Ladybugs are considered a godsend

Parasites - The Evolution Nightmare

To see a ladybug become a zombie is truly sad.

Ladybugs are highly developed predators. A single one can eat several thousand aphids during its life. In order to find prey, the beetle first uses sensors in its antennae to look for fragrances that plants release when insects eat on them. As soon as it has located these alarm signals, the ladybug turns its senses on molecules that are only given off by aphids. Then he sneaks up and hits his jaw in the louse.

Ladybugs themselves are well protected against most enemies. Their often black and red wing covers are a warning to other predators: “Better leave it alone!” Because when a bird picks up the ladybird, for example, the beetle secretes poisonous, bitter-tasting blood from its leg joints. Most birds then spit out their prey in disgust. They quickly learn to interpret the black and red color combination as a warning and to leave ladybugs alone in the future.

A predator protected from other predators: it seems like the ladybug could lead a wonderful insect life. If it weren't for wasps laying eggs in his body.

One such wasp is the brackish wasp, which is approximately three millimeters in size Dinocampus coccinellae. The females of this species sting ladybugs in the stomach and inject them with an egg. A tiny wasp larva will soon hatch from it and feed on the beetle's body fluid.

While the ladybird is already being eaten from within, it continues to hunt aphids. With each prey, however, it now indirectly feeds the parasite in itself. About three weeks later, the wasp larva is so large that it can leave its host and develop into a fully-grown insect. To do this, the larva forces its way out through a gap in the ladybird's shell.

NG animation: The fearless rat

The beetle's body is thus freed from the parasite, but its spirit remains trapped: the beetle remains in place while the wasp larva wraps itself under it in a self-spun silk cocoon. In it the larva transforms into the winged wasp. At this stage it would be easy prey for other predatory insects. But if such a predator comes closer, the ladybug hits with its little legs and drives away the attacker. He has become the bodyguard of the parasite and performs this function until the adult wasp hatches out of the cocoon after a week and flies away.

It is only now that most of the infected ladybugs usually die. They did their service to the parasite.

TED video: This is how parasites live

No horror writer came up with this story. All over the world, ladybugs in gardens, fields and flower meadows are turned from wasps into mindless bodyguard zombies. And it's not just the spotted ladybug that feels this way. Countless other insects, but also fish or mammals, become hosts of parasites and serve their purposes, whereby they often perish. But how does an organism come to secure that of its tormentor for the price of its own life instead of fighting for itself? And how do such astonishingly diverse behaviors come about that a parasite imposes on its host?

There is the parasite fly that lays its eggs in bumblebees. The fly larva develops inside the bumblebee, and in the fall, just before the larva pupates to transform into a fly, it forces the bumblebee to dig into the ground. There she is not only safe from predators, but also from the winter cold. In spring a new fly hatches out of the bumblebee's body.

Or let's look at the web spider native to Costa Rica Leucauge argyra. She goes to great lengths to meet the needs of Hymenoepimecis argyraphaga to satisfy another parasitic wasp. It starts with the wasp sticking an egg to the spider. A larva hatches from the egg. It drills holes in the spider's abdomen and sucks its blood. After a few weeks, the larva has grown. Now the spider becomes active: it tears up its web and builds a completely different one. It no longer consists of many thin threads in which insects get caught, but of a few thick strands that converge at one point. So the spider has had its day. After the wasp larva has sucked out and killed its host, it spins a cocoon that hangs on a thread under the center of the novel spider web. The cocoon hovers in the air, almost out of reach for hungry enemies, until the fully developed wasp hatches.

Parasites can also force a host to guard them while they are still living inside. An example is Plasmodium, the protozoan that causes malaria. Before it infects a person, it lives in the mosquito anopheles. The mosquito has to drink blood to survive. But this behavior is for Plasmodium not without danger: Under certain circumstances the harassed human victim kills the mosquito, then the parasite no longer has a chance to complete its life cycle in humans. As long as Plasmodium so is not yet ready to be transferred to humans, the unicellular organism creates an aversion to blood in its host. anopheles seeks fewer victims and gives up more quickly if she does not find a source of blood straight away.

But soon Plasmodium is ready to colonize a human host, it causes the mosquito to behave in the opposite way: it develops blood thirst, becomes foolhardy, visits numerous people every night and stings even when it is full. If the mosquito dies from the hand of a person, the consequences for Plasmodium are no longer serious: the parasite has long since moved to its new host.

Plasmodium manipulates and only varies the typical behavior of its host in order to reach the next stage of its life cycle. Other parasites cause more radical changes in behavior. As seen with the killi fish, colorful little tooth carps that aquarists like to keep. In nature they stay away from the water surface so as not to be eaten by birds. But if they are infected with parasitic flukes, they swim higher up and sometimes even turn on their backs. Her silvery belly glistens in the sun and signals: "Eat me!" Whatever happens. This is how the flukes get into the bird's intestines, where they reach sexual maturity and reproduce.

Similarly, another parasite manipulates the brains of terrestrial birds and mammals, often rats and mice. You become the host and victim of the protozoan Toxoplasma gondii. Spends a phase of his life Toxoplasma in the bodies of birds, rats and mice, where the parasite forms cysts in muscles or in the brain. It can persist for a long time in these cysts, but it must be used to reproduce Toxoplasma get into a cat's intestines. How is it supposed to get there from the brain of a rat? He has to get the rat to let the cat eat him. We know today that rats and mice that with Toxoplasma infected lose their natural fear of cats. Some become downright curious about the smell of cat urine - and thus easy prey. Plants in the cat's intestine Toxoplasma continues, forms stages of reproduction that are excreted by the cat and eaten again by other animals. The cycle can go on.

