Last week, agents from the U.S. Customs and Border Protection here in Honolulu caught an unwelcome stowaway in a container of granite and flagstone from Brazil: a 3.5 inch wandering spider (Phoneutria species). A second spider was found in another container from the same shipment four days later, and was likely another of the same species — though agents are not entirely sure because the worker unloading the container smashed the bugger beyond positive ID.
One spider is not so big a deal, but two is dangerously close to an introduction, and the Plant Quarantine Branch is now working with the importer to get the entire shipment returned to sender. But the close call begs a frightening question: could wandering spiders invade Hawaii?
Invasive Hot Spot
Hawaii has a lot of invasive species. Non-native animals and plants are everywhere on Oahu, as the diversity of habitats provide a wide range of ecological niches, and the native species that once roamed have been greatly reduced through hunting and habitat destruction. In addition, those native species evolved and diversified here in the absence of many threats, including predators and diseases, so they're not good at fighting off newcomers that either directly attack them or can pass along things that do.
Scientists estimate that new species show up in Hawaii every day, and the rate of introduced species colonization in Hawaii is 500 times the rate for the continental U.S. When the Hawaii Department of Agriculture inspected 100% of incoming air cargo at Kahului Airport on Maui, for example, they found an average of one new insect species arriving daily. Lowballing, scientists guess that 25 new species are introduced to Hawaii annually, but luckily, not all introductions take root and become invasive.
For example, there exists a small population of rock wallabies (Petrogale penicillata) in Kalihi Valley — definitely not a native species here in Hawaii, but not invasive either. To be considered invasive, a species must not only survive, it must cause or be likely to cause "economic or environmental harm or harm to human health." The wallabies have been up in that valley for about a century without spreading, causing damage to homes or crops, or otherwise causing trouble, so they're just introduced, not invasive.
Of course, we can only guess if the spiders that arrived in that shipment would have survived, reproduced and become established (or even have been able to — sex of the spiders was not mentioned). So if the inspectors had missed the Phoneutria in those shipping containers, no one can say whether they would have become "introduced".
However, it's pretty safe to say that if wandering spiders did establish here in any reasonable numbers, they would fit the term "invasive". There aren't any detailed studies on their diet, but they are large hunting spiders that would likely chow down on native species, potentially including the diversity of native — and often endangered — Drosophila. And even if they didn't have an ecological impact, they would likely have a pretty prominent human health impact. They are, after all, considered among the most venomous spiders on Earth.
According to the Guinness Book of World Records, Phoneutria are the most venomous spiders in the world because of their potent neurotoxins. The genus name translates to "murderess" in Greek, and murder they have: until the introduction of an antivenom, wandering spiders were to blame for at least ten deaths in Brazil.
But despite their name, bites from these spiders tend to be fairly mild. In a study of 422 bites, most patients only complained of local pain and swelling (89.8%), while few expressed moderate symptoms (8.5%), and 1.2% had no symptoms at all. Less than 1% of cases involved severe reactions, which can include life-threatening paralysis, pulmonary edema, and heart attacks.
Perhaps the most interesting venom side-effect, though, has garnered these spiders a new nickname: Viagra spiders. Scientists have found that their venom can cause priapism: painful erections that can last hours. Because of their ability to make male genitalia stand at attention, scientists have been mining their venom for potential pharmaceuticals, and recently, isolated one venom compound (PnTx2-6) that they think might become the next drug to combat erectile disfunction.
However, just because most bites are mild doesn't mean it would be a okay to let them make Hawaii their home. Doctors in Brazil and other South American countries are armed with antivenom which we don't have here, and even without killing they could still prove to be quite the health nuisance and cost thousands to millions in medical bills annually if they become commonplace on our ʻaina.
If The Habitat Fits
Scientists still debate what characteristics are most important when trying to predict the establishment and invasiveness of a species. Certainly, the most obvious and important factor for establishment is environmental suitability: does the new home have the right climate and habitat for the potential invader?
Hawaii certainly seems to be a nice place for large spiders to inhabit, as several have made these islands their home, including ones that are considered dangerous. Brown widows (Latrodectus geometricus), Asian spinybacked spiders (Gasteracantha mammosa) and Mediterranean recluses (Loxosceles rufescens) have all established populations on at least one of the main Hawaiian islands. In addition, cane spiders (Heteropoda venatoria) are nearly ubiquitous on the islands.
Like wandering spiders, these arachnids are active nocturnal hunters rather than web spinners and can grow to be quite large, thus their prolific spread in Hawaii may suggest that these islands would make a good home for wandering spiders, too. Of course, these other spiders may also serve as fierce competitors for the hopeful wanderers — we don't know yet what role competition plays in limiting their distribution naturally, or how well they would compete against other spiders.
