What to do about bumpy hibis­cus leaves

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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/

Re­mov­ing hibis­cus plant parts that are af­fected by the erinium mite can re­duce the in­sect’s pop­u­la­tion and im­prove the plant’s ap­pear­ance.

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