Ecophysiological Studies on Harmonia axyridis, Pallas (Coleoptera: Coccinellidae)

Abstract

Harmonia axyridis has been used as an efficient biological control agent worldwide but is causing concern because of adverse impact on native species biodiversity predominantly through intra-guild predation and competition. There is thus a need to understand the ecological, biological and physiological factors that primarily contributed to the accelerated invasiveness and establishment of this species over a wide range of agricultural systems. The main focus of this project was to investigate various life-history traits and their interaction with external environments cues. The first part of this project was designed to investigate various biological aspects of this ladybird under controlled environmental measures and fixed prey availability. First, it demonstrated the sexual body size-dimorphism and highlighted which traits showed greater dimorphism than others between sexes. In addition, the reproductive performance and the association between the body size and the life-time fecundity, egg hatchability and non-viable eggs was investigated. Several hypotheses were revaluated by this study by direct experiments. The developmental responses of the juveniles were performed under similar controlled measures by monitoring both the stage weight and duration in order to abstract the intra-specific variations among females as well as the significance of each larval stage relative to the final adult size. This species has gained additional advantage due to the presence of the 5th instar reflecting an elevation in fitness and the evolution of the predation and competitive proficiency. Although the genetic and evolutionary characteristics of this ladybird were only covered theoretically, a number of experiments has revealed the intra-specific variability among female body mass, nutrient allocation to reproduction strategies, and growth patterns that might be partially constrained by genetic background. Another important aspect of this project was to emphasize the influence of mating choice and frequency on lifespan, egg hatchability and viability of singly mated, multiply mated as well as virgin females. It was shown that H. axyridis is able to be fertile throughout their life-span from a single mating without significant costs on either longevity or life time fecundity. Nevertheless, multiple mating was found to impose some constraints on the proportion of fertile eggs. These reproductive features may help to understand the success of population establishment and persistence under constantly fluctuating environment. A novel method was described in this study in which the sex of pupae could be easily identified prior to adult emergence based on pupal pattern dimorphism when pupae were reared in the laboratory or directly collected from the field. This simple method is highly effective and could be implemented in several studies without manipulating adults although care must be taken when inspecting the patterns of colouration. Behavioural characteristics of this species that facilitated intra-guild dominancy and interference competitions were also investigated by simple methods. This was performed by testing the avoidance responses mediated by olfactory organs and semio-chemicals towards the presence of tracks of con- and hetero-specific-larvae and adults as well as con-specific adults. The results suggest that H. axyridis has a highly specialized sense organ that functions efficiently in selecting suitable patches and tends to refrain larvae or adults from foraging in contaminated patches by certain species. In the second part of this project, the characteristics of developmental and behavioural traits were also investigated by subjecting successive larval instars to a brief period of food manipulation protocols. This unravelled the compensation patterns, and short- and long-term trade-offs on life-history traits across generations. The results suggest that H. axyridis is able to exhibit phenotypic plasticity in several traits that are important for successful adaptation and colonizing a new habitat. In summary, the results of this study showed that H. axyridis is a highly competitive species owing to their reproductive tactics and unique immediate strategies in regulating reproductive performance and in optimizing off-spring fitness after food restoration. Females were able to optimize their fecundity after one mating and possess evolved defence strategies to avoid being attacked by other species. Nevertheless, the results of this study may contribute in understanding the evolution of life-history traits in this species and could be incorporated in intra- and inter-specific comparative studies. In conclusion, the study predicts that this species will continue to spread and become more widely distributed with more ecological consequences if effective control measures and protocols are not incorporated almost immediately.