Plutella xylostella  (Linnaeus)

Ringworm

General

• Insect probably originating in Europe, its distribution is today worldwide.
• Belongs to the order Lepidoptera and the family Plutellidae.
• Only attacks plants of the Brassicaceae family, in particular different species of cabbage (head cabbages, pak choï, pe-tsaï), ground watercress ( Barbarea verna ), etc.
• This insect has become an economically important pest due to its ability to resist many chemical molecules
• Observed in the open field as well as under shelters.

• Sensitive botanical family (s)

• Affected production areas :

• Organs attacked

Symptoms, damage

• Leaves with holes, cut after consumption of the blade by the larvae, considerably reducing yield.
• The entire leaf blade can be eaten in the event of a heavy infestation, only the main veins not being attacked.
• Death of seedlings and plants during particularly severe attacks 

Biology

• Development cycle :
  • Yellow colored eggs are deposited on the surface of the leaves in small groups of 2 to 10. Hatching takes place 3 days after laying in tropical climates.
  • Five larval stages follow each other in 6 to 10 days. The larvae are green in color and are about 1 cm long. They are very voracious and can consume a large part of the foliage.
  • Pupae protected in a cocoon, the pupal stage lasts about 4 days in the tropics.
  • The matings begin the same day of the emergence of the adults which measure approximately 1 cm, this at the time of the sunset. In the next 10 days, females will lay 160-288 eggs.
• Dispersion : This ringworm can disperse over very long distances using air currents.

Protection 

• Use tolerant head cabbage varieties (check with the seed supplier).
• Produce the seedlings in an nursery insect-proof .
• Check the sanitary quality of the plants before and during their introduction into the crop or shelter.
• Install "screen" cultures in inter-rows in order to reduce the pressure of insects, for example tomato plants (1).
• Install  canvases  insect-proof in the nursery, on the rows in the open field when planting, and at the openings of shelters when weather conditions allow.
• Spray a few minutes before nightfall in order to disrupt the flight of adults, mating and egg laying, and possibly knocking the larvae down to the ground.
• Install pheromone traps outside the shelter. (1)
• Favor natural enemies in field crops or under open shelters.
• Use biopesticides (2) taking into account authorized uses.

(1) The selective trapping of males with pheromones makes it possible to limit the parasitic pressure of this insect. This is also used to follow the evolution of the adult ringworm population and therefore to more precisely position the Bt-based treatments. Adult flights are generally followed by a peak of laying one to two weeks later, then d. '' peak larval hatching one to two weeks later (depending on the climate). Catches greater than 10 moths per week and per trap may be followed by a Bt treatment if the larvae are observed in the plot.

(2) The use of a Bacillus thuringiensis (Bt) -based product makes it possible to kill the young larvae of Plutella xylostella within 1 to 2 days after ingestion of the product. As the sun inactivates Bt-based formulations and the young caterpillars are especially active at night, applications at the end of the day will be preferred to ensure good treatment efficiency. Sprinkler irrigation of the plot should not be started after treatment, which could leach the preparation.