Diapause in Lucilia Sericata (MG.) Diptera

It is now well established that unfavourable environmental conditions such as cold or excessive moisture acting on larvae of the blowfly Lucilia sericata may produce diapause. Further, Cousin (1932) showed that adults of this species subjected to abnormal conditions produced eggs which, if not sterile, had a high mortality and the ensuing larval stage was longer than that of normal larvae.

In the course of several years’ work on L. sericata one of the present writers has often observed that many larvae collected in the field towards the end of the blowfly season enter diapause in spite of completing their larval feeding period under laboratory conditions favouring pupation. Larvae on small carcasses might experience adverse environmental conditions prior to collection, but this is not necessarily true for larvae taken off sheep suffering from myiasis. In the latter case, even in autumn, the larval environment is suitable for normal development. It would seem, therefore, that the diapause which occurred might be of maternal origin. This view was given further support by some preliminary experiments carried out in 1949 by one of the writers (J.B.C.) in collaboration with Miss B. A. Thurston. It was found that egg batches from wild flies caught in late summer, even when reared under laboratory conditions favouring normal development, gave a high proportion of diapausing larvae. The hypothesis was therefore carefully tested in the blowfly seasons of 1950-51 by comparing the diapausing tendency of field-caught L. sericata with that of laboratory controls ovipositing at the same time.

Female L. sericata were captured under field conditions, either in traps baited with the standard hydrogen sulphide-mercaptan attractant (Cragg & Thurston, 1950) or by being attracted to sheep (Cragg, 1950). The majority of the flies attracted to sheep were caught and taken to the laboratory, each in a separate 7·5 × 2·5 cm. tube containing a small lock of moist sheep wool. Some flies oviposited on this wool but most oviposited on meat in the laboratory. Occasionally flies were induced to lay on sheep, the egg batches being transferred to the laboratory. Control flies taken to the field station in tubes and then brought back to the laboratory gave larvae which behaved in the same way as those produced by the normal laboratory culture; thus the circumstances of collection did not cause diapause.

Both control and field-caught flies were kept individually in lamp glasses 16 cm. high and 12 cm. diameter, the tops being covered with muslin and the bases placed in trays containing damp sawdust. Each lamp glass contained a water vessel, sugar and meat. These lamp-glass cages were kept in a constant temperature room at 26° C. and 60-70 % R.H., and were illuminated for approximately 8 hr. per day by fluorescent lighting.

Egg batches were hatched on small pieces of rearing medium at 27° C. and 100 % R.H. After hatching, approximately 150 ist-instar larvae were transferred to 500 ml. conical flasks containing 55 g. of medium. Twenty-four hours later the medium was covered with a thick layer of slightly damp sawdust to absorb excess moisture. Rearing was carried out at 26° C. When larvae began to leave the medium (5-6 days after egg-laying) they were hand-sorted into honey-jars 10 cm. deep and 7 cm. diameter containing damp sawdust, to complete the larval and pupal stages at 22° C.

The rearing method and the breeding medium were modified from Lennox (1939) and Hill, Bell & Chadwick (1947). The medium contained: 100 ml. fresh slaughter-house blood, 6·7 g. Brewer’s yeast, 0·3 g. potassium (monobasic) phosphate and 0·5 g. agar-agar. It was autoclaved for 15 min. at 15 lb. pressure.

Controls were taken from a laboratory culture started in late July 1950 from larvae collected off sheep. As far as possible, equal numbers of field-caught and control flies were used for each set of observations. Only field-caught flies ovipositing within 24 hr. of capture were used ; in most cases eggs were laid within 2 hr.

Batches of L. sericata larvae, even from well-established laboratory cultures, usually give a small percentage of diapausing larvae (2-10 %). In the present investigation, therefore, the following criterion was used to measure diapause. A batch was considered normal if it showed 50 % pupation within 14 days of completing feeding. Those which failed to reach this level were regarded as diapausing batches. Of 147 normal batches 135 showed 80 % pupation within 10 days, whereas so-called diapausing batches never reached this level in less than 30 days and often required more than 100 days.

• (i) 1950. Table 1 and Fig. 1 show that the incidence of diapause in larvae of field-caught flies increased as the blowfly season approached its end. Some diapause occurred among control batches and the difference between control and field groups was only significant at the o-oi level in the last sample. Because of the small number of flies in both control and field-caught groups the Fisher-Yates test of significance described by Finney (1948) was used.

• (ii) 1951. As is clear from the table the trend shown in the 1950 results was established with a high degree of significance in 1951.

• (iii) Simmonds (1948) has shown that in Spalangia drosophilae and Cryptus inornatus the age of the mother can influence the amount of diapause. Since the Lucilia population at the end of the season may contain a high proportion of old flies, the increase in diapause might therefore be related to age. This possibility was tested by comparing larvae from 2-6-week-old control flies; ten batches were compared, no diapause being recorded in either group. Thus it is unlikely that age has a significant effect on the induction of diapause.

• (iv) In 1951, of twenty-one field-caught flies whose first egg batches gave diapausing larvae, sixteen gave normal egg batches after 10-16 days in the laboratory. Of fourteen whose first egg batch was normal, two gave diapausing larvae in subsequent batches.

• (v) There was no obvious correlation between the fluctuations shown in the figure and any one weather measurement.

Cousin (1932) and Mellanby (1938) have shown that unfavourable conditions acting directly on L. sericata larvae can induce diapause. The present investigation makes it clear that diapause in this species may also be of maternal origin. The physiological conditions inducing this type of diapause ceased to operate in most flies kept under laboratory conditions for 10-16 days. However, out of fourteen flies whose first egg batches were normal, two subsequently gave diapausing batches. This delayed occurrence of diapause suggests that the factors leading to its maternal induction take some time to affect developing oocytes.

Cousin claims to have reared L. sericata through eighty generations without the occurrence of diapause and she states that: ‘Tous les individus des pontes successives ont été suivis d’un bout à l’autre de leur développement.’ Such a complete absence of diapause has not been realized in the course of several year’s work in Great Britain. Here, even under optimal conditions, a small percentage of larvae may enter diapause. This might indicate that some larvae occur for which normal rearing conditions are not favourable. Another possibility is that in certain larvae the physiological state necessary for pupation is only slowly acquired.

Given suitable rearing conditions for both adults and larvae, diapause in L. sericata never shows a cyclical pattern such as Theodor (1934) described for Phlebotomus papatasii. It is, instead, clearly related to some seasonal change, and in view of the importance of temperature and photoperiod in other organisms (see reviews by Lees, 1950, and Andrewartha, 1952) their effect on Lucilia sericata adults should be studied. Dickson (1949) has shown that the photoperiod does not influence the amount of diapause in L. sericata larvae, but considering the normal habitat of blowfly larvae this result is not unexpected.

Diapause is often a method of surviving adverse conditions, particularly winter cold. The occurrence of maternal-induced diapause in L. sericata and perhaps in other blowflies over-wintering in the so-called prepupal state, ensures that the majority of larvae produced at the end of the blowfly season enter diapause in spite of the possible chance occurrence of ground conditions favouring pupation.

The writers wish to acknowledge the help given by Mr D. J. Finney, Lecturer in the Design and Analysis of Scientific Experiment, Oxford, for advice on the statistical treatment of their data. This investigation has been financed by a grant from the Agricultural Research Council.