Oviposition Behaviour in the Two Strains of the Rice Weevil, Calandra Oryzae Linn. (Coleopt., Curculionidae)

The original stocks of the two races of C. oryzae were kindly supplied by Dr O. W. Richards from the field station of the Imperial College of Science and Technology at Slough, Bucks. From these, stock cultures were set up and maintained as detailed elsewhere (Eastham & McCully, 1943; Eastham & Segrove, 1947), except that as C. oryzae proved to be more susceptible to humidity than C. granaria it was necessary to ensure that grain was sufficiently moist before being supplied to the weevils. Accordingly, the soft English wheat, which was used throughout the experiments, was packed in stoppered jars in which were also enclosed gauze-covered jars containing a quantity of saturated salt solution (in equilibrium with an atmosphere of 70% R.H.). By storage in this way for a week or two the wheat was brought to a moisture content suitable for the development of the weevils. No further precaution was taken other than keeping the culture jars lightly covered to prevent undue drying out. The weevils used for infecting the stock culture were left in contact with the wheat for a period of approximately 3 weeks and were then removed by sieving and sorting. Cultures were kept going continuously at 20 and 25 ° C., and the weevils reared at each of these temperatures were used only in experiments carried out at the same temperature as that at which they had been reared. All the experimental insects were thus the progeny of insects which for a number of generations had been reared under constant-temperature conditions. By this means was excluded any possibility of variation in the behaviour of individual insects in any particular experiment being caused by variations of temperature in the environment of the previous generations.

When the adult weevils began to emerge from the grain the stock cultures were carefully sieved and sorted daily, and the daily emergence groups were matured separately for a period of 3-5 days before being used in experiments. The groups were then sexed by examination of the rostrum under the binocular microscope, the coarsely punctured rostrum of the male being easily distinguished from the more sparsely punctate structure of the female (Richards, 1947). No failures were sub-sequently recorded as a result of relying solely on this method.

All the experiments were carried out with single pairs of weevils or with isolated females. In all cases at least fifteen replicates were set up, a procedure amply justified by the results, since the performance of individual weevils varied greatly. The insects were placed in in. glass tubes which were closed with cotton gauze held in place with rubber bands. They were supplied with a certain number of grains of wheat previously acclimatized to a particular humidity by the methods outlined by Eastham & Segrove (1947). The tubes were stacked in glass jars with tightly fitting fids containing potash solutions as humidity controls (Buxton & Mellanby, 1934), which were then transferred to constant-temperature rooms and incubated in the dark at the appropriate temperature. Every 14 days the weevils were supplied with fresh wheat in clean tubes, and this procedure was continued until all the female weevils had expired. Where pairs of weevils were being used and the male died first it was replaced by a male from the same age group. The used wheat was incubated for a further 14 days at 70% R.H. in order to permit the eggs to hatch out and thus to facilitate the subsequent counting, and was then stored in 50 or 70% alcohol according to the water content of the grain, the harder grain being stored in the weaker alcohol. After storage in this way the grain was of a soft cheesy consistency, and the number of contained larvae was determined by carefully slicing the grain under the binocular microscope with a razor blade.

Experiments were carried out at two temperatures and two humidities, viz. 20 and 25 ° C. and 50 and 70% R.H. The choice of the temperatures was dictated solely by the limitations of the available apparatus; in particular, it was found impracticable to employ temperatures below 20 ° C., the duration of many of the experiments precluding their restriction entirely to the colder months of the year. Regarding the humidities, C. oryzae, as previously mentioned, is more susceptible to moisture than C. granaria-, it survives for a period of days only at 40% R.H., and even at 50% R.H. mortality is high and the duration of life short. The upper limit of humidity, i.e. 70% R.H., was dictated by the difficulty of obtaining acclimatized wheat clear of fungal infection at any higher humidity.

