The 12-spotted ladybird beetle (Coleomegilla maculata), nicknamed "C-mac", is an important source of natural mortality to Colorado potato beetle eggs and small larvae in the US, and these predators may have the potential to contribute significantly to the management of this pest in Irish potato. However, under current management practices, overwintered Colorado potato beetle populations are typically so high that the naturally-occurring predation levels fail to prevent the development of damaging larval populations. If the size of the overwintered Colorado potato beetle population can be reduced using cultural practices (i.e., through crop rotation), the effectiveness of natural enemies may be increased. Additionally, a better understanding of the biology and ecology of predators such as C-mac is needed to identify potential cultural tactics (e.g., crop rotation) that would conserve and elevate their populations and enable them to have a practical impact on Colorado potato beetle management. Recently, we have investigated the basic ecology of C-mac in the potato-producing region of eastern North Carolina with the intent of identifying a specific crop rotation, mosaic or sequence that would lead to elevated C-mac populations in potato fields.
Our surveys indicated that C-mac populations were most abundant in wheat and corn fields and that these crops were the only significant sites for C-mac reproduction; C-mac adults were less abundant and reproduction was extremely limited in potatoes. During the period between corn harvest and the aggregation of C-mac at overwintering sites, C-mac were difficult to find. However, significantly more C-mac adults colonized overwintering habitats (edges of forests or hedgerows of trees) in October that were adjacent to harvested corn fields rather than soybean fields. Currently in North Carolina, growers rotate corn with a double crop of potatoes and late-planted soybeans, which should position the largest C-mac overwintering populations adjacent to potato fields.
In North Carolina, C-mac populations must be synchronized with the most damaging generation (1st generation) of Colorado potato beetle eggs and small larvae (i.e., late April through early May) to play an important role in significantly reducing populations of this pest. Although, larval populations of C-mac were synchronous with 1st-generation potato beetle eggs and small larvae, they were concentrated in wheat rather than potato fields because most potatoes had not yet emerged when the C-mac adults dispersed from their overwintering sites. The subsequent generation of C-mac tended to aggregate in corn rather than potato in late May and early June.
Perhaps, the major reason why predation does not appear to regulate Colorado potato beetle populations in North Carolina is that the major predator of potato beetle eggs and small larvae, C-mac, primarily colonizes wheat fields.
- Brian Nault and George Kennedy, North Carolina State University
(based on a poster presented at the 1996 Annual Entomological Society of America Meeting in Louisville, KY)
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