The Oviposition Responses of Calandra Granaria Linn

Once infestation of a stock of grain has taken place the rate at which it develops depends on the number of insects present and on the extent to which the conditions favour their multiplication. Of the factors governing multiplication may be mentioned the rate of oviposition, the length of time a female lives before she lays eggs and the length of her ovipository life. These factors are dependent in turn on the environment, and they have formed the subject of this investigation with reference to the grain weevil, Calandra granaria, in stored wheat.

In no earlier work have strictly controlled environmental conditions been applied in this connexion. For instance, Kunicke (1936) reports that at 26° C. a female lays 200 eggs over an unstated period of time and that at 24° C. adult Calandra could live for a year. Lavrekhin (1937) states that at 25–27° C. the females lived for about 130 days, and that the average rate of oviposition was 0·9 egg per day from the 14th to the 54th day, the rate then falling off. Since these results were obtained without any precise control of the atmospheric humidity they are of little value. With these defects in mind the features of oviposition rate, length of preoviposition period and of lengths of ovipository life in Calandra have been examined under a series of controlled humidities for each of several temperatures. In this way some comparison can fairly be made between the effects of temperature and atmospheric humidity separately and together.

Soft English wheat, previously sterilized at 80° C. for 3 hr., was used throughout the experiments. The water content of wheat varies considerably (Dendy & Elkington, 1920), and to stabilize this factor all grain used was first acclimatized to the atmospheric humidity intended to be used in experiment. This was done by exposing thin layers of grain on gauze trays in large desiccators containing appropriate solutions of KOH (Buxton & Mellanby, 1934) for periods of not less than 36 days. The water contents corresponding to the several atmospheric humidities as determined in the wheat by Dr O. W. Richards are given in the following table :

In this way it was ensured that wheat, by virtue of its water content, had no influence on the atmospheric humidity in any experiment, and conversely that the relative humidity of the air effected no change in the water content of the grain.

Temperature control in the chambers used was efficient, thermograph records showing the maintenance of a temperature constant to ±0·2° C.

Humidity of the atmosphere in the vessels was controlled by solutions of KOH, these being made up from data taken from Buxton & Mellanby (1934). Glass jars with cork-seated bakelite covers, capacity 750 c.c., were used, KOH of appropriate specific gravity in egg-cups being placed therein for humidity control.

A typical experiment was set up as follows. A pair of newly emerged weevils, male and female, was placed in each of fifteen gauze-covered tubes, size 3 by $12$ in., with ten grains of acclimatized wheat. In each of fifteen other similar tubes a pair of weevils was placed with five grains of wheat. The tubes were then packed into the jar together with the humidity controlling KOH and the jar then screwed up and placed in the temperature chamber in the dark and without any undue mechanical disturbance. In every experiment the weevils used were taken from stocks which had been reared under the same conditions as those about to be applied.

At regular intervals the grain was removed from the tubes for egg counts, the KOH renewed and new grain provided to the weevils after removal of any faecal matter, this having been shown by Dendy (1920) to be hygroscopic. At 20° C. grain was changed every 3 weeks, at 22·5 and at 25° C. every fortnight, and at 27·5° C. it was renewed every week. By the adoption of a suitable system of numbering the tubes, an estimation of the number of eggs laid by each individual female was possible for each stage throughout her life.

The following table sets, out the combined, conditions of temperature, humidity and water content of grain employed :

As the table shows, the water content of the grain is fixed for each relative humidity. Should this factor prove to be significant in oviposition, therefore, it will be possible to compare its effects against temperature but not against air humidity. In other words food with special reference to its water content represents a third variable which is not fully controlled. The difficulty of effecting a control of this factor is great, since weevils feed preparatory to oviposition (on the same grain on which they oviposit). If, for instance, at any single relative humidity of air, food grain containing a water content out of balance with the air humidity were inserted, no stability of either water content of grain or of air could be maintained.

While the moisture conditions were controlled and recorded in terms of relative humidity, it has to be pointed out that any single relative humidity through a range of temperatures corresponds to different saturation deficiencies of the environmental air, the saturation defidency increasing with the temperature (Buxton, 1931). Evaporation having been shown to be proportional to saturation deficiency, provided the air is moving at uniform speed (Ramsay,- 1935), it is clear.that important results are likely to emerge which comparisons between effects of relative humidities at different temperatures will not disclose. For these reasons, the saturation deficiencies in mm. mercury approximately equivalent to the relative humidities and temperatures employed have been indicated in the several figures.

