Earlier this week, I had a conversation with journalist Clare Proctor about some flies which were swarming over Chicago.
Earlier this month, Chicago was overrun with some kind of fly. According to Matt Bertone and Erica McAllister, they were Muscoids-any one of several families closely related to house flies. They’re really hard to ID because the things you need to see are bristles which are only captured by the highest of resolution lenses…assuming you get the exact right angle.
Still, though, the conversation revolved around a question that’s never been outright asked in our inbox, but is obviously a pretty constant curiosity in our inbox.
Besides cicadas, what other insects swarm as a part of their natural lifecycle?
So to answer this one, I’m going to stick to the continental US for the most part. There’s a lot of swarming critters all over the world, but I can’t really address each one of them.
March Flies/Love Bugs/Bibionids
Bibionid flies are a common insect we get a lot of questions about every spring. Every year, they emerge in huge numbers, and mate pretty much the entire time. Hence names like love bugs, two-headed flies, and the like.
They’re small flies whose larvae feed on grasses…specifically the dead grass which builds up underneath the living blades (the “thatch”). The adults don’t actually eat. They’re just here to mate and lay about 200-ish eggs before they die in less than a week.
Like cicadas, their strategy is a “safety in numbers” kind of thing where they make sure everyone’s too well fed to actually bother them.
Wooly Apple Aphids
Wooly apple aphids are aphids which appear late in the summer, and migrate to other plants in the fall. We typically see them when they’re in their migratory mode, which is why they dominate our inbox in the fall.
They get their name from the grey-white plumes of wax which cover their bodies. This wax makes it harder for predators to grab them, and harder for them to get stuck in spider webs. It’s not an uncommon defense mechanism across insects.
The reason people see them is because their hosts in cities are crabapple trees, which are a common ornamental species. Some are pests of fruit trees, but unless you’re eating the crabapples in your yard, they’re not going to do too much damage to your trees in their normal numbers.
They don’t swarm, but it’s more of a perception thing.
Mormon crickets are large…katydids, actually. They’re not crickets.
They can’t fly, but still grow to huge numbers and eat pretty much everything in their path…which makes them important rangeland pests. There was even a bit of a legendary war between them and early Mormon settlers, which was said to have been settled by a fortunate bunch of seagulls.
The story is an exaggeration, but it’s still a pretty cool legend.
Again, like the cicadas, this appears to be a “safety in numbers” strategy.
Midges are kind of a unique case here. They’re a massively diverse family of flies, numbering 10,000 species. They’re on par with birds, in terms of diversity.
Because midges are a family of flies, they’re almost always around. They breed in water, either lakes or streams, depending on the species, and emerge at various points during the warmer seasons. Although they look like mosquitoes, most species don’t feed as adults.
This is a unique case among the swarming insects, because there’s not really any strategy here. Any time people notice a swarm of midges, chances are, it’s a unique species. These insects are just incredibly common, and they’re not really large enough to be involved in a predator satiation strategy.
Typically, people notice them because of their mating behavior. They engage in a practice called “lekking” where males gather around a tall object and rely on the cloud to attract females. Very occasionally, the females do this to attract males.
People typically notice midges for either of these reasons. Sometimes, there’s mass emergences of several different species. Sometimes, it’s because of the lekking behavior of one or more species around their area. It’s more complicated than what we see in some of these other species, which I think makes it pretty interesting.
We’ll get to the locusts in a bit, but mayflies are probably the closest economic second to locusts on this list. It’s ironic, because they don’t actually eat. Instead, they reach economic “pest” status because they swarm so heavily that some states need to break out snow plows in order to sweep them off bridges to keep car accidents from getting out of hand.
Mayflies are their own kind of insect. They live in water as larvae, and emerge as non-feeding adults to mate, lay eggs, and eventually die. The lore around them says they live for only a day, but in actuality, it’s about a week (not unlike the lovebugs).
This is a case where the insect is suited for the habitat. Hexagenia is the mayfly which swarms, and it’s one of the hardier mayflies. They can’t live in water that’s too polluted, but they do better in polluted water than most mayflies. They live in burrows, filtering out stuff that floats in the water above. Huge numbers makes finding mates easier, and also likely provides a “safety in numbers” barrier to predation.
Rocky Mountian Locust
Locusts are grasshoppers. Cicadas are sometimes referred to as ‘locusts’, but that’s because both insects produce noise to find mates…and that’s their only similarity.
I shouldn’t say that. They both feed on plants, but cicadas feed on sap whereas locusts feed on leaves and other vegetation. Locusts are more harmful to the plants than the cicadas, and that’s an important difference.
Locusts swarm when conditions are right. This comes down mainly to weather and vegetation cover. When there’s enough of them, these conditions allow the insects to take on a whole other persona. The “swarming” form looks and acts completely different than the non-swarming form.
In an article focusing on swarming insects of the United States, it might seem a bit odd to talk about locusts. A lot of people think about locust swarms as an African or Middle-Eastern phenomenon. However, the US had its own species of locust, Melanoplus spreta, which went extinct in the early 1900s.
We talked about this on Twitter recently, and M. spreta, the Rocky Mountian Locust, is a really good cautionary tale. M. spreta was once the most abundant species on the planet and it went extinct as a result of a combination of forces which made its habitat too hard to live in. New livestock, forest fires, flooding, and ploughing related to agriculture all conspired to make its habitat too hard to live in.
Each of these things would have been enough for M. spreta to survive, but this combination of factors drove it to extinction.
The Bottom Line
Insects swarm for a variety of reasons. Sometimes, it’s to make sure the predators in an area are too full to eat everyone. Sometimes, it’s just because they’re the most common insects in the area. Other times, it’s not really because they’re common but because they feed on a plant that grows really close to people.
Periodical cicadas get people interested in swarming insects, and they really should. They’re beautiful insects…jet black, cherry red, and they’re proud enough to make themselves known by screaming from the rooftops. Frankly, I wish I had that confidence.
They even have the perfect name: Magicicada. They’re magic. They’re cicadas. Magicicada.
Still, though. Even though they happen in numbers high enough to show up on weather radar, we need to remember that insects just as numerous exist today, and some of which existed in higher numbers have gone extinct.
We get a lot of questions about swarming insects in our inbox…but a lot of these bugs are really quite vulnerable to extinction. So are the periodical cicadas, if we don’t pay attention to what they need to survive.