Why Some Invasive Plants Thrive

Invasive plant species sometimes flourish better in their new homes than in their place of origin, according to researchers from the Helmholtz Centre for Environmental Research (UFZ). Their research on Buddleia or the Butterfly bush (Buddleja davidii) and Oregon grape (Mahonia aquifolium) showed that genetic changes and the lack of insects that would normally munch on their leaves gives invasive plants like the Buddleia a striking advantage.

Buddleia was introduced to Europe about 100 years ago from China and has been cultivated since then. Although the blossoms of the Buddleia are aesthetically pleasing and provide a food source for butterflies, their beauty masks a dark side. The butterfly bush can easily go to seed and form dense populations - potentially displacing native species and becoming a safety risk along railway embankments due to its rampant growth.

In order to improve knowledge about the mechanisms responsible for the spread of invasive species, UFZ researchers compared ten populations of Buddleia in Germany with ten populations in its original homeland, the southwest Chinese province Yunnan. Although the climatic conditions are more favorable in China, the bushes were larger in Germany and produced more and heavier seeds.

"From the plants in the Chinese homeland, 15 percent of the leaves had been eaten by insects. By comparison, in Germany only 0.5 percent" said Susan Ebeling from the UFZ. "The intruder is not yet on the menu for our insects. Because there are no relatives of Buddleia in Central Europe, the insects need longer to adapt."

The two Asian insect species that were used in an attempt to control the bushes in New Zealand are not yet present in Europe.

The situation is different with another species that researchers investigated more closely. Originating in the western United States, the Oregon grape is an evergreen bush with yellow blooms; it is even the official flower of the state of Oregon. In Europe the Oregon grape has a similar relative: the European Barberry (Berberis vulgaris). Native insects had millions of years time to adapt to the European Barberry and could now comparatively easily "switch" to the Oregon grape.

The Oregon grape on the other hand could not develop any defense mechanisms against its herbivores.

Nevertheless, it prospers so well in Central Europe that the Swiss Commission for the preservation of wild plants requests owners of gardens to do without the cultivation of Oregon grape as a preventative measure: "Should one already have this species in one’s garden, one must absolutely prevent any further propagation, on the one hand by removing the infructescence, and on the other hand by constantly removing any young shoots."

In spite of herbivores, the Oregon grape is able to flourish, cover forest floors completely and consequently become a problem. "Its success obviously lies in cultivation. Through selection and hybridization it came to a genetic change, enabling the Oregon grape to grow larger in Europe than in its American homeland", said Harald Auge of the UFZ. The biologist and his colleagues from Halle had collected seeds from Oregon grape plants in the USA, Canada, Germany and the Czech Republic and had grown the plants under controlled conditions in a greenhouse.

-- LiveScience Staff

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Moths Remember Lessons Learned While Caterpillars

John Roach for National Geographic News

March 5, 2008

Adult moths can remember their "childhoods" as caterpillars, a new study has found.  Recently scientists trained tobacco hornworm caterpillars in the lab to avoid a nail polish-like odor delivered in association with a mild shock.  These bugs then entered the pupal stage and metamorphosed into moths. As adults, they also avoided the nail-polish odor—showing that they had retained their larval memory.  "We concluded that indeed the association does persist and is accessible to the adults in this artificial scenario," said study senior author Martha Weiss, a biologist at Georgetown University in Washington, D.C.  The finding also supports the idea that a piece of the caterpillar brain persists through metamorphosis, she added.  (Related: "Scientists Rethinking Nature of Animal Memory" [August 22, 2003].)  Weiss and colleagues report their research today in the Public Library of Science journal PLoS ONE.  Memory and Metamorphosis  Scientists have long wondered whether memory could survive the dramatic reorganization of the moth brain during metamorphosis, Weiss noted.  "The transition from a caterpillar to a moth or butterfly is really very dramatic," she said.  (See a picture of caterpillar larvae that look like bird poop.)  For example, caterpillars and moths move, eat, and sense the world differently—not to mention appear nothing alike.  The study also showed that memory retention depends on the maturity of the developing caterpillars' brains.  Caterpillars younger than three weeks old learned to avoid the nail-polish odor but could not recall the information as adults.  However caterpillars conditioned to avoid the odor in the final larval stage before pupation, called the fifth instar, retained the lessons.  Larvae trained during the third-instar stage also demonstrated an aversion to odor as fifth instars, but they did not avoid the odor as adults, the authors said.  The findings jibe with the idea that memories are retained in one lobe of a mushroom-shaped part of a caterpillar's brain that forms during the late stages of larval development and survives metamorphosis.  "Basically, the brain is not completely taken apart and rebuilt from scratch," Weiss said.  Reinhard Stocker, a biologist at the University of Fribourg in Switzerland, studies the nervous system of fruit flies.  In an email, he noted that Weiss team's data appear strong, but he said that the moths' memory retention only after the fifth instar was perplexing. "Metamorphic rewiring of the brain is generally thought to be completed only in later stages of metamorphosis, which for the moth would be well after the fifth instar," said Stocker, who was not involved in the new research. "Thus it should essentially not make any difference if one trains third- or fourth-instar larvae. I don't have any simple clue to interpret such an observation." Shock and Odor  The research may help explain how adult female moths that can eat a variety of food choose to lay their eggs on the same type of plant they fed on as larvae.  Study author Weiss describes it as an "if it was good enough for me, it's good enough for my kids" type of selection.  If the moths retain some memories from their larval stage, as this research shows, then they could remember what they ate as "kids."  Other researchers have theorized that moths have what's called a chemical legacy from their larval stage that could cue them what to eat.  "That could look like the larva is actually remembering something," Weiss noted.  Her research team was careful to show that larval memory was based on the formation of an association between a shock and an odor—not a chemical legacy, she said.  "For me, it is exciting to think that a learned association really can be transferred from one phase to another through this very radical transition."  Story

Bug's "Bird Poop" Disguise Decoded

February 21, 2007—This larval swallowtail butterfly's clever disguise (left) is literally a load of crap.

In its larval stages, the Asian swallowtail—also known as the Chinese or Japanese yellow swallowtail—mimics the appearance of bird droppings to prevent predators from gobbling it up.

In its last phase of development, the insect turns green (right) to blend into the leaves on which it lives.

A new study shows that a single juvenile hormone is responsible for switching on and off the "spectacular diversity" of the caterpillar's colors, Japanese researchers say.

"[Juvenile hormone] has been known to be involved in molt, metamorphosis, and some other events," study co-author Haruhiko Fujiwara of the University of Tokyo told National Geographic News in an email.

"But in this report, we found that [the hormone] regulates pattern change, which is an original finding."

Hormone levels drop when the caterpillar leaves the bird-droppings stage and begins its green color transformation.

The substance also has the power to shape the caterpillar's texture and pigment distribution, enhancing its disguise.

The study, conducted by Fujiwara and Ryo Futahashi, also of the University of Toyko, appears tomorrow in the journal Science.

—Christine Dell'Amore

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