Do Grubs Eat Bulbs?

Lesser Bulb Fly

• These tiny flies are about three-sixteenths to three-eighths of an inch long and are an iridescent blue with bronze tinting. Their maggots vary in size from one-sixteenth to three-eighths of an inch and are white to yellow-gray. Maggots burrow into the bulb and reduce its interior to mush as they feed. They can kill a bulb or damage it enough that only a few weak leaves sprout. Lesser bulb fly maggots attack a wide range of bulbs, from flowers such as lily, amaryllis, daffodil and iris to food crops that include onions, garlic and even potatoes.

Narcissus Bulb Fly

• The narcissus bulb fly uses camouflage to get close to flowers. It is large, about one-half inch long, and its black and yellow or orange stripes make it closely resemble a bee. These flies can't sting, however. Their larvae are one-sixteenth of an inch to three-quarters of an inch long and are white to yellowish gray. The maggots hollow out bulbs as they feed and often leave bulbs susceptible to rot. Affected bulbs are either killed or produce only stunted leaves. The narcissus bulb fly feeds mainly on flowers bulbs such as narcissus, hyacinth, lily and iris.

Bulb Treatments

• Once bulbs are severely infested, it is best to toss them out and start over. You can, however, prevent grub problems for your new bulbs. When you buy the bulbs, or before you plant them, check each entire bulb for soft or spongy parts. Avoid those bulbs if you find them because bulb fly larvae tend to attack such weakened bulbs. You also can boil new bulbs; if you store bulbs for winter, then you may boil them, too. To boil bulbs, place them in a pot of water for 40 minutes at a temperature between 109.4 and 111.2 degrees Fahrenheit. Follow that temperature guideline strictly to avoid injuring the bulbs.

Control

• Bulb fly females lay eggs in cracks and crevices in soil and in leaf litter. In autumn, mow down the leaves of your bulbs, and cover them with a layer of soil. Females locate the bulbs underground by scent; if the bulbs are covered, the flies will be unable to find them to lay eggs. An alternative is to purchase place an insect net over your flowers when the flies are active, which is usually during warm, May days. While that isn't the surest method of control, it can help. No insecticides are useful against bulb flies, making cultural control necessary.