SIR,-"Brain damage" occurs in infants and may or may not be causally related to immunisation procedures. Fortunately the number of cases is small; figures of between 1 in 10 000 and 1 in 100 000 are quoted. Assuming for the sake of the argument that immunisation is a cause-we know it is not the only cause-why do we blame the pertussis component of the vaccine? Practically all immunisations in the past 25 years have been triple immunisations; the diphtheria or the tetanus vaccine could be as guilty as the pertussis vaccine. We have no large-scale experience with either "pertussis only" or with "diphtheria-tetanus only" vaccinations and in view of the rarity of the alleged serious sequelae a prospective study of one or the other of these alternatives would have to be undertaken on truly enormous cohorts. I repeat, are there any facts, as distinct from hearsay and the irresponsible gossip of politicians and the media, that lay the blameif there is any blame-upon the pertussis vaccine ? G SCHOENEWALD

An examination is made of the effects of gamma radiation on laboratory strains of the grain and rice weevils, Sitophilus granarius (L.) and Sitophilus zeamais Mots., when reared at 26°C and 77 per cent relative humidity. Three criteria were used to determine radiation susceptibility: (1) the ability of immature stages to develop and emerge as adults from the grain, (2) survival after emergence and (3) the production of adult progeny. The effects of radiation on the complete life-history of S. granarius were examined at 24-hr intervals; each stage of development was subjected to 17 doses ranging from 250 to 20,000 rep. Similar observations on all stages of S. zeamais allow a comparison of the susceptibility of the two species. Periodicity in fertility in the grain weevil was examined at substerilising doses; dose levels evaluated for commercial disinfestation were tested against large populations.

The efficacy of 16,000 rads, recommended for industrial applications of gamma radiation for the control of insects in grain, is substantiated by the complete sterilisation and death of 10 million insects of a wild strain of grain weevil under conditions simulating bulk storage. Substerilising doses of 10,000–14,500 rads suppress weevil populations to a very low level and prevent their increase for 4 to 8 months; this period of “short-term” control is related to the size of the population and oxygen tension at irradiation. The reproductive potential of fertile and subfertile grain weevils is depressed when added to large irradiated populations; partial protection to the grain against reinfestation is afforded by the insemination of contaminants with sterile sperm, which remains competitive within the female and with fertile sperm subsequently inseminated, for periods greater than 4 months. A reduction in reproductive potential of weevils which may be underdosed at irradiation gives considerable flexibility to the requirement of dose uniformity in plant design for disinfestation of grain.

An examination is made of the effects of 8 culture densities and 2 post-irradiation treatments on the susceptibility of adult grain weevils to gamma radiation from 60Co. The lethal effect of gamma radiation is increased by culture densities which cause a substantial rise in metabolic temperature above 26°C during larval development. Densely crowded cultures in which temperatures during growth and maturation of larvae do not rise appreciably above 26°C fail to modify the adult's lethal response. Temperature fluctuations up to 36°C during development do not modify the susceptibility of the adult to radiation sterilisation. Accordingly, the efficacy of 16,000 rads evaluated for the control of grain weevils is unlikely to be reduced by population densities normally encountered in commercial storage.

No difference could be detected in the numbers of emrged adults, rate of emergence or stage at death in the grain when two types of infested wheat, containing mostly pupae, were treated with gamma radiation and accelerated electrons. Mature adults were more susceptible to killing and sterilisation by gamma irradiation. Possible explanations of the observed results are discussed. There is little likelihood of grain weevils providing a useful biological dosimeter in engineering studies for radiation disinfestation of grain.

Whilst reduced variability in the gene constitution of laboratory strains makes them useful for preliminary studies on radiation susceptibility, wild strains must also be examined in evaluating radiation levels for commercial application. Tests on five laboratory strains and 30 wild strains of Sitophilus granarius (L.) from various parts of the world showed marked differences in susceptibility to killing by irradiation but only slight differences in response to sterilisation. The dose level of 16,500 rep, previously evaluated for the sterilisation of large populations of a laboratory strain, may therefore be recommended for the commercial disinfestation of naturally occurring populations.

AbstractExperimental sources of cobalt‐60, made radioactive by neutron irradiation, have been used during recent years for investigating the radiation susceptibility of insect pests. Lethal and sterilising effects have been examined on species, strains and developmental stages to evaluate the dose levels required for commercial treatment. The technical feasibility of using radiation as an insecticide is now established.Results of research and problems of application are examined in relation to the two potential methods of controlling insect pests by irradiation; indirect control by the release of sterilised adults and the direct treatment of infested products. Application of the first method, exemplified by work on screw‐worm fly, is limited to very few species by a number of biological requirements. Direct treatment could have wider application for the control of all species infesting stored products. The most favourable products for radiation dis‐infestation are those handled in bulk at particular centres, and for this reason radiation treatment of grain has been examined in detail. The fundamental and applied problems of using γ‐radiation for this purpose are discussed and the relative merits of fumigation and radiation sterilisation are compared.It is concluded that among factors which may influence the success of radiation disinfestation, bulk handling and the incorporation of treatment into established handling procedures must receive primary consideration. γ‐Irradiation may eventually take its place as a useful means of insect control, but it is unlikely to displace conventional methods.

