It’s ant time again. The columns march en masse into our kitchens again and make a mess of sidewalks and terraces. Increasingly, it is not native ants that cause nuisance, but exotic ones. They can cause serious problems: in the built environment, but in the future perhaps also in nature reserves. And the risk of that is increasing.
That is why the Dutch Food and Consumer Product Safety Authority (NVWA) has had this problem mapped out. The results appeared on 7 June in the form of a so-called ‘risk scan’, carried out by Radboud University Nijmegen and EIS Kenniscentrum Insecten.
It was high time for such a risk scan, says Jinze Noordijk of EIS Kenniscentrum Insecten, one of the authors of the report. “We have known for decades that certain exotic ant species are on the rise,” he says. “But all that time those risks had not been systematically investigated. And the scale was not yet clear either. You have to have a clear picture of all these things if you really want to be able to do something about them.”
Rotterdam has a colony of more than 100 by 200 meters
Jinze Noordijk entomologist
No fewer than forty exotic ant species have been observed in the Netherlands. Of these, about a dozen can survive outside buildings, including the plague ant and the scorpion ant. A number of alien species form so-called supercolonies, with thousands of fertile queens and millions of workers.
“Those colonies can grow enormously,” says Noordijk. “In Wageningen there is a colony that extends over 200 meters, under a sidewalk. And Rotterdam has a colony of more than 100 by 200 meters, seven streets wide, reaching the top of apartment buildings.”
In the native road ant, which can be found in every garden and under every sidewalk, there is only one queen within a colony that lays eggs. Mainly workers crawl out of it. After a while she produces a number of new queens and males, all with wings. That generation flies out, in so-called nuptial flights. The fertilized young queens start their own colony elsewhere. A queen can live for twenty years. After her death, the colony dies out.
Supercolonies
“But things are different with these supercolonies,” says Noordijk. “The newly fertilized queens remain in the underground nest with their mother and sisters. They just move up a bit. And so the colony keeps getting bigger.” One queen produces thousands of workers and dozens of new queens per year. Each of them in turn… “In this way, the colony can eventually cover an entire residential area.”
This is the case with the colonies in Wageningen and Rotterdam the Mediterranean turning hole, a heat-loving species from southern Europe. It is so called because it can spray an irritating substance around with its abdomen, as protection against enemies. This species is one of the ant species identified as problematic in the new risk scan. In 2013, it was Noordijk himself who discovered this species in the Netherlands – close to his home. It is the Wageningen colony of 200 meters long.
It is no coincidence that the colony is just around the corner from him: as an insect expert, he simply pays attention to striking things in his environment. In this case, those were the typical, large sand craters on the sidewalk. “Undoubtedly they were already in many other places in the Netherlands at the time,” says Noordijk, “but no one had noticed that yet. Meanwhile, 55 colonies of this species have been discovered throughout the country. Plus a dozen of other species.”
Terribly useful
What’s so bad about that? Noordijk lists a whole series of reasons. “These exotic ants become so numerous that they can have a major impact on other animals and plants locally,” he says. “They eat everything they come across. This can threaten native insects, which are very important in our ecosystems. And they are food competitors of native ant species. If these exotic species end up in nature reserves, the misery will be incalculable.”
The ants can also undercut entire terraces and sidewalks. “And they invade houses,” says Noordijk. “Native ants sometimes do that too, but they usually have their nests outside. As a result, their numbers indoors remain limited. The non-native species sometimes also nest indoors.” For example, it could happen that people had their entire meter cupboard full of ants, eggs and pupae, he notes. “The ants are attracted to electric fields. Food and waste are also not safe.”
Now Noordijk would like to emphasize one thing. “Many people hate ants,” he says, “and these exotic invasions don’t make it any better. But our native ants are really no problem. On the contrary: they do terribly useful work. They clean up organic waste, make the soil airy and healthy, they form food for all kinds of animals, they disperse seeds… We really can’t do without it.”
Native ants will not often undermine the stability of your sidewalk or patio – so most of them don’t live in your home. They only come to look for food. According to Noordijk, you can solve this quite easily. First of all, by closing cracks and holes as much as possible, and by keeping your countertop and trash a little clean. “But also by brushing away the scent marks of the ants in your kitchen now and then with an all-purpose cleaner or natural vinegar,” he says. “Then you interrupt the route from the nest to the food source, and new scouts will first have to discover your counter.” So bait boxes are not necessary at all, he adds. “They are also rarely effective, because they often kill workers but not the egg-laying queens.”
You should be in such a colony… you don’t know what you see
Jinze Noordijk entomologist
In ants with a supercolony, poison can also have a counterproductive effect, the ant expert emphasizes. “If you tackle such a supercolony in the middle, the ants will move to the sides and the colony will only expand further,” he says. “That makes combat even more difficult. So far that’s just about it a few colonies succeededmaybe half a dozen, but in the meantime new colonies are being discovered at a faster rate.”
Successful control is only possible if you have detailed knowledge about the structure of colonies and the ecology of the species, says Noordijk. That is why EIS Knowledge Center Insects helps to develop effective control. “We have now been able to successfully combat colonies in a few places.”
But even more important than control, he says, is prevention: preventing these colonies from establishing themselves in our country. “Most introductions take place through garden centers,” he says. “The ants travel along in the clods of garden plants from Southern Europe, such as olive trees. But after that they also spread within the garden centres. In principle, they can settle in all plant pots.”
Southern European potted plants
Exotic ants have been found in more than half of the Dutch garden centres, Noordijk continues. “And it is expected that all garden centers will contribute to the transit of these ants. As far as we are concerned, this should be monitored much more strictly. That is one of the main messages from the report.”
Recognizing these species is not that easy: many of the exotic ant species are very similar to each other, and to their native relatives. But in any case, garden center managers should be alert to ant colonies in the root balls. And should they then treat all their southern European potted plants with poison? “Flooding for a week may also be sufficient,” says Noordijk, “but no research has ever been done into that. That is what we are arguing for: more research.”
Alertness among citizens is also important, according to Noordijk. “You really don’t have to call us for every ant colony,” he emphasizes. “Only if you think: hey, these are really a lot of ants, they really do move a lot of sand, or you see those big round craters, then I say: let a professional take a look at it. And don’t do anything else for a while.”
Millions of ants
Noordijk can imagine that people want to tackle the ants in their garden themselves, for example by excavating soil and replacing plants. He advises against that. “It is really not necessary for native species,” he says, “and with exotic species you absolutely have to let experts do it.”
In any case, make sure that you do not take garden plants, wood or other items infested with exotic ants to the landfill, he concludes. “This causes the ants to spread further and maybe they can end up in nature,” he says. “But you don’t want any further spread in built-up areas either. You should be in such a colony… you don’t know what you see. Millions of those swarming ants, all those craters, and columns that walk straight into the apartment buildings… Very fascinating, but I wouldn’t want to live in the middle of it.”
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