A Laboratory Method of Rearing Chiggers Affecting Man (Acarina: Trombiculidae)

Abstract

The work described was designed to establish a method of rearing Trombiculids for the provision of colonies for studying the transmission of tsutsugamushi disease (scrub typhus) and to provide larvae for testing acaricides and repellents under controlled conditions throughout the year. It was also desired to obtain information about the pest species of Trombiculids that infest man in the United States, viz., Eutrombicula alfreddugesi, Oudm., and E. masoni, Ewing, which are common, and E. batatas, L., which is restricted to the south-eastern States. The literature on the rearing of Trombiculids is briefly reviewed. The seven stages in the life-cycle are enumerated [cf. R.A.E., B 37 61], the protonymph and preadult being also called nymphochrysalis and imago-chrysalis, respectively. Work was begun between December 1945 and March 1946 in Panama, where a complete generation each of E. batatas and Trombicula alleei, Ewing, was reared by a method similar to Michener's [37 61-62], eggs of Anopheles albimanus, Wied., being used as food for nymphs and adults, with moderate success. However, mortality was still about 60 per cent. and production of second-generation eggs unsatisfactory. Therefore, during the following 12 months, when the work was continued in Florida, Texas, Maryland and Ohio with E. batatas, E. alfreddugesi and E. masoni, new methods and equipment were developed. The first species was reared to the second generation and the other two through four generations. The colonies were started with larvae collected from naturally infested vertebrate hosts on which they were allowed to complete engorgement and with unfed larvae collected on black boards and transferred to laboratory hosts. The most satisfactory laboratory host was the box turtle, Terrapene Carolina, although snakes were more attractive. It survived well when fed only between experiments, so that much of the trouble experienced with chicks and mammals through contamination of the collecting equipment with faeces was avoided, and it moulted much less frequently than snakes and lizards and was easier to handle. The larvae did not appear to cause any discomfort or damage to turtles or snakes as they do to birds and mammals. The host was kept in a wire mesh cage over a funnel through which the engorged larvae fell into a rearing jar. Engorgement of larvae on turtles in central Florida took an average of 8 days early and late in the season and 3-4 in July and August. The rearing jar was made by cutting the bottom from a glass screw-top jar with a capacity of 1 U.S. pint and substituting for it a layer of plaster of Paris about 3/8 inch thick and containing about 10 per cent. activated charcoal. This was covered when dry with a thin layer of activated charcoal over which was spread a layer of sterile humus or moist sandy soil 1/4 inch deep. A ring of vaseline inside the jar about 1/2 inch above the soil prevented larvae from crawling up the glass. The metal lid had a hole in the centre plugged with cotton-wool to provide ventilation and help prevent condensation. Temperature was maintained at about 25-35 degrees C. [77-95 degrees R] and relative humidity at 86-100 per cent. The soil was moistened daily and the excess moisture allowed to drain through the plaster into a petri dish. The larvae entered the nymphochrysalis stage after about two days, and nymphs emerged in about a week. Daily moistening of the substratum was continued and the same temperature and.humidity were maintained during the rearing of the nymphs and adults. These were fed once a week on eggs of Aedes aegypti, L., which were particularly suitable as they remained in good condition for a long time. Young nymphs had difficulty in piercing the shells of mature eggs, and at first only eggs removed from gravid females were used for them, but it was later found that moistening the mature eggs slightly was satisfactory. Excessive wetting caused them to hatch, and the larvae rotted and ruined the culture. Nymphs changed to the imagochrysalis stage in about 10 days, and adults emerged after a further 4-9 days. Adults two days old that had starved for 24 hours ate an average of 15 eggs each in 90 minutes. The optimum number of adults per rearing jar was 30-50. Most eggs were laid at temperature of 27-34 degrees C. [80.6-93.2 degrees F.]. The humidity was kept above 85 per cent. and the substratum quite moist. Eggs were laid singly on the surface or in the interstices of the soil. The average number laid per female per day in September was about ten, and the maximum, observed was 20. Individual females oviposited for more than six months. Newly hatched larvae were transferred to a turtle by suspending it over the opened rearing jar, whereupon the larvae crawled rapidly up the jar on to the host. The minimum length of time required to rear a generation through the complete life-cycle by this method was 50 days for E. masoni, 55 for E. alfreddugesi and 71 for E. batatas. The times occupied by the various stages are given for each species. The chief disadvantage of the method is the difficulty of obtaining the large quantities of mosquito eggs, for which no substitute has been found.