The female perforates the grain with her proboscis, turns round and deposits an egg in the hole so made and then seals the opening with a drop of fluid which sets hard to form an ‘egg plug’. Any part of the grain may be selected for oviposition, though there is a strong preference for the junction of the smooth and hairy parts at the end opposite to the embryo (Back & Cotton, 1926). In these experiments eggs were estimated on counts of egg plugs which were visible on the surface of the grain. A more accurate method would have been that recommended by Dr O. W. Richards in lit. Here the grain is soaked in 70% alcohol for 24 hr., the testa is stripped off and the eggs counted from the dissected grain. The concurrent experiments of this investigation made this last method impracticable. A comparison of the two methods reveals that about 8 % of the eggs are overlooked by the first method. It must therefore be recognized that the figures given in this work are inaccurate to this extent, but the general nature of the results given is claimed to present a true picture of the case.

The results obtained enable us to draw attention to the effects of saturation deficiency (S.D.) of the air and of temperature on the following: (a) rate of oviposition, (b) total number of eggs laid, (c) length of ovipository life, (d) total length of life, (e) length of preoviposition period; having regard always to the fact that a third variable food is involved, since for each atmospheric humidity the grain used had a particular water content.

There is considerable Variation in the performance of individual weevils. All were of similar age at the beginning of an experiment, and the uniformity of conditions applied to separate weevils in any one experiment, ‘together with the fact that all weevils had had applied to them, during their own develop-ment, the same conditions as those to which they were subjected in the experiment, forces one to the conclusion that the differences in behaviour mentioned are inherent in the weevils themselves.

This previous acclimatization to the conditions of treatment is important, for it has been shown that previous conditions, different from those of an experiment, can impose on animals a type of behaviour out of relation with the conditions subsequently applied (Mellanby, K., 1939, 1940).

The principal results are summarized in Figs. 1–3, these being arrived at by taking the average of behaviour of ten weevils from each experiment. In every case the results shown are taken from those experiments where a pair of weevils was placed with ten grains of wheat. Similar results, but of a lower order, were obtained in a parallel series of observations in which each pair of weevils was placed with five grains. For reasons of space the results of this latter series have not been recorded.

Fig. 1 shows the effects of a range of S.D. at each of four temperatures. The retarding effects of S.D. of increasing scale are obvious. That temperature also is an important factor is shown by comparing equivalent conditions of S.D. at different temperatures. The higher temperature is seen to be correlated with a higher oviposition rate.

For all the temperatures used there appears to be an optimum S.D. for rate of oviposition. Thus the following combinations of temperature and relative humidity are those equivalent to a common S.D. of 7·4 mm. mercury :

These are perhaps close enough to the relative humidities at which in Fig. 1 the highest ovipository rates are recorded for each temperature, to justify the conclusion that a S.D. c. 7·4 mm. mercury represents an optimum condition within this range of temperatures (Buxton, 1930). Ramsay(1935)showed that the higher rate of evaporation from an insect at any constant S.D. occurs at higher temperatures than at lower because of the increased rate of diffusion at those higher temperatures. While this is undoubtedly true it is difficult to attribute the increased rate of oviposition to such a cause. It is more likely to be due, to an increased rate of metabolism following additional ventilation of the tracheal system which results from a-rise in temperature (Wigglesworth, 1939)-

In 1937 Lavrekhin examined the fertility of Calandra, measuring it in terms of the number of progeny found in grain at the end of a month. His figures are lower than those recorded here, probably because of cannibalism- which occurs when two or more larvae compete for life in a single grain and also perhaps because he appears not to have ensured that all weevils were of the same age. He may well have used weevils which had passed their period of most rapid egg production. Lavrekhin further found that the highest fertility at 25° C. occurred after the 14th day and was maintained for a further 35 –40 days. All curves (not recorded in this paper for reasons of space) showing rate of oviposition in this work are sigmoid, and the highest rate occurred only after the 30th day in the 25° C. experiments, after which this steady high rate continued for 40 days. There seemed to be no special preference for full as against shrivelled grains for oviposition, though Mathlein (1938) states that weevils will not lay eggs in grain of lower water content than 10 %. It is not possible to say how far the latter differed from the driest grains employed by us with 10·56% water, since there is no information as to Mathlein’s method of estimating water content.

