As the citizens of Earth go about their daily business, most never notice the new species crossing their borders. They remain blissfully unaware as these exotic forces spread through their forests, swim up their streams, and sail over their mountains.
Scientists and extension educators in the college are among an intrepid band of Earth's defenders who believe that coordinated research and action must be taken now to beat back a number of these invasive species that--propelled by climate change and carried around the world by increased trade--threaten ecosystems, economies, and even human lives.
When a team of international researchers, including Matthew Thomas, professor and Huck Scholar in Ecological Entomology at Penn State, recently added up the long list of economic effects, from management costs to crop losses, they say invasive species--insects, plants, and animals--cost the United States about $120 billion a year. These costs, they suggest, could increase due to a fraying of nature's defensive shields, such as the geographic and climate barriers, that once kept species in place. And those are just the costs in dollars and cents.
According to Thomas, human intervention could be altering geographic and climatic filters. For instance, global trade and climate change allow pests to move around the world much more easily than in the past, as well as thrive in new environments that were previously inhospitable.
"One of the reasons that invasive species are such a growing problem is that this major dispersal constraint has been relaxed--you now have the free movement of people, trade, and commodities," said Thomas. "Historically, it was very difficult for potentially damaging pest species from, say, Japan or China to get to the East Coast of America, but not any longer. We are constantly being challenged by waves of invasive species; they are knocking on the door far more often than they used to because they no longer need to do it under their own steam."
Zika on the Move
Yellow Fever Mosquito (Aedes aegypti)
Origins: Africa; now found in tropical and subtropical regions throughout the world
Threat: Vector for transmitting several tropical diseases, including Zika virus, which has been linked to microcephaly and Guillain-Barré syndrome
Thomas, who led a team that received a $10.2-million grant in 2015 from the Bill & Melinda Gates Foundation for an innovative way to trap and kill mosquitoes trying to enter homes, is using his background in predicting pest movements to take on Zika, a virus that can cause birth defects and death. The sudden appearance of Zika, which burst into the headlines only a few years ago, is one of the deadliest examples of how high the stakes on invasive species prediction and control have become, he said.
The Zika epidemic is actually the result of two invasive species--the Zika virus and its mosquito vector. Mosquitoes carry Zika from victim to victim. If the mosquito ends up in new areas, outside of its normal habitat, and thrives there, the virus can soon follow, says Thomas. People infected with the disease who move around also increase the chances of an epidemic. A change in temperature, such as that occurring as a result of climate change, could mean that places that were never Zika threats--like the United States--may be at risk. Thomas says his team is building on approaches that they've used previously for malaria to better understand how environmental factors, especially temperature, influence the chance the virus could establish and trigger an epidemic.
Specifically, the researchers are using mechanistic models to examine the transmission potential of Zika. So far, they have found that the virus is most likely to be transmitted at temperatures between about 73 and 90 degrees Fahrenheit. "This implies that while year-round transmission is possible in tropical and subtropical areas (including parts of the United States), temperate areas are likely at risk for potentially very short transmission seasons only, and even then this requires that the necessary mosquito species are present," said Thomas.
With a recent grant from the National Institutes of Health, Jason Rasgon, associate professor of entomology and disease epidemiology, is investigating whether some of the most common mosquitoes present in the United States are capable of harboring Zika. "Aedes aegypti, which occurs in the United States, is thought to be the primary vector for Zika virus," he said. "And another American species, Aedes albopictus, also has been demonstrated to be a highly competent vector in laboratory studies. Yet, Zika virus has been detected in more than 25 species of mosquitoes. If other American mosquito species are competent to transmit Zika, the virus could potentially move into the United States beyond areas currently colonized by aegypti and albopictus."
Approximately 200 species of mosquito inhabit the United States. Sixty species occur in Pennsylvania. Rasgon plans to investigate the potential for some of the most widespread mosquito species in the United States to transmit Zika virus. He is particularly interested in examining the geographic variation among Aedes albopictus populations to transmit Zika virus. "Proactive knowledge about the potential role that North American mosquitoes may play in the introduced epidemiology of Zika virus is absolutely critical for the development of efficient Zika virus control strategies and risk management policies in the United States," said Rasgon.
Penn State Extension educators, too, are keeping Zika on their radars. They've always made mosquito control part of their monitoring duties, but now they are also on the lookout for signs that mosquitoes in Pennsylvania are carrying Zika. They also are educating residents about the virus and continuing to advise local officials about ways to spot mosquito breeding grounds to better control the bug's populations in those areas.