Four sets of fifteen replicates of pairs of weevils were set up, one set for each combination of the temperatures and moisture conditions employed, viz. 20 and 25 ° C. and 50 and 70% R.H. Each pair of weevils was supplied fortnightly with twenty fresh grains of acclimatized wheat. The mean number of eggs produced per female under the various conditions was as follows:

The mean rates of oviposition are shown graphically in Fig. 1. In calculating the means it has been thought advisable to reject certain results. Commonly in each experiment one or two females produced an abnormally low number of eggs, and though the cause of this has not been determined there is every likelihood that a proportion of insects was damaged by handling, even though this was reduced to a minimum. For instance, the total numbers of eggs produced by the individual weevils at 20 ° C. and 70% R.H. were as follows: 325, 323, 308, 289, 276, 245, 232, 208, 204, 192, 171, 146, 137, 76, 65. The last two figures are suspiciously low, and examination of the full tabulated results shows that the weevils concerned were short-lived and had low rates of oviposition. Such results have been rejected, and though the method adopted is necessarily somewhat arbitrary it is believed that the figures given are as a result less biased than would otherwise be the case. It may be noted, however, that the rejected results have invariably differed significantly from the mean of the remainder, taking P=0·05 as a test of significance. Throughout the paper the means have been expressed in terms of their standard errors calculated according to the formulae of Simpson & Roe (1939).

From the results it is clear that the general pattern of the insect’s behaviour is the same throughout the experiment. In each case the rate of oviposition rises rapidly to a peak, which is reached between 4 and 6 weeks after the emergence of the adult weevil from the grain. It then gradually declines, becoming zero about a week before the death of the weevil, the oviposition period being thus slightly shorter than the life-span of the imago. The apparently steady decline is, however, to some extent an artefact, due to averaging a series of weevils with varying life-spans. Individual weevils show a ‘middle’ period, following the peak, in which the rate of oviposition falls slowly, followed by a terminal period in which the decline is rather more abrupt.

Despite this general similarity in behaviour both humidity and temperature have important effects on oviposition. It is abundantly clear that 50% R.H. is approximating to the insect’s limit of viability, for at this humidity the weevil’s life (and therefore the oviposition period) is short, the rate of oviposition is low and hence fecundity is at a low level. At the higher humidity the weevil lives for a longer period and the rate of oviposition is considerably greater. It is noteworthy that temperature has little effect on the total egg production, for though the number produced at 25° C. is slightly less than at 20° C., both at 50 and 70% R.H., the differences are not significant. Temperature does, however, affect the rate of oviposition which is higher at 25 than at 20° C. at either humidity. The unaltered egg production at the higher temperature is thus due to a higher rate of oviposition coupled with a shorter oviposition period.

Four sets of fifteen replicates of pairs of weevils were set up at 20° C. and 70 % R.H. and supplied fortnightly with 5, 10, 20 and 40 grains of wheat respectively. The mean egg production of the four sets was: 5 g., 141·2±7·6; 10g., 156·3±12·7; 20 g., 235 + 17·4; 40 g., 269 ± 13·4. The differences between the means of the five- and ten-grain sets and between those of the twenty- and forty-grain sets are not significant, but all other differences are highly significant. It is quite clear, therefore, that the amount of available grain has a pronounced effect on the fecundity of the insect, higher egg production being associated with greater amounts of grain. The rates of oviposition are graphically expressed in Fig. 2, from which it may be seen that with five, ten or twenty grains of wheat available there is little variation in the behaviour of the insect. As in the preceding experiment oviposition rises rapidly to a peak and subsequently declines gradually to the end of the oviposition period. With forty grains of wheat available, however, the oviposition period is appreciably shorter, as also is the life of the insect, and the decline in the rate of oviposition is accordingly more rapid. It may be tentatively concluded that a high fecundity coupled with a high rate of oviposition has a tendency to shorten the life of the imago.

