Nature Notes | Convergent Ladybugs

National Park Service photograph, by Sam Prestigiacomo.
National Park Service photograph, by Sam Prestigiacomo.

Convergent Ladybugs: Getting to the Bottom of a Mystery at the Top of Texas

By Andrew Stuart

For this episode, we’re teaming up with “West Texas Wonders” – a reporting series where listeners ask questions and Marfa Public Radio finds answers. This question comes from listener Pam Gaddis, of Alpine.

“Why do the ladybugs gather up on the mountain peaks?,” Gaddis asks. “Somebody was trying to tell me, there’s more moisture – but why are they not 20 feet down the mountain? Why are they only on the top?”

Veteran West Texas hikers can appreciate Gaddis’ question. On Emory Peak in the Chisos, on Guadalupe Peak, on Mount Livermore in the Davis, and on many lesser summits, ladybugs often mass in the thousands, or tens of thousands. There’s a reason a group of ladybugs is called a “loveliness” – to encounter an assembly of the bright red beetles on a desert peak is magical. What’s going on here?

They’re certainly among the most beloved of insects. And the positive association is ancient, and seemingly universal. In Irish, Polish and Russian, traditional names for ladybugs translate as “God’s little cow.” The beetles have often been connected with the Virgin Mary – the “Lady” in their name – and a Yiddish appellation translates as the “little Messiah.” In children’s affections, they hold their own with butterflies.

Dr. Natalia Vandenberg is USDA scientist emeritus, stationed at the Smithsonian Museum in Washington, D.C.

“Children love them,” Vandenberg said, “and I just never outgrew it.”

Vandenberg specializes in ladybugs. The beetles aren’t “true bugs” – but Vandenberg doesn’t quibble with the common name.

There are more than 6,000 ladybug species around the world, and some 480 in North America. New research – to which Vandenberg is contributing – indicates the beetles first appeared in the Jurassic Period, more than 145 million years ago. But they exploded in diversity in the Cretaceous, with the advent of flowering plants, and with them, aphids. Aphids, and other plant-sucking insects, are the preferred food for many ladybug species.

Among these aphid-eaters is Hippodamia convergens, the convergent lady beetle. It’s this species we encounter on mountain peaks. Bright red or orange, they typically have 12 spots on their hardened wings. On the beetle’s pronotum – the area just behind its head – are two white, converging lines – the source of its common name.

Vandenberg has a connection with the species. Her mentor, Ken Hagen, did the pioneering research – including following migrating ladybugs in a hot air balloon.

In tropical latitudes, ladybugs stay busy year-round. Here, they’re active only in spring – March through May. Convergent ladybugs follow this pattern. But what they do at spring’s end is unusual.

“In late May and early June,” Vandenberg said, “when their bodies notice that the quality of food isn’t as good and things are drying out, they will fly. And they fly straight up in the air, and the winds blow them to the mountains.”

Hagen studied convergent ladybugs in California. Aided by convection currents, the beetles would rise more than 6,500 feet above their feeding grounds in the state’s Central Valley. When the air temperatures fell below about 50 degrees, their flight was inhibited, and they would drop down into a warmer layer of air, at which point they would begin to fly again. All the while they were being swept east – toward the Sierra Nevada.

There’s been no comparable study in West Texas, but convergent ladybugs here are almost certainly doing something similar.

Prevailing winds carry the beetles to peaks and high points. But they’re likely using other cues to come together after that. The conspicuous profile of a peak may play a role. The ladybugs may release pheromones. The beetles often gather in a patch of sunlight near a summit at day’s end.

Scientists call the gatherings “aggregations,” a term also applied to inanimate things. Vandenberg has her reservations about the terminology.

“If you go to your church, that’s a congregation,” she said, “but if you’re a ladybug, it’s an aggregation. I think it’s one of these things where people like to have a special term just for what they do, to make it sound more important. When people go to church, it could be for the same reason that ladybugs get into these groups: it helps them find a suitable mate, and also they feel safe in a community of like-minded people, and ladybugs are safe in a big group – they can defend themselves better when they’re in a group like that.”

It is, of course, ladybugs’ striking coloration that makes them so appealing. It’s an example of a phenomenon called “aposematic coloration.” Like a coral snake, a monarch butterfly, or a skunk, a ladybug’s eye-catching look is an advertisement to potential predators: Try to make a meal of me, and you’ll regret it.

In the ladybug’s case, the defense is a smelly one. When threatened, the beetles release a toxin from their knee joints. It’s malodorous, and intensely bitter.

An active ladybug can fly from a would-be predator. But a dormant ladybug can’t. By spending their dormant months in groups, individual ladybugs increase their odds of survival. A bird may eat one among their number – but after that bitter mouthful, isn’t likely to eat a second.

The ladybugs have other chemical adaptations. A mountain peak might not seem the most favorable place to sleep out the winter. But the beetles can handle the high-elevation cold.

“They’re not like us – we couldn’t just be surrounded by cold like that and survive very well,” Vandenberg said. “But they build up a kind of ‘antifreeze’ in their bodies – glycol – so they can take quite a bit of cold.”

Having “aggregated” in late spring, the convergent lady beetles tuck themselves into leaf litter, or cracks and crevices in rocks. Nine months later they awaken, and take flight. Wind currents carry them to lower elevations – where spring means new plant growth, and aphids to eat.

Because of their efficiency as predators, ladybugs have long been used as “biological controls” – deployed in fields to attack aphids and other agricultural pests. Wintering aggregations of convergent ladybugs are sometimes “harvested” and sold. But Vandenberg said the practice can harm ladybug populations – and often isn’t effective. The beetles typically move on quickly from the area where they’re released.

Non-native ladybugs have also taken hold here – including the multi-colored Asian lady beetle, and the seven-spot ladybird, from Europe. Natives – like the convergent ladybug – have suffered. In addition to insects, pollen is an important food source for many ladybugs. Vandenberg said we can support ladybugs with native-plant gardens, and by limiting our use of pesticides.

These tiny creatures are certainly part of the “loveliness” of the West Texas mountains.

Nature Notes is supported by the Shield-Ayres Foundation and produced by Marfa Public Radio with the Sibley Nature Center. The program can be heard each Tuesday and Thursday, at 7:45 a.m. and 4:45 p.m., Central time, on KRTS Marfa, 93.5 FM, and KXWT Odessa/Midland, 91.3 FM. This episode was written by Andrew Stuart.