Quantitative Relationships between Fleas and Rodents in a Hawaiian Cane Field!

Relative numbers of Xenopsylla vexabilis Jordan on Rattus eXlt/ans (Peale) and Mus musculus L. and in their nests were determined monthly in a field of maturing sugar cane at Kukuihaele, island of Hawaii, January 1962-March 1963, in a preliminary study using nest boxes and live traps. Mean numbers of fleas in active rat nests were most closely correlated with mean numbers of fleas infesting rats when they were cage-trapped in the previous month. Mean numbers of fleas in active mouse nests were most closely correlated with mean numbers of mice that had been recorded per mouse nest found occupied in the previous month. Ratios of mean number of fleas on trapped rats to mean number of fleas in rat nests the following month averaged .20 when means increased and .30 when they decreased. When the total flea index was substituted for mean number of fleas in nests, ratios averaged .32 when fleas became more abundant and .36 when they decreased in numbers. Further study of fleas in rat nests is needed before predictions of flea abundance are routinely made from fleas counted on trapped rats. To OBTAIN A MEASURE of rodent-flea abundance, the plague surveillance program in Hawaii has emphasized trapping rodents rather than excavating burrows for nests because monthly systematic excavation is not feasible in the fields of sugar cane that cover most of the enzootic plague area on the Hamakua Coast. Basic studies by Eskey (1934) on the epidemiology of plague were concerned with area-wide quarterly abundance of fleas on rodents, but not in nests. Flea and rodent populations can vary greatly in both space and time in Hamakua. Therefore, it would seem ideal to ascertain the mean number of fleas in nests in a small, relatively uniform area such as a cane field, and to relate this number in some way to the mean number of fleas on routinely trapped rodents in that field so that accurate predictions of total flea abundance or scarcity, or at least the mean 1 This investigation was supported in part by U. S. Public Health Service Research Grant No. AI02886-06TMP from the National Institute of Allergy and Infectious Diseases, Dr. C. M. Wheeler, Principal Investigator, and by the Department of Health, State of Hawaii. Manuscript received June 3, 1968. 2677 Deerpath Drive, Deerfield, Illinois 60015 (formerly with the Plague Research Unit, Department of Health, State of Hawaii, Honokaa, Hawaii). 70 number of fleas in nests, could be made for any particular field that is under surveillance by routine trapping only. Hence, the objective of this preliminary study was to obtain data sufficiently homogeneous for meaningful analyses of correlation and regression leading to identification of cause and effect relationships of predictive value in the Hamakua District, Hawaii. Success was limited; consequently, the results will have more academic interest for ecologists than immediate practical value to plague surveillance personnel. MATERIALS AND METHODS Location and Description of the Study Area Cane field 101b of the Honokaa Sugar Company, Hamakua District, island of Hawaii, was selected as the study area because the last isolation of the plague bacillus in Hawaii was made from fleas collected there in May 1957. This 15.4-ha field (Fig. 1) lies in the corner formed by the Pacific Ocean on the north and Waipio Valley on the west. The village of Kukuihaele borders on the east. Elevation ranges from 220-300 meters, and slope is 12-20 per cent. Roads and gulches fragment the field. The Quantitative Study of Fleas and Rodents-HAAS deepest gulch has many rock outcrops and trees; other gulches are less rocky and have a ground cover of grass, forbs, and shrubs. The soil is a reddish, granular humic latosol in the Lalakea silty clay loam series. Haas (1965b) described certain climatic and microclimatic conditions of the area and cited literature describing the district. Cultme of Sugar Cane After harvest in May 1961 of a third crop (second ratoon) from a planting made about 6 years previously, the field was plowed. A new crop (plant cane) was planted during the period June 3D-July 12, 1961, and the main part of this study was conducted during growth of this crop. In February 1962 a mixture of Dalapon, Karmex, and Atrazine in oil was applied to weeds among the cane plants. In March 1962 a fertilizer containing 61% K20 was applied. In May 1962 weeds were hoed by hand. There was no irrigation despite a severe drought. Harvest was in May 1963 when the cane was slightly over 22 months old.