Interstrain differences in the resistance of the silkworm larvae, Bombyx mori L., to DDT and Sumithion and the mode of inheritance of the resistance were studied. Responses to DDT and Sumithion varied markedly in 269 silkworm strains, and the LD₅₀ values (μg/g) of DDT in the usceptible and the most resistant strains were 0.2 and 40.0, respectively, whereas the LD₅₀ values of Sumithion in these strains varied from 0.3 to 13.2. When frequency of 269 strains was plotted against log LD₅₀ value of each toxicant, a normally distributed curve was obtained. The mean LD₅₀ values (μg/g) of DDT and Sumithion were 2.1 and 1.5, respectively, and both the insecticides seemed to have about the same toxicity to the silkworm. However, a significantly negative correlation was obtained (r=-0.42) between the LD₅₀ values of DDT and Sumithion. The results of study on the dose-mortality lines of resistant and susceptible strains and their F₁, F₂ and backcrosses indicated that DDT-resistant and Sumithion-resistant characters were inherited as in multifactorial genic systems.

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In order to establish a reasonable method for predicting the hatching date of Panonychus ulmi (KOCH) in spring, the intensity of diapause during the whole hibernating period has been investigated at Kuroishi, Aomori Prefecture. Winter eggs of this mite were collected at week intervals from an apple orchard of the Apple Experiment Station and incubated at different temperatures. It was found that their response to incubation changed progressively during the hibernation period. Except for a very few non-diapause ones, most eggs failed to hatch at 20° or 25°C in autumn, but their hatchability increased in the later season and reached its maximum between the end of January and the end of February. At the same time, the duration of incubation period and its variability decreased regularly, and became almost constant when the percentage hatching attained the maximum value. At an incubation temperature of 15°C, on the other hand, many eggs were able to hatch even at the beginning of hibernation. They hatched, however, over long periods in the early incubation but took less time in the late incubation. The incubation period thus reached its minimum by the middle of February. Exposure to 5°C followed by incubation at 21°-22°C gave essentially similar results, which suggest that the main factor for breaking diapause in the field is the accumulating action of low temperatures. From these results, it might be assumed that the diapause in P. ulmi can be regarded as terminated by the end of February at least from the view point of prognosis. In Aomori Prefecture, the daily mean temperature rarely exceeds the developmental threshold (ca. 7°C) for the mite eggs before March so that it seems reasonable to calculate the effective temperature summations from March for the purpose of predicting the hatching time.

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Nuclear polyhedroses of Malacosoma neustria testacea MOTSCHULSKY, Dasychira confusa BREMER, Ivela auripes BUTLER and Neodiprion sertifer GEOFFROY were described. Electron microscopic study was made on the nuclear-polyhedrosis virus of M. neustria testacea. Ultrathin sections of the infected tracheal epithelium and epidermis showed the presence, in the cell nuclei, of rod-shaped virus particles and polyhedra, within which virus particles were also present. The virus particles isolated from the polyhedra were, in most cases, found to be bundles consisting of two rods. In rare cases, however, the virus rods were found singly or as bundles of three rods. The rod was 300×50mμ in size.

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The percentage of the diseased female moths of silkworms always diminished with high speed in accordance with retardation of eclosion date irrespective of the difference of maturation days of larvae and their combination like that of the previous experiment (OHSHIMA et al., 1966c). In the case of male moths, their diminution velocity fairly decreased and became somewhat irregular. Comparing the result of the present experiment with that of the previous, there exist two great differences: (1) The percentage of diseased female and male moths of larvae that ripened early was inferior to that of the larvae that ripened late in the present experiment, while it was just the reverse in the latter. (2) Reversal in the percentage of diseased male moths occurred only on the day near the end of eclosion where their number greatly diminished in the former, if it happened, while it always occurred on an intermediate day where a great many moths eclosed. When a room was continuously lightened by the electric lamp every night from 0 hour until day break, the eclosion time of diseased female moths moved up and it happened earlier by repeated lightening, but in the case of diseased male moths, this phenomenon was not so conspicuous as that of the diseased female moths.

