Age structure and dynamics of house dust mite populations

Age structure-the relative numbers of eggs, immatures and adults-in populations of the house dust mites Dermatophagoides pteronyssinus and Euroglyphus maynei was investigated in four sequential monthly samples taken from mattresses in each of eight homes in Glasgow, Scotland. Additionally, age structure of D. pteronyssinus was determined in samples taken bimonthly for 6 months from nine quadrats of a double mattress. It was found that although age structure varied considerably with time, for D. pteronyssinus in different homes the most common structure was one in which immatures were dominant, then eggs and then adults (31% of samples). Immatures or eggs were dominant in 75% of samples. For E. maynei the age structure was quite different: the most common structure was one in which adults were dominant, then immatures and then eggs (69% of samples). In different quadrats of a double mattress, mean age structure of D. pteronyssinus underwent a shift towards higher proportions of immatures and then eggs during the sampling period, which reflected the increase in population density detected during this period. Life and fecundity tables were constructed for D. pteronyssinus and E. maynei using previously-available in vitro data on fecundity and survivorship rates and hypothetical values based on means derived from a number of studies. From the tables the stable age distributions were calculated and compared with the age structures of the natural populations. It was found that mean age structure of natural populations of D. pteronyssinus was fairly close to the predicted stable age distribution, but those of E. maynei indicated the populations were in decline during the sampling period, a fact confirmed by abundance data. The concept that the rate of increase of house dust mite populations can be estimated by determining age structure of mites isolated from dust samples was explored using the hypothetical population parameters of D. pteronyssinus. It was predicted that quite large differences in fecundity and mortality would not drastically alter the proportions of eggs, immatures and adults in stable populations. Eggs as components of the house dust mite population are considered seriously for the first time. Those of D. pteronyssinus and E. maynei were identified and differentiated by allometry. It is stressed that for D. pteronyssinus, during the sampling period, half or more of the mites in a dust sample may be represented as eggs, and to ignore them is to deliberately make a less accurate estimate of population density than could be otherwise achieved.