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Four Different Species Use the Same Odor to Exploit Each Other

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Chemical warfare. Hijacked communications. Stowaways. Eavesdropping. Sounds like the makings of a spy movie, but it’s just another day in nature. 

Plants have got to be fighters in this big bad world. When an animal that wants to eat them comes along to start chowing down, they can’t run or hide. They’ve got to fight back. Some of them have structural defenses like thorns or spines or nettles that jab at herbivores’ mouths. Some have waxy exteriors that make them too slippery for insects to land on, or produce resins and saps that create a sticky trap for bugs.

Others wage chemical warfare, sometimes in a roundabout way. Methyl salicylate (MeSA), also known as wintergreen oil, is produced by some plants when they’re damaged by herbivorous insects. The chemical doesn’t harm the insects directly, but acts like a mayday signal that attracts predatory bugs that come and eat the herbivores. In one case that University of Florida scientists have recently described, it’s also at the center of a web of species trying to exploit each other.

When citrus trees are damaged and release MeSA, it not only attracts helpful bugs, but also a jumping plant louse known as the Asian citrus psyllid (Diaphorina citri, top). The psyllids follow the chemical’s odor because a tree that’s already damaged is a good place to find food, places to lay eggs (which the psyllids can only do in new citrus shoots) and other bugs to mate with. 

Sometimes, though, there’s no meal to be had, because the tree has been infected by a bacterium called Candidatus Liberibacter asiaticus (Las). The psyllids are the bacterium’s primary vector for spreading from tree to tree. When Las infects a tree, it lowers the tree’s nutritional quality and also hijacks its odor production, forcing it to release MeSA as bait. When the psyllids show up and find there’s no less-than-ideal food, they move on in search of another tree—but not before the bacteria cling to them and hitch a ride to their next victim.

A group of entomologists in Florida, led by Xavier Martini, recently found that the trees, lice, and bacteria aren’t using these chemical signals in private, and that there’s another creature eavesdropping on them. The same odor that the lice use to find food, and that the bacteria exploit to lure the lice, also attracts a wasp called Tamarixia radiata. These parasites feed on the bodily fluids of citrus psyllids and also lay their eggs on the undersides of the immature insects. When the eggs hatch, the larvae attach to the psyllid and feed on their hemolymph (kind of the arthropod version of blood) until the host dies. Then, they crawl inside the mummified psyllids, where they’ll develop into adults and eventually break their way back out through the thorax or head. 

Martini and his team discovered that the wasps “eavesdrop” on the chemical cues produced by citrus trees to find their hosts, and were more attracted to the MeSA from Las-infected trees than non-infected trees. At trees where the bacteria was present, the wasps also parasitized five times as many psyllids as they did at the regular trees. 

That the wasps are attracted to the same chemical as the psyllids potentially complicates things for the Las bacteria. Martini found that the wasps sometimes arrive at an infected tree before the psyllids show up, which might drive them away or result in them being killed or parasitized before they can give the bacteria a lift to another tree. Then again, when the wasps show up after psyllids they might actually help the bacteria by causing the psyllids to disperse far and wide, which is exactly what the bacteria need to spread. 

A tree’s “cry for help” not only attracts more danger, but can also be hijacked by bacteria to help them spread infection, while an eavesdropping parasite uses the same odor to find a host that its children turn into a macabre nursery. Ain’t nature grand? 

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Animals
Watch a School of Humpback Whales 'Fish' Using Nets Made of Bubbles 

Just like humans, humpback whales catch many fish at once by using nets—but instead of being woven from fibers, their nets are made of bubbles.

Unique to humpbacks, the behavior known as bubble-net feeding was recently captured in a dramatic drone video that was created by GoPro and spotted by Smithsonian. The footage features a school of whales swimming off Maskelyne Island in British Columbia, Canada, in pursuit of food. The whales dive down, and a large circle of bubbles forms on the water's surface. Then, the marine mammals burst into the air, like circus animals jumping through a ring, and appear to swallow their meal.

The video offers a phenomenal aerial view of the feeding whales, but it only captures part of the underwater ritual. It begins with the group's leader, who locates schools of fish and krill and homes in on them. Then, it spirals to the water's surface while expelling air from its blowhole. This action creates the bubble ring, which works like a net to contain the prey.

Another whale emits a loud "trumpeting feeding call," which may stun and frighten the fish into forming tighter schools. Then, the rest of the whales herd the fish upwards and burst forth from the water, their mouths open wide to receive the fruits of their labor.

Watch the intricate—and beautiful—feeding process below:

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Big Questions
Why Do Dogs Love to Dig?
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Dog owners with green thumbs beware: It's likely just a matter of time before Fido turns your azalea bed into a graveyard of forgotten chew toys. When dogs aren't digging up your prized garden, they can be found digging elsewhere in your yard, at the beach, and even between your couch cushions at home. But what exactly is behind your dog's drive to turn every soft surface he or she sees into an excavation site?

According to Dr. Emma Grigg, an animal behaviorist and co-author of The Science Behind a Happy Dog, this behavior is completely normal. "When people say 'why do dogs dig,' the first thing that always comes to mind is 'well, because they're dogs,'" she tells Mental Floss. The instinct first appeared in dogs' wolf ancestors, then it was amplified in certain breeds through artificial selection. That's why dogs that were bred to hunt rodents, like beagles and terriers, are especially compelled to dig in places where such animals might make their homes.

But this tendency isn't limited to just a few specific breeds. No matter their original roles, dogs of all breeds have been known to kick up some dirt on occasion. Beyond predatory urges, Dr. Grigg says there are two main reasons a dog may want to dig. The first is to cool off on a hot day. When stuck on an open lawn with little to no shade, unearthing a fresh layer of dirt untouched by the sun is a quick way to beat the heat.

The second reason is to stash away goodies. Imagine your dog gets bored with chewing his favorite bone but knows he wants to come back for it later. Instead of leaving it out in the open where anyone can snatch it up, he decides to bury it in a secret place where only he'll be able to find it. Whether or not he'll actually go back for it is a different story. "There's a disconnect with modern dogs: They know the burying part but they don't always know to dig it up," Dr. Grigg says.

Because digging is part of a dog's DNA, punishing your pet for doing so isn't super effective. But that doesn't mean you should stand idly by as your yard gets turned inside-out. When faced with this behavior in your own dog, one option is to redirect it. This can mean allowing him to dig in a designated corner of the yard while keeping other parts off-limits, or setting up a raised flowerbed or sandbox especially to satisfy that urge. "You can get him interested in the area by burying a couple bones or some interesting things in there for him to dig," Dr. Grigg says. "I like the idea of buried treasure."

If your dog's motive for digging is more destructive than practical, he may have an energy problem. Dogs require a certain amount of stimulation each day, and when their humans don't provide it for them they find their own ways to occupy themselves. Sometimes it's by chewing up shoes, toppling trash cans, or digging ditches the perfect size for twisting ankles. Fortunately, this is nothing more walks and playtime can't improve.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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