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Brad Kelly, USDA Forest Service

Sticky Plants Protect Themselves with a Deadly 'Siren Song'

Brad Kelly, USDA Forest Service

In the forests of Northern California, a plant called Van Houtte’s columbine always draws a crowd. It’s a beautiful plant, though not quite as charismatic as a towering redwood or ancient bristlecone pine. But that’s fine, because its visitors aren’t people looking for photo ops, but hummingbirds and bees coming for nectar and pollen, and plenty of insects in search of a meal.

Some of these bugs are a problem for the plant, like the moth caterpillars that munch on its buds and flowers. Others are helpful, scavengers and predators like stilt bugs and assassin bugs that eat the caterpillars and other herbivores. And then there are the insects that show up to the plant seemingly for no reason at all. They don’t live on the columbine and don’t come to eat the plant or other insects, and all they get for their visit is a slow death. These visitors, or “tourists,” as entomologist Eric LoPresti calls them, come by only to get stuck and die in the fine, sticky “hairs” called trichomes that cover the plant. 

The plants are literal tourist traps, and LoPresti shows in a new study published recently in the journal Ecology that their victims don’t just show up and get caught by accident, but are lured in so that the columbines can attract all those predatory bugs that protect them from caterpillars. 

Quiet as they seem to us, many plants are chemical chatterboxes, communicating with each other and with insects through chemical signals. Some signals warn other plants of danger and prompt them to mount a defense. Others are used recruit helpful insects that act as bodyguards. LoPresti suspected that’s what the columbine might be doing, but in a more roundabout way. Instead of calling directly to predators for help, he thought, they lure innocent tourists to their deaths with a chemical “siren song” and then use their bodies as bait to entice predators to hang around.

COLLECTING HUNDREDS OF TOURIST CORPSES

To test that idea, LoPresti ran two experiments at a nature reserve where Van Houtte’s columbine is plentiful. First, he and his team wanted to see whether the dead tourist bugs did what they thought they did and attracted predators that helped the plants. Last July, they found an isolated group of columbines in a forest and pulled all of the dead bugs off of half of them every few days. The other half they left alone, letting them collect hundreds to thousands of tourist corpses. They did this for three months, keeping track of the number of trapped tourists, predators and caterpillar-damaged parts on each plant as they went along. 

They also tested whether the tourists just had bad luck and showed up on their own or if they columbine was really luring them. In a meadow where the columbines grow, they clipped some leaves and other pieces from the plants and put them in petri dishes covered in a plastic mesh. They laid these petri dishes out along the side of the meadow, alternating them with empty ones. A day later, they returned to see how many bugs got caught in the mesh.  

With both of their predictions, LoPresti and his team were right. In the forest, the plants with the trapped bugs on them had 74 percent more assassin bugs and other predators crawling on them than the ones that had their tourists removed. These bugs ate the caterpillars or scared them off, and the plants that hosted them had much less damage to their parts than the tourist-free ones. Meanwhile, the petri dishes that had columbine bits in them had 21 percent more bugs trapped in their mesh than the empty controls. Because the mesh hid the plant parts from view, LoPresti says, the attraction is very likely due to volatile chemicals the plants release. 

The researchers say their results demonstrate a “‘siren song’ indirect defense” that relies on drawing tourists that then draw predators. While direct attraction of bodyguards is a common plant defense, the team writes, a plant using a middleman like this is a first. 

LoPresti thinks that other plants could be using the same defense as Van Houtte’s columbine, but haven’t been reported by scientists. Looking through other studies, they identified sticky insect-trapping plants in 49 different plant families, most of them non-carnivores that wouldn’t benefit directly from attracting bugs and being covered in their corpses. The researchers want to do similar experiments with some of these other plants to see if they’re also defending themselves indirectly by inviting tourists to hang around, but never letting them leave alive. 

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One Small Leap: The Enduring Appeal of Mexican Jumping Beans
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In the fall of 1923, street vendors in Santa Barbara, California received an unexpected bit of attention regarding one of their more popular wares: The San Francisco Chronicle wrote about the sellers' “freakish little brown seeds” that “cavorted about to the edification and delight of children and grownups."

Those “freakish” seeds were (and still are) known as Mexican jumping beans. Part novelty item and part entomology lesson, they’ve been a staple of street vendors, carnival workers, and comic book ads for nearly a century, thanks to their somewhat inexplicable agility. Some early theories posited that the beans moved because of electrostatic charging, or because of tiny gas explosions inside—but in reality, it was a larva living in the bean. In Santa Barbara, the local Humane Society was concerned that the tiny caterpillar was somehow suffering in the heat; a police sergeant confiscated several of the seeds and took them home to investigate.

THE BEAN MYTH

In truth, the bean is not really a bean at all but a seed pod. In the spring, adult moths deposit their eggs into the flower of the yerba de flecha (Sebastiana pavoniana) shrub, which is native to the mountains of northwestern Mexico. The hatched larvae nestle into the plant's seed pods, which fall off the tree, taking the larvae inside with them.