Twenty replicates of pairs of weevils were set up at 25° C. and 70% R.H. and supplied fortnightly with twenty fresh grains of wheat. At the end of the first fortnight the males were removed from ten of the replicates, so that from then onwards the females were in isolation. The remaining males were removed from the other replicates at the end of the second fortnight. The mean egg production of the whole set of weevils was 247·2 ± 9·8. This figure is higher than that for pairs of weevils supplied with twenty grains of wheat under similar conditions (i.e. 217·6 ± 19·8), but the difference is not significant. Comparison of the rates of oviposition, however, shows a striking difference. From Fig. 3 it can be seen that in the case of the isolated females the rate of oviposition rises steeply to a high peak, again somewhere between the 4th and 6th weeks of oviposition, then rapidly falls away and ceases several weeks earlier than happens with the paired weevils. Thus though the isolated females do not produce a significantly greater number of eggs than the females paired with males they do deposit them in a considerably shorter space of time and exhaust themselves much sooner. This more rapid rate of oviposition is accompanied by a somewhat shorter life-span. These effects are still more striking if the two groups of isolated females are considered separately (see Fig. 4). In the case of the females isolated at the end of the first fortnight, the rate of oviposition rises to a maximum at the end of 4 weeks, when it is more than 50 % higher than that of the paired females. From this peak value it declines steadily. In the case of the females isolated after a month it can be seen that the rate of oviposition follows the expected course for the first 4 weeks (i.e. parallel to, though slightly higher than, the course followed by the permanently paired females), but that subsequent to the removal of the males the rate of oviposition continues to rise for a further 4 weeks before declining. The maximum rate of the second batch of females is considerably less than that of the first batch. As a result of this, maximum egg production is shown by the females isolated after 2 weeks, whereas those isolated after a month show a somewhat lower fecundity coupled with a rather different pattern of oviposition behaviour. These results appear strongly to indicate that the females possess, in the first few weeks of adult life, a very high egg-laying potential which can be realized only in a relatively short period. Any factor, such as in this case the presence of males, which tends to restrain the rate of oviposition and which is operative during this period has a permanent effect on the rate of oviposition, and possibly on the fecundity. Removal of the restraining factor after even a relatively short period (i.e. in this case 2 weeks) is not compensated by a subsequent rise in the rate of oviposition to the value which would have been reached in the early stages. Compensation does, however, occur and takes the form of an extension of the oviposition period. The difference between the two sets of isolated females in this respect is not great, but if either is compared with the permanently paired females the effect of the initial depression of the rate of oviposition on the overall pattern of oviposition is clearly indicated. It is not possible to say whether the maximum egg-laying potential has been realized in these experiments, since no data is available for the performance of isolated females in larger quantities of grain. It does seem rather unlikely, however, from the results of other workers. MacLagen & Dunn (1936) record a maximum rate of oviposition of 6·75 eggs/female/diem at a density of 1 weevil to 400 grains, as compared with a present maximum of 4·5 eggs/female/diem. Hinds & Turner (1911), according to these same authors, state that in maize a value of 15-16 eggs/female/diem is not uncommon. Neither of these results, however, must be interpreted as indicating that the total fecundity would exceed the values recorded here by anything like the same margin.

In the present experiments there was no appreciable increase in the number of infertile eggs at the end of the oviposition period. It does not therefore seem likely that the rapid falling off in the rate of oviposition observed is connected in any way with the exhaustion of sperm in the spermatheca of the female, though, unfortunately, this point was not checked. In some insects, e.g. Rhizopertha (Crombie, 1942), it is known that a single or several early impregnations are sufficient for unrestricted oviposition, subsequent copulations having no influence on fecundity or fertility. In Calandra such evidence as is available (Richards, 1947) indicates that in general the copulation of females which have been in isolation for considerable periods has little effect on oviposition. It has, however, been observed in the course of the present work that in most females the stimulus of copulation is necessary to initiate oviposition.

Oviposition in the small race of C. oryzae was investigated only at 25° C. At this temperature and at a R.H. of 70 % the mean number of eggs produced by 15 replicates of pairs of weevils supplied fortnightly with 20 grains of wheat was 148·8 + 13·7. This differs significantly from the number of eggs produced by the large strain under similar conditions, viz. 216 + 19·7, and agrees with the findings of other workers on the relative activity of the two strains (see Richards, 1944). There is little difference in the life-span of the two strains, and the lower egg production of the small strain is therefore attributable to a lower rate of oviposition, as may be seen by reference to Fig. 5, in which the rates of oviposition of the two strains are compared. Birch (1945a) records the figure of 265 for the egg production of the small strain at 25·5° C. and 70% R.H. with a weevil density of 1-10 grains of wheat, but in his case the grain was changed weekly. The extent to which this latter factor might explain his much higher figure is discussed in a later section.

During the counting of eggs in the foregoing experiments the numbers in individual grains were recorded in full. Examination of these results appeared to indicate that the distribution of eggs differed in the two strains of weevil, further analysis confirming this impression and yielding some evidence in explanation of the different behaviour of the two strains.

