The Effects of Temperature and Humidity on the Cheese Skipper, Piophila Casei (L.)

The larvae, pupae and adults used for the experiments were reared on common “cheddar” cheese. The cheese was cut up into lumps of about $12$ in. cube, and 1-pint milk bottles filled to about one-third with these lumps. The bottles were exposed to ovipositing females for 24–48 hours in a cage and then removed and stoppered with a plug of cotton-wool. The eggs hatched in a day or two and the larvae developed rapidly. Within a fortnight or less, according to the temperature, they were fully developed and began to migrate out of the cheese, which was now reduced to a soft mass, to find a drier situation for pupation. The migrating larvae came to rest in the cotton-wool plug into which they insinuated themselves and, if left there, they pupated.

If the cotton-wool plug was removed daily it might be assumed, for the purposes of the experiment, that all the larvae obtained daily from a given bottle were in a similar physiological condition. Pupae and imagines were obtained without any difficulty by keeping the migrating larvae in containers till they pupated.

For rough exposures to constant temperatures without controlled humidity the insects were placed in an incubator in a suitable container. When the humidity had to be controlled and the exposures were 24 hours in length or longer, the “fruit-jar “method of Buxton and Mellanby (1934) was used. For shorter exposures the apparatus shown in Fig. 1 was used. This consisted of a large wide-mouthed bottle (L) of about $112$ litres capacity filled with water and into which a cork (C₁) was fitted. This cork (C₁) was bored to receive a 6-in. boiling tube (B) which passed down into the water in the large bottle. The cork (C₁) had to be fitted very tightly so as to grip the boiling tube and hold it down into the water; it had, however, a small vent (V) to allow of changes in the volume of the water and air in the botde being compensated. At the bottom of the boiling tube was a short wide specimen tube (S), containing the humidity controlling mixture, with a suitable wire attachment to allow of its easy removal when desired. The cork of the boiling tube (C₂) was bored to receive a short thermometer (T) on the stem of which was a second tightly fitting cork (C₃). The actual container (G) for the specimens that were being used for an experiment was made of brass gauze, and it was held in the position shown by pushing its open end on to the cork (C₃) on the thermometer.

For use the apparatus, with a suitable H₂SO₄ solution in the specimen tube (S) to control the humidity, was placed in a constant temperature water-bath. When the thermometer (T) indicated that the desired constant ternperature was being maintained in the gauze container the cork (C₂) carrying with it the thermometer and the gauze container was removed from the boiling tube, the mouth of which was quickly covered by an unbored cork. The insects were placed in the gauze container which was then as rapidly as possible replaced in the boiling tube along with the thermometer. In actual practice it was found that, within 5 min. from the time of replacing the gauze container in the boiling tube, the temperature therein had returned to the constant level at which the apparatus had been running. In all the experiments the humidity was controlled with H₂SO₄ solutions prepared according to the directions of Buxton (1931a).

The eggs of P. casei hatched and the resultant larvae developed into apparently normal adults at temperatures up to 35 ° C. At and above this temperature a complication is introduced, since the fat contained in the cheese melted and many larvae were apparently drowned in it. Controlled humidity was of course unobtainable in the containers in which the larvae were bred owing to the nature of the cheese.

The thermal death-point of the larvae was determined for 1-and 24-hour exposures at relative humidities of o, 30, 60, 90, and 100 per cent. ; in all cases the larvae employed were mature specimens that had migrated out of the cheese into the cotton-wool plugs of the rearing bottles. The results of the 24-hour exposures are shown in Table I. Counts for the number of deaths, in each experimental group of 25–30 larvae, were made after allowing a recovery period of 12–24 hours at 25 ° C., a temperature which appeared to be very nearly optimal for the species.

The lowest temperature at which deaths occurred for a 24-hour exposure was 45 ° C. at a relative humidity of o per cent. The dead larvae were very shrunken and had undergone obvious “drying” ; the survivors were also very shrunken and would probably have died if the exposure had been prolonged. At this temperature, 45° C., there were no deaths at the other humidities, but the larvae exposed to a relative humidity of 100 per cent, appeared to be very “sick” and their recovery was very slow. No larvae survived for 24 hours at any relative humidity when subjected to a constant temperature of 48 ° C. The relationships of the effects of relative humidity at the other temperatures between 45 and 48° C. can be seen from the table which shows that the optimum relative humidity for the survival of P. casei larvae at high temperatures is about 60 per cent.

The thermal death-point of the larvae for 1-hour exposures was found to be 521 ° C., regardless of the relative humidity to which the larvae were subjected. The larvae survived for an hour at a temperature of 52 ° C.

These results correspond with those obtained by Mellanby (1934) in experiments dealing with a number of insects. It may, however, be noted that the temperatures required to kill the larvae of P. casei are about 10° C. higher than those fatal to the insects used by Mellanby.

Pupal life lasted 8 days at a constant temperature of 25 ° C., and this time was independent of the relative humidity to which the pupae were subjected. At 30° C. the time occupied was 6 days, and the same period was required by pupae subjected to a temperature of 35° C. Owing to the complexity of the physiological changes during the pupal stage and the difficulty of determining the death of a pupa the experiments carried out involved the exposure of pupae to given constant temperatures and humidities for the whole of their pupal life. It was found that, regardless of the humidity, at 35 ° C. too per cent., at 36° C. 50 per cent, and at 37° C. o per cent., flies emerged from the pupae after making allowance for the natural death-rate among pupae which, at lower temperatures, appeared to be between i and 3 per cent.

This effect was not solely due to temperature. Examination of the individual puparia showed that in all cases, after allowing for the natural death-rate, the puparia contained fully metamorphosed flies which, if not actually alive, had died as a consequence of their having failed to effect an emergence from their puparia. At the highest temperature (37° C.) approximately 10 per cent, of the flies had managed to push open the lid of the puparia but emergence had not been subsequently effected. At 36° C., as noted above, about 50 per cent, of the flies emerged, the remainder failing to do so. In nearly all cases these failures succeeded in displacing the puparial lid, and their heads and in some cases the anterior part of their thoraces protruded from the opening of the puparium. Often the flies could be seen making violent but ineffectual efforts to protrude themselves further from the puparium. During their struggles their ptylinal sacs were expanded to an enormous degree, and continued to be expanded and contracted long after it was certain that the fly was not going to effect its escape. Eventually such flies died in their puparia with the ptylinum usually incompletely withdrawn.

The exact cause of this failure to emerge was not determined. There were some indications that the proportion of weight lost by the pupae over the whole pupal period was less at the higher temperatures than at the low, leaving the imagines formed at the higher temperatures bulkier than they would have been had the temperature been nearer that at which development usually took place, this extra bulk being possibly sufficient to prevent their successful emergence from the puparia.

The paper gives the results of a short series of experiments carried out to determine the thermal death-point under conditions of controlled humidity of the larva and pupa of the Cheese Skipper, Piophila casei (L.). The larva is remarkable for the high temperatures it can withstand, namely 52° C., for 1 hour’s exposure and 45° C. for an exposure of 24 hours. The death of the pupa at a much lower temperature is shown to be due to a secondary effect of temperature on its physiology.

The work on which this paper is based was carried out in the Entomology Department of the London School of Hygiene and Tropical Medicine during part of my tenure of a Carnegie Research Scholarship. I am indebted to Prof. P. A. Buxton for his numerous suggestions and kindly criticism of the work as it proceeded.