The Narcissus Bulb Fly / Lesser Narcissus Bulb Fly

Merodon equestris, Eumerus strigatus and E. tuburculatus

Order: Diptera
Family: Syrphidae
Species: Merodon equestris, Eumerus strigatus and E. tuburculatus

Description and Life History: After pulling out some disappointing daffodil bulbs the other weekend, I figured that the Narcissus Bulb Fly would be a timely pest of the month for May. Since I’ve been here, I’ve noticed that a few of these gooey, mushy, worm-ridden, stinky bulb samples come into the MG clinic.

This is one of the few pestiferous syrphid flies; most flies in this family are good guys that munch on aphids. As an adult, the Narcissus bulb fly is a pretty neat looking fly. The adult Narcissus bulb flies, Merodon spp., are bumblebee mimics. These large (over ½”) fuzzy flies are striped with black and yellow/orange to trick you into thinking that it has a painful stinger. I hand collected one of these last year, and the fly even tried fruitlessly to sting me by tapping its butt against my hand. Even having a trained eye and being 99% sure that I caught a fly, not a bee, this stinging action made me think twice for fear I just grabbed a bumblebee. The lesser bulb flies, Eumerus spp., are smaller flies (~1/4”) that are colored dark blue with metallic/bronze iridescence.

Both the Narcissus and lesser bulb flies emerge around April and May. The Narcissus bulb fly has one generation per year while the lesser bulb fly can have two generations per year depending on climate. Adult flies seek out flowers and foliage of the respective host plants. Flies lay eggs near or on the foliage of the plant close to the soil line. Eggs hatch and larvae develop inside the bulb. These larvae are bullet shaped maggots that speed up the decay process and turn the bulb into soup. After feeding, larvae leave the bulb to pupate in the soil.

Damage: The Narcissus bulb fly attacks amaryllis, daffodil, Galtonia, Flanthus, hyacinth,iris, lilies, Leucofum, Narcissus, Scilla, tulips, and Vallota. The Narcissus bulb fly is large and usually attacks a bulb as a single or a few individual maggots. The primary area of attack is in the basal plate. Then the larvae will move up into the bulb to feed. Bulbs infested with this fly will rot over the next winter. If they are not completely killed, the following spring will produce very weak, spindly foliage and no flower (like mine did this year).

The lesser bulb fly can also attack the same types of bulbs in addition to onion, shallots, garlic, parsnips, potato tubers, cabbage roots, Calla elliottiana, Eurycies, Galtonia, Gladiolus, Scilla and Sprekelia formosissima. Larvae are much smaller and more numerous than the Narcissus bulb fly; 10-40 lesser bulb flies can infest a single bulb. Infestations of the lesser bulb fly cause the bulb to decompose rapidly, resulting in a mushy mess. The lesser bulb fly is less likely to attack healthy bulbs. These flies like to have some decay happening in the bulb prior to infestation. Both bulb flies are able to lay many eggs (over 100), so just having one in your neighborhood could spell “mush” for your bulbs.

Monitoring and Management: There are no pesticide recommendations available for these bulb flies. But that’s O.K.; we have many alternatives that we can use to avoid mushy bulbs. You should be thinking about trying these practices if you have a problem with bulb flies.

• In May, on sunny days look for large bumblebee-like flies hovering around your flowers. Bumblebees will have two pairs of wings while bulb flies will have one. Grab your handy insect net (you all have one, right???) and catch the critters before they can do too much egg laying. This sounds tedious, but is very effective for protecting small plantings of susceptible bulbs. Remember, each female fly can lay up to 100 eggs! Plus, if it is a nice sunny day, you should be outside admiring and tending your garden anyway.
• Adult flies use visual cues and smell to locate your delicious bulbs. After you have enjoyed your flowers, cover the bulb bed with a floating row cover, like Remay*. Another recommendation given suggests that you mow down the vegetative portions of your plant and gently cover the tops with soil. Female flies will be unable to locate the bulb. Once no new foliage is sprouting, remove and store the bulb through the off-season. If you do this, I do not know the impacts this will have on next year’s flower. That vegetation produces the bulb’s energy reserve that is needed for next year’s growth. Regardless, the earlier you can pull your bulbs out, the better chance that you will avoid bulb flies.
• Bulb flies are less active in open, windy areas. Plant your beds in exposed windy places, if your landscape provides this type of climate.
• Avoid any damage to the bulbs when handling and planting. The lesser bulb fly prefers damaged goods to healthy bulbs. Establishment of maggots is much easier if there are already rot producing organisms in the bulb.
• Plant your bulbs deep, if they can tolerate it. Bulbs planted 25cm (or about 10”) deep in the soil will evade attack by adult flies. I am unaware if planting this deep is practical.
• When the time comes to pull up the bulbs, check the basal plate of each bulb. When you purchase new bulbs, check the plate for any signs of squishiness and rot. If you find some rot there, do not plant them and discard the rotten bulbs.
• Infested or suspicious bulbs can be cleaned of maggots by soaking bulbs in hot water (43-44oC) for at least 40 minutes. Care must be taken to not exceed this temperature, because you will damage the bulb. This is a great way to kill other pests of bulbs, too.
• Finally, if the problem persists, the sure-fire way to avoid bulb flies is to buy your flowers at the store like all the non-gardeners and black-thumb’ers out there. If you don’t plant it, they won’t come. This option is the one that I’m going to take now.

** Mention of a product name, in text, does not constitute an endorsement by the USDA or WSU: or does not imply approval to the exclusion of other suitable products.