Understanding the mechanisms that result in the success of introduced species will contribute to predicting future invasions and managing invaded systems. We examined interactions between larvae of two predatory ladybird species recently introduced to North America, Coccinella septempunctata (CS) and Harmonia axyridis (HA), and two indigenous ladybirds, Coccinella transversoguttata (CT) and Hippodamia convergens (HC). By pairing young and old larvae in the laboratory at low and high levels of aphid availability, we assessed the degree of asymmetry in intraguild predation (IGP), the strength of competitive effects on growth and development of larvae escaping predation, and the nature of attack and escape behavior among the species. Interactions were generally asymmetric, with larvae of introduced species acting most frequently as intraguild predators and larvae of indigenous species serving most frequently as intraguild prey (the two Coccinella spp., however, preyed on each other at similar rates). Because they were especially aggressive and because other larvae were least successful in escaping their attacks, larvae of HA had stronger negative effects on larvae of the two indigenous species than did larvae of CS. Such negative effects, expressed most strongly when aphid availability was low, were especially adverse for the smaller of the two indigenous species, HC. In general, older larvae interacted with each other more strongly than young larvae did, and older larvae had especially strong negative effects on young larvae when interactions occurred between age classes. Our results suggest that HA more than CS may represent a threat to indigenous ladybirds as an intraguild predator, and that IGP in turn may play a stronger role for HA than for C. septempunctata in promoting the successful invasion of North America.

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Egg predation and cannibalism are common phenomena in predatory ladybirds despite the presence of defensive alkaloids. Consumption of heterospecific eggs negatively affects survivorship and development; however, intraspecific variation in quantities of alkaloids and post-ingestion responses to con- and hetero-specific alkaloids, are not well understood. We examined variation in the quantity of alkaloids in eggs of Harmonia axyridis (Pallas), Coccinella septempunctata L., and Hippodamia convergens (Guérin) using gas chromatography-mass spectrometry, and show a link between heterospecific alkaloids and their toxicity and/or costs by feeding high and low alkaloid eggs to first instar H. axyridis and C. septempunctata. The repeatability of alkaloid measurements in eggs in an egg cluster was high; however, the amount of alkaloids varied significantly between egg clutches within and among females. This variation affected egg consumption by C. septempunctata when fed H. axyridis eggs. Harmonia axyridis accumulated their own alkaloid by cannibalism and synthesized it de novo, but C. septempunctata lost some portion of the consumed conspecific alkaloids. Both species lost most of the consumed heterospecific alkaloids, but C. septempunctata died within 3 days. Most H. axyridis survived to the second instar, but C. septempunctata alkaloids led to a significant reduction in weight gain compared to an aphid control. In addition, ingestion of high alkaloid C. septempunctata extended development of H. axyridis compared to the aphid control or conspecific eggs. Harmonia axyridis had greater abilities to process ingested con- and hetero-specific alkaloids compared with C. septempunctata, which may, in part, explain their interspecific interactions in nature.

Reproductive potentials were compared for a large invasive lady beetle, sevenspotted lady beetle, Coccinella septempunctata L. (Coleoptera: Coccinellidae), and a set of four smaller native North American species that have been displaced from alfalfa, Medicago sativa L., fields in Utah. The invader rapidly attained predominance in these fields during years when aphid populations were high. In a laboratory experiment, females were provided with excess numbers of their principal prey in these fields, pea aphids, Acyrthosiphon pisum (Harris). Among the five species, both the number and total volume of eggs (number × mean egg volume) produced per day increased with increasing female size and were greatest for C. septempunctata. Rates of reproduction also increased with increasing female size within species. Similarly sized females of C. septempunctata and transverse lady beetle, Coccinella transversoguttata richardsoni (Brown), laid similar total volumes of eggs per day, but females of the invasive species had more ovarioles and laid larger numbers of individually smaller eggs. In summary, when feeding on abundant prey in a habitat in which it predominates, the invasive C. septempunctata gains reproductive advantage over native, North American lady beetles, from its large body size and its investment in many small eggs. The results support the generalization that high fecundity linked with large body size may often be one important factor that promotes the dominance of introduced species over native competitors in resource-rich environments.

Biological invasions have been accelerated by a variety of human activities. Propagule pressure, the number of introduced individuals and independent introductions, is probably to be influenced by these human activities and may be an important factor for successful range expansion in new environments. We tested whether the current distribution of the predatory ladybeetle Coccinella septempunctata in the introduced range (USA) is the result of multiple historical human introductions or natural range expansion from the first established populations in the USA. To test this hypothesis, we compared historical records of propagule size, propagule number, specific introduction locations and the date of each introduction, with estimates of genetic variation in mitochondrial DNA (cytochrome oxidase I). Our results indicated that genetic diversity in the introduced range was positively correlated with historical records of propagule size and number and negatively correlated with distance to nearest introduction point, suggesting that multiple human releases were successful. Higher genetic diversity in populations found near introduction points suggest that initial founder effects were limited, but lower genetic diversity found farther from introduction points is probably the result of serial founder effects during secondary range expansion. These results suggest that the current distribution of C. septempunctata in the introduced range is the result of a combination of human releases and short-range expansion from multiple established populations in the introduced range.