Are your dainty delphinium leaves turning brown? Are they plagued with black spots? Are the leaves wilting, curling or just looking like they may have been someone’s entrée at lunch? If you’re wondering what’s eating your delphiniums, search no more.
The answer is simple — it’s those pesky delphinium worms at work again!
Wondering how to get rid rid of those little pests?
Let’s dive in…
The Life Cycle of a Delphinium Worm
The scientific name for this green looper worm that affects delphiniums is Polychrysia Esmeralda, or the 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 in hollow delphinium stalks as eggs. Sometimes eggs are seen in curled leaves in the spring as well.
There is only one generation of this worm each year.
The caterpillars hatch and emerge in spring as the weather warms up and as the delphiniums begin to come back to life.
The caterpillars crawl up the delphinum stalks and feed on tender young foliage growth, their favourite being succulent delphinium buds.
This is the point at which we interface with these little critters…
Symptoms of Delphinium Worms
Here are four tell-tale signs you’ve got delphinium worms:
Wilted, dying delphinium leaves
Wilted leaves are the first sign you’ve got delphinium worms. The plant itself will just look out of sorts. This wilting is caused by caterpillars chomping on the stems.
Webby, curled delphinium leaves
The worms cement random leaves and upper leaves of the bud together to protect themselves as they feed on the juicy new green bud growth.
Black spots on delphinium leaves
Wondering what those black dots on the leaves are? Well, we don’t mean to be crass, but it’s probably frass! Frass, of course, is a fancy horticultural word for bug poo. If you’ve got delphinium worms, you’ll see round black dots on the leaves that look like specks of dirt sitting on the top sides of the leaves, interspersed around the curled leaves and near the buds.
Chewed up delphinium buds
Did you think delphinium worms were just any old worm? Well, if you did, you’d be wrong. These caterpillars have distinguished palates. Their appetites are for the tenderest, juiciest leaves available, which just happen to be the newly forming delphinium flower buds.
How To Get Rid of Delphinium Worms
So, what to do about this delphinium joy killer?
Here are the most effective ways to get rid of delphinium worms:
1. Pinch Back the Affected Area of the Delphinium
As soon as you see the webbing and frass, pinch off the affected area of the delphinium. This will hopefully get rid of the worm and any additional eggs that may be there.
If the infestation is quite bad, 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, do not – we repeat – do not put it the compost pile – because you will create a delphinium worm breeding station! No bueno.
So, why is pinching back delphinium buds so effective at getting rid of delphinium worms?
When the delphinium grows back (and it will), this developmental stage of the larvae will have passed and the newly emerged delphinium buds will be untouched by any other worms.
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 problem.
2. Safe Chemical Treatment for Delphinium Worms
There are a couple of chemical treatments available for Delphinium worms that we recommend. Both 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 – 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 bacillus.
- Insecticidal soap – applied as a spray. It is effective for treating a number of pests like aphids and is also effective for caterpillars. Spray it on the leaves regularly and it should help control these quite well.
3. Diatomaceous Earth for Delphinium Worms
A non-chemical treatment or deterrent for Delphiniums is called diatomaceous earth. Diatomaceous earth is made from crushed crustacean shells and is very abrasive to the soft bodies of the little caterpillars.
Applying this to the Delphinium leaves is helpful in discouraging the invasion of this pest.
Treating Delphinium Worms in the Fall
In the fall, be sure to cut back the Delphinium stalks flush even with the ground to make it more difficult for the eggs to overwinter by providing very little stalk surface for them to survive.
Also try placing fireplace ash or diatomaceous earth around the base of the Delphinium clump because this will make the environment more hostile to the emerging baby caterpillars.
Final Thoughts on Delphinium Worms
You don’t have to say goodbye to delphinium flowers because of the delphinium worm. By catching them early on in the spring, they definitely can be defeated! Good luck!
Still left with questions? We’d love to chat. Send us a line or give us a ring!
– by Sharon Wallish Murphy