Effects of pesticides on spin down and webbing production by the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae)

Spin down in response to pesticides is a known phenomenon. This study aimed at investigating the effects of varying acaricide concentrations and acaricidal combinations on the rate of spin down and thread length, the fate of mites after spinning down and, thus, the implications of spin down in the context of mite control and the relationship between webbing and the efficacy of ovicides. In response to cyhexatin at a concentration of up to 100 mg dm-3 the rate of spin down did not change, but in response to higher concentrations it occurred earlier and ceased before the end of the experiment. The mean thread length did not vary with the concentration of acaricide and, whilst the proportion of mites spinning down increased with a cyhexatin concentration of up to 10 mg dm-3, it did not increase further with higher concentrations thereafter. It is therefore postulated that at any given time only a certain proportion of mites have a propensity to spin down. In combination with cyhexatin, only amitraz (at low concentrations) increased the proportion of mites spinning down, but amitraz (at higher concentrations), flubenzimine and permethrin all inhibited spin down, which may be a useful attribute in pesticide formulations. With regard to the fate of mites after spinning down, they were able to climb back up threads, but the proportion doing so was too small to be of practical importance. Mites which spin down and remain at the end of a thread die, but those which reached another leaf were able to spin down again and were more likely to do so if the leaf bore a cyhexatin residue. Mites reaching another leaf are also able to lay eggs, the number being greater if the leaf which they reach is untreated. Spin down is therefore likely to be of significant advantage to Tetranychus urticae in avoidance of the effects of acaricides. Finally, some eggs were observed to be laid on webbing and not to come into direct contact with clofentezine residues. The use of a fine water spray to simulate dew increased the ovitoxicity of clofentezine, probably by resuspension of the pesticide, but not of amitraz, which inhibits webbing production. The avoidance of acaricides should be added to the several known advantages of webbing production for spider mites. It is concluded that, all other factors being equal, pesticide formulations which inhibit spin down and webbing production are likely to be more effective in control programmes than those which stimulate such behaviour.     

Differential Effects of Plant Species on a Mite Pest (Tetranychus Urticae) and its Predator (Phytoseiulus Persimilis): Implications for Biological Control

The influence of plant species on the population dynamics of the spider mite pest, Tetranychus urticae, and its predator, Phytoseiulus persimilis, was examined as a prerequisite to effective biological control on ornamental nursery stock. Experiments have been done to investigate how the development, fecundity and movement of T. urticae, and the movement of P. persimilis were affected by plant species. A novel experimental method, which incorporates plant structure, was used to investigate the functional response of P. persimilis. Development times for T. urticae were consistent with published data and did not differ with plant species in a biologically meaningful way. Plant species was shown to have a major influence on fecundity (P < 0.001) and movement of the pest mite (P < 0.01), but no influence on the movement of the predator. The movement of both pest and predator was shown to be related to the density of the adult pest mites on the plant (P < 0.001). Plant structure affected the functional response, particularly in relation to the ability of the predator to locate prey at low densities. The impact of these findings on the effective use of biological control on ornamental nursery stock is discussed.     

Prey preference of the predatory mite Neoseiulus womersleyi Schicha is determined by spider mite webs

Abstract:  We experimentally verified a proximate cause for the preference of prey species by the predatory mite Neoseiulus womersleyi , which prefers the eggs of Amphitetranychus viennensis Zacher to those of Tetranychus urticae Koch. To separate the effects of spider mite eggs and spider mite webs on the prey preference of the predatory mite, we manipulated combinations of eggs and webs of the two prey species. The number of eggs consumed by adult female N. womersleyi was compared with respect to two factors: eggs of the same species under webs of the different species and eggs of different species under webs of the same species. The results revealed that the prey preference of N. womersleyi was determined by the webs, and not by the eggs.     

Development of two spotted spider mite (Acari: Tetranychidae) populations on strawberry and raspberry cultivars

Five strawberry (Fragaria sp.) and five raspberry (Rubus ideaus L.) cultivars were evaluated for resistance to two spotted spider mite (Tetranychus urticae Koch.). Two methods of assessing the development of two spotted mite populations using detached leaves were compared. The number of eggs laid and mites which developed were compared. The strawberry cvs Hapil and Pegasus had significantly greater development of two spotted mite populations than the cvs Rhapsody, Symphony and Elsanta. The raspberry cv. Joan Squires had higher populations of two spotted mite whilst the raspberry cv. Leo the least, when compared with cvs Glen Clova, Glen Moy and Glen Prosen. Differences were observed in oviposition sites and mite distribution when comparing raspberries with strawberries. The method of assessing the populations development of two spotted mite which involved maintaining the cut leaf stem in water may be of potential use for studying population dynamics of both two spotted mite and possible predators over extended periods of time.     

