Aspects of the ecology of Scolypopa australis (Walker) (Homoptera:Ricaniidae) and its parasite Centrodora scolypopae (Valentine) (Hymenoptera: Aphelinidae)

The ecology, population dynamics and interaction, of natural populations of the univoltine passion vine hopper, Scolypopa australis Walker, and its egg parasite, Centrodora scolypopae Valentine, were studied in a bracken and blackberry filled gully near Hamilton, New Zealand, from 1980-85. The S. austral is egg population was estimated using a height frequency vegetation sampling method and the nymphal and adult stages by direct counting. The S. australis nymphal populations ranged from 200-400/m² in the first three generations studied, then fell to below 100/m² after a cold summer in 1982-83. S. australis overwintered as eggs laid mainly in blackberry and bracken stems. Parasitism by C. scolypopae was found to be the main cause of egg mortality, followed in importance by fungae, then stem boring insects. Egg mortality varied with height above ground and increased over winter. Host plant had a strong influence on egg survival, mainly because eggs were laid at greater depths in blackberry than in bracken, and thus were less susceptible to parasitism. Rate of S. austraiis nymphal development differed markedly between years and aspects in response to temperatures experienced. It was slower on blackberry than bracken. Time specific life table analysis showed that parasitism by C. scolypopae was the key factor determining fluctuations in the population of S. australis. The next most important factors were variation in natality and adult loss. Neonate loss was the largest cause of mortality and nymphal loss the smallest. No density dependent mortalities were found. The developmental stages of C. scolypopae are described. The larval instars all show a reduction in morphological differentiation when compared with other Aphelinidae. The first instar retains the egg chorion and lacks visible mouthparts, segmentation or tracheae. The third instar has four pairs of spiracles. Adult nutrition, internal and external reproductive systems and mating behaviour are described. Although C. scolypopae females were shown to discriminate between parasitised and nonparasitised host eggs, superparasitism was common in years of high percentage parasitism of host eggs. The progeny of the first oviposition is dominant and eliminates the younger individuals by physiological suppression. The low temperature threshold for S. australis egg eclosion was 10°C and the thermal requirement for completion of the life cycle was estimated between 1500-1900 day degrees. The low temperature threshold for C. scolypopae varied with the developmental stage. Egg eclosion required at least 10°C while 14•5°C was needed for the initiation of pupation and adult emergence. C. scolypopae was demonstrated to have a facultative diapause in the prepupal stage, that is influenced by both light and temperature. Life table analysis shows that mortality of overwintering larvae, caused by fungae, twig loss and stem boring insects, was the key factor in the population dynamics of C. scolypopae. The 'area of discovery' of C. scolypopae was related to both host and searching female density. The mutual interference constant was approximately 1, indicating high levels of interference between searching females. Parasite density was highly dependent on the number of available host eggs. The partial refuge of S. australis eggs in blackberry would increase the stability of the host-parasite interaction.