Evolutionary biologists have long been concerned with how such bizarre relationships can arise in nature. In his book "The Selfish Gene" Richard Dawkins gave answers to this question, which has been discussed in public for 40 years. Genes, says Dawkins, evolve in such a way that they can make copies of themselves as successfully as possible. Our bodies may be important to us. From the point of view of our genes, it is just a vehicle that transports the genes into the next generation. The totality of all the genes that make each of us what we are is called a genotype. The appearance and the abilities that this genotype has is called a phenotype.

Dawkins suggested that the phenotype need not be limited to the physical, but also include behaviors that are generated by genes. A beaver's genes make it look like a beaver and shape bones, muscles and fur. But they also create circuits in the brain that cause the beaver to build dams. The pond that is created then serves the beaver - and thus its genes - in many ways. For example, it is more difficult for predators to reach in its den surrounded by water.

If a mutation - a random change in genes - causes a beaver to build better dams, this particular beaver phenotype has an even greater chance of surviving and producing more offspring. All of which carry this new, modified gene again. The mutation becomes more and more common over many generations. From an evolutionary point of view, the dam and even the pond it creates are as much a product of beaver genes as the beaver's body itself.

Dawkins calls this "extended phenotype".

The next step is obvious: If the effect of certain genes can change the environment, can it not also change the behavior of living beings that are related to the carrier of the genes? For the benefit of these genes? Dawkins answered yes to this question. He named the parasites as a prime example. A parasite's ability to control the behavior of its host is inherent in its genes. If one of these genes mutates, this also has an impact on the behavior of the host.

Every mutation is initially random. It can either benefit or harm the carrier of the genes, in this case the parasite. If a flu virus mutates in such a way that its infected victims lock themselves in and starve, the pathogen can hardly spread to other hosts. This gene variant will soon disappear from the virus population. A mutation, on the other hand, which has a positive influence on the behavior of the host in the interests of the parasite and gives it more offspring, becomes more common. For example, if a wasp develops a mutation that can induce a ladybug to act as a bodyguard for its larvae, there will soon be more wasps with these genes.

When Dawkins developed this idea in the 1980s, it was way ahead of its time. A little more than 30 years ago, biologists only used a few examples to investigate the way in which parasites influence the behavior of their hosts. But now they can use new methods to study genes and their effects much better. And actually prove how parasites manipulate the brain of their hosts for their own benefit. Like the jewel waspAmpulex compressa.

The sting of this wasp turns a cockroach into a mindless zombie. The cockroach can move, but no longer does it of its own accord. Without resistance, the wasp pulls its prey into its burrow by the antennae. There she lays an egg on the cockroach's belly. It just stands there and waits for the wasp larva to hatch from the egg and dig into its abdomen.

The host insect lost its will when the wasp cleverly guided its stinger into the cockroach's brain and felt which areas control the movements. The jewel wasp flooded these nerve cells with a cocktail of chemical substances that act like psychotropic drugs. They precisely dampen the activity of those nerve cells that normally react to danger and cause the cockroach to flee.

The result agrees with Dawkins' theory of the extended phenotype: the genes that produce the poison molecules include the cockroach in the wasp's plan for survival and create an ideal nursery for the wasp offspring. Such genes that control host behavior have already been discovered. For example with baculoviruses. These viruses infect the caterpillars of several species of butterflies. The parasite invades the host's cells and causes them to produce new baculoviruses. The caterpillar looks completely normal on the outside. But the food she eats from now on serves as energy to produce other viruses. When their number is large enough, the caterpillar changes its behavior. Instead of hiding from enemies, it climbs up a tree and remains visible from afar on a leaf or on the bark. Either it will soon be eaten there by a bird that passes the virus on. Or it dies and disintegrates into a pulpy mass, from which the baculoviruses rain down on leaves on the ground, where they are eaten by other caterpillars.

In fact, biologists found the virus gene that causes the infected caterpillar to climb. It was named "egt". When the scientists switched off this gene, the parasites were able to penetrate the caterpillar's cells as before and multiply there. Over time, the caterpillars also turned into infectious mucus. But bacoluviruses without a functioning “egt” gene could no longer induce the caterpillars to climb up trees. The fact that a parasite manipulates the behavior of its host with a single gene is probably an exception. As a rule, the behavior of an animal is controlled by several genes. Presumably, most parasites direct their hosts through the interaction of several genes.

And what happened to the wasp Dinocampus coccinellae and its host, the ladybug? At the University of Montreal, the biologist Fanny Maure made an amazing discovery: the wasp, which makes its victim a docile bodyguard of its larvae, may even act under the control of a third living being. Because when the wasp lays its egg in a ladybug, it simultaneously injects a cocktail of chemical substances - and a virus that multiplies in the wasp's ovaries. There is evidence that it is this virus that gets the ladybug in place to protect the cocoon with the wasp's offspring from predators.

According to the principles of evolution, this serves the interests of the virus and wasp: If the wasp turns a ladybug into a bodyguard, more wasps are created. More wasps produce more viruses. Therefore, their genes work together to transform the ladybug into a puppet. So is Dinocampus coccinellae maybe not the actual puppeteer at all. There is probably another hiding in the wasp who is actually pulling the strings.

(NG, issue 11/2014, page (s) 88 to 107)