Until a few years ago, the butterfly known as the southern small white could barely be found north of the Alps. That was before a Europe-wide invasion that brought a huge increase in the insect’s distribution—at the same time as a rapid decrease in genetic diversity within the species.

It took a while for zoologist Daniel Berner to notice that a butterfly species that wasn’t local to his area had become established in his garden. Then, suddenly, he saw it everywhere: Pieris mannii—also known as the southern small white—with its wingspan of around four centimeters and white wings adorned with large black spots.

Indeed, until a few years ago, only a few small, localized populations of this principally Mediterranean species existed in Switzerland, in Valais and Ticino. But at some point in around 2005, the butterfly began its journey north and east—and it has now been identified at the North Sea and in the Czech Republic.

Comparison with museum specimens

With its expansion, however, came a significant loss of genetic diversity.

“We found that, as it invaded, the southern small white standardized local populations of its own species,” says Dr. Daniel Berner of the University of Basel. Together with researchers from the University of Greifswald and the Senckenberg German Entomological Institute, Berner has published a study in Current Biology investigating how the butterfly’s expansion has impacted diversity within its own species.

The researchers compared the genetic make-up of freshly caught butterflies with that of significantly older museum specimens—in other words, specimens that had been caught and preserved before the invasion began. In doing so, they demonstrated that the genetic composition of the researched local populations has changed considerably. Indeed, a large part of the original genetic make-up has now been replaced with that of the expanding population.

“If we hadn’t made the comparison with the museum specimens, we wouldn’t have spotted this genetic change,” says Berner.

For their analyses, the researchers were able to sequence butterflies from the collection of the Natural History Museum of Bern and therefore to characterize the insects genetically. They were very lucky that, over the decades, the lepidopterist (butterfly researcher) Heiner Ziegler had amassed an extensive collection of specimens of none other than the southern small white—and that they could use this collection in their research.

Favorite plants in gardens

Urbanization is helping the butterfly to spread rapidly. In fact, the southern small white doesn’t like to fly long distances. Instead, it spends its lifespan of approximately three weeks fluttering around within a modest radius of its birthplace, in which the caterpillars’ food plants also thrive—namely arugula and, above all, candytuft.

The latter is particularly widespread in gardens within settled areas. Accordingly, the increasing expansion of the built environment has given the southern small white the opportunity to spread far and wide.

Moreover, there are five or six generations of southern small white per year, rather than just one.

“This species can therefore quickly build up large populations in a newly settled area, favoring the settlement of new land over large distances,” Berner explains. It is very likely, he says, that the butterfly will expand further—provided that its food plants are available. “In any case, butterfly researchers in England are just waiting to spot the first one.”

Expansion and genetic mixing—loss or gain?

From a conservation perspective, the expansion of the southern small white is a double-edged sword. As the species uses largely human-designed habitats in the newly settled area, it is not expected to compete with indigenous butterflies. Moreover, expansion means that this butterfly species is now much more numerous overall, which tends to reduce its risk of dying out.

However, these positives are offset by the disappearance of genetic diversity that had built up over the course of millennia: “Although it’s the fate of living things that some local groups can die out, what’s special about the situation facing the southern small white is that the loss of original population diversity accompanies expansion of the built environment—and is therefore caused by humans,” Berner says.

As yet, the researchers don’t know why the southern small white in particular has embarked on major expansion or where exactly this process began.

“Presumably, nothing fundamentally new has happened on the side of the butterfly. So far, we haven’t found signs of major genetic change in the expansive population, and climate change doesn’t appear to play a key role in the situation,” says Berner.

The researchers want to continue looking into these questions, but they already have their suspicions as to the starting point: the invasion may have begun in eastern France.

In a new study, researchers at the University of Illinois Urbana-Champaign tackled a thorny problem: How do nutritional stress, viral infections and exposure to pesticides together influence honey bee survival? By looking at all three stressors together, the scientists found that good nutrition enhances honey bee resilience against the other threats.

Their findings are detailed in the journal Science of the Total Environment.

“Multiple stressors are often bad for survival,” said graduate student Edward Hsieh, who led the research with U. of I. entomology professor Adam Dolezal. “However, it is always context-dependent, and you have to be aware of all these factors when you’re trying to make broad statements about how interactive effects affect honey bees.”

Most studies focus on only one or two factors at a time, Hsieh said. They will explore the interplay of poor nutrition and pesticide exposures, for example, or pesticides and viral infections. But no previous studies have looked at how all three factors contribute to honey bee declines—probably because doing so is quite challenging.

Even understanding how bees respond to all the agricultural chemicals they encounter is a complicated task, Dolezal said.

“Some insecticides will work better against some insects than others, but they tend to be more lethal than fungicides or herbicides,” he said. “Some fungicides, however, are known to make insecticides more toxic to insects.”

For the new study, the team looked at pollen collected by honey bees visiting small patches of restored prairie bordering agricultural fields in Iowa. The researchers used the maximum insecticide and fungicide levels detected in bee-collected pollen grains as their guide to likely chemical exposures in the wild.

In a series of experiments, Hsieh exposed groups of caged honey bees to different dietary, viral and/or chemical treatments. The bees were fed either artificial or natural pollen. The agricultural pesticides included chlorpyrifos, an organophosphate; lambda-cyhalothrin, a pyrethroid; or thiamethoxam, a neonicotinoid.

Hsieh also infected some of the caged bees with the Israeli Acute Paralysis Virus, one of several viruses known to contribute to the collapse of honey bee colonies around the world.

The experiments yielded some obvious and some unexpected results, Dolezal said.

“What we found was that with the artificial pollen, if bees are exposed to the virus, a lot of them die. And if you expose them to the virus and pesticide at the same time, even more of them die,” he said. “However, if you do the exact same experiment but you give them better nutrition, you get a very different outcome.”

On the natural pollen diet, bees exposed to the virus still experienced higher mortality, the researchers found. But fewer bees died when they were also exposed to a mixture of chlorpyrifos and a fungicide.

“Bees have this inherent ability to deal with stress, and so if you give them a little bit of stress, like a low-level exposure to a pesticide, it may help them deal with a bigger stress from a pathogen like the virus,” Dolezal said. “However, it only works if they have the nutritional resources to do it.”

The researchers warned this doesn’t mean that chemical exposures don’t matter.

“Different pesticides have different molecular targets and do different things,” Dolezal said. “It’s not okay if bees get exposed to a little bit of any pesticide. It depends on the chemical.”

The findings offer some reassurance that providing high-quality prairie habitat near agricultural sites does not create an “ecological trap,” attracting bees to the flowers only to kill them with agricultural chemicals.

“The takeaway from this study is that bees are quite resilient even to the interaction of pesticides and viruses if they have really good nutrition,” Dolezal said. “However, we don’t want people to conclude that pesticides are not a big deal for the bees.”

Pesticides, alone or in combination with viruses, are in most cases detrimental to bees.

“But it is gratifying to know that providing high-quality habitat can at least increase their resilience to these stressors,” Hsieh said.