Effects of temperature and relative humidity on development times and ...

Experimental & Applied Acarology, 3 (1987) 279-289

279

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Effects of Temperature and Relative Humidity on D e v e l o p m e n t Times and Mortality of Eggs from Laboratory and Wild Populations of the European House-Dust Mite Dermatophagoides pteronyssinus (Acari: Pyroglyphidae) M.J. COLLOFF

Department of Zoology, University of Glasgow, GlasgowGI2 8QQ (Great Britain) (Accepted 18 December 1986)

ABSTRACT Colloff, M.J., 1987. Effects of temperature and relative humidity on development times and mortality of eggs from laboratory and wild populations of the European house-dust mite, Dermatophagoides pteronyssinus (Acari: Pyroglyphidae). Exp. Appl. Acarol., 3: 279-289. As part of a study on passive physical control of house-dust mites, a total of 6000 eggs from a population of Dermatophagoides pteronyssinus (Trouessart, 1897) from 17-year-old laboratory cultures were incubated at 60 temperature and relative humidity combinations between 10-35 ~ and 55-10O% RH. Eggs hatched at every combination, although mortality and development time increased between 10-20~ and 30-35~ and below 65% RH. Optimum conditions were 35~ and 80-85% RH. In temperate dry conditions, eggs from a wild population were found to be more resistant to mortality: they developed faster, with 7 times lower mortality than eggs from the laboratory population. This may have been because the laboratory population had become acclimated to the constant near-optimum conditions at which it was kept. Therefore it has been suggested that where laboratory cultures have been used in studies relating to passive physical control, caution should be taken in applying the conclusions to wild populations in the natural house-dust environment.

INTRODUCTION H o u s e - d u s t mites are well e s t a b l i s h e d as t h e source of p o t e n t a i r b o r n e allergens t h a t are r e s p o n s i b l e for the o n s e t o f a s t h m a a n d rhinitis in atopic individuals. Studies h a v e i n d i c a t e d t h a t r e m o v a l of live mites a n d the abiotic allergen pool of dead m i t e s a n d faecal pellets b r i n g s s y m p t o m a t i c relief ( S a r s field et al., 1974; M u r r a y a n d F e r g u s o n , 1982; W a l s h a w a n d E v a n s , 1986). S u c h strategies involve t h e p a t i e n t s a n d t h e i r families in a series of mite c o n t r o l a n d

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9 1987 Elsevier Science Publishers B.V.

280 avoidance measures which are not always completed (Korsgaard, 1982). Also there has been a reluctance amongst clinicians to advocate control measures, partly because they are regarded as draconian compared to immunotherapy and chemoprophylaxis (Whyte and Flenley, 1986). Clearly there is a need to establish procedures which are effective in controlling mites and removing allergens with the least possible inconvenience to patients and their families. Passive physical control by heating and/or drying has been proposed by Oshima et al. (1972) and Korsgaard (1983). However, the environmental tolerances of the different stages in the life cycle of the mites need to be known before such procedures can be effectively initiated. The effects of temperature and relative humidity on the growth and mortality of three species of the genus Dermatophagoides have been studied by several workers since 1967. The major conclusions of studies on Dermatophagoides pteronyssinus (Trouessart, 1897) are as follows: (1) The optimal conditions of temperature and relative humidity for growth of populations are 25-30~ and 75-80% RH (Spieksma, 1967; Koekkoek and Van Bronswijk, 1972; Murton and Madden, 1977; Dobson, 1979; Gamal-Eddin et al., 1983). (2) At the above conditions, development from egg to adult takes about one month. The egg stase lasts about 6 days; larva, 5-6 days; protonymph, 4-7 days; and tritonymph, 4-8 days. The adults live for about one month, the females tending to live slightly longer than the males (Dobson, 1979; Spieksma, 1967). ( 3 ) At optimum conditions, gravid females produce between 40 and 80 eggs in about 30 days (Spieksma, 1967). (4) The critical equilibrium activity was found to be equivalent to 73 % RH at 25 ~ for adult females (Arlian, 1975). Below this level feeding ceases, probably to minimise water loss (Arlian, 1977). (5) The thermal death point of D. pteronyssinus is 51~ after a 6-h exposure at 60% RH, or 46~ after a 12-h exposure (Kinnaird, 1974). One problem that inhibits the extrapolation of these laboratory studies to the natural house-dust environment is that nearly all mites used came from well-established stock cultures. It is normal practice to maintain stock cultures on highly nutritious diets, such as yeast and dried Daphnia, at near-optimum temperature and humidity. House dust mites in beds feed on skin scales, which have comparatively low nutritional status, and the mites are subject to lower temperatures and humidities for most of the day, although these parameters increase markedly when the bed becomes occupied (Koekkoek and Van Bronswijk, 1972; Heilsen, 1946). Because diurnal fluctuations are not imposed on laboratory stock cultures, acclimation to stable conditions may occur. As a result, the responses of laboratory populations to extremes of microclimate may be significantly different from those of un-acclimated wild populations. During a study on the effect of a wide range of temperatures and relative