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Some histopathological observations were made to realize the mechanism of sterilizing effect of metepa on male adults of the adzuki bean weevil, Callosobruchus chinensis L. Metepa was dissolved in acetone at the concentrations of 10, 20, 40 and 100μg per μl. One μl of test solution was applied to the dorsal side of the just emerged male adults. Treated weevils were left for 0-7 days. These dosages applied were the amounts completely inhibiting the hatchability of deposited eggs. No histopathological changes in testicular tissue were observed by the application of metepa at any dosage or at any elapsed time after application. The external shapes of eggs deposited by treated male×untreated female and by untreated male×untreated female were normal. Although a single untreated female did not deposit eggs on the adzuki beans in the Petri dishes, when 10 individuals of untreated female were introduced into the Petri dishes together with adzuki beans, they deposited their eggs on beans. The external shapes of eggs, however, were abnormal. From the results mentioned above, it could be considered that the spermatozoa of treated male maintained the ability for fertilization. This assumption could be proved by the histopathological examination about the eggs deposited by treated male×untreated female. The embryogeny of the eggs deposited by treated male×untreated female progressed until the early stage of cleavage, but the blastoderm was not formed. The necrosis of cleavage nucleus and plasmolytic atrophy of vitellus were observed in the eggs at 48 and 96 hours after deposition respectively.

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Intact eggs of the silkworm, Bombyx mori L., which are covered with chorion are suffocated in culture media because of inhibited gas exchange through respiratory canals. When the chorion was taken off, naked eggs developed well in culture media. And, when cultured eggs were infected with the virus, polyhedron formation was confirmed to occur in nuclei of various kinds of their tissues, including those in which polyhedron formation has never been reported at the larval and pupal stages: serosa, yolk cell, amnion, epidermis, tracheal epithelium, ganglion, fore-gut, hind-gut, silk-gland, muscle, fat-body, gonad, blood cell and mid-gut. This is remarkable, because polyhedron formation has never or scarcely been known in intact silkworm eggs, and definite results have not been obtained even by injection of virus in them. Diapause eggs also developed in culture media, though the growth of embryos was delayed 3 to 4 days as compared with cultures of non-diapause eggs. In accordance with this delay of growth, polyhedron formation was retarded or suppressed in cultures of diapause eggs. Culture of naked silkworm eggs can be applied as a simple technique to virus infection experiments under aseptic conditions and of use for studies of many problems of virus infection.

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Investigations were carried out at Asso, Wakayama Prefecture on the 1st, 2nd and 3rd generations of the southern green stink bug, Nezara viridula L., during the period of 1961-1962. With regard to the unparasitized eggs, there was no consistent relationship between the hatchability of eggs and the size of the egg mass, except that the small sized egg masses tended to contain a considerable number of unfertilized eggs. Parasitism is often responsible for an apparent regularity between the hatchability and the egg mass size, e.g. the 2nd generation in 1961 and 1962. It was considered that this was resulted from the bias relative to time both in the activity of parasites and the frequency distribution of egg mass size of the host. It was revealed by the key-factor analyses that the mortality during the 3rd instar affected mostly on the total mortality from hatching up to the 4th instar. And that the mortality of the 1st instar larvae was mostly constant irrespective of the size of the larval aggregation; that of the 2nd instars also remained almost constant except for the small aggregations which showed a higher mortality; whereas the mortality of the 3rd instars reduced with an increase in the size of aggregation. The predation of spiders, a “constant number” type of mortality (SOLOMON, 1964), was suspected to be one of the factors giving rise to such a differential mortality of the 3rd instars related to their aggregation size. The survival value of the aggregation of the 2nd instar larvae observed in the laboratory (KIRITANI, 1963) was not detected under the natural conditions. It is concluded that the larger the initial size of the larval aggregation, the higher the survival rate of the aggregation becomes. This means that the numerical contribution of the large aggregation is far greater than that expected merely from the survival rate.

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