J. R. Proffitt

The bionomics of Listronotus bonariensis (Coleoptera: Curculionidae) in Canterbury, New Zealand

A study was conducted in 1990–1995 in Canterbury, New Zealand to describe the phenology and seasonal abundance of all stages of the Argentine stem weevil Listronotus bonariensis (Kuschel), a New Zealand pest of Gramineae. Of note were major fluctuations in the densities of all stages from one season to the next which were not explained directly in terms of the availability of Neotyphodium lolii -free ryegrass tillers. The overwintering mortality rates of L. bonariensis were found to be linear and remarkably similar between seasons and locations in the absence of pathogens. This study also showed that care must be taken in the use of existing data to analyse the impact of the recently introduced South American parasitoid wasp Microctonus hyperodae Loan as a biological control agent of L. bonariensis ; errors may exist because of earlier sampling biases. In spite of the confounding effects of seasonal variation in the L. bonariensis population, reduction in the size of the L. bonariensis first summer generation egg and larval peaks coincided with a build up of M. hyperodae .

Population dynamics and larval establishment of Sitona discoideus (Coleoptera: Curculionidae) in New Zealand lucerne.

(1) Sitona discoideus Gyllenhal population data collected from young lucerne (Medicago sativa L.) in Canterbury, New Zealand in 1982-86 were analysed using life tables. Manipulated weevil densities in 1983-84 allowed intra-seasonal and inter-seasonal relationships to be studied. (2) In general, there was a high degree of density-dependent mortality, probably amongst the neonate larvae during their establishment in the root nodules. (3) There was considerable variation in the pattern of larval survival in wet compared with dry seasons. In the latter, density-dependent effects occurred after little cumulative egg laying (750-2400 eggs m-2) compared with higher levels (11 770 m-2) in the wet season. Under wet conditions, there was greater mortality amongst the later instars. (4) Egg laying in the field was modelled on laboratory-derived temperature/rate relationships and field air temperatures. Model predictions suggested that 30-50% of the maximum fecundity was reached, indicating some maladaptation. (5) The range of feeding habits of Sitona spp. is reviewed with regard to the degree of dependence of the genus on root nodules.

Host specificity testing and suitability of a European biotype of the braconid parasitoid Microctonus aethiopoides as a biological control agent against Sitona lepidus (Coleoptera: Curculionidae) in New Zealand

Abstract The European biotype of the parasitoid Microctonus aethiopoides Loan (Hymenoptera: Braconidae) is being considered for release against Sitona lepidus Gyllenhal (Coleoptera: Curculionidae) in New Zealand. Host specificity was evaluated in the laboratory using both endemic and introduced weed biological control curculionid species, with 12 no-choice and three choice experiments carried out comparing the S. lepidus and test weevils. Two further no-choice tests used the Moroccan M. aethiopoides biotype to compare attack rate between European and Moroccan M. aethiopoides, the latter released in 1982 to control the lucerne pest S. discoideus. Across all experiments, total parasitism of S. lepidus was 69% compared with 15% for the test weevils. European M. aethiopoides was able to develop in the native weevils Irenimus aequalis, Nicaeana cervina, Catoptes cuspidatus, Protolobus porculus and Steriphus variabilis with parasitism rates of 13, 28, 2, 7 and 8%, respectively. These levels were significantly less than those in the corresponding S. lepidus control. Total parasitism of I. aequalis and C. cuspidatus increased significantly in the presence of S. lepidus than recorded under no-choice conditions. The presence of European M. aethiopoides caused minor, if any, test weevil mortality prior to the onset of prepupal emergence and there was no significant reproductive suppression in parasitoid-exposed test weevils. Parasitism of the introduced weed control agent R. conicus by European M. aethiopoides was significantly lower (1.1%) compared to the Moroccan biotype (47.5%). Based on these and other experiments, should the European M. aethiopoides be released as a biological control agent of S. lepidus, its ecological impacts are likely to be less severe than those already exhibited by the Moroccan M. aethiopoides.

Multitrophic interaction between the ryegrass Lolium perenne, its endophyte Neotyphodium lolii, the weevil pest Listronotus bonariensis, and its parasitoid Microctonus hyperodae

Abstract Field experiments in Canterbury, New Zealand, investigated four levels of interaction involving the clavicipitaceous endophytic fungi Neotyphodium lolii measured as ppm of the alkaloid peramine in bulked samples of Lolium perenne, the weevil pest Listronotus bonariensis, and its braconid parasitoid Microctonus hyperodae. Weevil numbers were unusually low and the expected inverse relationship between ground densities and peramine concentration was not observed. There was, however, a highly significant tritrophic interaction whereby percent parasitism was inversely related to ppm of peramine. Possible mechanisms for this interaction are discussed and it is suggested that it may have been the result of reduced feeding intensity of L. bonariensis that led to a corresponding reduction in opportunity for the parasitoid to attack.