The Stink Bug Invasion... Well... Stinks
Brown Marmorated Stink Bug (Halyomorpha halys)
Origins: China, Japan, Korea, and Taiwan
Threat: This insect is becoming an important agricultural pest in Pennsylvania. It has produced severe losses in some apple and peach orchards. It also has been found feeding on blackberry, sweet corn, field corn, and soybeans. In neighboring states it has been observed damaging tomatoes, lima beans, and green peppers.
At present, Zika does not pose much of a threat in Pennsylvania. The brown marmorated stink bug, on the other hand, is here now and is causing trouble.
Originally from Asia--and, according to Greg Krawczyk, associate professor and extension tree fruit entomologist, not even a particularly stinky stink bug compared to other Asian species--the brown marmorated stink bug is thought to have traveled to the United States by stowing away in cargo containers from China, Japan, or Taiwan. When the insects arrived, they found not only a suitable climate but also few natural enemies to halt their march inland.
Stink bugs use their long proboscises to jab into fruits and vegetables and suck out the juices, causing blemishes and deformations. That can lead to lower prices at the market--and multimillion-dollar losses for farmers.
Penn State scientists and extension educators are coordinating a two-pronged counter-attack against the stink bug that includes research and management. For example, Krawczyk and colleagues are investigating a method of monitoring the insects that involves luring them into a trap. They found that a stink bug pheromone combined with a synergistic chemical does the trick better than either of the chemicals alone. Use of the concoction revealed that stink bug abundance peaks in the Mid-Atlantic region in early September, indicating a sustained threat in the late season shortly before or during the harvest of many crops.
Gary Felton, professor and head of the Department of Entomology, also is using the insect's own biology against it. He and colleagues developed a method to extract stink bug saliva and identify the major protein components, which could lead to new pest control approaches.
"Until now, essentially nothing was known about the composition of stink bug saliva, which is surprising given the importance of these insects as pests and the fact that their saliva is the primary cause of feeding injury to plants and crop losses," said Felton. "Other than using synthetic pesticides, there have been few alternative approaches to controlling these pests. By identifying the major protein components of saliva, it now may be possible to target the specific factors in saliva that are essential for their feeding and, therefore, design new approaches for controlling stink bugs."
Felton and his colleagues presently are in the preliminary stages of using RNAi, a naturally occurring process in which RNA molecules inhibit gene expression, to suppress a specific salivary gene in brown marmorated stink bugs that is involved with feeding. "We hope the technology might one day be used to interfere with successful feeding by stink bugs," said Felton.
While researchers study ways to manage or eradicate stink bugs, Penn State Extension educators are in the field, monitoring the situation, gaining more information, and implementing management plans. By all accounts, it's a successful collaboration.
In 2010, a team of extension educators, including Krawczyk, joined a regional group of researchers to monitor stink bug populations and to study the bug's behavior and biology. By carefully monitoring infestations, they learned that fruit growers can precisely time their pesticide applications, returning the number of sprayings back to near pre-invasion levels. That's important because farmers and growers went from spraying insecticides four to five times a year to ten times a year right after the infestation. The extra money they spent on insecticides is less important, however, than the costs the insecticide has had on the environment. Reduced use of the poisons minimizes those environmental impacts.
Spraying, even in large quantities, does little to control another invasive insect from Asia. Once maple forests are infested with the Asian longhorned beetle, the only way to mitigate the problem is to chop down the trees, according to Kelli Hoover, professor of entomology. In places that support a lucrative maple sugar industry, that's an expensive option. "The beetle is mainly a problem right now in maples in a section of western Massachusetts, southeastern Ohio, and several places in New York, and, for obvious reasons, the U.S. Department of Agriculture, is interested in eradicating them," said Hoover.
Folded up in a skeletal-like sarcophagus, the pupa of the Asian longhorned beetle looks intimidating. But, by the time the soon-to-be beetle has reached this pupal stage, it's already done some of its scariest damage to trees. The beetle can reach nearly one inch long and has antennae that stretch as long as--or longer than--its body. Females lay their eggs in the bark of the tree. Once they hatch, the caterpillar-like larvae begin dining on the wood, boring so deep into the tree that even cold New England winters won't bother them.
Hoover and her colleagues have discovered a pheromone produced by female Asian longhorned beetles that could be used to manage the pest. The researchers found that when the proper ratio and amount of pheromone chemicals is produced by females and deposited on the surface where they walk, signaling that they are fertile, males come.
