ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02386.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

Metatetranychus ulmilays eggs of two types. The summer eggs, which are laid on the leaves of the host plant (e.g. apple), are of the non‐diapause type and develop without interruption. The winter eggs, which are deposited predominantly on the bark, enter diapause at the blastoderm stage of development. ‘Summer’ and ‘winter’ females lay eggs of only one type if exposed to constant environmental conditions. Diapause is facultative. Over seventy successive summer generations have been reared under diapause‐preventing conditions. Winter females appear in the first post‐diapause generation in response to stimuli which induce diapause.Three environmental agencies are capable of evoking diapause, namely photoperiod, temperature and nutrition. Mites feeding upon undamaged young or mature leaves obtain a plentiful food supply and the incidence of diapause is then determined solely by photoperiod and temperature. However, if the food supplies are restricted, winter females appear even when photoperiod and temperature are such as to prevent diapause. Such deficiencies, which are probably of a quantitative nature, are manifested when the diet consists of the cell contents either of senescing leaves or of ‘bronzed’ foliage previously damaged by the feeding punctures of large mite populations.With daily photoperiods lasting from 6 to 13 hr., and at medium temperatures (c.15° C.), only winter females develop. The incidence of diapause falls to zero at 15–16 hr., and only summer females are produced in continuous illumination. About 40% of summer females appear in the absence of light. The mites respond to the absolute duration of the cycle of illumination and not to increasing or decreasing photoperiods.Provided the illumination exceeds a threshold of 1–2 f.c., the response is independent of light intensity.Radiation in the near ultra‐violet, blue and blue‐green regions of the spectrum is photoperiodically active. Maximum sensitivity occurs in the blue region. Wavelengths above 550 m μ, i.e. in the orange, red and infra‐red regions, are totally inactive even if the energy level is high.The mites are influenced directly by the photoperiod, not indirectly through the medium of the host plant.High temperatures (e.g. 25° C.) tend to prevent diapause even if the photoperiod is short. Low temperatures (e.g. 10° C.) induce some diapause even with a long photoperiod. Temperature activity seems to be mainly confined to the dark phase.Developing mites are indifferent to photoperiod, temperature and nutrition until the deutonymphal instar. This is also the period of greatest sensitivity, but egg‐laying females, if exposed to antagonistic conditions, can still be caused to ‘switch‐over’ to the alternative egg type. Eggs intermediate in character may be laid during the period of reversal.Winter eggs in diapause never hatch at 18 or 25° C. Diapause can be broken by chilling the eggs at 1, 5 or 9° C. for 150–200 days.InTetranychus telariusphotoperiod, temperature and nutrition play a similar role in controlling the onset of diapause; and females in diapause can be caused to feed and oviposit if chilled. The tropical speciesMetatetranychus bioculatusis without diapause and photoperiod has no influence on development.These experimental findings have been used in an interpretation of the life cycle and phenology ofM. ulmi.In orchards where mite populations remain small throughout the season the first winter females appear at a time when food supplies are still plentiful. This is a

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02387.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

The daily cycle of illumination is one of several agencies which control the onset of diapause inMetatetranychus ulmi.Both light and dark phases in the cycle are concerned in the determination process.In general, a long light phase tends to suppress and a long dark phase to induce a diapause. In any combination, the path of development is decided by the balance between diapause‐preventing (light phase) and diapause‐inducing (dark phase) stimuli. However, as their effectiveness does not increase linearly with duration, the existing balance changes with the phase duration.The effectiveness of the light phase in suppressing diapause increases most rapidly between 8 and 16 hr.; that of the dark phase rises very sharply between 8 and 12 hr. Longer dark periods of up to several days duration also induce diapause but are no more effective than a 13 hr. phase. The inclusion in the cycle of very long periods of light or darkness may also influence diapause by reducing the number of complementary phases experienced by the mite during the sensitive period of development.M. ulmiis highly insensitive to the interruption of effective light and dark phases by short intervals of darkness or light—a further indication of the slow inception of the light‐and dark‐phase reactions.These findings are discussed in terms of hypothetical mechanism involving cumulative synthesis and removal of some active substance, but the experimental results cannot yet be fully reconciled with a simple hypothesis of

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02388.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

Successive spraying with DDT suspensions of the adults of a wild colony ofDrosophila melanogasterand the progeny of survivors enhanced the resistance to that insecticide.The rate at which resistance increased depended on: (1) the relative proportion of resistants to susceptible individuals, or (2) the intensity of selection as measured by the concentration of DDT, the proportion killed or on both. The resistance of the populations of the insects fluctuated considerably whether subjected to successive sprayings or not, and in one sprayed series there was some indication of a rhythm with peaks of susceptibility occurring at regular intervals.Enhanced resistance may show a change of slope in the probit log concentration regression line, leading to different relative values at different levels of mortality, or by a parallel shift of the regression line. The former appears to be a preliminary stage of selection and indicates a change in the frequency distribution within a population.Increasing the concentration of DDT, slowly or rapidly, may have enhanced resistances at an increased rate, but the series sprayed with the lower initial concentration reached finally the same end point, as judged by the values of loglc. 50.During the course of these experiments the insects developed sensitivity to carbon dioxide (used in anaesthesis). Its bearing on our work is considered in Part II.