Each larva is quite content to remain in its little biosphere until it enters its pupal stage and eventually bores a hole to continue life as a moth. (But only when it’s good and ready: If the pod develops a hole before then, the caterpillar will repair it using natural webbing it makes.) The pod is porous and the larvae can eat the interior for nourishment. Metabolic water creates moisture for the larva, but it never needs to pee. Essentially, it's the ultimate in downsized efficiency living.

A Mexican jumping bean store display
Mike Mozart, Flickr // CC BY 2.0

When it's in the pod, the larva isn’t exactly dormant: It twists and contorts itself to create encapsulated movement, almost like the snap of a rubber band. When it moves, so does the pod. No one is exactly sure why they do this, though some believe it's to keep the pod from settling on a hot surface (as high temperatures can be deadly to the insect).

The larva will keep up this activity for six to eight weeks. If a pod appears lifeless and rattles when shaken, it’s probably dead. If it lives, it will go dormant in winter before creating an escape hatch in the spring and flying off to begin life as a moth.

CHEAP THRILLS

It’s hard to know who exactly first decided to begin hawking the “beans” for amusement purposes, though some credit an enterprising man named Joaquin Hernandez with popularizing them in novelty shops in the 1940s. Later, in the 1960s, Joy Clement of Chaparral Novelties noticed the beans after her husband, a candy wholesaler, brought them home from a business trip. Though she was initially confounded by their appeal, Clement agreed to distribute the pods and watched them grow into a significant success: Between 1962 and 1994, Chaparral shipped 3 to 5 million of them each year, and saw the bean transition from sidewalk dealers to major chains like KB Toys.

“There's not much you can buy at a retail store that can give you this kind of satisfaction for under a buck," one bean dealer told the Los Angeles Times in 1994. "It's one of the last of the low-end entertainments available in the world.”

Interest in the beans seems to come in waves, though that can sometimes depend on the weather in Mexico. The jumping bean's unusual insect-crop hybrid stature means that farmers in Álamos, Sonora—where the pod is harvested and remains the area's major export—rely heavily on ideal conditions. Lowered rainfall can result in lower yields. Álamos typically handles more than 20,000 liters of the pods annually. In 2005, thanks to unfavorable weather, it was just a few hundred.

BEAN PANIC

There have been other issues with marketing hermetic caterpillars for novelty purposes. A UPS driver once grew nervous that he was transporting a rattlesnake thanks to a shipment of particularly active pods. Bomb squads have been called in on at least two occasions because the noise prompted airport workers to believe a ticking explosive device was in their midst. And then there was the Humane Society, which remained dubious the beans were an ethical plaything. (Since the caterpillars repair breaches to the pod, the reasoning is that it seems like they want to be in there, though no one can say whether the insects enjoy being handled or stuffed into pockets.)

You can still find the beans today, including via online retailers. They’re harmless and buying them as "toys" is probably not harmful to the caterpillar inside, though the standard disclaimer warning owners not to eat the beans remains. The police sergeant in Santa Barbara found that out the hard way: After taking his nightly prescription pill, he felt an odd sensation and went to the hospital. After physicians pumped his stomach, they noted that he had accidentally consumed a jumping bean. In his digestive tract, it was leaping to get out.

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DIY Tips for Preventing 5 Household Bugs from Infesting Your Home
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Most American homes—whether they're houses, apartments, or something in between—have bugs. A 2016 study estimated that there are more than 100 species of creepy crawlers in the average house. Pest Web suggests the global insect pest control market will hit $17.3 billion by 2022.

Bed bugs, cockroaches, termites, ants, and mosquitoes are some of the most prevalent intruders—and they can damage your health, your building’s structure, and your wallet. Fortunately, there are DIY ways to prevent these household pests from getting in the door. Grab your sponge and sealant: This is a long war.

1. BED BUGS

Bed bug on a piece of white fabric
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Though they’re not known to transmit disease from one person to another, bed bugs—which pierce exposed skin to suck blood, causing itchy, red welts—are still bad news. They can sneak into your home via used furniture, luggage, or, if you live in an apartment, from your neighbor's place. And infestations are on the rise.

“Everyone is really concerned with bed bugs because they’ve made a real resurgence in the U.S. in the last 20 years,” Dr. Jim Fredericks, chief entomologist at the National Pest Management Association, tells Mental Floss. In 2015, 99.6 percent of exterminators treated bed bugs during the year. That number was just 25 percent in 2000.

With all pests—but especially with bed bugs—the best treatment is prevention. A little time and money up front can save a huge headache later on, because professional bed bug treatment can run from $1000 to $10,000. Bed bugs aren't microscopic (and they leave behind markers like reddish stains or dark spots) so a periodic inspection of your home, especially your bedroom, is key. Apartment renters with nearby neighbors should be extra vigilant.

When you return from vacation, wash and dry all your clothes, towels, and bags from the trip. Drying on high heat for 30 minutes will kill all live stages of bugs that may have hitchhiked home with you. (If any garment can’t be washed or dried in a dryer, experts suggest storing the items in bags for a few months and, if possible, storing in direct sunlight or in a freezer, which can dramatically decrease the storage time needed.)

