Maintenance of diapause variability in the two-spotted spider mite,tetranychus urticae, in a heterogeneous and stochastic environment

A simple nongenetic mathematical model analyzed the processes responsible for the variations in the diapause percentage among populations of the two-spotted spider mite,Tetranychus urticae Koch. This model incorporates the following assumptions. 1) Mites have diapause (DD), non-diapause (NN), and “plastic” (DN) populations (plasticity exists in the phenotypic expression of diapause in response to habitat conditions at the time of diapause induction). 2) A heterogeneous mite habitat consists of microhabitat L, in which all the non-diapausing mites die during the winter due to the lack of winter host plants, and microhabitat O with winter hosts capable of supporting some of the non-diapausing mites overwinter. 3) Temporal fluctuation of winter conditions which affect the survival and reproduction of non-diapausing mites. Using these assumptions, we compared the fitness functions of the three populations and analyzed the conditions under which each population is favoured over the other two, thereby elucidating the processes involved in the maintenance of variability in diapause. Our analysis revealed: 1) frequent mild winters are of primary importance for the non-diapause trait to be maintained, 2) the existence of winter hosts is also important for the nondiapause trait to be favoured, and this importance depends greatly on the degree of the adaptive diapause expression in the DN mites in response to habitat conditions, i.e., the better the phenotypeenvironment matching in DN, the higher the probability that DN will be favoured, 3) The combined effect of the temporal and spatial variation enhances the maintenance of variablity in the diapause trait of the mites.     

Photoperiodic induction of diapause in the spider mite Tetranychus urticae: qualitative or quantitative time measurement?

Abstract. Insects and mites may measure photoperiods eitfier by classifying them as long or short relative to a critical value (qualitative time measurement) or by using the absolute value (quantitative time measurement). The spider mite Tetranychus urticae is thought to use a qualitative mechanism of time measurement. In this paper we present the results of experiments with an inbred line of the spider mite (to keep genetic variation in photoperiodic responses small), to test whether quantitative aspects also play a role. Differences in diapause incidence in different long-night photoperiods at different temperatures may be an indication of quantitative responses to photoperiod. The effect of temperature on the photoperiodic response curve was studied at 16^oC, 19^oC and 22^oC. The response curves appeared to be similar at 16^oC and 19^oC, with a critical nightlength between 10 and 11 h. At 22^oC, diapause induction was less than 100% in all long-night regimens and die critical nightlength had shifted to 12 h. Maximum diapause induction (93%) occurred in a light-dark cycle with a 16 h dark phase (LD 8:16 h). Diapause induction was lowest in long-night photoperiods with dark phases of 20 h and longer. The number of light-dark cycles needed for 50% diapause induction at 19^oC varied. between 12.1 and 14.7 for LD 6:18 h, between 10.9 and 12.5 for LD 8:16 h, between 10.6 and 11.6 for LD 10:14 h, and between 10.1 and 10.7 for LD 12:12 h. Independent of die light-dark regimen, diapause induction took place in some individuals after receiving 8 cycles and virtually all individuals entered diapause after 16 cycles. No effect was found of the photoperiodic treatment during prediapause development (LD 6:18 h, LD 8:16 h, LD 10:14 h, LD 12:12 h) on diapause duration. The average diapause duration at LD 10:14 h and 19^oC was 61 days over all four treatments. We explained the results by hypothesising that nightlengths are assessed qualitatively and mat the photoperiodic clock operates more accurately near the critical nightlength.     

The effect of powdery mildew on the number of prey consumed by Typhlodromus pyri (Acari: Phytoseiidae)

Abstract:  Prey consumption by Typhlodromus pyri Scheuten was studied in the presence and absence of apple powdery mildew, Podosphaera leucotricha (Ell. and Everh.) under constant laboratory conditions. Eggs of Tetranychus urticae Koch were offered to predatory mites as a prey. Seven densities ranging from five to 100 T. urticae eggs per arena were used. Mildew conidia (approximately 0.5 mg) were added to half of the arenas by brushing them from infected apple leaves. A single adult female of T. pyri was introduced onto each arena and number of prey eggs consumed was counted 12 h later when the predator was offered new T. urticae eggs again and the procedure was repeated once. Data showed that predators consumed in both experimental periods nearly all prey in experiments with densities up to 40 eggs per arena and no mildew. However, the number of eggs consumed decreased more than twofold when mildew conidia were supplied, even at high prey densities. Differences in predation rate between treatments with and without mildew were highly significant. The results thus indicate that availability of mildew as an alternative food can reduce prey suppression by T. pyri . Possible implications of these findings in biological control of spider mites by generalist predatory mites are discussed.     