If a number of eggs are deposited at random in a given number of wheat grains the probabilities of obtaining grain containing 0, 1,2, etc., eggs may be calculated from the expansion of the binomial ⁠, where n is the number of grains and r the number of eggs. As, in general, a few terms only are required the formula ⁠, given by Stoy (in appendix to Salt, 1932) and derived from the general term of the expansion, may be conveniently used. This gives the probable number of grains of the total number of n which will contain x eggs when the total number of eggs deposited is r, and is calculated for values of x = 0, 1,2, …, etc. For instance, the random distribution of 10 eggs in 20 grains gives the following values:

At 25° C. and 70% R.H. weevils of the small strain, during the course of the experiment, laid between 9 and 11 eggs in 20 grains in a 14-day period on six occasions ; weevils of the large strain under the same conditions did likewise on five occasions. The numbers of grains containing o, 1, 2, etc. eggs from the aggregates of these results are shown in Table 1, together with the expected frequencies of random distribution calculated from the table above. (The figures for the small strain have been reduced by one-sixth in order to equate them to those for the large strain.) The agreement between the three sets of figures is extremely close and would seem to indicate that at this rate of oviposition (i.e. 10 eggs/20 grains/14 days) both strains of the weevil deposit eggs at random. It is, nevertheless, doubtful whether these results are anything more than fortuitous, for at higher intensities of oviposition a pronounced difference in the behaviour of the two strains becomes apparent.

The distributions,of 250 eggs in 200 grains by each type of weevil (aggregates in each case of 10 cases where 24-26 eggs were laid in 20 grains in a fortnight) are shown in Table 2 and depicted graphically in Fig. 6. It may be seen that in the case of the small strain there are fewer ‘empty’ grains, more grains with one egg, and fewer with two or more than is to be expected on random distribution, while the reverse is the case with the large strain. In neither case, therefore, are the eggs distributed at random. Similar results are obtained with the cases in which 20 eggs were laid in 20 grains.

The ‘over-distribution ‘by the small weevil appears to indicate a tendency on the part of the insect to avoid grain in which eggs have already been deposited or in which larvae are present. This conclusion will be discussed later in relation to the results of other workers. The ‘under-distribution’ by the large strain seems, on the other hand, to be correlated with the size of the wheat grains and the tendency for the weevil to select the larger grain for oviposition. This has been demonstrated by the examination of one fortnightly batch of 300 grains supplied in the usual way to 15 weevil pairs in lots of 20. Previous to the counting of the eggs by the slicing technique each individual grain was weighed ; any grains showing more than slight signs of being eaten were rejected and the remainder were divided into heavy and light halves. From the aggregates of the heavy and light halves the following figures were obtained: the 134 heavier grains contained 321 eggs, 13 grains containing no eggs; the 134 lighter grains contained 189 eggs, 42 grains containing no eggs. This result is, of course, partially explicable in terms of the larger surface area available for oviposition in the heavier grains. Calculation shows, however, that after due allowance is made for this factor a disproportionately large fraction of the total eggs occurs in the heavier grains, whilst the discrepancy is even more apparent in the distribution of the empty grains. It is evident that the weevil prefers the heavier, and therefore larger, grain for oviposition. In addition, calculation indicates that in the heavier grain the distribution of eggs is at least as good as random with the possibility of over-distribution (as found in the small strain), though the numbers involved are too small to be conclusive. It appears, therefore, that the difference between the results with the two strains is related to their preferences for different sizes of grain and that the average size of the experimental grain fell below the optimum of the large strain. It should be added that the suitability of grain for oviposition is determined by its shape as well as by its size or weight. Grains with a characteristic plump shape are avoided, whatever their size, as meticulously as the very small ones, though the reason for this avoidance is not clear.

