ARS | Publication request: Potential causes and consequences of decreased body size in field populations of Coccinella novemnotata

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 21, 2011
Publication Date: January 12, 2012
Citation: Losey, J.E., Perlman, J., Kopco, J., Ramsey, S., Hesler, L.S., Evans, E., Allee, L., Smyth, R. 2012. Potential causes and consequences of decreased body size in field populations of Coccinella novemnotata. Biological Control. 61:98-103. DOI: 10.1016/j.biocontrol.2011.12.009.

Interpretive Summary: The nine-spotted ladybeetle (C9) was historically one of the most common ladybeetles across the US and southern Canada. In the 1980s it became extremely rare in eastern North America and apparently throughout other regions of North America. Reasons for its decline are unknown. However, anecdotal observations of the size of adult C9 beetles collected from field populations in Oregon and South Dakota in 20008 indicated that they were smaller than that of the species historically, and this provoked a series of experiments to determine whether decreased size was due to genetic or to environmental factors. The first study quantified size of field-collected C9 and size of seven-spotted lady beetles, a congeneric introduced species that was collected in the same habitats and has been implicated in causing of C9 decline. The sizes of field-collected individuals of both species were compared with that of historical specimens and individuals reared in the laboratory. Field-collected C9 adults were significantly (20%) smaller than specimens bred in captivity and specimens from collections, indicating that this species still has genetic capacity to reach historically larger sizes but is prevented by environmental factors. To determine if size differences could have been caused by prey limitation, adult C9 size was quantified after immature stages were raised with varying prey availability. Prey availability affected resulting adult sizes across treatments and bracketed the range found in the field. Low-fed larvae were significantly smaller than high-fed larvae. The feeding study data suggested that feral C9 may be eating fewer than five aphids daily, a level associated with over 77% mortality. While the results do not definitively point to any single explanation for C9 decline, they are consistent with expectations for competition between C9 and C7.

Technical Abstract: Coccinella novemnotata, the nine-spotted ladybeetle, was historically one of the most common ladybeetles across the US and southern Canada. In the 1980’s it became extremely rare in Eastern North America and has remained rare, as it is now also throughout North America. In 2008 adult C. novemnotata were collected from field populations in Oregon and South Dakota. The observation that the mean size of these individuals seemed smaller than the mean size of the species historically 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 C. septempunctata, a congeneric introduced species which 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 were 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 indicating that this species still has the genetic capacity to reach historically larger sizes but is prevented from doing so by current environmental factors. To determine if these size differences could have been caused by prey limitation, we quantified adult size of C. novemnotata larvae raised with varying 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. The feeding study data suggest that C. novemnotata found in nature may be eating fewer than five aphids daily, a level that is associated with over 77% mortality. 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.