Causal agent(s) and transmission
Unlike insects which have three body sections and three pairs of legs, myriapods (millipedes and centripedes) have numerous body segments and many legs. Centripedes have fewer legs than millipedes and have only one pair of legs per body segment, their hind legs extend backwards and they move rapidly.
The symphylans (Scutigerella immaculata), also known as garden centipedes, are myriapods, characterised by the presence of many pairs of legs (up to 12 pairs in their last stage of development). Adults are small, 5 to 8 mm long, elongated creatures, with a brilliant white translucent colour on their soft-shelled body. They have two long segmented antennae (photo 1).
In case of large populations of symphylans, significant damage may be observed in the field, usually localised in foci (photo 2) in which reduced plant growth, or even losses on plant emergence, can be seen. Failure to emerge or limited foliage growth may be related to a reduced root system, caused by symphylan bites (photo 3). In some cases, the tubers may be gnawed.
Symphylans can also cause damage to other crops, such as beetroot or maize. Their development is favoured by light and organic soils, a temperate climate and simplified soil cultivation methods. An infected soil may remain contaminated for a long time, as the adults may live for several years. Eggs cannot be laid below 10°C from spring (the optimum laying time) until autumn, when a second generation may occur.
Millipedes, the most common species of which is the spotted snake millipede (Blaniulus guttulatus), are hard-shelled myriapods. The millipede B. guttulatus is between 10 and 18 mm long (photo 4) and has a thin, grey, elongated body (with brownish spots on the sides), consisting of 50 similar segments (with 2 pairs of legs on each), and is often curled up on itself when disturbed or dormant (photo 5).
These millipedes are mainly found in soils which are rich in organic matter, as they feed on decaying plant debris. However, when in large quantities, they may bite and damage underground plant organs: roots, stems and stolons. On the potato, the main damage is caused by superficial gnawing of the tubers.
Eggs are laid in holes beneath the soil surface. Larvae gradually grow through several moults and become adults which then overwinter in the soil. The duration of the cycle is about 3 years.
Control
High levels of myriapods are favoured by light soils, a temperate climate, absence of soil insecticides in the rotation and simplified soil cultivation methods or permanent pasture.
Millipedes seldom need to be controlled and there is no specifically authorised chemical product for the control of myriapods in potato crops, but most of the broad-spectrum soil insecticides (such as organophosphates and synthetic pyrethrinoids) are effective against a large number of these arthropods.
Cultivation methods such as soil preparation also reduce the populations of most of the myriapods, especially in favourable environments such as grass and fallow land.