Effect of Euphorbia esula on growth and mortality of migratory grasshoppernymphs

摘要

Most herbivores in North America avoid the invasive exotic leafy spurge (Euphorbia esula L.), possibly because E. esula contains high concentrations of terpenoids and condensed tannins. Our objective was to determine if plant parts from undefoliated and previously defoliated E. esula shoots, mixed in different proportions with grass hay, limit growth and increase mortality of nymphs of Melanoplus sanguinipes F. Condensed tannins, a secondary compound, were present in all E. esula plant parts, increased seasonally, and were highest in stems from previously defoliated shoots. For E. esula harvested in July, nymphs presented with leaves from previously defoliated shoots were heavier than those presented with other plant parts, indicating that the high nutritive value of leaves of E. esula in July offset any negative effects associated with secondary compounds. For E. esula harvested in August, increasing proportion of E. esula in the diet reduced nymphal weight, indicating nymphs were sensitive to higher concentrations of secondary compounds when nutritive value of this plant declines in late summer.