Sex allocation in haplodiploids is mediated by egg size: evidence in the spider mite Tetranychus urticae Koch

Haplodiploid species display extraordinary sex ratios. However, a differential investment in male and female offspring might also be achieved by a differential provisioning of eggs, as observed in birds and lizards. We investigated this hypothesis in the haplodiploid spider mite Tetranychus urticae, which displays highly female-biased sex ratios. We show that egg size significantly determines not only larval size, juvenile survival and adult size, but also fertilization probability, as in marine invertebrates with external fertilization, so that female (fertilized) eggs are significantly larger than male (unfertilized) eggs. Moreover, females with on average larger eggs before fertilization produce a more female-biased sex ratio afterwards. Egg size thus mediates sex-specific egg provisioning, sex and offspring sex ratio. Finally, sex-specific egg provisioning has another major consequence: male eggs produced by mated mothers are smaller than male eggs produced by virgins, and this size difference persists in adults. Virgin females might thus have a (male) fitness advantage over mated females.     

一种新型叶螨饲养器的设计与制作

分析了现有的叶螨个体饲养或定量饲养的方法或器具的缺陷。根据酢浆草如叶螨Tetranychina harti(Ewing)及其寄主植物红花酢浆草(Oxalis corymbosa DC)的特点,设计、制作了一种新型饲养器,解决了常用的离体湿基叶片法中叶片表面温湿度与设定的环境条件不一致、叶螨生活状态不自然和活体叶片法中镜检观察不方便的问题,取得了较好的效果。本设计在其他叶螨或微小昆虫的饲养中也可以作为借鉴。     

Sex dimorphism of life-history traits and their response to environmental factors in spider mites

Sex dimorphism is ubiquitous in the animal kingdom and can be influenced by environmental factors. However, relatively little is known about how the degree and direction of sex difference vary with environmental factors, including food quality and temperature. With the spider mites from the family Tetranychidae as subjects, the sex difference of life-history traits in responses to host plant and temperature were determined in this meta-analytic review. Across the 42 studies on 26 spider mite species (N?=?8057 and 3922 for female and male mites, respectively), female spider mites showed longer developmental duration than the males in all except two species. The direction of sex difference in development was consistent regardless of temperature and host plant. The 16 spider mite species in 33 studies generally showed female-biased longevity, with an overall effect size of 0.6043 [95%CI = 0.4054–0.8031]. Host plant significantly influenced the sex difference in longevity, where the males lived longer than females below 22.5 ℃, but the reverse was true at higher and fluctuating temperature. Host plant also influenced the magnitude of sex difference in longevity, with females living longer than males when reared on herbs but not on trees. This study indicated that life-history traits are highly variable between sexes under temperature and host plant influence, highlighting that environmental conditions can significantly shape the direction and magnitude of sexual dimorphism of life-history traits.

     

Pesticide-induced mite outbreaks: pyrethroids and spider mites

Factors inducing outbreaks of spider mites (Acari: Tetranychidae) following use of pyrethroid insecticides are reviewed. Differentials in direct toxicity between spider mites and phytoseiid (Acari: Phytoseiidae) predators provide one explanation. Wide variation exists between pyrethroids in their direct toxicity toTetranychus urticae. The acaricidal action of pyrethroids is largely controlled by the amount of irritancy or repellency induced by the respective chemicals. Laboratory assays for repellency reflect field results. Repellent activity induces spider-mite dispersal to either recolonise plants free of residues or leave the treated habitat. Pyrethroids may also affect reproduction and development rates. Residual activity is likely to vary, so limiting any general theory of spider-mite responses to pyrethroids. Variable responses to pyrethroids, strain variation and development of behavioural resistance also add complications to understanding outbreaks. Future research needs are identified.     

Why size matters: altering cell size

Several genes involved in growth control have lately been demonstrated to exhibit more potent effects on cell size than on cell proliferation. Many of these genes direct protein and ribosomal synthesis, highlighting the interdependence between cell size and macromolecular content. The failure to maintain normal cell size when these genes are deregulated suggests that, in certain contexts, cell growth and division are not coupled or coordinated. Several physiological repercussions of altering cell size have been identified.     