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02389.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

During the selection of a stock ofDrosophila melanogasterfor resistance to DDT, in which carbon dioxide was used for purposes of anaesthesis, a sensitivity to this gas developed. The phenomena closely paralleled those shown by the CO2‐sensitive ebony stock isolated by L'Héritier and his co‐workers. An experimental analysis of its effect upon DDT sensitivity was made. It was found that a stock selected for CO2resistance gave the same probit regression line as the original stock. A CO2‐sensitive stock, whether anaesthetized with nitrogen or carbon dioxide, gave the same regression line at a temperature of 25° C. at which CO2sensitivity disappeared, or at 15° C. if adjustment to the proportion of deaths in the control was made. The effect of CO2was therefore to limit the population from which selection is made for DDT resistance, rather than to alter the distribution of DDT resistance within t

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02390.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

Adult apterae ofMyzus persicae(Sulz.), were fed, after a period of fasting, on leaves containing radioactive phosphorus. The weight of sap imbibed by the aphids after varying feeding times was estimated by relating their radioactivity, at the end of each feeding period, to the activity per unit fresh weight of the leaf lamina on which they fed. The calculations were made on the assumption that32P is uniformly distributed in the leaf tissues.The mean rates of uptake so estimated were about 10 μg. of sap for the first hour of feeding; 40 μg/hr. between 1 and 4 hr. feeding, and 17 μg./hr. between 6 and 24 hr. feeding. The decrease in apparent rate of uptake with the longer feeding times is attributed to loss of32P in nymphs born during the feeding period.When aphids were fed on seedlings raised in water culture solution containing32P no activity was detected after 5 min. feeding and an insignificant fraction after 15 min., but when the isotope was introduced by immersing the leaves for several days in the culture solution, aphids fed for 5 and 15 min. became appreciably active.The increase in rate of uptake after 1 hr. of feeding indicates that aphids do not start to feed normally until they reach the phloem, but the activity after short feeding times suggests that previously starved aphids feed to some extent on other tissues, possibly only on the epiderm

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02391.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

The reported low toxicity of schradan to honey‐bees has been confirmed. This has led to the consideration of the possibility that this systemic insecticide might be present in nectar and subsequently appear in honey in an unchanged form.Using radioactive schradan labelled with32P, it has been shown that spray applications of this insecticide to mustard and borage plants result in the contamination of nectar. A series of nectar samples taken over a 4‐week period following spraying showed on radio assay a progressive decrease in total32P content and also in the amount of schradan present in proportion to the decomposition products. The highest figure recorded for the schradan content of nectar was 21 p.p.m.Tests on the stability of schradan in contact with the honey stomach of the bee and also in contact with the enzyme invertase,in vitro, showed that no appreciable breakdown occurred. Furthermore it was shown that schradan was stable in contact with honey over a period of 2 1/2 months.It is concluded that this systemic insecticide may appear in an unchanged form in the honey obtained from the nectar of plants which have been sprayed less than 4 weeks previou

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02392.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

Beans inoculated with tobacco necrosis virus were kept in the dark at different temperatures for 1 hr. before and 1 hr. after inoculation; in this experiment the number of lesions increased with temperature over the range 55–82° F.The effect of 30 min. periods of darkness before or after inoculation depended on the time of day, the number of local lesions usually being decreased. Prolonging the night period before inoculation sometimes increased the number of lesions.Light appeared to be more important than temperature in controlling the daily variation in susceptibility. However, in a test over a 30 hr. period this variation continued even when plants were placed under constant conditions before and after inoculation.When plants that had been kept in the dark were exposed to light of about 800 f.c. intensity for 1 min. immediately before inoculation the number of local lesions was doubl

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02393.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

Italian ryegrass and a late‐flowering red clover were grown together, with abundance of water and nutrients for both. It was found that even a small number of ryegrass plants reduced the growth of clover by 30%. This effect varied very little with increasing density of the clover crop.The presence of clover reduced the ryegrass crop by an amount diminishing as the density of the ryegrass was increased. In a sparse crop of ryegrass, clover reduced the growth of the grass considerably more than did barley under comparable conditions.There is no evidence of any specific effect of the roots of one plant on the other. When ample nitrogen is available the clover tends to take some that would otherwise be available for the grass and does not provide the grass with additional nitroge

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02394.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY

摘要:

The interaction of certain soil saprophytes andFusarium udum, the wilt organism of pigeon‐pea, with special reference to their effect on pathogenicity, has been studied. The filtrates ofAspergillus niger, Rhizopus nigricansand mixed filtrates of all the saprophytes inhibited the growth ofFusarium udumon solid medium. This inhibition ofF. udumhas been shown to be due to unfavourable reaction of the medium rather than to food exhaustion or the presence of toxic substances. The culture filtrates after passage through soil beds failed to affect adversely the growth ofF. udumbecause of the change in pH. Inoculation experiments have indicated that onlyRhizopus nigricansis effective in reducing the incidence of wilt because of its faster rate of growth. The mixed inocula of the organisms and mixed filtrates of all the saprophytes have also been observed to be effective in reducing wilt incidence.Aspergillus terreusappears to enhance the virulence ofFusarium udu

ISSN:0003-4746    

DOI:10.1111/j.1744-7348.1953.tb02395.x

出版商:Blackwell Publishing Ltd    

年代:1953

数据来源: WILEY