Allowing for some unaccountable discrepancies in the results set forth in this figure, it appears that at any given temperature total egg production varies inversely as the saturation deficiency. That this conclusion is valid emerges from a Consideration of the effects of a rise in S.D. through a range of temperatures. For example, the columns indicating an experimental condition of 60% relative humidity at the temperatures from 20 to 27·5° C. demonstrate that the total egg production falls as the S.D. rises in the following order: 7·5, 8·4, 10·0, 11·5 mm. mercury. On the data given it is impossible to estimate accurately the effects of temperature under a condition of constant S.D. By interpolation, however, one can indicate the expected total egg production at such a constant S.D. through the several temperatures. The line AB (Fig. 2), for instance, passes through such expected points of egg production at the common S.D. of approximately 11·0 mm. mercury, and it is seen that the egg production goes up as the temperature rises. The same thing is shown by the line CD which passes through points of expected egg production at the common S.D. of 8·0 mm. mercury. (A comparison between these two lines enables us to confirm the conclusion already expressed that egg production varies inversely as the S.D.)

The effect, however,-is not solely one of temperature, for by choosing a common S.D. in this way by interpolation, we have passed, up the scale of relative humidities from 40 to 60% and therefore we have at the same time been treating of weevils whose food has varied in its water content, this water content rising from 10·56% at the lowest tempera-ture to 13·26 % at the highest temperature. How far such differences in water content of food grain are effective in this aspect of weevil behaviour is impossible of exact analysis. In view of the findings of Robinson (1925), who claims that Calandra gains weight and thrives more readily on wheat with higher water content, and of Ewer & Ewer (1942), who in Ptinus found an increase in egg production when this beetle had access to drinking water, it appears likely that the total egg production in Calandra is directly proportional to both temperature and water content of the grain on which this insect feeds.

On a matter of detail Muller’s records (1927) for total egg production give lower figures than those indicated here.

Several conclusions can be drawn from the results embodied in this figure. For instance, weevils live longer at lower than at higher temperatures. This fact emerges when one views the data as a whole and more forcibly when comparisons of temperature effects are made under conditions of S.D. which are approximately the same. For each temperature it is similarly obvious that length of life varies inversely as the S.D. The quantitative effects of S.D. are not the same, however, at all temperatures. We can, for instance, by interpolation, indicate the expected length of life of weevils under S.D.’S of a uniform range in the several temperatures. The lines AB, CD, EF, and GH (Fig. 3) express graphically the expected differences in age of weevils kept under a common scale of S.D. ranging from 10·5 to 5·5 mm. mercury at each temperature. As the temperature rises the difference of age of weevils, associated with this constant scale of S.D., becomes less. This un-expected result can be illustrated in another way, viz. by comparing the length of life of weevils under an increasing difference of S.D. through the temperature scale.

By comparing the length of life of weevils at the several temperatures under the relative humidities of 50 and 70% (Fig. 3), we are in fact examining the age factor under an increasing range of S.D. Table 1 sets out the. average longevity in days at each temperature and relative humidity and at each corresponding S.D. In the last two columns of this table are given the differences in age of weevils kept at 50 and 70 % relative humidities together with the differences in S.D. associated therewith. The difference in age is greatest where the difference in S.D. is least and vice versa. If an increase in S.D. is the handicap in the life of a terrestrial insect as may logically be assumed, then a priori we would expect results of an inverse order to those recorded here, viz. that the greater difference in S.D. should be associated with greater difference in age. We suggest that the solution of this problem lies in the interplay of the two factors, temperature and S.D., in the power of. one factor near the optimum to mask the full effects of the other.

The lowest temperature of 20° C. represents (by comparison with the higher temperatures) an adverse condition at which a particular S.D. is able to produce its most marked effect. The higher temperatures, the more favourable as shown by the weevil’s behaviour in oviposition rate and total egg production, on the other hand, appear to compensate increasingly for the adverse effects of S.D., especially where this temperature is near the optimum as 27·5° C. appears to be.

It is worth noting that for each of the relative humidities (50 and 70%) under consideration, through the rising scales of S.D. and temperature, there was employed food grain with constant water content.

A matter of general interest emerges from a consideration of Figs. 1 –3 together. Those conditions of temperature which are conducive to an increased oviposition rate are in general the same as those which lead to shorter life. In view of the not dissimilar performances in total egg production at the several temperatures (Fig. 2) it would seem that weevils have a potential for egg production which is realized in a longer or shorter time according to the temperature applied, provided of course that the conditions are not too severe.

Those conditions appropriate to an increased span of life are such as to allow the animal to cdntinue living after its ovipository powers are spent. Under conditions which shorten life, however, the animal dies when it has ceased to lay eggs. This may be given a different emphasis by assuming that under some conditions the weevil ceases to lay eggs some appreciable time before death, and under others it continues to lay eggs until it dies. At 20 and at 22·5° C. the majority of weevils stop ovipositing about 10 –20 days before death while at 25 and 27·5° C. the majority continue to lay eggs until they die. Much variation was noted in these matters, and the results are incapable of any precise analysis.

