PHYSIOLOGICAL AGE OF IXODID TICKS - ASPECTS OF ITS DETERMINATION AND APPLICATION

This review analyzes 30 yr of data concerning physiological age of ixo did ticks. The level and state of nutritional reserves in unfed ticks has been proposed as an index of tick physiological age. The dynamics of the use of these substances reflect tick aging. Thus, physiological age of ixodid ticks is determined by estimating the irreversible chan ges caused by natural vital activity in the body of unfed ticks, which inevitably raises the probability of tick death. The 4 age groups use d in most studies correspond to different periods in the life of unfed ticks: the postmolting development, the commencement of active life ( = questing activity), the midperiod of active life, and the final peri od of active life. The aging of adult ixodid ticks has been studied in natural populations and laboratory colonies of different species belo nging to several genera. The dynamics of age composition through the a ctivity season observed in adult Ixodes persulcatus, I. ricinus, and D ermacentor reticulatus from field populations correlated well with the known data on the life history of these species. Physiological age wa s successfully used as an indicator of the patterns of tick activizati on (= beginning of questing activity). Tick susceptibility to acaricid es was shown to increase with tick aging. The success of maintenance, reproduction, and transmission of pathogens was found to be strongly i nfluenced by tick physiological age. The physiological age can provide information about the present physiological status of an individual t ick or a group of ticks and, on this basis, predictions can be made (f or example, life expectancy, degree of response to certain factors). T he physiological age does not give reliable information about the past life of ticks, in particular their calendar age. In this respect, the meaning of the physiological age of ixodid ticks corresponds to the b iological (physiological, functional) age of animals, as used in geron tology, and differs from the physiological age of mosquitoes and other bloodsucking dipterans. Balashov's histological method of tick age de termination successfully used by many authors, remains the most approp riate, until now. However, there is ample room for further advancement of methodologies of tick age assessment. The future methods should be based on the quantitative estimation of a number of characters (test- battery) related to different organs or systems.