The Narcissus Bulb Fly / Lesser Narcissus Bulb Fly
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 Ive been here, Ive 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 thats 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.
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