One of the most striking features of the results in general is the great variation in the performance of the individual weevils. This in part is doubtless to be attributed to genetic variation in the stock and can be dealt with only by conducting experiments on the largest possible scale if statistically significant results are to be obtained. Experience has shown that it is not possible to circumvent this problem by selecting weevils showing an average performance over a short period, a practice frequently employed in short-term experiments. Fecundity in the early weeks of adult life is no sure index of total egg capacity, low initial fecundity being followed on occasion by maintained oviposition, high initial fecundity by rapid decline and a relatively low egg total. A further difficulty arises when the procedure adopted in the present work is employed, viz. of rearing stocks at the temperatures it is intended to apply during the experimental work. In such circumstances it is impossible to test the females under identical conditions before commencing the experiments, and careful determinations of average performances under different conditions would have to be made if the final results were to be in any way comparable. In view of these facts it is clearly desirable to eliminate, as far as possible, the contribution of the environment to the induction of variability in the stock. Two points seem worthy of note. In the first place Richards (1944) has shown that grain size influences the size of the adult insect, and also (1947) that egg production is related to the size of the weevil. A more uniform size of weevil and therefore a more uniform oviposition performance may be expected if grain of a standard size is used in stock cultures. In the second place it seems desirable to avoid the use of mass cultures in rearing the experimental insects. In such cultures accumulation of carbon dioxide or frass and overheating in the interior of the mass are all factors calculated to produce non-uniformity, whereas if the grain were spread out in thin layers, say in desiccators, such effects could be entirely eliminated.

From the results of the individual experiments a number of points emerge which require brief discussion. The effects of temperature on oviposition will first of all be considered. In an earlier paper (Eastham & Segrove, 1947) it was shown that an insect could be subjected to approximately equivalent moisture conditions at several different temperatures by employing throughout a fixed relative humidity, and that by so doing it is possible to isolate the effects of temperature as an environmental factor. In the present work this has been done (see § 1 above), and the effect has been to demonstrate that there is no significant difference between the total eggs deposited at temperatures of 20 and 25° C. under equivalent moisture conditions. It may be concluded that the main effect of temperature changes is to alter the general tempo of the life of the insect. The results of Eastham & McCully (1943) on oviposition in C. granaria are in broad agreement with this conclusion if due allowance is made for variability in the weevil stock on a similar scale to that encountered in the present work.

The second point to be considered concerns the distribution of eggs between grains. The results of the present work have established with reasonable certainty that the small strain of C. oryzae exhibits a marked tendency to avoid laying eggs in grain already containing eggs and/or larvae, and that though the large strain of weevil exhibits an apparent inverse effect, i.e. tends to overcrowd its eggs, the result is to be ascribed to its greater preference for a larger-sized grain, and its behaviour in reality is no different from that of the small strain. Moreover, although it has not been established that there is an optimum size of grain for each weevil, since it was not possible to show that the smaller weevil was not also making more use of the larger grain, it is nevertheless clear that the smaller weevils utilize the smaller grain far more readily than the larger weevils. The most likely explanation for this is that the females of the larger strain have more difficulty in manipulating the small grain in the act of oviposition. This point is of some interest, since Ewer (1945), working with C. granaria, failed to establish any substantial difference in the performance of selected small and large female weevils when presented with unselected grain. Both showed a preference for large grains, and the smaller weevils showed no less preference than the larger ones, and Ewer concludes that the behaviour of the animals is influenced by physico-chemical differences in the grain rather than by size. This author also showed that the female weevils tend to avoid laying in grain containing a fourth instar larva, but otherwise found no evidence of oviposition being inhibited by the presence of eggs or larvae. One of her experiments, quoted in more detail by Richards (1947), does nevertheless support this idea.

In this particular experiment a mixture of large and small grains—the proportion of each being such that the total surface areas of each size of grain were equal—were presented for a short period to a group of weevils. In the subsequent analysis it was found that more eggs had been laid in the large grains than in the small, and that there were fewer grains with o or 2 eggs per grain and more with 1 or 4 than could be expected on random distribution. Richards contends that the abnormal distribution is due to the admixture of two sizes of grain ‘in either of which alone oviposition would have been normal’, i.e. random. I believe, however, that it is a clear case of over-distribution of eggs, and that even the slight excess of 4-egg grains is explicable if a somewhat different method of analysis is employed. Strictly speaking it is permissible to compare the actual distribution of eggs with the calculated random distribution only when the eggs are partitioned between the two types of grain in proportion to the numbers of grain of each type (or alternatively in proportion to their aggregate surface areas), and this is manifestly not the case in Ewer’s experiment. An alternative procedure is to make separate calculations of the random distribution for the two types of grain and compare the sums of the two sets of results with the experimental data. It is not difficult to show that this second method is the better one to apply in the present case.