Photoperiodic time measurement in spider mites

Summary A model has been developed which describes in mathematical terms the incidence of diapause in the spider miteTetranychus urticae under different photoperiodic regimens. The model has been derived from Beck's (1974a, b, 1975, 1976a) Dual System Theory of photoperiodic time measurement, by means of a number of essential alterations and modifications. The spider mite's model is composed of two hour-glass timers, one of which starts at lights-off and measures the length of the night, whereas the other is initiated both by the onset of the first hour-glass timer and by lights-on. This second hour-glass defines the time at which the first hour-glass is read off, the state of the first hour-glass at this particular time being decisive for the developmental alternative (diapause or nondiapause) to be determined. The model may be classified as a form of internal coincidence according to the terminology of Pittendrigh (1972), since it is based on the interaction of two internal systems rather than on the coincidence of light with a particular light-sensitive phase of the timing mechanism, as in the case of external coincidence (cf. Saunders, 1978). Good agreement is attained between diapause incidences predicted by this model and incidences observed in spider mites, both in the diapause induction response curve and in asymmetrical skeleton photoperiods.The authors are grateful to Dr. S.D. Beck for the program printout of the Dual System Theory put at their disposal. It is also a pleasure to thank Miss C. van Ruth and Mr. G. van de Berg for technical assistance, and Mr. H. Bos for drawing the figures. The investigations were supported by a grant to the first author from the Netherlands Organization for the Advancement of Pure Research (ZWO).     

Web-mediated interspecific competition among spider mites

Some spider mites, such as Tetranychus spp. and Amphitetranychus spp., create complicated webs (CWs), whereas others, such as Panonychus spp., produce little webs (LWs). We verified whether interspecific competition occurred between CW and LW mites via habitat arrangement under laboratory conditions. The complicated webs produced by CW mites clearly inhibited juvenile development in LW mites, whereas there was no effect of LW mites on CW mites. In oviposition site choice tests, both CW and LW females preferred the lower surface of leaves to the upper surface. The preference of LW mites for the lower leaf surface, even in the presence of CW mite webs, suggests that the costs of amensalism are outweighed by the possible benefits, such as avoiding rain. These findings show that the shift in mite species composition from LW to CW mites can occur as a consequence of the interspecific association between spider mites via their webs, without pesticide applications or the presence of natural enemies.     

作物重要叶螨综合防控技术研究与示范推广

叶螨是一类重要的农业有害生物,近年来,在我国各地危害呈上升趋势,严重制约了粮棉果蔬等产业的可持续健康发展。在公益性行业(农业)科研专项叶螨项目资助下,课题组明确了东北果园、华北果园和北方蔬菜田叶螨种群消长规律,开展了朱砂叶螨Tetranychus cinnabarinus(Boisduval)、二斑叶螨Tetranychus urticae Koch和柑橘全爪螨Panonychus citri(McGregor)的种群遗传结构研究,在共生菌对叶螨生殖影响方面取得了重要进展,在生物防治和药剂防治方面已取得可喜的成绩,组建了3个叶螨的综合防治技术体系,示范面积共达3万多亩。     

Organelle size regulation: length matters

The control of flagellar length can be easily studied in the model genetic cell Chlamydomonas. Recent work has revealed that the mutant gene in a long-flagella mutant encodes a protein kinase.     

Induced resistance against spider mites in cotton: Field verification

Cotton plants that had been damaged by spider mites (Tetranychus spp.) or by mechanical abrasion at the cotyledon stage were less likely to develop infestations of spider mites compared to controls early in the season. These are the first field results to demonstrate that induced resistance can reduce pest populations in an agricultural system. Differences in mite populations early in the season caused by induced resistance did not translate into differences in plant growth or cotton yield at harvest.

