Search

We report the complete mitogenome of Sitona obsoletus, an agricultural pest in New Zealand and some European countries. Like other Sitona mitogenomes, the 6 tRNA gene box is ordered RNSAEF, supporting the hypothesis that this signature is common to, and potentially diagnostic, of this genus. The Trojan Female Technique (TFT) is a genetic pest control strategy that exploits mitochondrial DNA alleles that affect male, but not female fertility and fitness. The complete mitogenome is an essential first step in exploring the utility of TFT for the control of S. obsoletus.

The clover root weevil Sitona lepidus Gyllenhal (Coleoptera: Curculionidae) became an economically important pasture pest in New Zealand shortly after it was discovered in the Waikato region in 1996. A classical biological programme was initiated and an Irish biotype of Microctonus aethiopoides Loan (Hymenoptera: Braconidae) was released at four sites in the North Island in late summer 2006. These sites in Waikato, Hawke's Bay and Manawatu (two sites) regions were monitored monthly and parasitoid establishment confirmed at all sites within four months. In the winter of 2007, parasitism exceeded 70%. A widespread North Island drought in summer 2008 had a severe impact on S. lepidus populations at the Waikato and the two Manawatu release sites, resulting in parasitism below detection levels in the following summer. However, populations recovered by autumn. Within three years at the Hawke's Bay site, M. aethiopoides appears to be suppressing S. lepidus populations and has dispersed naturally over 60 km. [ABSTRACT FROM AUTHOR]

The clover root weevil, Sitona lepidus Gyll. is a pest of clovers ( Trifolium spp .), particularly white clover ( T. repens). Larva feeding severely impairs the capacity of white clover to fix atmospheric nitrogen, by attacking the nitrogen-fixing root nodules. Development of the egg stage was studied at six different constant temperatures ranging from 9 to 30°C to improve the basis for phenological forecasts. Development occurred over the entire range of temperatures, although hatching percentage at 30°C was only 24.4% compared to 90.1% at 28.5°C. Developmental time decreased with increase in temperature from 9 to 28.5°C but increased markedly at 30°C. At 28.5°C, the embryo development period was the shortest (10.35 days); whereas at 9°C, this parameter was the longest (55.08 days). Using linear regression, the lower threshold temperature was calculated 4.38°C and the thermal constant was 236.45 degree-days. The relationship between constant temperature and developmental rate was evaluated using nine models. The suitability of the models was evaluated based on eight indicators ( R, RSS, R, AIC, BIC, AICC, W, and Z). Of the nonlinear models, the Logan-6, Sharpe and DeMichele, and Lactin models were the most accurate at calculating t of 27, 27.50, and 28°C, respectively. The results suggest that of the three models (Lactin, Briere, and Hilbert and Logan), that estimated critical temperatures, ( t, t, and t), Lactin gave the most suitable fit of data. This response of S. lepidus to temperature can be used for developing phonological models to predict the timing of egg hatch which are important for management programs targeting the weevil. [ABSTRACT FROM AUTHOR]

Abstract: Arbuscular mycorrhizal fungi (AMF) are known to reduce the growth of generalist root-feeding insects, but whether the same is true for a specialist insect is unknown. White clover (Trifolium repens) was inoculated with the AM fungi Glomus fasciculatum and Glomus mosseae individually and in combination, and larvae of the clover root weevil (Sitona lepidus) reared on mycorrhizal and non-mycorrhizal plants. On emergence, adult weevils were weighed and the percentage of larvae surviving to adulthood was calculated for each treatment. Larval survival to adulthood was increased by both species of fungi, but weight was unaffected. Larval feeding reduced foliar biomass, but had no effect when two fungi colonized the root system. Although larval survival was greatest in the dual fungal treatment, the proportion of grazed root nodules was lower, suggesting that AMF may improve root quality for the herbivore. Root feeding caused an increase in arbuscular colonization in the dual fungal treatment, and this may have enabled plants to tolerate herbivory, through enhanced mycorrhizal benefit. We conclude that a specialist root feeder is less affected by the presence of AMF than are generalist species. However, AMF enable a plant to tolerate the effects of root loss, and this is dependent on the number of mycorrhizal species in the root system. [ABSTRACT FROM AUTHOR]