281

humidities on the development times and mortality of eggs from a long-established laboratory population of D. pteronyssinus (Colloff, 1985), it was suspected that acclimation may have taken place. To test this hypothesis, a comparison was made of the responses of eggs from the laboratory population with those of eggs from a l-month~old culture of wild mites to warm, semihumid and temperate, dry conditions. METHODS

Mites had been regularly sub-cultured at the University of Glasgow since 1970. They originated from specimens found in house dust in the southeast of England and had been grown at the M A F F Pest Infestation Laboratories at Slough for some time previous, although probably not before 1968 (Cunnington, 1971). A wild population was obtained from beds in a t e n e m e n t flat in Glasgow about I month before the start of the study, and maintained on house dust at 15~ and 65% RH. Development times and mortality of the eggs of D. pteronyssinus from the laboratory population were investigated over a series of combinations of tem~ perature and relative humidity, from 10 to 35 ~ at 5 ~ intervals and from 55 to 100% R H at 5% intervals. At least 30 males and 30 females were maintained on yeast granules in 53 • 25 • 3 mm perspex micro-culture chambers at 25 ~C, 75% RH. Eggs of known age from the time of oviposition (minus a maximum of 12 h ) were removed from micro~cultures every 12 h, using a fine nylon brush, until 100 eggs per treatment had been collected. The eggs were placed at labelled positions on glass microscope slides, which were inserted into slotted perspex racks inside Kilner jars and incubated at the required temperature and humidity. Humidity was controlled using saturated solutions of various inorganic salts ( W i n s t o n and Bates, 1960). The eggs at temperatures of 20-35 ~ were examined every 12 h, and eggs at temperatures of 10-15 ~ every 2 days. This longer interval was required because preliminary observations showed that eggs kept in cool conditions often took several weeks to hatch. Development time was defined as the period from oviposition (minus a maximum of 12 h) to eclosion (minus a maximum of 12 or 48 h, depending on temperature t r e a t m e n t ) . Causes of mortality were classified as egg dehydration (the egg lost water, shrivelled and died with no visible signs of embryonic development), abortion ( the embryo developed but died before eclosion ) and inviability (the egg failed to develop or dehydrate and remained in more or less the same condition as when it was first oviposited). Observations were also made on the number of larvae that died of dehydration during or soon after eclosion and had thus failed to emerge completely from the eggshells. The time intervals between observations on larvae were the same as those for the eggs. To compare the responses of eggs from the two populations, about 30 male

282

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Fig. 1. Mean development times (days) of eggs from laboratory populations of Dermatophagoides pteronyssinus incubated at different combinations of temperature and relative humidity. Vertical bars = standard deviation.

and 30 female mites from laboratory and wild stock cultures were maintained in micro-cultures at the same conditions as the stock cultures and eggs were collected as before, until 100 eggs from each population per t r e a t m e n t had been obtained. Two treatments were used: 20 oC, 60% RH and 30 oC, 80% RH. The eggs were examined every 12 h.

Statistics One-way analysis of variance was used on the data on development times of eggs from the laboratory population. The data from eggs incubated at 10-15 oC and 20-35 ~C were treated separately because of the time differences between observations. Two-way analysis of variance was used on the mortality data after arc/sin transformation. Location of significance within analyses of variance was done using t-tests. Chi-squared tests were used for the comparison of mortality and development times of eggs from laboratory and wild populations.

283 RESULTS

Effects of temperature and relative humidity on development time of eggs from the laboratory population (Fig. 1). Temperature had a highly significant effect on development time between 10 and 15~ ( P =