While it must be admitted that the work was not designed to investigate this point fully, such observations as were made are not without interest.

When pupation is ended weevils remain inside the grain husk for a certain length of time. Seldom does emergence from the grain coincide with emergence from the pupal exuvium. Assuming, however, that weevils pass the same length of time between pupation and leaving the grain, the preoviposition period can be measured as the period passed between emergence from the grain and the laying of the first egg. At 20°C. this period is about 21 days, at 22·5° C. about 15 days, at 25° C. 8 days, and at 27·5° C. about 4 days. We may assume from this that the higher temperature has hastened the development of the gonads.

There is nothing to show that air humidity has any marked effect on the duration of this period, though one would expect the water content of the food to produce significant effects. Muller’s (1927) claim, that there is a regular preoviposition period of about 3 weeks, provided the temperature is above 12° C., is not substantiated by us. That temperature affects the rate of maturation of gonads in insects has been similarly demonstrated, by Bishopp, Dove & Parman (1915) in the case of the housefly, and by Wille (1920) in Blatella germánica. For other references, Uvarov (1931) should be consulted.

The environmental factors employed, in these experiments, temperature, S.D. and water content of food may be said to act on the animals in two main ways.

The food provides fully -or incompletely the necessary material to sustain life and produce eggs. Temperature and air humidity acting through the nervous system affect the general activity of the animal, the first by altering the rate of metabolism, the second by way of the spiracular closing mechanism, imposing on the animal the need under certain circumstances, to conserve water (Mellanby, K., 1934a). It is the integration of these factors in the life of weevils which determine their behaviour, setting a limit to the length of life, rate of oviposition and therefore to the total number of eggs produced during a lifetime.

Little work seems to have been done on the oviposition responses of Calandra. Such work as exists refers mostly to observations on animals for which not all environmental factors were under experimental control. For instance, Back & Cotton (1926) record, without humidity control, seasonal oviposition activity, taking note of the daily mean temperatures. Lavrekhin (1937) stresses the importance of water content of grain but does not mention any control of air humidity nor the acclimatization of grain to it. Ln a short review by Richardson (1925) the oviposition responses of insects generally in relation to temperature, moisture and nutrition are recorded, but in the works quoted no single environmental factor is treated with reference to others under control. For instance, Detouches (1921) notes a lengthening of the oviposition period with a reduction in temperature, and further finds an increase in the total eggs laid by Galleria, but to what extent moisture in the air or in the food were concerned does not emerge.

Within the range of temperatures here used the results obtained confirm those of earlier workers. Increase in temperature by raising the rate of metabolism and hastening the maturation of gonads reduces the length of the preoviposition period. Bishopp et al. (1915) agree in this. Also the effect of a rise in temperature is to shorten the life and increase the rate of egg production. This in the main confirms the majority of authors quoted by Uvarov (1931), Schubert (1928), for instance, finding a steady increase in the number of eggs laid by Piesma quadratum in a given time as the temperature rises. That the higher rate of oviposition due to temperature does not always compensate for the shorter life is demonstrated by Titschak (1925) in.the clothes moth. Disregarding for a moment the important effects of S.D. of air, Calandra, under the temperatures used here, appears to lay about the same number of eggs at low as at high temperatures, though the time taken to lay them may vary as widely as from 250 days at 20°C. to 100 days at 27·5° C.

It is possible and indeed probable that temperature affects the rate of oviposition or other type of behaviour for reasons other than those concerned with metabolism, the actual process of laying being delayed or hastened according as the temperature is low or high. This possibility was examined by Bliss (1926), who showed that a temperature applied before oviposition (during maturation) is more important in this respect than that applying at the time of ovi-position. In view of the observations of H. Mellanby (1937) that a temperature of 20°C. (low for the tsetse fly) caused pregnant tsetse flies to retain their larvae for periods longer than the normal, the possibility that temperature may have an effect on oviposition directly must not be overlooked.

Reference has already been made to Ramsay’s (1935) interesting experiments where the rate of evaporation, at a constant S.D. of air, is increased as the temperature rises because of changes in diffusion rate. Such an effect, however small, would be that of placing increasing difficulties before the animal as the temperature rose. It is not possible to say how far such an environmental factor is of material significance, but it would seem likely from Ramsay’s work that no appreciably harmful effect would arise until temperatures higher than those employed here had been used.

