Impacts of exotic species invasion and refuge habitats on the persistence of native lady beetle species

Background/Question/Methods

The diversity and abundance of native lady beetle (Coccinellidae) species in the US is declining.  This decline has been attributed to the intentional and unintentional introduction of exotic lady beetles, including Coccinella septumpunctata and Harmonia axyridis.  However, most of the studies documenting negative effects of exotic species on the community of native lady beetles have been done in agricultural systems, whereas grasslands are suggested as potential refuge habitats for native species from the pressures of exotics.  Our objectives were 1) to document the impact of the introduction of exotic species on the community of native lady beetle species occurring in Missouri using a historical dataset (1893-present), and 2) to determine whether different grassland habitats provide similar refuges for native species by comparing the community of native and exotic lady beetle species in remnant native tallgrass prairies, restored native tallgrass prairies, and non-native tall fescue fields.   Samples of adult lady beetles in each habitat type were taken monthly throughout the summers of 2010 and 2011.

Results/Conclusions

Using a dataset from museum and personal collections with over one hundred years of haphazardly-collected lady beetles, we documented a change in the composition of the native lady beetle community in Missouri after the introduction and establishment of C. septempunctata and H. axyridis.  Changes in the abundances of two native species, Coccinella novemnotata and Cycloneda munda, are significantly correlated to the introduction of the exotic species, with C. novemnotata declining and C. munda increasing in abundance.  We also found that lady beetle communities in contemporary grassland habitats vary, with the communities found in remnant and restored native tallgrass prairies more similar to each other than to the community found in agricultural tall fescue.  Surprisingly, agricultural tall fescue had a greater abundance and diversity of native lady beetle species than either native prairie habitat, whereas all grassland habitats had similarly low abundances of exotic beetles.  Given the well-documented displacement of native lady beetles from many agricultural habitats, these results support the importance of grassland refuge habitats for the persistence of native lady beetle species in the landscape.