Fly infestation on common milkweed - Ask Extension
Hi,
I noticed a weird trend on a patch of milkweed I planted next to my driveway. Probably at least one hundred flies (blue or amber metallic bodies...
Knowledgebase
Fly infestation on common milkweed #808503
Asked August 29, 2022, 11:11 AM EDT
Hi,
I noticed a weird trend on a patch of milkweed I planted next to my driveway. Probably at least one hundred flies (blue or amber metallic bodies) buzzing around on the milkweed. I have the same species planted just 10 feet uphill at the top of the driveway with no such infestation, maybe just a handful of flies. I first noticed it because as I walk up the driveway, I brush some of the flopping milkweeds and a cloud of flies blows up and out.
Do you know what might be the cause of this phenomenon, and why it affects one area more than another? I have even seen some fly carcasses stuck to the undersides of the leaves, and there is somewhat of a foul odor present, but I didn't find any dead animals. I read this might be driven by aphid honeydew, but why then are the flies not on the other milkweed plants where other honeydew is present? Also, I see more milkweed bugs on the uphill patch than on the fly-infested patch.
Would love your help with this mystery! They've been around for going on 4 weeks. First pic shows several flies close up, second pic shows the uphill milkweed with few flies and a *maybe* Virginia Tiger Moth (spilosoma virginica is my best guess). Third pic is fly carcasses on underside of leaf.
Montgomery County Maryland
Expert Response
The flies are likely attracted to the honeydew, though we don't know why one patch of milkweed interests them more than another, unless it's the amount of honeydew present (or how much sooty mold is covering the honeydew and making it inaccessible to them).
We see what looks like the young larva of a syrphid fly (also called flower fly; different than the metallic ones) among the aphids on one of the photos, which is good because they're an aphid predator.
The odd behavior of the dead flies might be due to a fungal infection. Some fungi specifically infect insects (called entomopathogenic fungi) and some within that group cause the insects to behave in such a way that it puts them in a position to better disperse the fungal spores once the insect dies. Often, this entails reaching a high or prominent position on a plant so that the spores will be blown away on the wind (or splashed off in rain), or the spores will simply drift down via gravity onto future insect victims below. If this is the case with the flies, those fungi won't necessarily infect any caterpillars or other types of insects on the milkweeds, but you can pluck them off if you're concerned. If they remain, you might see a fungal fuzz developing on their bodies next time it rains or stays very humid, which would be the fungus producing spores in order to spread.
Miri
We see what looks like the young larva of a syrphid fly (also called flower fly; different than the metallic ones) among the aphids on one of the photos, which is good because they're an aphid predator.
The odd behavior of the dead flies might be due to a fungal infection. Some fungi specifically infect insects (called entomopathogenic fungi) and some within that group cause the insects to behave in such a way that it puts them in a position to better disperse the fungal spores once the insect dies. Often, this entails reaching a high or prominent position on a plant so that the spores will be blown away on the wind (or splashed off in rain), or the spores will simply drift down via gravity onto future insect victims below. If this is the case with the flies, those fungi won't necessarily infect any caterpillars or other types of insects on the milkweeds, but you can pluck them off if you're concerned. If they remain, you might see a fungal fuzz developing on their bodies next time it rains or stays very humid, which would be the fungus producing spores in order to spread.
Miri
