Many invasive plants have escaped from their native enemies. EICA hypothesis predicts that invasive populations have evolved to have reduced allocation to anti-enemies defense and increased allocation to growth or reproduction. Several studies support this prediction. However, previous studies compared only constitutive defense (always active defense) between native and introduced populations. In contrast, there is very little quantitative data on the change of induced defense. We predicted that introduced plants decreased constitutive defense compared to natives but maintain the inducible defense. To test these predictions, we compared the levels of the constitutive and induced defense levels of ragweed Ambrosia artemisiifolia in introduced and native populations against their native enemy, ragweed beetle, Ophraella communa.

When we reared the beetle by un-damaged plants, mortality of the beetle reared by introduced plants is lower and dry weight is heavier than the beetles reared by natives. On the other hands, when we reared the beetles by damaged plants, mortality of beetles reared by both introduced and native plants increased and dry weight of the beetles decreased compared to un-damaged plants. These results suggest that introduced ragweed decreased the levels of constitutive defense but they maintain the inducible defense reaction.