Exotic Species are non-native species that are introduced to a new location. Invasive species are exotic species that develop large populations, displace native species and alter the ecosystem. Not all exotic species become invasive species. Why do some exotic species fail to become invasive species? A variety of climate and biological factors are important.
Natural enemies are one important factor. Invasive species often leave their natural enemies (predators, parasitoids and diseases) behind them in the “old country”. In their new location, released from their enemies, populations of the invasive species can “explode”. This population explosion can be dampened by the presence of native predators, parasitoids or diseases in the new location that can keep populations of exotic species in check. The populations exotic insects and their enemies are often in delicate balance and subtle factors can make a difference between an invasive pest and a non-invasive exotic species.
The Immigrant Green Leaf Weevil is an exotic pest that has established in Michigan and Wisconsin. I found these weevils on my recent road trip to Michigan. I also found evidence of predation on the Immigrant Green Leaf Weevil by ants. Ants are common in the trees of Michigan where they collect honeydew from aphids feeding on the leaves. Honeydew is rich in sugar, but low in protein. As predators of other insects, ants can get a more balanced diet of protein (insect meat) to accompany the sugar (honeydew).
The Immigrant Green Leaf Weevil does not move rapidly through the foliage of trees. It does not appear to be well defended against ants that are common. Are ants a major mortality factor for this weevil? The answer would require study of the rates of ant encounters and predation. We commonly think of ants as pests, because they tend aphids that can damage the trees. If those same ants are natural enemies of other tree pests, the relationship becomes more complex.