The effects of larval herbivory by clover root weevil (CRW, Sitona lepidus) on seven white clover (Trifolium repens) and two red clover (Trifolium pratense) progenies from plants showing tolerance of CRW in field trials were tested in two controlled glasshouse experiments. CRW larvae recovered from red clover plants were, on average, fewer (6.3 larvae/g root dry weight) and shorter (5.7 mm/g root dry weight) than those from white clover (16.7 larvae, 7.1 mm length).This confirmed previous findings that red clover is an inferior host for this pest. In the first experiment, progeny of four of the white clover selections for CRW tolerance showed high tolerance (shoot dry weights -2.9% to +3.1% different from their respective weevil-free controls) compared with a 13-24% shoot weight loss for two white clover cultivars. These results confirm field trial observations that there is variation for CRW tolerance in white clover and also that CRW tolerance is hereditable in at least some populations. The second experiment, on a smaller subset of white clovers, showed little effect of CRW on either the tolerant clover selections or the cultivar controls. An index of larval feeding pressure (larval numbers×larval projected areas/root dry weight) gave much lower values for the second experiment (12-78 mm2/g) than for the first (69-102 mm2/g). The absence of negative effects of CRW on the cultivar controls in experiment 2 was assumed to result from low CRW feeding pressure, allowing compensatory growth responses. A small but consistent increase in shoot: root ratio in the +CRW treatment in experiment 1 provided supporting evidence for increased allocation to shoots as a mechanism for plants to reduce the impact of root herbivory. [ABSTRACT FROM AUTHOR]

The egg, larval, pupal and adult abundance of the clover root weevil Sitona lepidus Gyllenhal (Coleoptera: Curculionidae) was monitored at three sites for the first ten years following the discovery of this exotic pest in the Waikato region of New Zealand. The species went through an initial boom and bust cycle at two sites, with populations reaching up to 1800 larvae m-2. Thereafter, winter larval populations were relatively stable, ranging between 450-750m-2. Unlike in the Northern Hemisphere, S. lepidus was found to have two generations a year in the Waikato region of New Zealand. Pasture white clover content at the time of peak adult numbers was positively related to the subsequent peak larval populations for each generation. The factors contributing to the emergence of S. lepidus as one of the most important pasture pests in New Zealand are discussed. [ABSTRACT FROM AUTHOR]

The effects of root herbivory at five densities of Sitona lepidus larvae (overall means between 4 and 333 larvae m-2) were assessed over 2 years on newly established perennial ryegrass-white clover swards in a small plot trial. Initial larval establishment in autumn 2003 was positively related to clover content in plots, and there was no significant impact on clover herbage yield in the first year. Nodule damage in winter 2003 increased with larval density, and results suggested an overcompensatory response in nodule production. A 34-35% reduction in clover yield between highest and lowest S. lepidus densities was recorded for both cultivars in the second year, with greatest losses in spring 2004. This coincided with reductions in clover root and stolon weights. Plant parasitic nematodes and grass grub larvae were most abundant in the plots with lowest weevil numbers. These results confirm field observations that S. lepidus is a major pest of pastures. [ABSTRACT FROM AUTHOR]