Turning to the effects of atmospheric moisture on. these aspects of behaviour in Calandra, it is clear that increasing S.D. of the air puts a brake on the animal’s egg-laying activities. The higher the S.D., the lower the oviposition rate, the fewer the eggs laid and the shorter the life that the insect enjoys provided the temperature is constant. In other words, within the ranges of temperature and S.D. employed these two environmental factors are antagonistic. But while temperature would appear to exert its effects metabolically, with concomitant effects on the oviposition responses of the animal, S.D. seems not to operate in this way.

That humidity of the air has no effect on metabolic rate in insects has been shown by a number of workers (Buxton & Lewis, 1934; Mellanby, K., 1932, 1934c, 1935 a, b;Gunn & Cosway, 1942 ). It would seem therefore that where an insect lives for a shorter time at a higher S.D. than at a lower, the shorter life is not attributable to changes in metabolic rate. Factors other than temperature are known to be concerned in the determination of thermal deathpoints, i.e. longevity of insects (Mellanby, K., 19346). Thus starved Culex cannot survive in temperatures as high as those which have been fed. Further, the thermal death-point is higher in atmospheres of low S.D. than where the air is drier. Here a high S.D. is a limiting factor.

There is some evidence for the existence of an optimum S.D. at each temperature for oviposition rate. It is not possible to point to any such relation between S.D. and longevity. Both a higher temperature and a higher S.D. are conducive to shortening the span of life, but whereas the former acts so as to enable a full potential of egg production to be realized in a minimum time, the latter reduces the span of life, inhibiting at the same time the full expression of oviposition. This inhibition is almost certainly due to the animal’s reaction of closing its spiracles so as to reduce water loss, by which action the effects of higher temperatures towards increased ventilation of the spiracles are countered. This interplay of temperature and S.D. has already received comment with reference to length of life (Fig. 3). That this same interplay is effective in oviposition rates and in relation to the total number of eggs laid, emerges in a similar manner from a consideration of Figs. 1 and 2.

Though the chief controlled factors in these experiments have been temperature and S.D., the third, factor of water content of the grain, imperfectly controlled for reasons already given, cannot be ignored. Any condition of food which renders it less acceptable and less easily masticated and digested must serve as a deterrent to.egg production. Such a condition is low water content. On the other hand, food containing more water is softer, and easier of mastication and digestion. The weevil in respect of the types of behaviour here dealt with cannot escape the impact of this part of its environment. It is claimed therefore that for length of life, rate of oviposition and number of eggs laid, the low figures associated with the greater S.D.’S are also in part attributable to low water content of grain, though this effect is incapable of quantitative analysis.

When researchers have examined the lengths of time which insects can survive under certain applied conditions they have very commonly eliminated the food factor by working on starving animals. In whatever manner the conditions have affected the animal’s ability to survive in such experiments, the fact remains that in many cases death was due to starvation following either the exhaustion of food reserves or the inability to use those reserves as the other conditions of life became severe. Though here the food factor is not possible of precise analysis the conclusion can safely be drawn that when weevils die under the more severe temperature or S.D. conditions, starvation is not the cause of death, though malnutrition owing to inadequate water in the food is a contributory factor. Where starvation conditions apply, any conditions conducive to long life are advantageous. But, as in this work, where a female with access to food lays roughly the same number of eggs whether the temperature be 20 or 27·5° C., but lays them at different rates, there seems to be no reason for assuming that a weevil gains any special advantage from either a long life or a short one.

From the economic point of view, however, a short life with full ovipository capacity leads to more rapid infestation of grain stocks, and the conditions which determine these two results are clearly of the greatest economic importance. They lead to rapid increase in numbers and are therefore advantageous to the species. They are the conditions which should be avoided in the storage of grain if the attempts to keep the pest to harmless proportions are to meet with success.

1. The oviposition responses of Calandra granaria as manifested by the rate of oviposition, total eggs laid, length of life, length of ovipository life and length of preoviposition period, have been investigated under controlled conditions of temperature and saturation deficiency of air, grain having been acclimatized in its water content to the relative humidity of the air.

2. Calandra lives for a shorter time under high than under low temperatures but lays eggs at a greater rate, thus compensating for the shorter life.

3. There is evidence for the existence of an optimum saturation deficiency at each temperature for oviposition rate.

4. Weevils are shorter lived at high saturation deficiencies than they are at low.

5. The total number of eggs laid by weevils is smaller at high than at low saturation deficiencies of air.

6. Water content of the food grain contributes to these results in that dry food is conducive to low rate of oviposition, low total egg production and shorter life.

The work was made possible by a grant from the Royal Society to whom our thanks are accorded. We wish also to thank Dr O. W. Richards for help in matters of technique, for estimating the water content of samples of the grain used, and for testing the validity of the method employed to estimate the number of eggs laid in the grain.