Response of Neoseiulus fallacis Garmen and Galendromus occidentalis Nesbitt (Acari: Phytoseiidae) to Tetranychus urticae Koch (Acari: Tetranychidae)-Damaged Hop Humulus lupulus (L.) (Urticales: Cannabaceae)

Abstract 1 The response of Neoseiulus fallacis Garmen and Galendromus occidentalis (Acari: Phytoseiidae) to Tetranychus urticae Koch (Acari: Tetranychidae)‐damaged and undamaged hop, Humulus lupulus (L.), plants was tested using a Y‐tube olfactometer. 2 Neoseiulus fallacis but not G. occidentalis was attracted to volatiles from T. urticae‐damaged hop plants when paired with undamaged plants. 3 The response of N. fallacis to these volatiles was stronger for plants severed at the soil surface than for intact plants. 4 There was no difference in the response of N. fallacis to severed or intact hop plants that had no spider mite damage, indicating that artificial wounding by severing alone does not elicit the production of attractive volatiles detectable to N. fallacis. These results are consistent with the existence of cross‐talk between signalling pathways initiated by feeding damage and artificial wounding that result in elevated levels of predator‐attracting volatiles.     

温度对拟小食螨瓢虫发育和繁殖的影响

拟小食螨瓢虫是热带地区香蕉、番木瓜、橡胶和木薯等作物上有害叶螨的重要天敌。在室内条件下,以皮氏叶螨Tetranychus piercei为食物,观察了不同温度(16℃,20℃,24℃,28℃,32℃)对拟小食螨瓢虫生长发育及繁殖的影响。结果表明:拟小食螨瓢虫卵、幼虫和蛹的历期在32℃下最短,分别为2.38,4.26和3.04 d;16℃时最长,分别为12.48,25.10和16.12 d,雌成虫寿命在16℃时长达142.53 d,而在32℃时为95.46 d。世代发育起点温度和有效积温分别为13.29℃和227.63日度。成虫产卵量在28℃最高,达595.49粒/雌,在16℃最低,为125.05粒/雌。内禀增长率(rm)在32℃达到最大(0.2063),而在16℃时最小(0.0342)。结果表明拟小食螨瓢虫有较好的高温耐受性,研究结果可为该瓢虫人工繁育的温度条件提供依据。     

Inheritance of sex ratio in the Kanzawa spider mite,Tetranychus kanzawai Kishida

Population Ecology - The frequency distributions in sex ratios of offspring from 4 fieldTetranychus kanzawai populations were examined. No significant difference was observed between average sex...     

Control of sex ratio by female spider mites

Control of sex ratio in the progenies of mated female spider mites was investigated in two laboratory experiments. In Experiment I, a single strain of Tetranychus urticae Koch was reared in four different environments. In Experiment II, 4 different geographic strains of T. urticae were reared in a single environment.Results indicated that spider mite females were able to control the sex ratio of their progeny at two levels: an initial control of mean population ratio according to ovipositional environment. In addition, a secondary control resulted in an approximately uniform daily ratio. Although each parental female produced a varying number of eggs from day to day, a more or less constant fraction of those eggs were fertilized every day. It is concluded that both of these characteristics are probably important to the colonizing ability of the species.

