Growing Concerns 197: Columbine pests and environmental friendliness

I have several columbines, which bloom beautifully each spring (well, late May and June anyway). After they are done blooming, the foliage is attacked by small green worms. One season I was told to spray with a garlic water solution. I must have made it too strong, it turned the leaves black. I have used some chemical treatment but really would prefer not to do that if possible. However, it is them against me and I really want to win.

Is there anything that can be done now when the plants are just coming up to prevent this or is there some environmentally friendly thing that I can do when they are up and blooming.  I am not concerned with the creature that makes the white trails on the leaves. I don't like that but apparently they don't hurt the plant, just make it look weird. - L.K. -

"Environmentally friendly" implies that natural systems register human values such as friendliness. However, gardening isn't one big happy family collaborating to make flowers and fruit. Players in an ecosystem use each other ruthlessly. The columbine sawflies that deposited their eggs in your garden probably see neither friendliness or rancor in what we are about to write, just business as usual.

Suspect #1: Columbine Sawfly

The columbine sawfly, a gnat-like insect, lays its eggs on columbine leaves. Before you even think to look, tiny green worms are eating and growing. At first, an individual larva doesn't eat much and because it blends perfectly with the leaf color, you don't notice it. As the larvae grow, however, they eat more and more until a fully grown larva (think "teenager" because they're about full sized, just not reproductive age yet) is putting away a leaf a day. Then, you notice!

At this point in the sawflies' lives, they are about to quit eating, drop off the plant and wiggle into the soil where they'll rest and pupate... until the next spring. Then, they'll emerge as adults just about the time columbine flower buds are swelling, lay eggs on the leaves of their host plant and start the whole cycle again.

You've made its acquaintance, now you can be a formidable enemy to this pest. The pupae can't be killed with stomach poison chemicals because they are not eating. Meanwhile, the soil and the pupa case protect each larva from skin-cracking chemicals and materials such as soaps and diatomaceous earth. So forget chemical controls once the sawfly damage is at final stages.

Focus then on physical control -- smashing, destroying. Do it by cultivating the soil around those plants to expose and smash the pupae, or leave the tiny pupae on the surface where hungry birds can find them. Alternatively, or in addition, watch for adults to appear the next spring, and you can kill them with insecticidal soap before they lay eggs. You'll see the gnats hovering around the plants. A physical approach is possible at this point to, in that you can cover your columbines with floating row cover to screen out the adults. Row cover is a lightweight, porous fabric also called Reemay.

Know the pest well to gain the upper hand

Options are legion for the informed. If you anticipate sawflies you will see the worm-like larvae before they inflict much damage. Then, they can be killed with almost any contact insecticide such as Malathion, a drying agent such as diatomaceous earth or any insecticidal soap, including a tablespoon of Murphy's oil soap in water. Just be ready. Attack before the damage mounts.

A gardener can also imitate a lion which moves in on jackals' hard-won kill. That is, you can steal the sawflies' food. Faced with a heavy infestation, we've cut established, blooming plants down to the ground, saved the flowers for our vases and disposed of all the leaves in the compost. Resident insects die with the foliage. Meanwhile, the plant generates new foliage and sometimes more flowers. Don't do this to brand new plants, only to those which have had a year or more to build up reserves in their roots for just such an eventuality.

A pest worth keeping: Columbine skipper butterfly

The columbine duskywing, or columbine skipper, also lays its eggs on columbine leaves. It's far less common than the columbine sawfly but like the sawfly it results in tiny green "worms" eating the leaves. However, these larvae grow into hairy, white-specked caterpillars which drop off, pupate in the leaf litter, and emerge later to repeat the process. There may be one, two or three such cycles a year. The last crop overwinters as caterpillars in garden debris near the host plant, pupates in early spring and hatches in late spring -- June. The butterflies are an inch wide, brown with dark spots and a bit of purplish-blue on the outside of each upper wing.

If you must control skipper caterpillars you can dust the plants with Bt -- Bacillus thuringiensis bacteria, available in products such as Dipel. It's a bacteria that infects and kills only caterpillars that eat it.

Keep an eye out for columbine leaf miner

While you war against sawflies, columbine leaf miners may build to truly harmful levels. Keep an eye on this.

Leaf miner - an unremarkable, tiny fly -- lays its eggs on columbine in spring, too.. The young live and eat between the leaf tissues -- "mining" the insides of the leaf -- until they're ready to emerge. Leaf miner can be worse than sawfly larvae in one way, since disfigured foliage remains. At least sawflies eat clean, and new foliage can fill the void.

Within the leaf, miners can't be reached by spraying contact insecticide or conventional stomach poisons. Instead, we ambush the adults before they can lay eggs, or use a systemic insecticide* such as imidacloprid which poisons all parts of the plant, including those inner tissues being scraped away by the miners. Neem, an organically derived insect growth regulator found in Bio-neem and Neem-Away that kills by preventing maturation, may also be effective if sprayed on and absorbed into the foliage being eaten.

In covering such tactics, we've been chided for ruthlessness as well as for straying from the ecologically-conscious path. We've also been dismissed as irrational, chemical-avoiding "greenies."We are glad to be assigned this wide range of labels -- it may mean we are achieving our goal of being thinking gardeners.

Everything we do in a garden has wide effect. We try to use narrowly-targeted tactics but even those can create collateral damage. We always record what we do so we have a fighting chance of noticing all the effects. Chemical compounds in garlic that are repellent or sickening to some insects and fungi can be as harmful to non-target organisms as synthetic pesticides. You saw proof of this when garlic oil burned your columbine. Better to test every new remedy on one leaf and wait a day to check the plant's response.

            *What we hope to do, and hope to help others do, is think before acting and limit pest control to the narrowest possible target. The last time we used a systemic insecticide, for instance, we cut all the flowers off the treated plants, to avoid killing bees, butterflies, or beneficial insects with tainted pollen and nectar.