Causal agent(s) and transmission
Cutworms, also called nocturnal moths, are insects of the order Lepidoptera which, in their larval stage (caterpillars), can damage potatoes. There are different species of cutworm, of the genus Agrotis, whose caterpillars develop mainly in the soil (terricolous cutworms), on the foliage (defoliator cutworms) or in the stems (borer cutworms: photo 1). The colour of the larvae is usually dull, grey or brown (especially the terricolous cutworms, more commonly called grey worms: photos 2 and 3), glabrous or fairly smooth. They are 4 to 5 cm long, with black dots on each segment; the numbers and patterns of these vary according to the species.
Cutworms are polyphagous insects which develop mainly in light and moist soils. The adults and caterpillars of nocturnal moths (photo 4) are mostly active at night. Hibernation can occur either in the larval stage or as a chrysalis; egg laying can be abundant when the weather is favourable.
Damage caused by cutworm to the potato is relatively rare and results from large caterpillar populations in the fields (sometimes only in a few foci) which feed on root systems or cut stems, thus resulting in reduced plant cover and even losses or failure on emergence (photo 5).
Damage to tubers is less frequent and consists of wide irregular cavities dug by the caterpillars. Finally, defoliation caused by the cutworms (photos 6 and 7) is relatively rare and has limited impact on the yield.