"It is possible that a synthetic version of the pheromone could be used in combination with a pathogenic fungus," said Hoover. "This fungus can be sprayed on a tree, and when beetles walk on it, they pick up the fungus, which infects and kills them. By also applying the pheromone that female beetles use to attract males, we can trick the male beetles into going to the deadly fungicide rather than to a fertile female."
Hoover serves on the scientific advisory group to the technical panels involving forest pests of the United Nations International Plant Protection Convention, a global group that is trying to find new ways to stop invasive species, particularly ones that are using wood containers and pallets as their ride of choice, the most likely way the Asian longhorned beetle entered the United States.
Hoover says that the best way to prevent invasive species from becoming a problem is to take away the bugs' free ride pass to the United States--and to stop the American pests from traveling abroad. The pine wood nematode, a roundworm with a voracious appetite for pine, for example, has spread from America to Japan and parts of Europe, such as Portugal, where it is wreaking havoc on the native and introduced pines.
"It's not all one sided," said Hoover. "We've done some exporting ourselves--and those countries aren't too happy with us."
To stop potentially damaging beetles and bugs from burrowing into wood pallets and containers, international organizations require treating the wood before it is transported. Pallets and other containers that could harbor insects must be treated in industrial kilns for hours before they can be stamped for use in international trade.
"There are two ways to treat the wood to meet regulations: you can heat the wood in huge ovens, which takes a lot of time and is expensive, or you can use methyl bromide fumigation, which is a pretty nasty poison," said Hoover.
Hoover and her team are developing the intellectual property behind a radio-wave-powered method to create bug-free wood. It's similar to creating a bigger, better microwave oven, Hoover said. She explained that when you are heating a burrito in a microwave, you may notice it can be a little cold in the center if your hunger pangs force you to remove it before the timer sounds, which shows that microwaves can only penetrate a few inches. Radio waves, on the other hand, can go much deeper--in some cases, meters--into material where burrowing bugs may be hiding. "It would be quicker and more effective, and you wouldn't have to use poisons," said Hoover.
We've Met the Enemy and It's Us
Spotted Wing Drosophila (Drosophila suzukii)
Origins: Native to southeast Asia
Threat: Fruit crop pest and a serious economic threat to soft summer fruit (e.g., cherries, blueberries, raspberries, blackberries, peaches, nectarines, apricots, and grapes)
Winning the war on invasives does not necessarily mean that researchers are winning the peace. After an invasion--whether it's plant, pest, pathogen, or a combination--the delicate ecological balance may be altered for good, and efforts to return it to its prior state may be difficult, if not impossible, said Thomas. Invasive species can wipe out native plants and animals, even reshaping the land and changing how water flows. "The trajectory of that system has changed," he said. "Once it's out, you might not be able to put it back in the box." Repeated pesticide applications can also kill beneficial bugs that may have fed on harmful pests.
While researchers at Penn State and around the world are doing their best to battle invasives, they know that this is just the beginning. They must not only keep an eye on current threats but also predict the next wave of infestations because other species--ones we would never imagine becoming a problem--could strike at any moment.
For example, Krawczyk is monitoring the spotted wing drosophila, a fruit fly that is native to Asia and was first spotted in the northeastern U.S. in 2011. The fly not only adds blemishes to fruits but also lays its eggs in them. "We want to try to make sure that when people bite into a blueberry or some other soft fruit, there's no extra protein--in the form of fruit fly maggots," said Krawczyk.
Trying to get ahead of a potential problem, Krawczyk and others already have started warning small fruit growers--the state's blueberry and strawberry farmers, in particular--and are preparing them for a response. Extension educators have produced a series of educational flyers, which cover identification, monitoring, and management of spotted wing drosophila, and distributed the flyers to the growers, in person, as well as posted them online.
Krawczyk said that Penn State Extension's quick reaction to the appearance of the spotted wing drosophila could help growers avoid potentially devastating financial losses from the fruit flies and limit environmental damage from excess pesticide use.
Hoover, whose lab is filling up rapidly with invasive bugs, says another relatively new creature to the United States, the spotted lantern fly, concerns researchers here in Pennsylvania. Graduate student Erica Smyers is studying this invasive species that attacks a broad range of plants and is capable of producing so much of an excretion called honeydew--a sappy material--that it requires the researchers to wear raincoats when they are around these pests.
According to Thomas, researchers are learning new ways to better predict the spread of pests and mitigate potential problems, but predicting infestations and epidemics is not easy. "A few years ago, a lot of people probably never even heard of Zika, let alone predicted that it would be this big of a problem, and that raises the question, what's the next one? Hopefully, by understanding the factors that cause these invasions, we can better predict them in the future and be better prepared to handle the response."
