Delphiniums are a reliable, tall and stately perennial for Edmonton gardens adding a nostalgic cottage flair. Their blossoms come in 4 main colours of white, and shades of pink, purple, and blue. Delphiniums come in a variety of heights ranging from about 2 feet to over 6 feet, depending on the variety. One of the main attractions to Delphiniums is their tall spires. Another allure is their blue blossoms. Not just blue as in purple (yes, they have shades of purple) but true blue -and there aren’t many flowers out there that have true blue blossoms. Within this basic four colour mix there are many variations. Delphiniums are double flowered – they have an outer flower and an inner flower. This inner flower is called the ‘bee’. Some have lighter bees, others have the same colour bee, and some bees are darker. Delphiniums work very well as cut flowers gracing arrangements for inside the home and out.
But if there is anything that makes delphinium care frustrating, it’s the worm that they are sometimes plagued with each spring. The scientific name for this green looper worm is Polychrysia esmeralda; syn. P. moneta. Another name is Golden Plusia Moth.
One of the keys to defeating any garden pest is to understand the details about its life cycle. The Delphinium worm overwinters in the soil and hollow delphinium stalks as eggs. The caterpillars begin to emerge in spring as the weather warms up and the Delphinium begins to come back to life. The caterpillars feed on the tender young growth, their favorite being the succulent bud growth. The worms cement the upper leaves of the bud together and feed on the juicy new green bud growth. This area looks webby and you will see black specks dotting the webbing and leaves. These black dots are called frass, which is a fancy horticultural word for bug poo. You will see this happening by the middle of June at the latest, depending on the year.
The pupation process begins in late June. Adults emerge in July and begin laying eggs at the base of the Delphinium plants in late summer. This garden pest fortunately only produces one generation per year.
So what to do about this Delphinium joy killer? There are a few options:
As soon as you see the webbing and frass, pinch off the affected area of the Delphinium or cut the Delphinium stalk back to about 4- 6″ above the soil. This is actually very effective.
Place the pinched plant material in a plastic garbage bag, don´t put it the compost pile – because you will create a Delphinium worm breeding station here.
The reason pinching off the bud works is that when the Delphinium grows back (and it will) this developmental stage of the larva will have passed and the newly emerged Delphinium buds will be untouched. Remember, there is only one generation of this bug every year. Usually it´s effective enough to pinch off the affected area but if the plant is short and young, cutting it back completely will take care of this worm.
There are a couple of chemical treatments available. Both kinds are safe to use, are applied topically, and need to be reapplied after a rain because it does get washed off:
- BTK, also known as Bacillis thuringiensis, is a naturally occurring bacterium found in the soil. When a caterpillar ingests this bacterium, it blocks the system that protects the worm’s stomach from its own digestive enzymes and it dies from being poisoned by its own gastric contents. Neither birds, nor fish, nor mammals are susceptible to this bacillis.
- Insecticidal soap – is also effective for caterpillars.
In the fall, be sure to cut back the Delphinium stalks even with the ground surrounding them to make it more difficult for the eggs to overwinter. Also try placing fireplace ash around the base of the Delphinium clump because this will make the environment more hostile to the emerging baby caterpillars.