Objective  Effects of Chenopodium ambrosioides (L.) essential oil on the exoskeleton structure and the insecticide permeation of Plutella xylostella (Linn.) were studied to determine if the insecticidal efficacy on the moth could be improved by treatment with the essential oil.

  Method  Changes on the exoskeleton ultrastructure of diamondback moth (DBM, P. xylostella) larvae upon treatment of the wormseed essential oil (WEO) or Azone (laurocapram) were examined under a scanning electron microscope. The permeabilities of subsequent separate application of 3 insecticides into the larvae by percutaneous absorption and penetration were determined by GC-MS.

  Result  Using distilled water as control and acetone for comparison, WEO or Azone treatment produced significant alterations on the spines and the epicuticle waxy layer structure of DBM. The hair-like piles became sparse, the cuticle surface smoothed, and the lower part of spines folded or cracked. Prolonged treatment significantly increased the penetration of the 3 subsequently applied insecticides through DBM exoskeleton. In 24h, the transdermal absorption rate of fipronil had reached 99.34%, which was significantly higher than control（55.12%）.

  Conclusion   The enhancing effect of WEO pretreatment on the transdermal penetration of subsequently applied insecticide on DBM was similar to that of Azone. The enhancement on fipronil penetration was also found on trichlorfon and chlorpyrifos permeation. Similar results were, therefore, expected for the cases involving other organophosphorus, phenylpyrazole, and thiophosphate insecticides.