1 Root-feeding insects are now considered to play a greater role in ecosystem processes than previously thought, yet little is known about their specific interactions with host plants compared with above-ground insect herbivores. Methodological difficulties associated with studying these insects in the soil, together with the lack of empirical and theoretical frameworks, have conventionally hindered progress in this area. 2 This paper reviews recent empirical and theoretical developments that have been adopted for studying root-feeding insects, focusing on the non-invasive techniques of X-ray tomography and acoustic field detection and how these can be integrated with new mathematical modelling approaches. 3 X-ray tomography has been used for studying the movements of several insects within the soil and has helped to characterize the host plant location behaviour of the clover root weevil, Sitona lepidus. Acoustic detection of soil insects has been used in various managed systems, ranging from nursery containers to citrus groves. 4 Mathematical modelling plays a complementary role for investigating root-feeding insects, illustrated by a number of published models. A model is presented for the movement of S. lepidus in the soil, which suggests that these insects undergo Lévy movements, similar to those recently demonstrated for above-ground organisms. 5 The future directions and challenges for investigating root-feeding insects are discussed in the context of the wider ecosystem, incorporating both above and below-ground organisms. [ABSTRACT FROM AUTHOR]

Red clover ( Trifolium pratense L.) is becoming increasingly important in grassland systems because of its high productivity, protein content and nitrogen-fixing ability, but its use has been constrained by losses due to pests and diseases, and this contributed to the decline of red clover usage allowing white clover ( T. repens L.) to become the dominant legume of UK grasslands. One of the major pests of red clover is the clover root weevil ( Sitona lepidus Gyllenhal, Coleoptera, Curculionidae) which attacks both the shoots and roots, particularly the N-fixing root nodules, of clover plants. This current work investigates the feeding preferences of S. lepidus with respect to 11 varieties (Pawera, Kuhn, Astra, Norseman, Norseman low, Norseman high, Marcom, Merviot, Milvus, Britta, Sabtoron) and 5 lines (AA30, AA31, AA4493, AA4494 and AA4495) of red clover in order to identify relationships between the variation in attractiveness of different red clover varieties for both adult and larval stages of the weevil. Of those tested Norseman high showed potential resistance, being less favoured by both adults and larvae of the weevil. This may be attributed to the potentially high phyto-oestrogen levels in this variety. [ABSTRACT FROM AUTHOR]

Abstract: White clover (Trifolium repens) is an important pasture legume but is often difficult to sustain in a mixed sward because, among other things, of the damage to roots caused by the soil-dwelling larval stages of S. lepidus. Locating the root nodules on the white clover roots is crucial for the survival of the newly hatched larvae. This paper presents a numerical model to simulate the movement of newly hatched S. lepidus larvae towards the root nodules, guided by a chemical signal released by the nodules. The model is based on the diffusion–chemotaxis equation. Experimental observations showed that the average speed of the larvae remained approximately constant, so the diffusion–chemotaxis model was modified so that the larvae respond only to the gradient direction of the chemical signal but not its magnitude. An individual-based lattice Boltzmann method was used to simulate the movement of individual larvae, and the parameters required for the model were estimated from the measurement of larval movement towards nodules in soil scanned using X-ray microtomography. The model was used to investigate the effects of nodule density, the rate of release of chemical signal, the sensitivity of the larvae to the signal, and the random foraging of the larvae on the movement and subsequent survival of the larvae. The simulations showed that the most significant factors for larval survival were nodule density and the sensitivity of the larvae to the signal. The dependence of larval survival rate on nodule density was well fitted by the Michealis–Menten kinetics. [Copyright &y& Elsevier]

