Yukie Kajita

Asymmetric larval interactions between introduced and indigenous ladybirds in North America.

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.

Effect of conspecific and heterospecific feces on foraging and oviposition of two predatory ladybirds: Role of fecal cues in predator avoidance

Growing evidence suggests a flow of chemical information from higher to lower trophic levels that affects foraging and oviposition of ‘prey’ in response to potential risks from predators. This was investigated in two species of ladybird predators of aphids, Harmonia axyridis and Propylea japonica. H. axyridis is known to be the stronger intraguild predator and P. japonica to be the more frequent intraguild prey in interactions of these two species. These ladybirds share aphid prey on mugworts, hibiscus, and Italian ryegrasses in fields of northern Japan but largely avoid each other on the same plant. Fecal cues of these ladybird predators were found to contribute in their assessment of predation risk from conspecific and heterospecific competitors in common habitats. Gravid females of H. axyridis reduced rates of feeding and oviposition when exposed to feces of conspecifics, but not when exposed to feces of P. japonica. In contrast, gravid females of P. japonica reduced feeding and oviposition when exposed to feces of both H. axyridis and its own species. Females of both ladybird species exhibited similar behavior in response to water extracts of feces. For P. japonica, the influence of heterospecific feces was greater than that of conspecific feces. Our results demonstrate that feces of ladybirds contain odors that have the potential to deter the feeding and oviposition activities of conspecific as well as heterospecific ladybirds. Such deterrence allows these insects to avoid predation risk. Differences in responses of the two predators are discussed.

Interactions between introduced and native predatory ladybirds (Coleoptera, Coccinellidae) : factors influencing the success of species introductions

Abstract.  1. Why some exotic species invade successfully while others do not is poorly understood. This study focuses on the inability of the exotic predatory ladybird Adalia bipunctata (L.) (Coleoptera, Coccinellidae) to establish and spread in Japan. In particular, this study considers the interactions between A. bipunctata and three native Japanese ladybird species, Harmonia axyridis Pallas, Coccinella septempunctata Linn., and Propylea japonica Mulsant.

Relationships of Body Size, Fecundity, and Invasion Success among Predatory Lady Beetles (Coleoptera: Coccinellidae) Inhabiting Alfalfa Fields

ABSTRACT 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.

A population genetic signature of human releases in an invasive ladybeetle

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.

Alfalfa fields promote high reproductive rate of an invasive predatory lady beetle

Invasive insect species often may attain high fecundity in agricultural habitats, thereby contributing to their establishment in new geographic regions and their displacement of similar native species. Such may be true for predatory lady beetles (Coleoptera: Coccinellidae) that have been introduced to North America in recent decades, raising concerns of adverse impact on native lady beetles. In northern Utah, Coccinella septempunctata L. first appeared in 1991, and is now predominant among lady beetles especially in alfalfa fields. We assessed the suitability of alfalfa fields as breeding habitat for females of C. septempunctata and the native, similarly sized Coccinella transversoguttata richardsoni Brown. The timing and amount of egg production differed significantly between C. septempunctata and C. transversoguttata as populations of aphids increased through spring and early summer. Reproduction by both species conformed to the egg window hypothesis, with populations of the predators producing most eggs before aphid numbers peaked. But consistently among fields and years, females of C. septempunctata produced more eggs, and did so earlier in the spring, than C. transversoguttata females even at low prey density. Furthermore, C. septempunctata females were more successful than females of C. transversoguttata in approaching their maximum body weights and reproductive output as measured in the laboratory under ideal conditions. The strong reproductive success of C. septempunctata may contribute to its displacement of C. transversoguttata in irrigated alfalfa in the generally arid Intermountain West of North America and to its establishment as an abundant species in this region of North America.

