The grass isdamp, and the slugs are on the move again.
Why are slugsespecially active when it has rained? And where do they keep themselves whenthey are not out in the open?
Bjørn ArildHatteland is a researcher at the Norwegian Institute of Bioeconomy Research(Nibio). Some of his work has been with slugs.
He tells us whythey thrive in moist environments. The reason isn’t terribly surprising,considering the slimy properties of slugs.
Protective house
Slugs need dampenvironments “simply because their problem is that they can dry out, especiallyslugs that don’t have a house on their back,” says Hatteland.
They really dependon finding a place to hide when it gets dry outside.
Slugs withhouses – or snails – on the other hand, carry their own hiding place with them.You can sometimes see them hanging on a wall in the mid-day sun.
“Snails aretypically smaller and not as easy to see,” says Hatteland.
And yet, snailslive in the harshest environments.
In a scientificarticle from 1971, the researchers write that some desert snails only diewhen they are exposed to temperatures of 50 to 55 degrees Celsius for longerperiods.
In the heatthey hibernate, while during the more humid winter they eat and mate.
Other snails areable to cope well with the cold.
Found above tree line
If you didn'tthink of slugs as mountain dwellers, Hatteland has news for you:
“You can find slugsat elevations of 1200 to 1300 metres above sea level. Some of them canwithstand a lot of cold. We're talking about well below 0 degrees.”
When thetemperature drops below zero, the slugs go into a kind of hibernation, somewhatin the same way as other insects and small animals, the researcher says.
“Some slugs usea kind of antifreeze in their bodies so that they don't have too much icebuild-up. Other species supercool themselves and can tolerate ice forming betweenthe body's cells.”
They areinactive when it gets too cold, but on a mild winter's day along the coast you mightcome across an active slug. According to Hatteland, they don’t need more than afew degrees above freezing to take advantage of the opportunity to eat something.
Then they findtheir way back to their cosy hiding places when the cold returns.
Where are the slugs when we don't see them?
But where do slugsgo when we don't see them crawling around after a rain and at night?
“They hole upin cavities in the ground, in taller vegetation and other places that retainhumidity,” says Hatteland.
Small slugs,for example, can burrow a bit into moss or the top soil layer. They also staythere in the winter.
Some slugs havefixed hiding spots, says Hatteland.
“A lot of slugsreturn to the same hiding places when the day dawns and it starts to get dry.”
The Roman snailis used in the popular escargot dish in France. It has been released andis spreading in some places in Norway as well, although it is unwanted here.
Smells its way to food
The slugs witha fixed spot have a home range that they stay within when they move around tofind food.
You can seethat for yourself in your own garden. Woodpiles or compost piles are often favouriteplaces for the slugs to seek shelter.
“Then you canfollow them and possibly combat them,” says Hatteland.
“They don'thave very good vision, but can detect light and tell whether it’s cloudy orsunny. They signal where they are and largely use their sense of smell. They canalso smell their way to flowers and plants that they want to eat.”
With the helpof slime trails, slugs can find their way to members of their own species, suchas happens when slugs have their mating season.
Mostly plant and mushroom eaters
We don't knowmuch about the slugs' social life otherwise, says Hatteland.
Slugs sometimesgather in large numbers in the same place, but probably not because they seekeach other's company.
“They’re notsocial in the same way that certain insects are. They’re looking more for thesame things, like food or a place to hide.”
That is, exceptfor the aggressive ones, like the leopard slug or giant garden slug. Theydeliberately go after other slugs to chase them away. Other species preyexclusively on other slugs.
Otherwise,slugs are largely herbivores, says Hatteland.
“Many slugsprefer slightly weakened or withered and dying plants. They act as decomposers.”
Other species ofboth slugs and snails have other food preferences. They opt for mushrooms,lichen and moss.
Hard to predict Spanish slug occurrence next year
Slugs likemoisture, but the summers can get too wet, especially with torrential rain.
“Flooding isn’t good for air-breathingslugs. They shouldn’t be down in the water,” Hatteland says.
During the last decades the Spanish slug has invaded large parts of Northern Europe, causing frustration for many a gardener. The next generation of Spanish slugs are hatching in the late summer, and autumnand winter will impact how many survive.
You can alsoaffect the population by making your garden less slug-friendly.
Piles of grassor plant debris in your garden are good places for brown slugs to lay theireggs.
You might findthe egg clusters when you clean up the garden. The eggs are white and abouthalf a centimetre wide. Each cluster can consist of about 20 to 60 eggs.
After theirlate summer egg-laying, most adult slugs die.
Still spreading
In the longterm, slugs are certainly likely to spread even more, and Spanish slugs are nowfound all the way above the Arctic Circle in Northern Norway, says Hatteland.
Spanish slugscan be present even if you don't see them: in dry periods it can look like theyare pretty well gone. Then it gets damp and it can look like an invasion.
Admittedly, conditionsvary from season to season. Nevertheless, Hatteland does not expect that placesthat already have Spanish slugs will see any dramatic increase.
“We can expect themto establish themselves further inland and north,” says Hatteland.
———
Read the Norwegian version of this article at forskning.no
