A large swarm of insects was recently captured by weather radar as they flew from the San Gabriel Mountains across southern California towards Mexico.

No! That’s not a huge rainstorm! A massive migration of adult lady beetles -- “ladybugs” -- (blue and green cloud), was captured by weather radar over southern California as they headed towards Mexico on 4 June 2019. (Credit: National Weather Service / public domain.)

No! That’s not a huge rainstorm! A massive migration of adult lady beetles — “ladybugs” — (blue and green cloud), was captured by weather radar over southern California as they headed towards Mexico on 4 June 2019.
(Credit: National Weather Service / public domain.) NATIONAL WEATHER SERVICE VIA A CREATIVE COMMONS LICENSE

Ladybird, ladybird, fly away home
Your house is on fire and your children are gone
All except one, and that’s Little Anne
For she has crept under the warming pan.”

If you are a weather watcher in southern California, then you were recently greeted by a remarkable phenomenon when you watched the evening news: despite clear and cloudless skies overhead, local weather radar recorded a large blob, or “echo”, over the San Gabriel Mountains headed down towards San Diego. This echo looked like a giant raincloud. Except it wasn’t: it turned out that this weather radar echo was a swarm of lady beetles that measured roughly 80 miles by 80 miles.

“The large echo showing up on SoCal radar this evening is not precipitation, but actually a cloud of lady bugs termed a ‘bloom’,” the National Weather Service tweeted.

This mass insect migration occurred in the evening on 4 June. Weather radar clearly showed that millions upon trillions of lady beetles, which are popularly known as “ladybirds” or “ladybugs”, were flying south over the desert in a large swarm. This mass migration typically occurs in springtime due to increasing daylength and warming temperatures that surpass 18 degrees Celsius (65 degrees Fahrenheit), which trigger these insects to awaken from diapause — a physiological state in insects that is similar to hibernation in mammals.

Upon awakening, the insects take to the skies and begin searching for something to eat so they are well-nourished enough to produce and lay their eggs. Ladybirds are an important natural enemy of aphids, scales, thrips, and other soft-bodied insects that damage food crops and flowering plants, such as roses.

Radar is designed to measure the shape, reflectivity, and altitude of flying objects, and can also be used to estimate speed and direction. But it cannot identify species. So how did meteorologists know this cloud on their weather radar was composed of droplet-sized insects instead of droplets of rain?

First, the sky was clear. Second, a human NWS weather spotter located near Wrightwood high in the San Bernardino Mountains actually saw the multitudes of ladybirds flying south over San Diego towards Mexico.

What did these swarms of flying insects look like?

“I don’t think they’re dense like a cloud,” Joe Dandrea, the meteorologist with the NWS in San Diego who asked the spotter to describe what he was seeing, told The Los Angeles Times. “The observer there said you could see little specks flying by.”

Mr. Dandrea noted that the ladybirds weren’t in a concentrated mass, but rather, they were flying in a loose aggregation spread throughout the sky between 1,525 and 2,745 meters (5,000 and 9,000 feet) in altitude, and the most concentrated mass was about 16 kilometers (10 miles) wide.

Yet, despite the stir that they caused, as twilight faded into nighttime, the lady beetles disappeared into the darkness as they moved towards Mexico.

The convergent ladybird likely formed this massive swarm

About 200 species of ladybug species live in California, so which species probably formed this mass movement?

“By far the likeliest species is the convergent ladybird, Hippodamia convergens”, said entomologist, Richard Comont in email. Dr. Comont, who studied the lady beetle for his doctoral work, now heads the Bumblebee Conservation Trust’s science program, and he is a member of the UK Ladybird Survey and Garden Bioblitz teams.

This likely species identity is based on knowing a bit about the behavioral ecology and natural history of lady beetles: although they are solitary by nature, adult convergent lady beetles are known to seasonally migrate between agricultural areas and mountainous regions. Perhaps the largest of these seasonal ladybug aggregations in North America occurs in the Sierra Nevada Mountains of California.

Aggregation of adult convergent lady beetles, Hippodamia convergens, on a tree in Alamo Peak, Otero Co., NM. (Credits: Jerry Oldenettel / Institute of Food and Agricultural Sciences Extension, University of Florida)

Aggregation of adult convergent lady beetles, Hippodamia convergens, on a tree in Alamo Peak, Otero Co., NM.

“This is a species which overwinters in the mountains in huge aggregations (sometimes covering entire houses) and in spring comes down to the lowlands in search of food in swarms — though not normally this size!” Dr. Comont added in email.

As springtime conditions return, the insects then migrate down the mountains and back to grasslands, agricultural fields and other rural areas where they feast on aphids and other soft-bodied insect pests, and lay their eggs.

“The only other possible candidate really would be the Harlequin ladybird, Harmonia axyridis, which also overwinters in big aggregations, but it doesn’t show this swarming behaviour”, Dr. Comont explained in email.

The Asian ladybird beetle, Harmonia axyridis, is an invasive lady beetle pest species in North America. It differs from the native convergent lady beetle by aggregating in houses and other human-made structures instead of in natural areas.

But why did this large swarm occur this year? It is possible that recent high levels of moisture helped huge numbers of lady beetles survive the winter, where they diapause under snow cover, and this then allowed this exceptionally large swarm to occur.

But this swarm of lady beetles are certainly a little bit of good news for California’s agricultural sector.

“It would be good for people to understand that these ladybirds aren’t coming to cause havoc — they’re aphid-eaters and help gardeners and farmers by eating huge numbers of plant pests (psyllids and coccids as well as aphids).”


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