Sequestration of cucurbitacins by Diabrotica balteata larvae and its effects against the attack of natural enemies

Cucurbitacins are bitter tetracyclic triterpenoids produced by Cucurbitaceae plants and protect them against several insects and pathogens. Cucurbitacins are known to be sequestered by adult Diabroticina beetles and it has been speculated that the larvae too sequester them as a chemical defense. In order to test this, we study the ability of Diabrotica balteata larvae to sequester cucurbitacins and whether this protects them against natural enemies. We first profiled cucurbitacins in shoots and roots of four commercial varieties of cucumber (Cucumis sativus). D. balteata larvae were then fed on these plants and we subsequently measured cucurbitacins in the insects. We found qualitative and quantitative variation in cucurbitacin content in the different plant and larval tissues, with surprisingly low levels in the roots. The results show that D. balteata larvae indeed actively sequester and transform cucurbitacins. To study if the sequestered cucurbitacins provide protection against natural enemies, we fed D. balteata larvae on plants with or without cucurbitacins. The larvae were then exposed to the entomopathogenic nematode Heterorhabditis bacteriophora or to one of the predatory insects Atheta coriaria, Orius laevigatus or Chrysoperla carnea. Surprisingly, larval mortality was similar independent of the cucurbitacin content in the food source or insect tissues. A good understanding of this complex chemical defense strategy may provide new insight into the ecological interactions between plants, Diabroticina herbivores and their natural enemies, with possible implications for the development of sustainable pest control strategies