Furocoumarin resistance in the larvae of Phytomyza spondylii (Diptera: Agromyzidae) feeding on Heracleum lanatum is associated with the enzymatic breakdown of 8-methoxypsoralen

The larvae of Phytomyza spondylii feed and produce characteristic tunnels or mines on the leaves of Heracleum lanatum, a plant which is rich in both angular and linear furocoumarins. These molecules, in the presence of near ultraviolet light, photosensitize cells by producing DNA adducts which, unless repaired, arc lethal. Thus, many plants belonging to the Umbelliferaceae are often considered to be resistant to insect foraging because of the presence of furocoumarins. The larvae of Phytomyza spondylii rapidly break down 8-methoxypsoralen into photobiologically inactive compounds. This is an enzymatic reaction requiring the presence of an electron generating and accepting system not unlike that seen with the mixed function microsomal oxidases of mammalian liver microsomes. This reaction may be specific as it was not observable in the fall webworm (Hyphantria cunea) on red alder or in Papilio zelicaon feeding on dill or parsley. It is postulated, therefore, that the presence of this detoxifying system in Phytomyza spondylii is related to the host specificity and distribution of the insect.