Adult clover root weevil Sitona lepidus show a feeding preference for white clover Trifolium repens over red clover Trifolium pratense. The effects on S. lepidus of three red clover T. pratense lines, selected for high, medium, or low levels of the isoflavone formononetin in foliage, were compared in three experiments using white clover as a control. In a no-choice slant board experiment, weevil larval weights were greater for larvae feeding on white clover roots than those feeding on roots of the red clovers. The effect of larval root herbivory on plant growth was similar for all four clovers. Following root herbivory, a large increase in root and shoot formononetin levels was observed in the high-formononetin selection of red clover but little change in the low-formononetin red clover. In a no-choice experiment with sexually mature female adult weevils feeding on foliage of the four clovers, all the red clovers had increased weevil mortality. Female weevils eating the high-formononetin red clover laid fewer eggs than weevils eating white clover. The red clover diet caused a large accumulation of abdominal fat and/or oil in the weevils, whereas weevils feeding on white clover did not accumulate fat/oil. When sexually immature adult weevils were given a choice of foliage from all four clovers, white clover was eaten preferentially, and the low-formononetin red clover was preferred to the high-formononetin red clover. The results suggest that formononetin and associated metabolites in red clover may act as chemical defences against adult S. lepidus and that distribution in forage legumes can be manipulated by plant breeding to improve root health. [ABSTRACT FROM AUTHOR]

The Palaearctic parasitoid Microctonus aethiopoides Loan (Hymenoptera: Braconidae) has been introduced to North America for biological control of weevils in the genera Sitona and Hypera (Coleoptera: Curculionidae) and to Australia and New Zealand for control of Sitona discoideus Gyllenhal. Various geographic and host-associated populations of M. aethiopoides have exhibited differences in host preference, host range, and adult morphology. These differences have generally been interpreted as indicative of genetically differentiated biotypes of M. aethiopoides, but direct genetic evidence of biotypic variation has been lacking. Nucleotide sequence data were generated from the gene regions COI, 16S, 28S, and β-tubulin to assess genetic variation among M. aethiopoides reared from various host species collected in Australia, Iran, New Zealand, the United States, and 10 European countries. Ten adult morphological characters were also measured to validate the identity of the specimens and to assess morphological variation among the geographic and host-associated populations. Parsimony and maximum likelihood analyses of the COI, 16S, and β-tubulin sequences provided strong support for the presence of at least two M. aethiopoides biotypes, one associated with Hypera species and the other with Sitona species. There was also evidence for genetic divergence among parasitoids associated with different Sitona species. Morphological variation was also closely correlated with host species, but the occurrence of morphological variation in the absence of genetic variation suggested morphological characters should be used cautiously with M. aethiopoides biotypes. [Copyright &y& Elsevier]

One of the most significant pests of white clover (Trifolium repens L.) is the clover root weevil (Sitona lepidus L. Coleoptera; Curculionidae). The adult of this species feeds on the leaves of the plant. However, root feeding by the larvae is more insidious. An experiment is described that was designed to test the hypothesis that root nodule herbivory by early instar larvae of S. lepidus reduces the assimilation of C by plants of white clover.White clover plants were grown (± larvae) in individual growth chambers and the net carbon (C) assimilation by each plant was estimated by monitoring CO2 flux in the chambers. White clover plants which had been infested with larvae had a significantly (P<0.01) reduced biomass when compared with the control plants and tended to have a smaller root:shoot ratio (0.68 versus 0.78). The number of nodules on the clover roots were significantly (P<0.05) reduced by the weevil infestation. The diurnal pattern of the mean C flux during the experiment shows that 44±2.5% less C was assimilated during the light period, and 39±3.3% less C was lost in respiration during the dark phase, by the infested plants. Significant treatment differences in net C accumulation were evident only towards the end of the study period with the control plants showing a significant (P<0.05) net gain of C from day 19 onwards. This study demonstrates the initial impact of specialised nodule herbivory on the clover plants. [Copyright &y& Elsevier]

We report the complete mitogenome of Sitona obsoletus, an agricultural pest in New Zealand and some European countries. Like other Sitona mitogenomes, the 6 tRNA gene box is ordered RNSAEF, supporting the hypothesis that this signature is common to, and potentially diagnostic, of this genus. The Trojan Female Technique (TFT) is a genetic pest control strategy that exploits mitochondrial DNA alleles that affect male, but not female fertility and fitness. The complete mitogenome is an essential first step in exploring the utility of TFT for the control of S. obsoletus. [ABSTRACT FROM AUTHOR]