The Effect of Patch Size and Persistence of Host Plants on the Development of Acaricide Resistance in the Two-spotted Spider Mite Tetranychus Urticae (Acari: Tetranychidae)

Spatial and temporal characteristics of host plants can influence the population biology of the herbivores feeding on them. In this study, I examined the effect of variation in host plant characteristics on the development of acaricide resistance in the two-spotted spider mite Tetranychus urticae, a widely distributed agricultural pest. This investigation examined the geographic variation in the degree of resistance to two new types of acaricide, pyridaben and fenpyroximate. From mortality tests at field-level concentrations of the acaricides, many populations collected from fruit trees and roses had a high frequency of resistant individuals for acaricides while almost all populations collected from herbaceous crops had low frequencies of resistant individuals. These results, combined with those from a previous allozyme study, indicate that patch size and persistence of host plants regulate the population structure of the mites including gene flow between populations and, by extension, the development of acaricide resistance.     

Laboratory and glasshouse evaluation of entomopathogenic fungi against the two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae), on tomato, Lycopersicon esculentum

Forty isolates of entomopathogenic fungi from six genera were assessed against the two-spotted spider mite, Tetranychus urticae, in a single dose, direct application laboratory bioassay on tomato leaflets. Only three isolates caused greater mortality than the control: these were Metarhizium anisopliae 442.99, Hirsutella spp. 457.99, and Verticillium lecanii 450.99. These isolates were assessed in a multiple dose bioassay, together with three isolates cultured from commercial biopesticides as follows: Beauveria bassiana 432.99 (cultured from 'Naturalis-L', Troy Biosciences, Phoenix, TX, USA); Hirsutella thompsonii 463.99 (cultured from 'Mycar', Abbott Laboratories USA); and V. lecanii 19.79 (used in 'Mycotal' Koppert BV, The Netherlands). Beauveria bassiana 432.99, H. thompsonii 463.99, M. anisopliae 442.99, and V. lecanii 450.99 were all pathogenic to T. urticae in this bioassay. In addition, it was found that the mortality caused by B. bassiana 432.99 and Naturalis-L was increased when the mites were exposed to tomato leaflets sprayed previously with conidia suspensions, compared to spraying the mites directly. In a glasshouse experiment, sprays of B. bassiana 432.99, H. thompsonii 463.99, M. anisopliae 442.99, V. lecanii 450.99 and Naturalis-L reduced T. urticae populations in a tomato crop grown according to commercial practice. Naturalis-L reduced T. urticae numbers by up to 97%. In a second glasshouse experiment, single sprays of Naturalis-L and the acaricide fenbutatin oxide (Torq) were compared as supplementary treatments to release of the predatory mite, Phytoseiulus persimilis. Supplementary sprays of fenbutatin oxide reduced the numbers of T. urticae nymphs (80% reduction), while Naturalis-L reduced numbers of T. urticae adults, nymphs and eggs (98% reduction in all three cases). It is concluded that Naturalis-L has the potential to be used against T. urticae on glasshouse tomato crops.     

Ethanol Oxidase Respiratory Chain of Acetic Acid Bacteria. Reactivity with Ubiquinone of Pyrroloquinoline Quinone-dependent Alcohol Dehydrogenases Purified from Acetobacter aceti and Gluconohacter suhoxydans

Membrane-bound, pyrroloquinoline quinone-dependent, alcohol dehydrogenase functions as the primary dehydrogenase in the respiratory chain of acetic acid bacteria. In this study, an ability of the enzyme to directly react with ubiquinone was investigated in alcohol dehydrogenases purified from both Acetobacter aceti and Gluconobacter suboxydans by two different approaches. First, it was shown that the enzymes are able to reduce natural ubiquinones, ubiquinone-9 or -t0, in a detergent solution as well as a soluble short-chain homologue, ubiquinone-I. In order to show the reactivity of the enzyme with natural ubiquinone in a native membrane environment, furthermore, alcohol dehydrogenase was reconstituted into proteoliposomes together with natural ubiquinone and a terminal ubiquinol oxidase. The reconstitution was done by binding the detergent-free dehydrogenase at room temperature to proteoliposomes that had been prepared in advance from a ubiquinol oxidase and phospholipids containing ubiquinone by detergent dialysis using octyl-β-D-glucopyranoside; the enzyme of A. aceti was reconstituted together with ubiquinone-9 and A. aceti cytochrome a₁ while G. suboxydans alcohol dehydrogenase was done into proteoliposomes containing ubiquinone-10 and G. suboxydans cytochrome o. The proteoliposomes thus reconstituted had a reasonable level of ethanol oxidase activity, the electron transfer reaction of which was also able to generate a ‘membrane potential. Thus, it has been shown that alcohol dehydrogenase of acetic acid bacteria donates electrons directly to ubiquinone in the cytoplasmic membranes and thus the ethanol oxidase respiratory chain of acetic acid bacteria is constituted of only three membranous respiratory components, alcohol dehydrogenase, ubiquinone, and terminal ubiquinol oxidase.     