Ovarian dynamics and oosorption in two species of predatory lady beetles (Coleoptera: Coccinellidae)

Abstract Ovarian development and oviposition dynamics of two species of lady beetle, Coccinella septempunctata L. and C. transversoguttata richardsoni Brown, are examined in laboratory experiments in which pea aphids (Acyrthosiphon pisum Harris) are provided ad libitum and then removed. Both species respond to prey removal by stabilizing at lower body weights, and by laying progressively fewer and smaller eggs of reduced viability, until oviposition ceases altogether after several days of starvation. Dissections of females after prey removal reveal similar patterns of oosorption in both species. However, C. septempunctata reduces oviposition more rapidly after prey removal than does C. transversoguttata. When prey are again provided, C. septempunctata soon lay as many eggs as previously but C. transversoguttata lay fewer. Females, especially of C. septempunctata, that stop producing and resorb eggs in the absence of prey lay more eggs subsequently than do females that feed continually on prey provided ad libitum and lay eggs throughout the experiment. Thus, although both species are responsive to a rapid change in prey availability, C. septempunctata appears to be especially responsive. Rapid responses to changes in prey availability may contribute to the greater abundance and reproductive success of this introduced species relative to the native C. transversoguttata in western North American alfalfa fields that exhibit widely varying pea aphid densities. However, both species engage in oosorption as a means of reserving resources under poor prey conditions and enhancing future reproductive effort when prey conditions improve.

Reproductive responses of invasive and native predatory lady beetles (Coleoptera: Coccinellidae) to varying prey availability.

ABSTRACT As adults, many predatory insects must adjust to a constantly changing prey environment while balancing between survival and reproduction. Two laboratory experiments were conducted to compare reproductive responses of females of two species of lady beetles, invasive Coccinella septempunctata L. and native C. transversoguttata richardsoni (Brown), in Utah alfalfa fields to varying availability of prey. When both lady beetles were placed immediately on experimental diets after being collected from the field (first experiment) and when they were provided excess prey for 14 d before being placed on experimental diets (second experiment), C. septempunctata produced more but individually smaller eggs than C. transversoguttata. Overall, however, in both experiments, C. septempunctata and C. transversoguttata responded similarly when they consumed pea aphids in varying amounts, by laying fewer and less viable eggs when fewer prey were consumed. In particular, the experiments provided no evidence that C. septempunctata converts pea aphids into eggs at a relatively higher rate than C. transversoguttata under limited prey availability. However, C. septempunctata had greater ability than C. transversoguttata to maintain body weight, even as they were producing eggs at low rates. This suggests that low aphid availability is less stressful for C. septempunctata, perhaps because it has more physiological ability than C. transversoguttata to assimilate pea aphid nutrients at low aphid availability. Such ability might contribute to the numerical dominance of the introduced C. septempunctata in alfalfa fields, which have supported low numbers of aphids in recent years.

Do Defensive Chemicals Facilitate Intraguild Predation and Influence Invasion Success in Ladybird Beetles

Egg predation and cannibalism are believed to be common phenomena among many species of aphidophagous predatory ladybird beetles despite the presence of alkaloid based defensive chemicals in all life stages. We identified defensive chemicals from eggs of three congeneric species, one introduced into North America (Coccinella septempunctata L.), and two native (C. transversoguttata richardsoni Brown, and C. novemnotata Herbst), and examined the effects of ingested defensive chemicals on first instars. Ingested congeneric alkaloids were not toxic to first instars, likely because the three congeners produce the same principal alkaloids, precoccinelline and coccinelline, in similar amounts. First instars of the three congeners accumulated alkaloids ingested through egg cannibalism and congeneric predation. Egg consumption doubled the amount of alkaloids in first instars when they fed on conspecific or congeneric eggs, in comparison to a pea aphid diet. No detrimental effects of ingested congeneric alkaloids on development or survival of first instars were observed among these congeners. Chemical defenses of eggs are therefore not likely to be important in favoring the invasive species, C. septempunctata, in interactions with these native congeneric species. Because the invasive species is the most aggressive predator, having the same types of alkaloids may facilitate disproportionate intraguild predation on native congeners by C. septempunctata thereby potentially enhancing the invasion success of this introduced species.