Fates of Rare Species under Siege from Invasion: Persistence of Coccinella novemnotata Herbst in Western North America alongside an Invasive Congener

Abstract

Species invading new geographic regions may threaten continued existence of similar, indigenous relatives, particularly those species whose rarity may reflect an already tenuous existence. The spectacular colonization of North America in recent decades by Coccinella septempunctata L. has generated widespread concern over potentially adverse effects on population viability of native species of ladybird beetles (Coccinellidae). Coccinella novemnotata Herbst in particular has been hypothesized to be at great risk, as this species apparently dwindled in numbers across much of North America during the twentieth century. Here results of sampling diverse habitats over three decades are examined to address the fate of C. novemnotata in the intermountain region of western North America following the establishment of C. septempunctata during the 1990s. Alfalfa fields have served as a major habitat for C. novemnotata in the intermountain west. Sweep sampling in the late 1980s and early 1990s demonstated that C. novemnotata and C. septempunctata were both rare members of the alfalfa lady beetle community. Subsequent sampling in alfalfa over the next two decades revealed that populations of C. septempunctata increased greatly, while populations of C. novemnotata remained low but persistent. Similarly, modest numbers of C. novemnotata were found continuing to persist, often alongside large numbers of C. septempunctata, in a variety of other habitats, including sagebrush steppe and weed-infested rangeland and riparian sites. Morphological comparison of these individuals of C. novemnotata with museum specimens collected throughout the twentieth century revealed no significant difference in mean body size between these recently and previously collected individuals, nor any significant long-term decrease in body size following the arrival of C. septempunctata, as might reflect increasing food limitation for larval C. novemnotata. Collectively these results indicate that even in the face of invasion by a dominant competitor, C. novemnotata has maintained an ecological foothold in the intermountain west of North America, occurring in diverse habitats including both those supporting low and high numbers of the invader. The availability of these diverse habitats across a heterogeneous landscape may promote persistence less tenuous, even as confronted with invasion, than the low abundance of C. novemnotata alone might suggest.

Interactions Among Native and Non-Native Predatory Coccinellidae Influence Biological Control and Biodiversity

“…Is a reduction in the biodiversity of predatory coccinellids feeding on aphids given these environmental conditions, to be expected? Natural and/or seminatural environments could be important refuges for native species ( Bahlai et al 2015 , Evans 2017 ). For example, Coccinella novemnotata has been the focus of several papers due to its decline in North America (e.g., Harmon et al 2007 , Tumminello et al 2015 ).…”

Comparison of native and non‐native predator consumption rates and prey avoidance behavior in North America and Europe

“…Likewise, A. bipunctata showed stronger declines in the presence of H. axyridis compared to other native lady beetle species, in Europe (Roy et al., 2012). In contrast, C. novemnotata maintained an ecological foothold in the face of invasion by the equally sized C. septempunctata (Evans, 2017). Thus, we suggest that lady beetle body size might be a good predictor for their invasion potential in areas beyond their native ranges.…”

Exotic ladybirds for biological control of herbivorous insects – a review

“…The net result is that by the early 21st century, C. septempunctata and H. axyridis had become two of the most common and most widely distributed species of Coccinellidae throughout the USA. As in Europe, C. septempunctata is especially abundant in the USA in agricultural habitats (e.g., Evans, 2000Evans, , 2017Finlayson et al, 2008;Hesler & Kieckhefer, 2008). So too is H. axyridis, which in addition occurs in the Nearctic as well as the Palearctic in large numbers on trees, including in urban settings (LaMana & Miller, 1996;Hon ek et al, 2016).…”

“…As in Europe, C. septempunctata is especially abundant in the USA in agricultural habitats (e.g., Evans, 2000, 2017; Finlayson et al, 2008; Hesler & Kieckhefer, 2008). So too is H. axyridis , which in addition occurs in the Nearctic as well as the Palearctic in large numbers on trees, including in urban settings (LaMana & Miller, 1996; Honěk et al, 2016).…”

“…This suggests that these native species may persist regionally but no longer colonize spring alfalfa fields in large numbers as aphid densities are kept in check in these fields by C. septempunctata . Mobile native ladybirds, which disperse as adults frequently and widely across landscapes in tracking ephemeral populations of their prey, instead may persist by colonizing and occupying other habitats (Evans, 2017). During the summer, when C. septempunctata numbers often are low and aphid populations may reach high densities in alfalfa fields of northern Utah, native ladybirds at times outnumber the introduced species.…”