Linear patterns of dispersal and build up of the introduced parasitoid Microctonus hyperodae (Hymenoptera : Braconidae) in Canterbury, New Zealand

The dispersal of Microctonus hyperodae Loan, an introduced parasitoid of the South American grassland pest Listronotus bonariensis Kuschel, was measured in Canterbury, New Zealand. Considering all directions, the mean annual dispersive increment was 1.9 ± 0.9 km year –1 as measured in the winters of 1993, 1994 and 1995. The parasitoid’s ground distribution suggested that its movement was biased towards the south-west indicating probable wind-borne dispersal. The overall relatively low rate of dispersal was thought to be related to the inhibitory effects of M. hyperodae parasitism on L. bonariensis flight. A generalized linear model fitted to percentage of L. bonariensis parasitized took a simple form, with a quadratic increase in weevil infection, that with time, gradually decreased. The build-up of parasitism at the release site was significantly greater than the rates at the other sites measured in this study ( P c . 140 km 2 .

Listronotus bonariensis (Coleoptera: Curculionidae) flight in Canterbury, New Zealand

Flight by Listronotus bonariensis (Kuschel) in Canterbury, New Zealand varied greatly both within and between seasons; no winter flight was detected between mid-May and mid-August. Predisposing conditions for flight were found to be >19degC, rh –1 . Three flight phases with different characteristics were identified when approximately equal numbers of weevils were trapped. These were between mid-August and early December (overwintered weevils), between early December and mid-March (first summer generation weevils) and mid-March and mid-May (first and second generation weevils). The proportion of individuals with developed flight musculature was c . 8% in the first phase and 30% in the last phase; muscle development was delayed in the mid-phase. First phase weevils with flight muscles were c . 20 times more likely to fly than those in the last. A very weak inverse relationship was observed between developed flight musculature and the presence or absence of eggs. Similarly, there was a very weak inverse relationship between weevil ground densities and the proportion with developed flight muscles. Levels of parasitism by Microctonus hyperodae Loan amongst trapped weevils were far less than that on the ground, indicating that it inhibits flight and may explain the relatively slow dispersal of the parasitoid. It was concluded that the adaptive implications of flight could be linked to escape responses from desiccating microclimates rather than dispersal from crowded habitats.

Curculionoidea (Insecta: Coleoptera) of New Zealand agricultural grassland and lucerne as potential non‐target hosts of the parasitoids Microctonus aethiopoides Loan and Microctonus hyperodae Loan (Hymenoptera: Braconidae)

Abstract The parasitoids Microctonus aethiopoides Loan and Microctonus hyperodae Loan (Hymenoptera: Braconidae) have been introduced into New Zealand to control the adult stage of the forage pests Sitona discoideus and Listronotus bonariensis (Coleoptera: Curculionidae), respectively. These parasitoids have been chosen to examine the environmental impact of introduced biological control agents on native and non‐target species. A survey of the Curculionoidea fauna of pasture, lucerne, and modified native grassland in parts of the southern South Island, Canterbury, and the northern North Island of New Zealand, where these parasitoids are present, was carried out in order to identify Curculionoidea with taxonomic and ecological affinities with the target hosts, and hence, potential non‐target hosts. At a total of 155 sites, 85 species of Curculionoidea were identified, of which 64 (75%) were native species. Three tribes in the subfamily Brachycerinae (broad‐nosed weevils) accounted for 61 species, and 7 trib...

Effect of host condition and photoperiod on the development of Microctonus hyperodae Loan, a parasitoid of the Argentine stem weevil (Listronotus bonariensis (Kuschel))

Abstract Microctonus hyperodae, a euphorine parasitoid of Listronotus bonariensis, was found to enter photoperiodically induced diapause independently of the seasonally determined reproductive condition of its host. Indications were that the critical daylength may be c. 12.3: 11.7 L:D, which represents a photoperiod of 13.6: 10.4 L:D.

A Study of the Effects of Sitona discoideus (Coleoptera: Curculionidae) Larval Feeding on the Growth and Development of Lucerne (Medicago sativa)

(1) Plant growth analysis techniques were used to investigate the continual effects of Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) larval root-feeding on lucerne growth during the 1983-84 season in Canterbury, New Zealand. From data collected at about weekly intervals, continuous time trends and their errors were derived from orthogonal polynomial functions fitted to the natural logarithms of root dry mass and components of foliar growth derived from both affected and unaffected plants. The same approach was taken with total plant mass. (2) The high level of correspondence of estimated S. discoideus-induced yield loss using curves fitted by this technique, to the actual yields measured at mowing for hay, shows that the fitted trends are probably reliable. Indications were that after the first cut for hay, the damaged plants became abruptly dormant with heavier roots than their undamaged counterparts. In general, these plants exhibited many of the characteristics of plants under severe drought stress. (3) It is speculated that the abrupt cessation of lucerne growth observed in this study may somehow be related to the equally abrupt appearances of larval damage thresholds reported elsewhere in the literature.

Use of an electric blanket for winter field collection of Argentine stem weevil, Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae)

Maintenance of large populations of the parasitoid Microctonus hyperodae Loan demanded high numbers of its host the Argentine stem weevil Listronotus bonariensis (Kuschel). To fulfil this requirement under winter conditions a very effective technique using an electric blanket was devised to collect up to 450 weevils day - 1 irrespective of weather conditions. Under warmer spring conditions fewer weevils were trapped and there were indications that the method may have biased catches towards weevils that had been opportunistically parasitised by the Sitona discoideus Gyllenhal parasitoid, Microctonus aethiopoides Loan. Possible reasons for these results are discussed, as is the potential of the method as a surveying tool.