Slugs and Salal - Bugs, Diseases: Narcissus Fly

NARCISSUS FLY

Q. My King Alfred daffodils were something of a disappointment this past spring.

It was only their second year in the garden. They produced plenty of foliage, but almost half the plants were bloomless.

A. This problem turns up in my mail faithfully every year, as gardeners are puzzled by a poor show of flowers and sometimes spindly foliage in their daffodil plantings.

Though poor growing conditions of wet or impoverished soil will cause a rapid decline in the bulbs, it is more probable that your planting of daffodils is infested with the Narcissus bulb fly, a pest that lays eggs at the bases of daffodil foliage. This plant is their favorite target, though hyacinth, snowdrops and amaryllis sometimes serve as hosts for the pest too.

The eggs hatch into grubs that move down into the soil and burrow into the base of the bulb, to feed there, reducing the bulb's centre to a rotten mass. In the process, the embryonic flowers that would have unfolded the following spring are destroyed.

In the spring the grubs emerge as adult flies to mate and begin the egg-laying process again. In the case of the greater Narcissus bulb fly, there is usually just one fat grub to a bulb. A lesser Narcissus bulb fly infestation results in a mass of tiny white grubs in a bulb.

Depending upon the warmth of the season, the flies hatch out somewhere in the mid-April to early May period, when temperatures warm to 20 C/70 F. Egg laying continues for roughly six weeks, during May and June. The greater fly is easily recognized by its resemblance to a bumblebee both in appearance and its light buzzing sound.

Prevention

A primary line of defense against the Narcissus bulb fly is to dig and remove no-show or suspect bulbs before the maggots mature and hatch into adult flies in the spring. These bulbs will be soft and squishy, with a telltale opening in the base. Destroy the bulbs. Do not compost them.

Some gardeners routinely dig their daffodil bulbs after the plants die down in the spring, trash the infested ones and store sound bulbs in a dark, cool place for replanting in September. At that time, you're into a whole new Narcissus game with a planting of clean bulbs, and there are several methods for preventing further infestations.

One is to cover the planting after bloom is over with a lightweight floating garden cover, fine netting, or old curtains that are light and sheer. Such materials bar access to the flies. Keep the planting covered until July.

Another precaution, one you might begin planning for now, is to plant a close-knit ground cover over the bulbs, as the flies prefer to lay eggs in earthy accesses. Creeping phlox, periwinkle and ivy are some possibilities. You may have to use a post-bloom cover for the first year until the ground cover grows enough to form an effective barrier between fly and soil. Bulbs growing in grassy areas enjoy some natural protection.

Applying diatomaceous earth, available at garden centres, around the bases of the plants every 10 days during May and June helps to control freshly hatched grubs. Pyrethrin or rotenone dusts may also be used.