Movement of the twospotted spider mite, Tetranychus urticae, among hosts in a corn-peanut agroecosystem

A 3 year study of spider mites in a corn-peanut agroecosystem indicated that the persistance and pest status of mite populations depended on dispersal among a succession of temporarily suitable cultivated and non-cultivated hosts. In the spring, mites crawled from overwintering sites in vegetation along field margins into both peanut and corn fields, but became established only in corn. Initiation of aerial dispersal from corn was coincident with and appeared dependent upon mites moving to the top of the corn canopy, where they were exposed to the wind. Mites were first observed in peanut immediately following the onset of aerial dispersal from corn, and these aerial dispersers were the primary source of subsequent mite infestations in peanut. As populations grew in peanut, mites dispersed into vegetation along peanut field margins and became the nucleus of populations which persisted over the winter. This pattern of host utilization was reinitiated in the spring if corn was planted adjacent to infested field borders. This pattern commonly occurs since corn is planted following peanut in the predominant crop rotation sequence in this area.

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Résumé Des populations d'acariens ont été étudiées pendant 3 ans sur maïs, arachide et végétation spontanée, dans 7 fermes commerciales de Chowan Co., Caroline du Nord. Les déplacements d'acariens parmi ces plantes ont été notés en fonction de la croissance de la population et de la phénologie des hôtes. Un schéma cohérent de croissance de la population d'acariens et de leurs mouvements en découle, dont seule la taille change suivant les fermes et les années. Les acariens provenant de populations hivernantes sur la végétation spontanée bordant les champs migrèrent dans les champs cultivés adjacents quelle qu'en soit la nature de la culture. Cependant, à cette époque, les populations d'acariens ne s'établissaient que dans le maïs. Dans le maïs, les acariens se répandirent à travers le champ et se multiplièrent sur des pieds particuliers jusqu'à ce qu'ils eussent atteint le sommet de la canopée du maïs. Les premiers acariens transportés par le vent ont été capturés la semaine suivant la première apparition d'acariens au sommet de la canopée du maïs et aussitôt après la découverte des premiers acariens dans les arachides voisines. Ceci suggère que les acariens dispersés par le vent sont à l'origine des contaminations ultérieures des arachides voisines. Plus tard dans la saison, les acariens se dispersèrent à partir de l'arachide vers les sites d'hibernation dans la végétation spontanée poussant autour du champ d'arachide. Puisque des champs semés en arachides sont généralement semés en maïs l'année suivante, les acariens se dispersant au printemps depuis les sites d'hibernation rencontrèrent souvent du maïs qu'ils colonisèrent. Ce cycle répétitif d'utilisation de l'hôte et de dispersion est considéré comme fortement responsable de la pérennité des acariens et de leur danger potentiel dans les agroécosystèmes à maïs-arachide, caractéristiques du canton de Chowan (N.C.).Les problèmes potentiels posés par les acariens sur arachide ont paru être liés à l'intensité de l'attaque d'acariens sur le maïs voisin, qui sert de réservoir pour les populations d'acariens. Ceci suggère que le contrôle des populations d'acariens sur maïs peut être une stratégie viable pour limiter les infestations ultérieures sur arachide et, en dernier lieu, les populations hivernantes.

     

Enzymatic Reactions by Five Chalcone Synthase Homologs from Hop (Humulus lupulus L.)

The enzyme activities encoded in five cDNAs for chalcone synthase (CHS) homologs from hop were investigated. Only valerophenone synthase (VPS) and CHS_H1 showed both naringenin-chalcone and phlorisovalerophenone forming activity. Narigenin-chalcone production by VPS was much lower than by CHS_H1. Therefore, it is highly possible that flavonoid depends mainly on CHS_H1, while bitter acid biosynthesis depends mainly on VPS and CHS_H1.     

Demographic examination of sex ratio in the two-spotted spider mite, Tetranychus urticae

Primary sex ratio of arrhenotokous tetranychid mites depends on the age-specific schedule of ratios of progeny produced by individual females. Since primary sex ratio is conditional upon female age and since the proportion of females of a given age in a population is influenced by the rate of increase, both the agespecific ratio of males to females and the overall ratio of males to females depends on the rate of increase. We given an expression for this dependence and compare it to experimental results for the two-spotted spiter mite. These results imply that there is no standard sex ratio of spider mites because of interpopulation variation in rates of increase.

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Résumé Le taux sexuel des acariens tétranychidés arrhénotoques est défini comme le rapport de la somme des nombres de mâles sur la somme des nombres de femelles pour chaque âge de la distribution en âges. Comme le taux sexuel primaire dépend de l'âge de la femelle et comme la proportion de femelles d'un âge donné dans la population est influencée par le taux d'accroissement, alors le rapport mâle/femelle spécifique d'un âge déterminé et le rapport global mâle/femelle dépendent du taux d'accroissement. Nous avons cherché à exprimer cette dépendance et à la comparer aux résultats expérimentaux pour Tetranychus urticae. Ces résultats signifient qu'il n'y a pas de taux sexuel caractéristique de Tétranyques par suite des variations du taux d'accroissement suivant les populations.