Plant Health Problems
The disease problems on larkspur are similar to those of delphinium. See Delphinium for a detailed discussion of these diseases.
Insect Problems:

Aphids, Brachycaudus rociadae.
This species causes larkspur leaves to cup downwards. Another species, the green peach aphid, also attacks delphinium. They may be managed by spraying with insecticidal soap or ultrafine horticultural oil, which are among the compounds registered for control of this pest in Connecticut. Imidacloprid can be applied as a soil drench for season-long, systemic control. Consult the labels for dosage rates and safety precautions.

Blackened leaves.
Leaves and flowers are badly deformed, leaves are brittle and thickened with black or brown spots or streaks. This trouble is caused by cyclamen mites. The cyclamen mite, Phytonemus pallidus, is a translucent, microscopic mite that often infests the new leaf and blossom buds, and can proliferate on spent blossoms. Characteristic injury includes dwarfed, thickened and wrinkled leaves. On azalea, dahlia, and other plants with stems, the distance between internodes becomes drastically shortened. Infested plants do not bloom, or blossoms may be misshapen. Cyclamen mite is not easily controlled when mature plants are badly infested: effective miticides are not available to homeowners, so a commercial applicator may be required for effective miticide applications. Effective restricted use products are abamectin (requiring multiple applications), dicofol, and endosulfan. Consult the labels for dosage rates and safety precautions.

Cutworms, Feltia subgothica.
New transplants are often cut off at ground level by this worm. Other species crawl up the stem and eat foliage. They feed at night and hide in the soil during the day. These cutworms are large and easily found if you dig in the soil around the base of the plant. They can then be destroyed.

Ground mealybug, Rhizoecus falcifer.
This species infests larkspur. After correct diagnosis, a drench of imidacloprid or malathion, which are among the compounds registered for use against this pest in Connecticut, will provide control. Consult the labels for dosage rates and safety precautions.

Larkspur leafminer, Phytomyza delphiniae.
This is a blotch miner causing large areas of the leaf blade to become blackened, distorted and shrunken. Larvae pupate in a case that hangs from the leaf. Handpicking and destroying affected leaves as well as all plant material left in the fall will reduce their populations. Imidacloprid applied as a systemic to be taken up by the roots or spinosad applied as a foliar spray, may provide control. Abamectin is an effective restricted use product. Consult the labels for dosage rates and safety precautions.

Stalk borer, Papaipema nebris.
This borer infests an occasional stalk of many kinds of herbaceous plants. As a rule its presence escapes notice until the plant begins to wilt. Then it is too late for the plant to recover. The larva tunnels up and down inside the stem and the top portion usually wilts and later dies. There is one annual generation. The moths emerge in September and October and lay eggs on the stalks of their food plants, in which stage the insect passes the winter. The eggs hatch in May or early June. The young larva begins to feed on the leaves of the nearest food plant, and later tunnels in the stem. The mature larva is nearly 1 ½" long, grayish brown with one white stripe on top and two white stripes on each side. On the front half of the body the lateral stripes are interrupted, and the lower brown stripe extends forward onto the side of the head.

Burning all the old stalks, if allowed, and destroying weeds at the edges of the garden helps control this insect. When needed, methoxychlor, which is among the compounds registered for use against this pest in Connecticut, applied as a dust, in June, should control this pest. Consult the label for dosage rates and safety precautions.