A comparison of Hawaiian and Brazilian climactic variables like average temperatures, temperature variation, rainfall and humidity suggests that the survival of wandering spiders is certainly possible. There isn't a lot of data on what wandering spiders prefer, but we can assume that since they live and bite in Rio De Janeiro, those conditions are favorable enough.
Temperatures in Honolulu fluctuate just slightly less and are on average about 2.5 degrees F hotter, which isn't likely to be enough to kill off the spiders. However, Honolulu on average has almost 2 feet less rainfall every year and is almost 20% less humid, which might mean the spiders will have trouble keeping hydrated. But of course, those are averages which include very dry areas on Oahu as well as the rainforests of the mountains.
My guess is that there is definitely suitable habitat, particularly in the mountains and on the windward side of the island, where the spiders would do just fine. And if we compare Hilo, Hawaii instead, the climate variables line up even better — more rainfall, only 10% less humid, and less than a degree cooler on average.
So could they survive in Hawaiian climates? I'm willing to bet on it. The rainforests of Hawaii would provide ample habitat for the spiders to live and feed. There are plenty of bromeliads, which wandering spiders are known to hide in during the day. There are plenty of insects and small vertebrates to hunt.
In fact, if the spiders developed a taste for green dart frogs or coqui frogs (they've been known to feed on frogs in their native range), they could do quite well for themselves on Oahu and Hawaii respectively while helping manage non-native species. But crossing our fingers that they'll eat introduced species over native ones is naive — we've introduced species intentionally with such high hopes before, and those have proved disastrous.
But What Are The Odds?
Just because a species hypothetically can live somewhere doesn't mean it will. Other factors that scientists consider when trying to predict invasions are variables like whether the same or similar species have become invasive in other areas. In the case of wandering spiders, there's not much evidence that they're good at invading.
Wandering spiders have managed to find their way around the globe through agricultural commodities. In 2014 alone, wandering spiders surprised a family in Staffordshire, had to be vacuumed away in Colchester, dropped in on a man in Quebec City, shut down a nursery in Rochdale, and were delivered to a family in London. Though some doubt the IDs of these worldly spiders (which is fair, given that spiders found in bananas and other groceries are often misidentified), several of the hitchhikers — like the ones in Hawaii — have been checked by entomologists.
Arachnologists have confirmed at least six cases in which Phoneutria boliviensis were intercepted in bananas between 1926 and 2011, and a specimen of Phoneutria nigriventer that was shipped to Pennsylvania in 2009. Thus it seems like Phoneutria species have gotten around the globe pretty well. And yet, Phoneutria have never established outside of their homeland of Central and South America.
It's possible that the spiders have been caught every time they enter a new area, and thus the propagule pressure, or number of individuals introduced into a potential invasive range, is essentially zero, but that's a pretty optimistic view of our inspection abilities.
It's more likely that these spiders have made their way around the world undetected many times, but for whatever reason, simply don't take. Why wouldn't they establish outside their native homeland? It's possible that these spiders are just poor at spreading into new areas. They might strongly prefer a food item not found elsewhere, or are very picky about the species they inhabit. Maybe they're finicky reproducers — people have had trouble getting them to mate and produce offspring in captivity.
So my guess is that even if last week's shipment had been overlooked, these spiders wouldn't have established here in Hawaii. But that's just a guess, and invasive species control cannot count on hunches. Ultimately, there are a lot of unknowns about wandering spiders, including their environmental tolerance ranges, diet, reproductive output and frequency, and interactions with other species.
We know surprisingly little about the biology and ecology of these species that are considered so deadly, and thus we cannot predict what would happen if they were introduced outside their native range, especially somewhere like Hawaii.
We know they've arrived in U.K., Canada and mainland U.S. — but climate-wise, Hawaii is far more like their homeland than anywhere we know they've been shipped. Maybe the only thing that has kept Phoneutria from invading is that they haven't set foot in a tropical paradise. Hadn't, anyway. Let's hope the inspectors found them all.
UPDATE 4/22/15: Apparently the drama isn't over. The company which imported the contaminated containers — Selective Stone LLC — is so far refusing to return them to Brazil, reports KHON2. The containers are quarantined off of Lagoon Drive until a decision is made as to what will happen to them. Co-owner Kevin Nip told KHON2 that the material in the containers “was about 70 percent sold, so that is definitely a loss of business if these containers have to be returned," and that “we had to pay $12,000 in sea freight to bring them here and we will have to pay $7,000 to send them back to Brazil."
According to Nip, the Brazilian company doesn't want them back, and feels that "it should be standard procedure here in the state of Hawaii... to fumigate the containers.” However, the Department of Agriculture has replied saying
“There is no pesticide that has label instructions for treatment of Brazilian wandering spiders. So there is no way that anyone can determine that any pesticide will kill this particular spider and its eggs. Therefore, the shipment remains quarantined and will not be opened in Hawaii. We will continue to work with the company to have the shipment sent out of state.” [emphasis mine]
I bet there isn't.