The actual experimental situation lies somewhere between two clearly defined limiting situations, which are : that the weevils either show an equal preference for the two types of grain, or, at the other extreme, confine their attention (as regards oviposition) strictly to one type or the other. In the former case it would be correct to determine the random distribution for the whole grain sample, but in the latter contingency the calculation should concern only that fraction in which the eggs are laid, i.e. the two types of grain should be treated quite independently in the calculation. An appreciation of this point is of fundamental importance to the whole argument, for the two sets of values are widely different, as comparison of lines 1 (or 2) and 3 in Table 3 will show. My contention is essentially that in the second limiting situation the environment of the insect is divisible into two distinct sub-environments, only one of which is being utilized for oviposition and which therefore should be treated of as a separate entity as regards this particular activity. However, once the insect begins to utilize the other type of grain the two sub-environments begin to overlap or coalesce, and do so completely when the insect ceases to dis-criminate between the two sizes of grain. Hence it would appear that to subject the two sub-environments to separate analysis in such intermediate situations would inevitably be more or less incorrect. One could obviously make an estimate of the relative importance of the two parts of the environment from the partition of eggs between them and then decide to which limiting situation the experimental situation more closely approximated in order to choose the better method of analysis. Fortunately this is unnecessary, for the position is far more precise than appears on the surface. The order of the numbers involved in the present case is such that even the hypothetical situation where both sorts of grain are equally acceptable to the weevils, i.e. where there is a single uniform environment, may be analysed in two separate parts with little loss of accuracy. This will be apparent from a comparison of the two sets of values in lines i and 2 in the table, which have been arrived at in the following manner. Line 1 is a straightforward evaluation of the random distribution for the total eggs in the total grain. In line 2 the total of 164 eggs has been divided between the large and small grains in proportion to the numbers of each type of grain present, and yields values of 63 eggs to the 116 large grains and 101 eggs to the 185 small grains. Separate evaluations of the random distributions for each type of grain have then been made, and the two sets of values have been added together to obtain a single set of figures for the whole of the grain. The figures obtained by the two methods of calculation are almost identical (see lines 1 and 2 in the table). This means, in effect, that even when the insect is shewing no discrimination whatever, independent analysis of the two types of grain introduces a negligibly small error. Furthermore, this error reduces to vanishing point by this method of treating the results as the experimental situation approaches the other extreme where the insect makes exclusive use of one type of grain.

The random distribution for the experimental result has therefore been evaluated by calculating separately the random distributions in each type of grain in terms of the numbers of eggs actually laid in each type, and then summing the corresponding figures. Thus since 101 eggs were laid in 116 large grains and 62 eggs in 185 small grains in the actual experiment, these two pairs of figures have served as the basis for the calculation of two random distributions. The values obtained from the sum of these distributions are shown in line 4 of the table. Comparison of them with the experimental results in line 5 reveals appreciable differences. The numbers of grains with more than two eggs are too small to be significant, but the excessive number of 1-egg grains in the experiment, and the corresponding deficiency of empty and 2-egg grains, leave little room for doubt that the eggs are not deposited at random. One can only conclude from this more effective distribution that the weevils exhibit a definite tendency to avoid grains in which eggs have already been deposited.

This conclusion, if the validity of the argument be admitted, is apparently at variance with other of Richards (1947) data. For example, in a detailed analysis of five cultures in unselected wheat he finds that eggs are distributed at random (though a closer inspection indicates that the result actually shows slight under-distribution). Elsewhere, by plotting percentage grain attacked against life per 100 grains, for a large number of cultures, he produces further evidence of random distribution. Both results, however, are explicable in the light of the evidence from the experiment analysed above and the results given earlier for the large strain of C. oryzae, viz. of the existence of two opposing tendencies in the weevil—to distribute the eggs widely by avoiding grains with life on the one hand, to crowd the eggs into the more suitably sized grains on the other. Under certain circumstances, when the grain available to the weevil is of some particular average size, these opposing factors balance one another. The resulting distribution is superficially a random one, and its spurious character is concealed rather than made evident by certain methods of analysis. It is clearly necessary to distinguish between a true ‘random distribution ‘due to genuine random behaviour on the part of the insect and an ‘effectively random distribution’ which may arise from other causes. It is quite feasible, and I think probable, that these points provide an explanation of Richards’s results, and that further investigations in which use is made of grain of different sizes will confirm the findings of the present paper.

