G. Taylor

REARING SPODOPTERA EXEMPTA ON SEMI-SYNTHETIC DIETS AND ON GROWING MAIZE

Larvae of S. exempta can be reared very successfully on a diet containing dried maize‐leaf powder renewed at about weekly intervals. The larvae pass through either five or six instars, as shown by measuring their head capsules. Sterilized soil should be provided for pupation. The moisture content of the soil does not appear to influence the survival, fecundity or fertility of the adults.

The fumigant action of formaldehyde incorporated in a semisynthetic diet on the granulosis virus of Pieris brassicae and its evaporation from the diet

Abstract If Pieris brassicae eggs contaminated with granulosis virus 1 in the form of intact inclusion bodies are exposed to formaldehyde gas under suitable conditions, the virus can be inactivated without preventing the eggs from hatching. The formaldehyde may be derived from formalin solutions, but it is more convenient to enclose the 1-day-old eggs with the diet containing formaldehyde on which the young larvae are intended to feed and to open the jar the day before they hatch. This involves less handling than washing the eggs. As would be expected, transovarially transmitted infection is not reduced. Colorimetric assays show that at 20°C the equilibrium concentration of formaldehyde gas in the air space over the diet in a closed jar is low but that it evaporates relatively quickly from diet in open containers. As a consequence, under the condiditions in which diets are used to feed larvae, their antiviral activity declines rapidly, and it must be presumed that the preservative action of the formaldehyde is also of short duration.

The stability of a purified granulosis virus of the European cabbageworm, Pieris brassicae, in dry deposits of intact capsules

Abstract The stability of a granulosis virus of Pieris brassicae in the form of dry deposits of the intact capsules has been investigated. It is shown that in the dry form, the virus lost a significant amount of activity in 2 days at 20°C and that the rate of loss of activity increased with temperature, but that a wide variation in relative humidity of the air to which the virus was exposed when dry was without effect. When suspensions and dry deposits were compared at temperatures up to 30°C, activity was lost more quickly by the dry virus.

The effect of reducing the content of certain ingredients in a semisynthetic diet on the incidence of granulosis virus disease in Pieris brassicae

Abstract The incidence of granulosis virus disease among Pieris brassicae larvae feeding on a synthetic diet and modifications of this diet containing less of certain ingredients has been examined. The content of each of the following ingredients was reduced in different batches of the diet: calcium, magnesium, potassium, mixed minor elements, choline chloride, sugar, casein, ascorbic acid, mixed vitamins and the preservatives Aureomycin, and methyl p -hydroxybenzoate. The virus was either fed to the larvae or transmitted to them transovarially. Only when either the sucrose content or the casein content was reduced was there an effect, i.e., an increase, in the incidence of virus deaths. Further work is in hand to obtain an explanation of these results.

The stabilizing effect of insect hemolymph on a granulosis virus held in darkness as dry films of the intact capsules

Abstract The stabilizing effects of either insect hemolymph, gelatin, or glucose on a granulosis virus of Pieris brassicae held in darkness, as dry films of the intact capsules, have been investigated. The hemolymph solids exert a significant stabilizing action; gelatin is significantly less effective; and glucose almost ineffective. The protective action of hemolymph, which occurs in films deposited on glass and leaf surfaces, is probably due to the proteins present since viruses are known to be stabilized by certain types of foreign proteins. This explanation would also account for the high level of stability of insect viruses in dried films of crude preparations held in darkness. (In daylight there is also a screening action which excludes some UV radiation.)

Formaldehyde as an antiviral agent against a granulosis virus of Pieris brassicae

Abstract A granulosis virus of Pieris brassicae failed to produce infection in second-instar larvae when applied to the surface of a synthetic diet containing 0.04% w v of actual formaldehyde (H·CHO). On the other hand, tests in which the diet containing formaldehyde was fed to the larvae which had already fed on virus afforded no evidence that it could prevent the development of symptoms. In the case of transovarially transmitted infection, also, the incidence of primary virus disease was as low among larvae fed on diet without formaldehyde as on diet containing formaldehyde. In the case of P. brassicae the low incidence of virus observed among certain stocks feeding on diet compared with sibling larvae on fresh cabbage leaves cannot, therefore, be ascribed to the presence of formalin in the diet.

Bioassaying an insect virus on leaves. II. The influence of certain factors associated with the larvae and the leaves.

Abstract The importance of certain factors which act through the larvae or through the leaves when a virus suspension is bioassayed by applying a known volume in droplets to a leaf of standard size has been investigated. The most critical was the age of the second-instar larvae. In 24 hr their resistance to infection increased significantly. On the other hand, starving the test larvae for 6 hr to assist randomization of factors that influenced the area of treated leaf consumed did not produce a measurable effect on mortality. A 2°C increase in the temperature reduced the incubation period of the disease but produced no effect on the final kill due to the virus. The practical implications of these results are discussed in relation to the bioassay procedure. The method allows a 5-fold change in the virus concentration to be detected with certainty and is adapted for investigating problems concerned with the stability and persistence of the virus on leaves under practical conditions.