HORMONALLY MEDIATED INSECT-PLANT RELATIONSHIPS - ARTHROPOD POPULATIONS ASSOCIATED WITH ECDYSTEROID-CONTAINING PLANT, LEUZEA CARTHAMOIDES (ASTERACEAE)

An extensive zoocenotic study of arthropod populations was performed o n a Siberian plant Leuzea carthamoides, which was recently introduced to Central Europe as a medicinal crop. Because it contains a very high concentration of insect hormones (ecdysteroids) (300-1,000 ppm of 20- hydroxyecdysone equivalents in the leaves), the plant was thought to b e resistant to attack by non-adapted arthropod herbivores (effective c oncentrations 25-100 ppm in insect diet). Two consecutive seasons of a nalysis revealed that, in spite of the high ecdysteroid content, the c rops of L. carthamoides contained a well-established and consolidated arthropod fauna. 126 species of arthropods were observed on these plan ts during 1993 and 1994. Of this number, 74 were feeding on the leaves , and 33 of these could complete their adult development on the plants without apparent difficulties. There were also 52 parasitoid and pred atory arthropod species of secondary importance. The results revealed the following four observations that are relevant to the possible resi stance of L. carthamoides to phytophagous arthropods: (a) the plant is by no means universally resistant to all phytophagous arthropods; (b) the most abundant and, perhaps, most resistant herbivores that can co mplete their life cycle on the plant are from groups in which ecdyster oid action is little known or unknown, e.g., spider mites, collembolan s, thysanopterans, psocopterans or Exopterygota with sucking mouthpart s; (c) Endopterygote insect pests, which are the most sensitive to ecd ysteroid-containing diet, were represented by only a few species and r elatively low abundances; and, (d) all arthropods colonizing this plan t belonged to polyphagous or oligophagous species with a relatively wi de range of different host plants. Composition of arthropod fauna with in plantations of L. carthamoides was similar to that observed in stud ies of the sugar beet (Beta vulgaris), which is also an introduced pla nt. Physiological conditions associated with possible resistance of in sects to dietary ecdysteroids are discussed in terms of the following two points of virtual insensitivity: (a) use of molecules other than t he plant-contained ecdysteroid as the endogenous hormone, and (b) elim ination of the exogenous dietary ecdysteroid by excretion during feedi ng period.