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Vérification au champ de la résistance du coton induite par les acariens

Résumé Les plants de coton qui ont été endommagées, au stade cotylédons, par des acariens ou par une abrasion mécanique, ont moins de chance que les témoins, de présenter ultérieurement des dégats dus aux acariens. C'est la première fois que des résultats montrent qu'une résistance induite peut réduire les populations dans un agrosystème. Les différences ultérieures dans les populations d'acariens, ne transparaissent pas au niveau de la croissance du coton ou de la production au moment de la récolte.

     

Sex allocation and clutch size in parasitoid wasps that produce single-sex broods

The parasitoid wasp genus Achrysocharoides (Eulophidae) is unusual in that many of its species lay male and female eggs in single-sex clutches. The average clutch size of female broods is always greater than that of male broods, and in some species male clutch size is always one. We constructed models that predicted that severely egg-limited wasps should produce equal numbers of male and female eggs while severely host-limited wasps should produce equal numbers of male and female broods (and hence an overall female-biased sex ratio). Theory is developed to predict clutch size and sex ratio across the complete spectrum of host and egg limitation. A comparison of 19 surveys of clutch composition in seven species of Achrysocharoides showed a general pattern of equal numbers of male and female broods with a female-biased sex ratio (suggesting host limitation) although with considerable heterogeneity amongst collections and with a number of cases of unexpectedly low frequencies of male broods. Using a previous estimate of the relationship between fitness and size in the field, we predicted the maximally productive (Lack) clutch size for female broods of Achrysocharoides zwoelferi to be three. Of clutches observed in nature, 95% were equal to or smaller in size than the predicted Lack clutch size. When we manipulated local host density in the field, and as predicted by our models, clutch size and the proportion of female broods of A. zwoelferi decreased as hosts became more common, but the absolute frequency of male clutches was lower than expected. Copyright 1998 The Association for the Study of Animal Behaviour.     

Temperature-mediated stability of the interaction between spider mites and predatory mites in orchards

The nonlinear behavior of the Holling-Tanner predatory-prey differential equation system, employed by R.M. May to illustrate the apparent robustness of Kolmogorov’s Theorem when applied to such exploitation systems, is re-examined by means of the numerical bifurcation code AUTO 86 with model parameters chosen appropriately for a temperature-dependent mite interaction on fruit trees. The most significant result of this analysis is that there exists a temperature range wherein multiple stable states can occur, in direct violation of May’s interpretation of this system’s satisfaction of Kolmogorov’s Theorem: namely, that linear stability predictions have global consequences. In particular these stable states consist of a focus (spiral point) and a limit cycle separated from each other in the phase plane by an unstable limit cycle, all of which are associated with the single community equilibrium point of the system. The ecological implications of such metastability, hysteresis, and threshold behavior for the occurrence of outbreaks, the persistence of oscillations, the resiliency of the system, and the biological control of mite populations are discussed.     

Cardinium symbionts cause cytoplasmic incompatibility in spider mites

Intracellular symbiotic bacteria belonging to the Cytophaga-Flavobacterium-Bacteroides lineage have recently been described and are widely distributed in arthropod species. The newly discovered bacteria, named Cardinium sp, cause the expression of various reproductive alterations in their arthropod hosts, including cytoplasmic incompatibility (CI), induction of parthenogenesis and feminization of genetic males. We detected 16S ribosomal DNA sequences similar to those of Cardinium from seven populations of five spider mite species, suggesting a broad distribution of infection of Cardinium in spider mites. To clarify the effect of Cardinium on the reproductive traits of the infected spider mites, infected mites were crossed with uninfected mites for each population. In one of the populations, Eotetranychus suginamensis, CI was induced when infected males were crossed with uninfected females. The other six populations of four species showed no reproductive abnormalities in the F(1) generation, but the possibility of CI effects in the F(2) generation remains to be tested. One species of spider mite, Tetranychus pueraricola, harbored both Cardinium and Wolbachia, but these symbionts seemed to have no effect on the reproduction of the host, even when the host was infected independently with each symbiont.