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Résumé L'étude de la régulation du sexe de la descendance de femelles inséminées d'acariens est basée sur deux expériences de laboratoire. Dans l'expérience I, une lignée de Tetranychus urticae a été élevée dans deux conditions différentes de qualité des feuilles pendant le développement des parents, d'une part, pendant leur ponte et le développement de la descendance d'autre part. Dans l'expérience II, quatre souches géographiques différentes de T. urticae ont été élevées dans les mêmes conditions.Il est apparu nettement que les femelles dans des conditions différentes ont donné des descendances avec des taux sexuels variant dans certaines limites. Ces différences étaient statistiquement significatives dans les duex lots de données expérimentales. Les taux sexuels des populations d'acariens variaient aussi avec les conditions écologiques. Dans les bonnes conditions, la proportion de femelles dans la descendance était la plus élevée. Les taux sexuels des populations variaient aussi entre les quatre souches géographiques.Les analyses ont aussi révélé que les femelles d'acariens sont capables de contrôler le taux sexuel à deux niveaux: une régulation initiale du taux de la population en fonction des conditions écologiques ou de la lignée géographique et un contrôle secondaire qui s'est traduit par un taux quotidien approximativement uniforme.En utilisant un argument mathématique simple incorporant la fécondité caractéristique des acariens, nous avons conclu que ces deux caractéristiques de la rulation sont probablement d'une importance capitale pour le pouvoir colonisateur de cette espèce.

     

Effect of water stress on abundance of twospotted spider mite on soybeans under greenhouse conditions

A rope wick technique was used to establish different levels of water stress in potted, greenhouse-grown soybean, Glycine max (L.) Merrill, plants. Water stress reduced, within a single generation, the overall abundance (number of mites/cm² leaf) of the twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) and particularly influenced female mites and eggs. Mites were most numerous in the upper strata of soybean plants on youger and possibly more nutritious leaves.

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Influence de l'arrosage sur l'abondance de Tetranychus urticae sur soja, Glycine max en serre

Résumé Un arrosage par capillarité contrôlé a été utilisé pour maintenir différentes teneurs en eau dans des pieds empotés de G. max. Le stress hydrique a réduit, en une génération, la densité de T. urticae (nombre d'acariens/cm² de feuille), et particulièrement celle des femelles et des oeufs. Les acariens étaient plus nombreux sur les feuilles les plus jeunes et peut-être les plus nutritives des strates supérieures des pieds de soja.

     

Seasonal dynamics of economic injury levels for Tetranychus urticae Koch (Acari, Tetranychidae) on Cucumis sativus L.

Abstract:  Economic injury level (EIL) for Tetranychus urticae Koch on glasshouse cucumbers was established in two ways: EILs based on the number of T. urticae introduced to cucumber early in the growing season and EILs based on mite density and feeding durations (i.e. cumulative mite-days) in each of four growing seasons. The relationship between yield loss and T. urticae infestation level varied among four growing seasons. One mite introduced to cucumber early in the growing season could reduce the cucumber yield by 5.03, 3.20, 1.12 and 2.86 g per plant in spring, summer, autumn and winter, respectively. One mite-day (i.e. one mite feeding in a day) reduced cucumber yield by 0.0123, 0.0074, 0.0035 and 0.0073 g per plant in spring, summer, autumn and winter, respectively. When the seasonal dynamics of market values of cucumber was considered, EILs were the lowest in spring and the highest in autumn. When four seasonal data were combined, EILs can be calculated as EIL = C /(0.00276 ×  K ) for the number of T. urticae introduced early in the growing season and EIL = C /(6.8342 × 10⁻⁶ ×  K ) for cumulative mite-days, where K is efficacy of control measure and C the management cost. This study demonstrated the seasonal dynamics of EILs for T. urticae on glasshouse cucumbers, suggesting that decision making for T. urticae management on cucumber needs to be seasonally different under the glasshouse condition.     

Diapause‐inducing signals prolong nymphal development in the two‐spotted spider mite Tetranychus urticae

Female two‐spotted spider mite Tetranychus urticae are grown under different photoperiods and the photoperiodic regulation of diapause is examined. The photoperiodic response curve for diapause induction was of the long day–short day type, with critical day lengths (CDLs) of 2 and 12.5 h; diapause was induced between these CDLs. The preimaginal period is significantly longer in diapausing females than in non‐diapausing females; moreover, a significant positive correlation is detected between diapause incidence and deutonymphal period. Diapause incidence is high when long‐night photoperiods are applied against a background of continuous darkness in the stages including the deutonymph; this stage appears to be the most sensitive to photoperiod. These observations suggest that diapause‐inducing conditions inhibit nymphal development, particularly in the deutonymphal stage when photoperiodic time measurement for determination of reproduction or diapause is carried out.