Date of this Version
Biological Control 61 (2012) 98–103; doi:10.1016/j.biocontrol.2011.12.009
Coccinella novemnotata, the nine-spotted lady beetle, was historically one of the most common lady beetles across the US and southern Canada. In the 1980s it became extremely rare and has remained rare. In 2008 adult C. novemnotata were collected from field populations in Oregon and South Dakota and initial observations suggested that these individuals seemed smaller than the mean size of the species historically. These observations led to a series of experiments to determine if there had been significant decrease in size and if any decrease found was due to a genetic change or to environmental factors. In the first of these studies we quantified the size of C. novemnotata collected in the field and the size of Coccinella septempunctata, a congeneric introduced species that was collected in the same habitats and has been implicated as a cause for C. novemnotata decline. The size of these field-collected individuals of both species was compared with the size of historical specimens and individuals reared in the laboratory. Field-collected C. novemnotata adults were significantly (20%) smaller than specimens bred in captivity and specimens from collections. To determine if prey limitation alone could yield the range of sizes observed we reared larvae across a range of prey availability. There was a significant effect of prey availability and adult sizes across treatments bracketed the range we found in the field. Low fed larvae are significantly smaller than high fed larvae. While these results do not definitively point to any single explanation for the decline of this species they are consistent with expectations for competition between C. novemnotata and C. septempunctata.