Further observations on an insecticide-baitbox method for the control of sylvatic plague vectors; effect of prolonged filed exposure to DDT powder

A field experiment was conducted from July 1958 to February 1959 in the San Francisco Bay region to test the effect on flea populations of prolonged exposure of the native meadow vole, Microtus californicus, to 5% DDT powder in insecticide-bait-boxes. Previous studies had shown that two important vectors of sylvatic plague, Malaraeus telchinum and Hystrichopsylla linsdalei, could be controlled both on their hosts and in the host nests by this method. However, the question of possible resistance was postulated since the control of H. linsdalei on its host was inferior to control of other flea species.

Bait-boxes with DDT powder were maintained in the field for 6 months. Records of flea populations on Microtus during this period showed equally good control of all flea species. Observations on vole nests at termination of the study revealed deposits of DDT and good control of all flea species in them. During the period of study, the evidence suggested that none of the flea species involved exhibited DDT resistance.

The major portion of the field routine was done by Messrs A. R. Kinney and R. L. Martin. Flea identifications were made by Mr H. E. Stark. Analyses of nests and rodents for DDT were performed by Mr C. Cueto, Technical Development Laboratories, CDC, Savannah, Georgia.

Experience in the eradication of enzootic plague in the north-west part of the Caspian region of the USSR

The writer recounts the history of plague control in the north-west part of the Caspian lowlands from 1924 to 1956. Plague has long been enzootic in this area, the main rodent involved being the little sisel. Until 1932, sisels were exterminated only in small areas where epizootics occurred, but from 1933 to 1941 an extensive extermination campaign was waged, covering up to 7 million hectares annually. The excellent results achieved by these wholesale measures suffered a setback during the Second World War, but effective rodent control was re-established in 1947 and it is thought that complete eradication of enzootic plague from the area can now be expected.

A confirmed case of DDT-resistance in Xenopsylla cheopis in India

The authors describe a technique, developed in their laboratory at Poona, Bombay State, for estimating the susceptibility of the oriental rat-flea, Xenopsylla cheopis, to insecticides. Using this technique, they compared the susceptibility to DDT, gamma-BHC and dieldrin of a field population of X. cheopis from a locality in Poona District which had been sprayed with DDT 28 times, with that of a similar population from a locality in North Satara District which had been sprayed only three times. The Poona strain proved to be highly resistant to DDT and moderately resistant to BHC, which has never been used in Poona District, but remained susceptible to dieldrin, although slightly less so than the Satara strain. The authors consider this a clear-cut case of the development of physiological resistance as a result of continued DDT spraying, and suggest that the rat-fleas in any area where DDT has been used on a large scale for disease control should be tested regularly for susceptibility so that, in the event of an outbreak of plague, the insecticide most likely to destroy the vector could be applied from the outset.

Wild-rodent-flea control in rural areas of an enzootic plague region in Hawaii; a preliminary investigation of methods

Preliminary field tests were carried out to develop methods for controlling fleas on wild rodents in the enzootic plague region of the Island of Hawaii. Among several designs, a metal hood-type DDT bait-box was developed which was simple in design, low in cost, easy to transport to and maintain in the field, protected the insecticide from the elements, and attracted all species of field rats inhabiting a gulch in the Hamakua District. These rats-Rattus rattus (and its subspecies), R. norvegicus, and R. hawaiiensis-readily fed in the DDT bait-boxes and were dusted with 10% DDT powder (in pyrophyllite) on contacting hanging cloth sacks, containing the insecticide, when entering and leaving the bait-boxes. The results indicate that a reduction of the plague-vector fleas, Xenopsylla cheopis and X. vexabilis hawaiiensis, may be obtained both on rats and within their burrows and nests, by the use of DDT bait-boxes. It is suggested that the use of such bait-boxes for wild-rodent-flea control may constitute a valuable method in the prevention of epizootics of plague in the Hamakua District, Hawaii.