Malia asks: The leaves on my hibiscus shrub have developed little bumps. Someone said it might be an insect issue. Do you know what it could be and how I can get rid of it?
Tropical Gardener Answer: The small bumps that you see are actually a response to the feeding by the tiny erinium mite (aceria hibisci). Bumps that look like blisters can appear on leaves, buds and stems of affected hibiscus varieties. That is why the insect is often called the hibiscus blister mite.
Controlling these mites can be difficult. The erinium mite has a narrow host range, limited to a few hibiscus varieties. The only other plant it may attack is okra. A list of the hibiscus varieties that have been tested at UH for vulnerability and resistance are listed in their downloadable publication at https://www.ctahr.hawaii.edu/oc/freepubs/pdf/ip-7.pdf.
Removing the damaged plant parts will reduce the mite population. A tiny, fast moving predator mite will arrive when you have erinium mites. Though this predator’s presence will reduce the blister mite population, it will not get rid of the existing bumps on the leaves. Those must be removed and disposed of far from any hibiscus plants. Go easy though. Pruning more than one third of the plant in a three-month period can drain the plant’s energy reserves, making it more vulnerable to insect and disease attacks.
Mite damage usually occurs on new growth. You can limit new growth by reducing the amount of nitrogen fertilizer you apply.
Some success can result from spraying miticides such as Avid or Cinnamite. These are somewhat toxic and the directions must be read and followed. These insecticides will also kill the predators of the erinium mite.
The blister mites move on air currents or on the bodies of insects or birds. Though you cannot prevent their arrival, you can minimize the attack by keeping the plant open allowing air flow within the branches, which will discourage the mites.
Healthy plants are somewhat resistant to insect and disease attacks. Be sure your hibiscus is getting adequate sun, water and nutrition to keep it healthy. Be aware, however, that too much water or fertilizer can be as harmful as too little. The soil should drain well so that the plants roots don’t sit in a bog. Fertilizing with a balanced product every three months at the recommended amount described on the label is advised. Over-feeding can make the leaves succulent and very attractive to pests. Under watering or lack of fertility can cause plant stress. Weak plants are unable to resist pest and disease pressure. Go for a little “tough love” to get the proper balance so your hibiscus will not attract the mites and if they come the plant will be strong enough to fight off the attack.
Email plant questions to konamg@ctahr.hawaii.edu for answers by Certified Master Gardeners.
Some questions will be chosen for inclusion in this column.
Diana Duff is a plant adviser, educator and consultant living in a dryland forest north of Kailua-Kona.
Gardening Events
Wednesday: “Hands-On Composting Workshop,” 8:15 a.m.-12:30 p.m. at Organic Matters Hawaii at 73-887 Kupaloke St., just north of Kailua-Kona. Registration required with Gina at 322-4892 by Monday at 4:30 p.m. Overview of composting and mulching, including biochar and NRCS cost share programs. Come dressed for outdoor composting activity. Water, snacks, some shade and port-a-potty provided.
Saturday: “Work Day at Amy Greenwell Garden,” 9 a.m.-12:30 p.m. Meet at the Garden Visitor Center across from the Manago Hotel in Captain Cook. Bring a brown bag lunch. Water and snacks provided. Info: Peter at 323-3318.Farmer Direct Markets
Wednesday: “Sunset Farmers Market,” 2 p.m.-sunset at the HPM parking lot.
Wednesday and Friday: “Hooulu Farmers Market,” 9 a.m.-2 p.m. at Sheraton Kona Resort &Spa at Keauhou Bay
Friday: “Pure Kona Market,” 9 a.m.-5 p.m. at Amy Greenwell Garden in Captain Cook
Saturday: “Keauhou Farmers Market,” 8 a.m.-noon at Keauhou Shopping Center
“Kamuela Farmer’s Market,” 7 a.m.-noon at Pukalani Stables
Sunday: “Pure Kona Green Market,” 9 a.m. to 2 p.m. at Amy Greenwell Garden in Captain Cook
Tuesday through Saturday: “U-Pick greens and produce,” 10a.m.-4 p.m. Tropical Edibles Nursery, Captain Cook.
Plant Advice Lines
Anytime: konamg@ctahr.hawaii.edu
Tuesdays and Thursdays: 9 a.m.—noon at UH-CES in Kainaliu, 322-4892
Mondays, Tuesdays and Fridays: 9 a.m.-noon at UH-CES at Komohana in Hilo:981-5199 or himga@hawaii.edu