Results of Eight Years' Examination of the Habitats of Residual Urban Norway Rat Populations after Eradication

In Budapest (population 2 million) 33 out of 100 buildings were rat-infested in the early seventies. Thus general deratization was decreed to cover the whole of the city in 1971 to 1972. As a result of the global eradication method applied, the rate of rat-infested premises could be reduced below 0.5% annually. After control the sparsely occurring rats could freely choose any of the habitats released for settling. The habitats of Budapest most preferred by Norway rats are presented after a wide-ranging investigation carried out over 8 years. Our examination data support and in some cases reveal the requirements of Norway rats living in Central European large cities as regards to the environment as well as their ethological features. This knowledge may help in improving deratization and especially maintenance operations, thus increasing the efficiency of the fight against rats. AREA EXAMINED Budapest, the capital of Hungary lies in an area of 525 km with a population of 2 million. Its 22 administrative boroughs include 220,000 premises. Although pest management before 1970 had effected a certain decrease in the number of rats, still the problem remained unsolved due to treatments having covered only some parts of the territory and completed at different times. In order to determine the number of live rats and the losses incurred by them, the Public Health Authorities of Budapest effected assessments in 100 industrial units of various sizes and characters allowing the extrapolation of the data obtained to the whole of Budapest in proportion to the area. On the basis of this calculation the number of rats was estimated at approximately 2 million and the losses incurred by them at 300 to 400 million forints (6.4 to 8.5 million $US) yearly (Hercegh 1969, Gács 1974). In Budapest the Norway rat (Rattus norvegicus Berk.) predominated, while the black rat (Rattus rattus L.) played a minor role. The size of rat-infested premises were determined by an objective method based on the global eradication techniques,i.e., both assessment and placement of baits were carried out in each of the buildings. Detections of bait consumption by rats indicated a 32.8% rat infestation level in the premises on an average. While rat infestation in the centre of Budapest was approximately 100%, that of the suburbs was considerably lower. An organized deratization programme covering the whole of Budapest started after long and thorough preparations in spring 1971. In the course of the rat control action carried out by Bábolna Pest Control Centre, baits containing the anticoagulant ingredient coumatetralyl were placed in all of the premises and the appurtenant sewerage system simultaneously. The project consumed 1,632,000 kilos of bait. After cessation of consumption repeated inspections and replenishments of bait were effected. As a result of thorough work the rate of rat infested premises could be reduced below 0.5% by the end of the year 1972 (Burgert 1972, Gács 1974, 1977). Already during the period of deratization arrangements were made for the organization of follow-up treatments (prevention, maintenance). Maintenance of rat-free state was also done by the Bábolna Pest Control Centre commencing on January 1, 1973. As a result of continuous maintenance applications, which increasingly consisted of preventive measure, the annual rat infestation level continued to diminish (Bajomi 1980a, 1983a). The rate of diminution and the total number of rat occurrences in the 13 years are shown in Figure 1. Figure 1. Yearly index of the Budapest rat population as judged by the number of infested premises. 1986, Proceedings Twelfth Vertebrate Pest Conference (T.P. Salmon, Ed.). Printed at Univ. of California, Davis, Calif. 66 EXAMINATION OF HABITATS Thoroughly planned maintenance applications, which are implemented by only one company, enable a reliable inquiry of rat populations remaining after eradication. Searching of infestation foci, regular and directed assessments, as well as comprehensively and exactly kept registers, all being part of maintenance techniques, allow the analysis of rat occurrences and of the typical character of habitats. As a result of the deratization of Budapest, the size of the remaining rat population is exceedingly small in relation to the carrying capacity of the environment. Thus living space much larger than necessary is available for the rats. Consequently, the rats may, on principle, choose any vacant space free from any intraspecific competition (Szeky 1975). It may be supposed that in such a case the most favourable areas (habitats) will be colonized by the animals tested. By examining the occupied habitats, important information can be gathered concerning the requirements and habits of rats. Good use can be made of such knowledge in practical rat control. METHOD OF EXAMINATION To detect the habitats occupied by rats, the notifications received from the population and the public institutions were checked. Moreover, inspections were effected periodically in the sewers, the major factories, apartment buildings, and in the so-called "barrier zones." The presence of rats in the individual habitats could be best determined by measuring consumption from the poisoned bait. Survey of the habitats preferred was facilitated by the fact that if there was evidence of rats, rodenticides were placed within the buildings, around them, and in the appurtenant sewers (maintenance operations). In the sewers specific bait wax-blocks were used on which the gnawing marks of rats could be clearly recognized. The Health Authorities, who effected supervisory work monthly, also helped in getting acquainted with the habitats preferred. As standard of investigation served the so-called complex habitat. This represents the smallest part of the rats' living space, in which environmental conditions are the same but they are different from those of other habitats (Bajomi 1980a,b). Accordingly a complex habitat is, for instance an apartment building, having more than one story, its appurtenant cellar, courtyard and the sewers within the building as well as its flats, staircases and garret. This habitat is typical and differs, for instance, from the complex habitat of a family house or from that of a food manufacturing plant. At the same time rats live occasionally in a confined area (e.g., cellar of an apartment house) within such habitats of greater size like an apartment building. As a consequence, one complex habitat can be divided in several micro-habitats of different types, the latter being in connection with each other. The most important and best utilizable consequences can be drawn by studying these micro-habitats preferred by rats. In order to provide an exact registration and analysis of the relatively great number of rat incidences appearing over several years, a code system distinguishing 45 micro-habitats was set up. By means of this procedure, first the complex habitats of 3,355 rat incidences covering the period 1975 to 1978 were evaluated (Bajomi 1983b). On the basis of our experiences gained, the code system was reworked to achieve a more exact evaluation; thus a new code system including 95 micro-habitats within six main groups was established (Table 1). On repeated investigations on site and in accordance with the types of premises and areas of detection, each of the rat occurrences was given as many code numbers as micro-habitats had indicated the presence of rats. When, for instance, consumptions by rats were noted at bait-points in the cellar, in the courtyard and in one burrow with a family house then this occurrence was given the code numbers 22, 26, 27. RESULTS The present processing completed on computer covers the analysis of 3,550 rat occurrences of the period 1978 to 1985. In Budapest Norway rats occurred most frequently in the apartment building complex habitat at 30.03% (1,066 cases) and in the family house complex habitat at 17.21% (611 cases) in the 8 years under examination as per Table 2. Rats occurred in nonfood manufacturing plants at 15.24% (541 cases), in food manufacturing plants at 13.35% (474 cases), and in public institution complex habitat at 13.13% (466 cases). The remaining 392 rat incidences were not detected in complex habitats but in other areas (sewers, riversides, etc.). The fact that Norway rats did more frequently become established in apartment building complex habitats than in other ones is most remarkable! It is also interesting to notice that nonfood manufacturing plant complex habitats were frequented by rats to a greater extent than food manufacturing plant complex habitats. These peculiarities will most probably depend also on the number of the different types of complex habitats. For this reason these establishments can be generalized only with great caution! A more interesting conclusion can be drawn by studying those micro-habitats which are preferred by Norway rats living practically without any intraspecific competition. According to our investigations 3,550 rat occurrences were detected in 5,795 micro-habitats. Naturally there is a possibility for one rat to visit even several micro-habitats simultaneously. Considering that the rats were allowed to visit the individual micro-habitats and to settle at choice, this will be characteristic of their requirements towards the living-space and of their behavioural habits in urban environments.