check. There are many species of parasitic wasps and tachinid flies that naturally control sawfly larvae.
When hand-squashing isn’t possible, turn to an organic product control with the active ingredient spinosad. I use these products only as a last resort, when the damage is extreme and the health of my plants is truly at risk — which is seldom the case with perennial hibiscus, even when infestations are severe. The damage they cause is mostly aesthetic.
Spinosad is a fermented bacterial product that is labeled for use on many common garden pests. Always follow label instructions carefully, and apply any products only when bees are not active (early morning or late evening). For spinosad to work against sawfly larvae, the tops and bottoms of all leaves must be covered.
The preferred time to move your perennial hibiscus is in the early spring, just after cutting the plant back. They have a number of thick tap roots, so try to keep as much soil on the roots as possible when moving them, and dig a root ball that is about twice as wide as the cluster of plant stems. Be sure to keep the newly transplanted plant well watered throughout its first season of growth.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio.
Send your gardening or landscaping questions to [email protected] or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212.