Spatial proximity between two host plant species influences oviposition and larval distribution in a leaf beetle

Everything else being equal, insect herbivores can be expected to oviposit on host plants that provide the qualitatively and quantitatively best food for larvae. However, the selection of a plant for oviposition may be influenced by such ecological factors as natural enemies, host distribution, host patch size or host patch density. We performed a field study to test whether spatial proximity between two host plant species influences the oviposition patterns and larval distribution of the alpine leaf beetle <i>Oreina elongata</i>. In the population studied, <i>O. elongata</i> oviposits and feeds on two host plants, that belong to the same family (Asteraceae): <i>Adenostyles alliariae</i> and <i>Cirsiumspinosissimum</i>. The first species contains pyrrolizidine alkaloids that are sequestered by the beetle as a chemical defence, whereas the second plant does not contain any alkaloids but has hairy and spiny leaves that might give some mechanical protection to beetle larvae. <br> During two consecutive summers, we quantified oviposition and larval distribution on randomly chosen <i>C. spinosissimum</i> that grew spatially isolated from <i>A. alliariae</i>, on <i>C. spinosissimum</i> that grew in leaf contact with <i>A. alliariae</i> and on <i>A. alliariae</i> that grew in leaf contact with <i>C. spinosissimum</i> (isolated <i>A. alliariae</i> was not considered, because it is rare in the study population). In both years, more eggs were laid on <i>C. spinosissimum</i> than on <i>A. alliariae</i> and more on those <i>C. spinosissimum</i> that were growing close to <i>A. alliariae</i> than on those growing isolated. Large numbers of larvae moved from <i>C. spinosissimum</i> to <i>A. alliariae</i> during the season. Patch size did not influence egg and larval numbers. Eggs survived better on <i>C. spinosissimum</i> than on <i>A. alliariae</i> in the field. The data suggest that <i>C. spinosissimum</i> may provide eggs with better protection against stormy weather. In a separate study of the same population, we found that larval performance was better on <i>A. alliariae</i> than on <i>C. spinosissimum</i>. Our present data suggest that <i>O. elongata</i> preferentially oviposits on plants of the species that maximizes egg survival and that grow in close proximity to plants of the species that provides better food and chemical defence.