Potential benefits of iridoid glycoside sequestration in Longitarsus melanocephalus (Coleoptera, Chrysomelidae)

Whenever potentially noxious plant compounds are taken up and recycled by herbivorous insects, a protective function of these sequestered compounds is assumed. The flea beetle Longitarsus melanocephalus sequesters iridoid glycosides from its host plant up to a concentration of 2% DW, yet so far it remained unknown whether the insects gain protection from natural enemies by sequestering plant compounds at these comparatively low concentrations. Here we tested whether iridoid glycosides might deter or inhibit various soil and litter-dwelling potential enemies and pathogens. In choice experiments presenting L. melanocephalus pupae together with Tribolium castaneum pupae, the predator Lithobius forficatus was deterred by the iridoid glycoside containing pupae, while Forficula auricularia as well as the nematode Heterorhabditis bacteriophora were not deterred. L. forficatus also avoided artificial baits doted with 2% iridoid glycosides while F. auricularia showed no aversion to iridoid glycosides at these concentrations and H. bacteriophora did not suffer any toxic effect. Of the pathogens tested, the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae were not inhibited in their growth by iridoid glycosides ranging up to 2%. However, an inhibitory effect could be observed against the entomopathogenic bacterium Bacillus thuringiensis, even at the relatively small concentrations that are common in L. melanocephalus. The antibacterial effect might thus be another important selective value of iridoid glycoside sequestration in this species. Immer wenn potentiell abschreckende, pflanzliche Stoffe von Insekten eingelagert werden, liegt die Vermutung nahe, dass sie diese Stoffe zu ihrem eigenen Schutz gebrauchen. Der Flohkäfer Longitarsus melanocephalus lagert bis zu 2% seines Trockengewichts an Iridoidglykosiden ein, welche er aus seiner Futterpflanze Plantago lanceolata aufnimmt. Bisher ist jedoch unklar, ob er durch diese vergleichsweise geringen Konzentrationen irgendeinen Schutz erlangt oder nicht. Um dies zu klären, haben wir einige boden- oder streubewohnende potentielle Feinde und Pathogene daraufhin getestet, ob sie durch Iridoidglykoside gehemmt oder abgeschreckt werden. In Futterwahlversuchen wurden den Prädatoren Lithobius forficatus und Forficula auricularia, sowie den entomopathogenen Nematoden Heterorhabditis bacteriophora Puppen von L. melanocephalus und Tribolium castaneum angeboten. Während L. forficatus die iridoidglykosidhaltigen