Monday, September 19, 2016

Canna Alert!

One of the great joys of Memphis summers is the lengthy blooming season, and from May into fall, my yellow canna (Canna ‘Ra’) offer a majestic six-foot splash of color at the southwest corner of my front porch.  They are tall enough to offer some shade for mophead hydrangeas stressed by late afternoon sun.  The canna may have been spreading there for decades.   Early this summer, when the canna seemed most thriving, I thinned one section, transplanting some plants to the alley and others to pots for the Master Gardeners' Spring Fling plant sale.  Other plants in the front lawn had suffered in recent years:  the gorgeous pomegranate tree (death by borer), the gardenia (near murder after a cold winter), azaleas (die back), and the large crepe myrtle (scale, of course).  I rejoiced in my seemingly indestructible canna, so tough a specimen that it was once called “cast iron plant.”

But alas, as the poet says, “nothing gold will stay.”  By July, the many colorful blooms could not hide a growing disfiguration of the foliage.  The edges of unfurled leaves were stuck together forming brown stalks frequently bent at an angle from the tall stem.

Manny Pailet, Greenhouse Manager at the Memphis Botanic Garden, diagnosed the problem easily by unrolling one of the leaves and exposing a worm amidst a thick layer of black worm feces (or more technically, frass).  My canna had become infested by the lesser canna leafroller, the most common canna pest.  This yellowish caterpillar feeds inside rolled leaves tied with short silken threads.  It doesn’t kill the plants, but it certainly makes them unsightly, and the evidence is all over town.  For pictures of the moth and worm, click here.  

So I looked into ways to control or eliminate the leafrollers.  I sprinkled a bit of Sevin on a few of the infected leaves, but knew it couldn’t penetrate to where the worms were and also that it was toxic to many beneficial insects. Those leaves have remained healthy, however.  Manny suggested a thuricide concentrate, which he had used at the Botanic Garden.  This product contains a naturally occurring bacterium and is applied as a spray and works by causing target pests not to feed, so they starve. I admit that the main reason I chose not to use this product was I would have to purchase a sprayer and mix the product myself. Although it is advertised not to kill non-target insects such as honeybees and other pollinators, I was also reluctant to use a spray so close to other blooming plants covered with bees and butterflies.  I wasn't yet convinced of its safety.

Responding to my preference to avoid a spray, Urban Earth recommended a Tree & Shrub Systemic Insect Drench, which contains an imidacloprid compound related to the systemic I was using for crepe myrtle scale.  Hoping for similar results, I poured this mixture around the canna roots in several beds.  It wasn't until later that I realized imidacloprid is in the class of pesticides known as neonicotinoids which are highly toxic to all insects.  Maybe I should have used the thuricide!

Further research confirmed this conclusion.  Extension agents generally recommend the biological insecticide containing Bacillus thuringiensis (Bt) for the very good reason that it is least toxic to beneficial insects. This product promises to work well to manage not only the canna leafroller but also other destructive worms and caterpillars. Moreover, it should have no effect on earthworms, birds, honeybees, and ladybugs.  I found several brands of Bt at Dan West Garden Center in various formulations: as a dust (like Sevin), as a concentrate (which Manny had used), and as a ready-to-use spray.  I chose the ready-to-use spray as simplest for me to use.

In mid-September, the adult leafrollers (small moths) were still visible flying around the canna, searching for the best spot to lay their eggs, so I decided not to delay spraying the plants. I shook the container well, both before I started spraying and throughout the process.  My goal was to spray all surfaces, above and below and especially down and around any curling or already curled leaves.

It is too soon to report results, but I am hopeful that the lesser canna leafrollers will be done in by the double whammy of a systemic application and an insecticide spray.  I know I will have to spray again, perhaps weekly, but I am not relying on chemicals alone.  Whenever I deadhead, I also remove blighted foliage and, I hope, the worm within.  Because my canna were so thick, they may have been more susceptible to infestation, so I will continue to thin and clear out the dense areas.  A necessary part of management of the leafroller is cutting down and disposing of dead canna plants in winter since the pupae overwinter in this material. When my plants die back this winter, I will undertake a major cleanup.  And next spring when the plants emerge, I will be watchful and ready to spray before the leafrollers take over.

And, of course, I will not take canna to the Spring Fling plant sale.

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