Gastropods belong to the phylum Mollusca and include snails (genera Helix and Helicella) and slugs (genera Limax and Arion).
These animals have a soft body, protected by a shell in snails and very reduced or absent in slugs. They are mainly nocturnal and can spread fungal diseases. They seek out damp, shady places, as moisture is essential for their survival and movement.
They are hermaphrodites, but they usually mate to reproduce, mainly in spring and autumn when the weather suits them. After mating, they lay eggs (20 to 50), often translucent, in clusters in the soil in cool, shady spots. Incubation depends on humidity and temperature. There can be several generations a year, for example in a heated greenhouse where conditions are ideal.
The main species found in France are:
-the garden slug = Arion hortensis
-the marsh slug = Deroceras laeve (the most common in greenhouses)
-the grey field slug = Deroceras reticulum
-the large red slug = Arion rufus
-the garden snail (petit gris) = Helix aspersa
-the banded snail = Cepaea sp. (several fairly similar species)
–Trichia striolata (tiny snail)
Snails and slugs are polyphagous, meaning they attack a wide range of plants. Begonias can therefore be infested by them, especially when grown in a greenhouse, but also when plants are put outside during the summer.
Symptoms
As well as leaves or other parts of the plant being nibbled, gastropods are easy to detect from the silvery trails they leave behind, caused by the mucus they secrete to move. Sometimes a few droppings can also be seen. As these animals are rather discreet, most of them being nocturnal, these few clues are what should alert you.
Treatments
Before considering treatments, it is essential to start with mechanical control: collecting the snails and slugs and destroying them. To do this, you can set out “shelter traps” such as boards or tiles, under which these animals will gather during the day. A similar technique can be used for greenhouse or terrarium growing: a freshly cut slice of carrot placed on the substrate attracts small gastropods (snails with conical or flat shells), which come to “hide” underneath. However, these techniques have their limits when the growing area is large. It is also very useful to use cultural
control, which means limiting hiding and egg-laying places near the plants by removing weeds and keeping potting compost and other composts stored well away. Finally, it can be worth creating welcoming spaces for natural predators (hedgehogs, toads, ground beetles…), arranging the growing area so they can play their part in keeping the population in check.
At present, biological control remains the best solution against these pests. As well as traditional methods such as beer traps or ash, several options are now available to everyone. The simplest is to scatter Ferramol pellets between the plants. The active ingredient, ferric phosphate, is completely harmless to animals and beneficial insects. It is an environmentally friendly product, easy to find in garden centres and inexpensive. Finally, it is worth mentioning that there is a nematode (a microscopic worm) that parasitises slugs, called Phasmarhabditis hermaphrodita. This treatment can be used against heavy attacks on large crops.