The evolution of chemical defenses along invasion routes: Harmonia axyridis Pallas (Coccinellidae: Coleoptera) as a case study

Abstract

The evolution of increased competitive ability (EICA) hypothesis (Blossey &Nötzold,1995) postulates that escaping from coevolved enemies increases invaders fitness byenergy reallocation from defenses and immunity to growth and reproduction. In thiscontext, we evaluated the evidence of evolutionary change in invasive populations ofHarmonia axyridis Pallas (Coccinellidae: Coleoptera). We measured egg defenses-cocktail of hydrocarbons on the egg's surface flagging egg toxicity and the concentration of the main alkaloid harmonine-in individuals from three populations alongthe invasion route (Japan: native, United States: introduced more than 30 years ago,South Africa: introduced in the early 2000s) in a common garden experiment. Ourresults support the EICA hypothesis: We found changes along the invasion route inthe profiles of the hydrocarbons coating the eggs' surface and a decrease in the concentration of harmonine in eggs from the most recent invasive South African population compared to the long established in the United States and the native Japaneseones.