And don’t let the “bed” in bed bugs fool you—they don’t always need fabric to make themselves at home. Bed bugs can also hide behind loose wallpaper, wall hangings, the corners where ceiling meets wall, and electrical outlet covers. Follow the U.S. Environmental Protection Agency’s rule of thumb: If a crack can hold a credit card, it could hide a bed bug. Do a sealant sweep of the house to keep unwanted visitors at bay.

If prevention fails, it’s time to call in the big gun exterminators. They have specially designed equipment that will heat up your house enough to kill bed bugs and eggs.

2. COCKROACHES

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Cockroaches come in two main sizes: big and small. American cockroaches (which are actually native to Africa) are one of the heavyweights. This large breed typically lives outside, and there are things you can do to keep it that way. For example, don’t store trash or wood close to the exterior of your house, and if you’re bringing firewood inside, tap it on the ground before crossing the threshold to shake off any hangers-on.

German cockroaches—which migrated to the United States long ago—fall into the small set. They can stealthily slip into your abode with everyday movement, like in a package fresh from the delivery truck. Once they’re inside, their population grows rapidly. Of all the pest roaches, German cockroaches have more eggs, more successful hatchings, and the shortest time from hatching until sexual maturity, which speeds up their reproductive cycle. In just a year, it's possible to go from one egg-laden female German cockroach to 10,000.

To keep these pests at bay, maintain a neat interior and don’t forget to clean regularly behind the stove and fridge. Watch for grease buildup in sneaky spots like the hood over your stove, and clean the bathroom drain. Though you may prefer not to think about it, hair can be a food source if it collects gunk.

If you live in an apartment, there’s another consideration. Heavy rain can cause the sewer line to fill up with water, and cockroaches of any size living inside will rise to the top of the sewer and move to someplace dry. Sometimes when this happens—particularly in large cities—they’ll start moving into buildings through the pipes.

In your home, look for pipes that attach the sink to the wall. If you see a gap, close it with a surface sealer like Poxy Paste. You can also get a small mesh screen to put in the drain so cockroaches can’t get through.

3. TERMITES

Termites eating rotten wood
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Termites, which are hardwired to seek out wood for food, can often go undetected for years, by which point (depending on the size and age of the colony) they've already done a lot of damage. So don’t give them a reason to get close: Keep logs, wood piles, and mulch away from your exterior walls. Be on the lookout for raised tubular trails around the base of your house’s foundation, which indicate that a termite network has already arrived; shredded cardboard boxes in the garage or basement are also telltale signs of termite infestation.

Though physical termite barriers—plastic or metal guards that prevent termites from burrowing into the house's foundation, which can last up to 50 years—are often installed when a house is built, a chemical barrier can also be installed along the foundation of any existing structure for extra protection. They'll last five to 10 years before the pest control company needs to upgrade.

Since termite damage can have devastating consequences on buildings, think seriously about professional help if you fear an infestation. “Let’s say you have a support beam in the center of your house that’s been damaged—you need to have that repaired,” Dr. Angela Tucker, a Tennessee-based Terminix entomologist and manager of technical services, tells Mental Floss. “At some point you’re going to have an issue with the foundation of your house. It’s the same thing with floors and walls.”

4. ANTS

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Ants can appear in and around your home even if you're not prone to picnicking. Once inside, they can contaminate food, and carpenter ants can cause structural damage by nesting in soft or weakened wood.

If you’re eating outside, always clean up so you’re not attracting ants to the building. Keep them outside where they belong by filling cracks and crevices with weatherproof sealant.

Inside your home, store food in airtight containers. Original packaging isn’t necessarily bug-proof, and ants are savvy at finding those food sources. And rinsing cans and plastic food containers before disposing of them can go a long way toward repelling ants. “You’re doing a good thing, you’re recycling your soda cans,” Orkin entomologist Chelle Hartzer tells Mental Floss. “But the last few drops of soda in there can build up in the bottom of your bin and be attractive to cockroaches, ants, and other pests.”

5. MOSQUITOES

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Contrary to popular belief, mosquitoes don't just bite at night—they can be active outside day or night. Beyond the exasperatingly itchy bites they cause, mosquitoes can carry a slew of serious diseases, including the West Nile virus and the Zika virus—which might explain why, in 2016, mosquito control services were among the fastest-growing pest segments.

When a virus-carrying mosquito is looking for a watery place to breed, “it doesn’t even need to be as big as a saucer,” Tucker says. “They need as little as a bottle cap with water to get the eggs in it.”

To keep mosquitoes out, confirm that all of your window and door screens are intact—look for rips or worn-out rubber seals and replace them if needed. If you keep plants right outside the door, check the saucer underneath for stagnant water. In fact, make sure there are no areas of standing water—birdbaths, patio décor, or children's toys in the yard—near your home.

According to Mosquito Squad pest control group, if mosquitoes do infiltrate the house, place a small bowl of water in the corner and add a camphor tablet. The odor will drive mosquitoes away.

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