The reproductive seasonality and flight capability of Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) and its pattern of larval establishment in Canterbury lucerne

Abstract

In order to improve the level of understanding of the reproductive seasonality and flight potential of Silona discoideus Gyllenhal, samples were taken at regular intervals from lucerne (Medicago saliva L.) stands throughout the 1982-83 and 1983-84 seasons. Changes in the weevil's reproductive condition and flight muscle morphology were monitored by dissection. This work confirmed that the univoltine adult weevils entered aestivatory diapause some three to four weeks after the eclosion of the new generation in December-January each year. Flight to aestivation sites remote from the host crop, was preceded by a vigorous feeding phase during which time the indirect flight muscles developed. The existence of aestivatory diapause was further supported by an observed suppression of reproductive development, absence of mating, increase in fat content of the weevils and changes in the proportional representation of several fatty acid constituents of the total fatty acid complement. During aestivation there were also changes in the indirect flight musculature of S. discoideus. Following flight to aestivation sites, the indirect flight muscles atrophied. Subsequently, the muscles regenerated, enabling reinfestation flight to the lucerne to commence.

Aestivatory diapause was found to last some seven to eight weeks. However, in late eclosing weevils, the predetermined time period for diapause development was not always completed before cool winter conditions limited flight muscle regeneration. Hence, throughout this study, some S. discoideus were able to be located at aestivation sites. Laboratory experiments revealed that depleted metabolic reserves in these weevils ultimately made muscle regeneration impossible irrespective of temperature, thus precluding a spring invasion of lucerne by these laggard aestivators.

Generally reproductive activity in adult S. discoideus was found to commence with the successful termination of aestivatory diapause. Oviductal eggs were located in the lateral oviducts from mid March onwards, while egg populations became apparent in the field in mid April.

The importance of the winter months for larval establishment of S. discoideus was determined in Canterbury. Chlorpyrifos applied at 0.6kg a.i. ha⁻¹ on lucerne field plots was found to effectively eliminate adult S. discoideus populations but left their eggs unaffected. A field experiment was thus established whereby populations of reproductive weevils were eliminated at 5-weekly intervals throughout the 1983 egg laying period. The importance of the different egg laying time spans was thus determined by sampling the resultant larval populations in spring. There were found to be two main peliods of S. discoideus larval recruitment: the first in early autumn and the second, more important in spring. The current recommendation to prevent S. discoideus larval damage to lucerne is to spray insecticide against the weevils in late autumn, subsequent to the post-aestivatory reinfestation flights but before the onset of the bulk of their reproductive efforts. The present findings suggest an insecticide spray applied to susceptible stands as late as mid August could usefully reduce spring larval populations in most years.

Analysis of head capsule widths of both field-collected and laboratory-hatched s. discoideus larvae confirmed that this species possesses five larval instars. Probit analysis allowed for more accurate statistical definition of the head capsule size range of each particular instar, despite some size overlap between successive instars. Such accurate definition of instar sizes subsequently pelmitted detection of both intra- and interseasonal variations in the within-instar head capsule sizes of S. discoideus larvae duling each of the 1982-83, 1983-84 and 1984-85 seasons. Such variations were attdbuted to intra- and interseasonal variations in the rhizobial roqt nodule availability/accessibility to the larvae, i.e., changes in food availability.