The larval parasitoid guild of Chrysophtharta agricola (Coleoptera: Chrysomelidae) : host - parasitoid ecological and developmental interactions

Chrysophtharta agricola Chapuis (Coleoptera: Chrysomelidae), an economically important pest of plantation eucalypts in Tasmania, is parasitised in its larval stage by the solitary primary parasitoids Eadya paropsidis Huddleston and Short (Hymenoptera: Braconidae), Paropsivora australis (Macquart) and Ralde striatum gen., sp., nov., (Diptera: Tachinidae: Blondeliini). Hyperparasitoids included in the guild are Perilampus tasmanicus (Cameron) (Perilampidae), Mesochorus sp. (Ichneumonidae) and possibly Meteorus sp. (Braconidae). A taxonomic key was developed for the adults and pupae of all parasitoid species, and brief notes on their biology provided Formal taxonomic descriptions are provided for two species in the guild. One species is a new species of tachinid fly with a new genus (Balde) erected to accommodate it whilst the other species P. australis, is redescribed. The taxonomic status of the third primary parasitoid E. paropsidis was unambiguous and not dealt with further. The lower developmental thresholds for E. paropsidis, P. australis and B. striatum were experimentally determined and estimated at 5.9 °C, 294 DD (egg to pupa), 6.2 °C, 384.6 DD (egg to adult), and 6.3 °C, 344.8 DD (larvae to adult) respectively. The developmental thresholds of all three parasitoids were lower than that of their host, and all three required fewer day-degrees for development than their host. The developmental threshold for Mesochorus sp., a hyperparasitoid of B. striatum, was 9.7 °C, which was higher than B. striatum, the primary host and C. agricola, the ultimate host. The thermal constant of 333.3 DD for Mesochorus sp. was also lower than its primary and much lower than its ultimate host. These thermal thresholds were used to successfully predict the phenology and thermal constraints of all three parasitoids in the field. E. paropsidis and P. australis overwinter as pupae within the plantation in the soil near the tree where the host larva fed, but B. striatum did not. Despite there being no thermal constraint to a second generation within the host's field-active larval period E. paropsidis had an obligate pupal diapause and hence only one genera ion per year. The two tachinid flies both had second generations but second generation B. striatum adults always emerged in the same season even when pupation was very late in the season. P. australis were found to be bet-hedging, with approximately half of the population emerging same-season, and half remaining as pupae in the soil until the following season. E. paropsidis displayed a preference for early instar hosts with attempts at ovipositor insertion being twice as long and only half as successful when attacking large hosts compared to small hosts. Apart from a taking slightly longer to develop from first instars, small hosts offered the best potential for E. paropsidis to maximise fitness via increased body size and reduced host handling time. Both tachinid flies, which have different oviposition strategies, also showed strong host stage preferences. P. australis, which is the larger but less fecund of the two tachinids, shows a preference for fourth instar hosts, and B. striatum prefers second instars, in laboratory and field tests. The stage of host preferentially attacked was found to be constrained by the reproductive biology of the flies, and by interactions between life histories and host defensive behaviour. The developmental strategy of P. australis, which attaches unembryonated macrotype eggs onto the integument of its host, favoured body size over developmental rate, with delays in development from all host stages except the ultimate instar. Male and female P. australis developed at the same rate, and were roughly equal in size. In contrast, B. striatum, which is ovolarviparous and deposits its larvae onto the underside of the host, exhibited a trade-off between body size and developmental rate. Male B. striatum were smaller and developed more rapidly than females. Finally, the effectiveness of all three parasitoids as biocontrol agents in plantations is discussed and practical recommendations for leaf beetle management strategies that may enhance the effectiveness of all three parasitoids in the field are explored.