The reproductive seasonality and morphology of Argentine stem weevil Hyperodes bonariensis Kuschel (Coleoptera: Curculionidae) and the effect of host grasses on its pre-reproductive development

Abstract

In order to improve understanding of the phenology of Argentine stem weevil (Hyperodes bonariensis Kuschel), samples were collected at regular intervals from the field and the changes in their reproductive morphology monitored by regular dissection. This demonstrated that adult Argentine stem weevils enter reproductive diapause in early March and remain in this state until early August when reproductive activity resumes. The existence of diapause was further supported by the observed resorption of terminal oöcytes leaving residual β-carotene crystals at the base of the ovarioles, cessation of mating, an increase in body fat, changes in the fatty acid composition and an apparent drop in the titre of juvenile hormone in the haemolymph. Evidence was collected that indicates that photoperiod is the most important factor controlling this seasonality.

The dates of onset and termination of diapause appear highly predictable and independent of local climatic variation. This predictability therefore presents opportunities for improved control by the accurate timing of insecticidal and managerial control approaches. This was substantiated by an observed tenfold reduction in egg and larval infestation in autumn sown. Tama ryegrass drilled so as to coincide emergence with the predicted end of egg laying. Furthermore the diapauses seasonality of Argentine stem weevil renders the pest ideal for phonological modelling. An initial investigation into the subject showed that day-degrees accumulated above a development threshold temperature of 10°C can be used to predict accurately development during the reproductive phase.

Argentine stem weevil flight, monitored daily by a 300mm Johnson and Taylor suction trap and six sticky traps, was found to be largely dependent on prevailing weather conditions. Flight and mating were not interrelated. Furthermore Argentine stem weevil was observed to fly both in the reproductive and diapause state and it can be concluded that flight was of a trivial dispersive nature rather than a migratory response to adverse conditions.

Studies on the interaction between the host grasses and Argentine stem weevil were conducted in both the laboratory and field. These demonstrated that, in general, ryegrass susceptibility to larval attack appeared to be related to the closeness to Lolium multiflorum parentstock. Of the other grasses tested, cocksfoot and Timothy were susceptible, although the former showed some signs of antibiosis. Yorkshire fog and tall fescue were resistant. It is suggested that the hairiness of the former tended to deter oviposition as did the toughness of tall fescue. Larval survival of between 30-40% was estimated to occur between the egg and first instar stage in most grasses. Survival was more variable between the first and second instar stages ranging from 7% to 23%. The less succulent grasses resulted in lower survival rates. From the second instar onwards 80-100% survival rates were estimated in all grasses except cocksfoot.

Through the dissection studies it was found possible to differentiate the males from the females by the forms of their seventh sternites. In general, the reproductive morphology of Argentine stem weevil was similar to that of other Curculionidae. Of particular interest however was the observation that during copulation the evaginated internal sac of the aedeagus was secured in the bursa copulatrix by an internally reinforced lobe. It is proposed that the terms “anchoring lobe” and “anchoring sclerite” be adopted for these structures to eliminate current confusion over the terminology of similar structures in other curculionids.