The aerial dispersal of the Pseudococcid vectors of virus diseases of cacao in Ghana and the possible influence on it of wind speed was studied at Tafo. Seasonal variations in wind speed in Ghana are slight, particularly inland in the cacao-growing areas. Observations at Tafo during 1955 showed that air movement was maximal between 12.30 and 3.0 p.m. and minimal during the hours of darkness. Daily wind speeds in the open, from April to November 1955, averaged 1·1 m.p.h., 4 ft. above ground. About 80 per cent, of wind speeds measured in one-minute periods on two days in February 1955 were within the range 1–5 m.p.h. and 11–12 per cent, between 5 and the maximum recorded speed of 7 m.p.h.In cacao, wind speed is reduced by a factor of 10–20 times; daily speeds averaged 0·09 m.p.h., at 8 ft. above ground during April and May 1955. The highest daily averages under a closed canopy (400–650 ft./hr.) were recorded 2 ft. above ground-level; speeds fell to a minimum (25–125 ft./hr.) in the canopy at 15–25 ft., and rose again above the canopy at 30 ft. to a speed (250–350 ft./hr.) comparable with that just below the branches at 10 ft. At breaks in the canopy caused by the removal of diseased trees, daily averages were lowest (125–150 ft./hr.) 2 ft. above the ground. They rose to a peak (400–600 ft.) on a level with the lower branches of the canopy, dropped markedly (250–450 ft./hr.) at the level of the middle canopy and rose to a maximum (650–1,000 ft./hr.) at 30 ft.All instars of Pseudococcus njalensis Laing walking on pieces of cacao wood in the laboratory withstood removal at air speeds of 10 m.p.h., but these and corresponding stages of three other species could be dislodged by gently tapping the wood. The late nymphs and adults of Ps. njalensis and Ps. gahani Green were more easily removed than their ‘crawlers’, though this difference was not found between the developmental stages in Planococcus citri (Eisso) and Ferrisiana virgata (Ckll.). Amongst the four species tested, first-instar nymphs of Pl. citri were most easily dislodged, and those of Ps. njalensis or F. virgata more so than those of Ps. gahani.Airborne mealybugs were caught on adhesive traps, on bait twigs pinned to mature trees, and on cacao seedlings. Eight vector species became established on cacao after dispersal by air currents.Under a closed canopy, more airborne mealybugs were caught at two and ten feet above ground than at levels in and above the canopy. At breaks in the canopy, catches averaged about 13 per cent, of those obtained under a continuous canopy and were insufficient to show changes in aerial density with height. The distribution of catches over the surface of traps would suggest that mealybugs drop from the branches, are carried by air currents when falling, and become laterally dispersed at levels a few feet above the ground.In a clearing where cacao had been removed to simulate conditions following the routine cutting out of virus-infected trees, airborne mealybugs became established on seedlings at a distance of 45 ft. from infested cacao trees. The ratios of boxes of seedlings which became infested by aerially dispersed Ps. njalensis at increasing distances from infested standing cacao, in relation to those beneath it (unity), were: 0·86 at 10–20 ft., 0·57 at 30 ft. and 0·14 at 40–100 ft. Under conditions of high insolation, the maximum recorded distance of mealybug aerial dispersal from surrounding vegetation to cacao seedlings was 340 ft. Aerial catches on seedlings 40 to 165 ft. from cacao showed an over-all decrease with distance.Aerial dispersal is more pronounced during dry conditions, particularly during the main dry season, December–February, and to a lesser extent during the brief dry period experienced in July or August.The infestation rate of cacao seedlings by windborne mealybugs (predominantly Pl. citri and F. virgata) was increased by 50 per cent, when plants were protected from weather by artificial shade. These traps, insulated from ants, failed to become infested by Ps. njalensis.Out of 64 young cacao trees, 22 per cent, became infested by airborne vectors during the five-month period May to September 1955; when, for similar trees, normal dispersal was augmented by an initial artificial colonisation with Ps. njalensis, the corresponding figure was 41 per cent. The infestation rate, after either augmented or natural dispersal, was not significantly affected by attempts to establish on the trees nests of the ant, Grematogaster striatula Emery, or by affording protection from the weather in the form of artificial shade. It was evident, however, that the presence of mealybug-attending ants is almost essential for infestations of Ps. njalensis to develop, but there must be other limiting factors, since establishment failed on 58 per cent, of trees on which coccidophilic species were present.The part played by airborne vectors in extending infection by radial and ‘jump spread’ is discussed, together with the possible use of insecticidal measures to prevent their establishment on the trunks of healthy cacao.

AbstractA preliminary study has been made of the effects of three dose levels of γ‐radiation on the physical, biochemical and baking properties of English wheat. Products made from irradiated grain have been examined for organoleptic changes by a taste panel of selected members. The slight changes induced by irradiation are considered of little commercial significance.