A Knotty Problem with Some Wicked Weeds
Combining "mind" with "might" yields successful weed management.
Pennsylvania is ground zero for two related plants that recently have moved their ranges. The state already has a few native species of the weed--known as pigweed--and farmers have dealt with them for decades. "They are a group of plants that are prolific, can be very competitive, and can produce lots of seeds--it's not uncommon for pigweed to have 100,000 seeds per plant," said Bill Curran, professor of weed science.
As if farmers didn't have enough on their hands with the native species, in the last five years, two more species of pigweed--one from the Midwest and one from the South--began their march on the state, Curran said. These species, though, are resistant to common herbicides, including the active ingredient in Roundup herbicide.
The first, Palmer amaranth--or Palmer pigweed--started as a weed that grew in soybean and cotton fields in the southern United States before becoming resistant to herbicides there and spreading north. In a Tennessee study, Palmer amaranth decreased soybean yields by 78 percent. High densities of the weed also have been shown to reduce corn yields by up to 91 percent.
Researchers first became aware of the weed in Pennsylvania when it showed up in a Lancaster County soybean field about five years ago. The plant, which now has been identified in more than 30 different locations, mostly in southeastern Pennsylvania, tends to grow late in the season, making it difficult for farmers to spot initially.
"You don't really notice the weed until late summer, unless you're really looking for it," said Curran. "So, what happens is, by August, farmers have these weeds that can grow 5 to 7 feet tall growing above the soybeans, which are usually 3 feet tall, and at that point there's no practical way to manage them."
Curran said that Penn State researchers and extension experts are promoting several campaigns to tackle Pennsylvania's pigweed problem. They distributed identification and management guides on pigweed and, because early detection is critical to management, are reminding farmers to "scout early and often." They also are providing farmers with large paper bags for pigweed removal. The bags are intended to nip the problem in the bud when it first begins. Because the seeds are so small and the plant is so prolific, pigweed must be carefully removed, placed in the bag, and--either through burning or burying--destroyed.
Meanwhile, Penn State researchers are investigating other ways of containing pigweed, including testing new and alternative herbicides and using cover crops early in the season to curb the growth of the late-blooming weed, according to Curran. "The take-home message is to diversify the weed management program, not only by rotating herbicide sites of action and using effective herbicide mixtures, but also relying on integrated weed management tactics that include effective cultural and mechanical control measures."
Curran and colleagues at the Pennsylvania Department of Agriculture hope to classify pigweeds in the near future as "Pennsylvania Noxious Weeds" to help increase awareness of the problem and encourage landowners to prevent their spread.
While farmers and landowners keep a watchful eye on their fields and yards in Lancaster and other southeastern counties for pigweed, another invader is using the smartest camouflage--human ignorance and, in cases, indifference--to sneak through Pennsylvania, especially in the state's numerous rivers and waterways.
Green plants, like invasive knotweed, may be green and lush--and even beautiful to the unbiased observer who hasn't been fighting the weed for a few decades. "Most people who interact in the settings where we work don't perceive a problem," said Art Gover, a research support associate working on the Department of Plant Science's Wildland Weed Management project. "It's full of plants. It may look green, but it's not really providing all of the ecological services that a plant community should."
Gover, who has been working to keep knotweed--a plant originally introduced in the 1800s from Asia that can grow 10 feet tall--in check for decades, says the plant can seem harmless, but that opinion changes when boaters and campers want to access the area where the species has thickly taken root, especially in state parks.
The knotweed's rhizomes--masses of underground stems--can be torn from the plant and float downstream, where they create a new stand. The plant soon chokes out most of the other species in the area. Like other invasive species, the takeover can affect insects and animals in the area.
"If you look at a plant community as a dynamic, interconnected system, what invasive species do is throw that balance off," Gover said. "But, more important, they are not providing benefits to the other part of the food web."
Managing this prolific plant requires equal parts brainpower and muscle power. Teams of researchers, extension educators, and workers need to find the plant and determine the best way to handle it in a particular setting. Then they have to walk in, or in many cases, float in, to cut, dig, and spray the area clean of knotweed.
Penn State has led several teams of workers and researchers into the field at various state parks, most notably Ohiopyle State Park, with its 13-mile stretch of the heavily recreated Youghiogheny River, while teaching them management techniques so eventually they can take over knotweed removal efforts, according to Gover.
The effort to control knotweed, like actions against pigweed, has no foreseeable end. The war on weeds, the researchers pointed out, is a never-ending battle.