Abrupt Change in Mix of Ladybug Species
The assortment of ladybug species living in North America has shifted dramatically in the last 20 years. Of the 179 species introduced to the continent, many deliberately to control aphids on farm crops, 27 species have permanently settled in United States and Canada. At the same time, several native North American species have become markedly less common. Suddenly scarce, for instance, are the native two-spotted lady beetle (Adalia bipunctata) and the nine-spotted lady beetle (Coccinella novemnotata).
A review of 36 studies on lady beetles finds that prior to 1985, native species made up 95% of all ladybugs collected. Since then, only two-thirds of trapped ladybugs are indigenous species. An abrupt transformation of ladybug populations occurred in 1987, and has persisted ever since. Most often the local ladybugs are replaced by two of the exotic insects: seven-spotted lady beetle (Coccinella septempunctata) and multicolored Asian lady beetle (Harmonia axyridis).
There is little overall pattern, however, in the changes occurring to ladybug diversity that can collectively be gleaned from the research. Density of ladybugs in some cases dropped when introduced species became established, but in other cases ladybug density actually increased.
In some studies, species diversity increased, while in others, the number of species in an area decreased. The density of native species has declined overall, though, by 16%, but again results among studies vary.
No general conclusions, then, can be made on how introduced ladybugs have affected native ones. Despite the lack of general trends measured, scientists suspect that introduced ladybugs, particularly the two most prevalent species, have precipitated huge declines of North American ladybugs.
Jason P. Harmon, Erin Stephens and John Losey. 2007. The decline of native coccinellids (Coleoptera: Coccinellidae) in the United States and Canada. Journal of Insect Conservation. 11(1): 85-94.