MacLagen & Dunn (1936), also working with C. oryzae, are the only workers who appear to have suspected that the weevil is capable of detecting and avoiding grain containing life, but their reasons for doing so are somewhat obscure. Using a series of different densities of weevils/grain they found that the frequency curve of egg distribution altered from a marked positive skewness at low densities to something approaching a normal curve at high densities. They suggest that whilst the former may indicate what I have termed ‘over-distribution’, the latter undoubtedly means that the weevils use the grain indiscriminately at high densities. I can agree with neither finding. Marked skewness of the frequency curve at low concentrations of eggs/grain is not of itself any proof that the distribution is other than random, as reference to Fig. 6 in the present paper will show. In the second place, the frequency curve automatically becomes less skew and approaches normality as more and more eggs are crowded into a given number of grains, and it becomes increasingly important to show that the deviation from a normal distribution is statistically significant before any reliable conclusion can be drawn.

The main reason for this attempt to establish the existence of a non-random behaviour on the part of the weevil in its choice of oviposition sites is that if proved it becomes yet another factor to be considered in the interpretation of experimental results. For if the insect is influenced to deposit eggs in other grains by the presence of eggs or larvae, the occurrence of these in appreciable numbers seems likely to depress the rate of oviposition. This may, for instance, be a partial explanation of the discrepancy between Birch’s (1945 a, b) results with the small strain of C. oryzae and my own, to which previous reference has been made. Birch’s figure for the fecundity of the weevil under the same conditions of temperature and humidity and the same density of weevils/grain as employed in the present work, is nearly double that recorded here. Whilst this may be due to a variety of causes—genetic differences in the stock, different strains of wheat, etc.—the mere fact of his changing the wheat with greater frequency, actually weekly instead of fortnightly, may have contributed substantially to the production of the higher figure. Not the least effect of this difference in technique would be to relieve the congestion of grains by eggs, and thus to ameliorate a condition likely to inhibit oviposition. Alternatively, of course, the rate at which this congestion builds up can be reduced by increasing the quantity of grain. The present series of experiments, and those of MacLagen & Dunn (1936), have shown how fecundity rises as more grain is made available (vide § 2 above). In part, this is undoubtedly to be explained by the reduced probability of the male disturbing the female in the act of oviposition in a larger volume of grain (as the experiments which isolated females clearly emphasize), but may also be due to the effect under discussion. More intensive experiments with isolated females in different quantities of grain would probably enable the relative importance of the two effects to be determined ; the present data, unfortunately, is insufficient to decide the point. It may be added that such experiments would also shed some light on the results of MacLagen & Dunn (1936). They found that a maximum rate of oviposition was obtained with a density of weevils/grain of 1 : 400, but that under these conditions only I grain in II was used in oviposition.

Finally, a brief reference must be made to a second important point which emerges from the experiments with isolated females. The marked rise which occurred in the rate of oviposition in these experiments following the removal of the males was found ultimately to be unaccompanied by any statistically significant increase in the total eggs laid. This serves to emphasize a danger which is inherent in short-term experiments, viz. of translating the short-term effects of changed conditions into terms of an overall change in performance. Where, therefore, an accurate knowledge of the fecundity is required, as, for example, in the computation of the ‘biotic potential’ of a population (see Birch, 1945b) there is no alternative to the complete investigation of the insect’s reproductive span.

The oviposition behaviour of the large strain of Calandra oryzae has been investigated at 20 and 25° C. and under moisture conditions equivalent to 50 and 70% R.H. The small strain of the weevil has been investigated at 25° C. and 70% R.H.

At either temperature the lower humidity shortens the life of the insect and depresses the rate of oviposition so that fecundity is of a low order. Changes of temperature with a fixed humidity alter the rate of oviposition but have little effect on the total egg production. Under all conditions the pattern of oviposition remains the same, the oviposition rate rising to a peak in the earlier weeks of maturity and subsequently declining.

Increasing the amount of grain leads to increasing egg production. It is unlikely that conditions for maximum fecundity were realised in the experiments.

Isolating females in the early weeks of maturity leads to a high initial rate of oviposition, followed by a more rapid decline and little overall increase in fecundity.

At 25° C. and 70% R.H. the fecundity of the large strain is of the order of 50% higher than that of the small strain.

The distribution of eggs between grains differs in the two strains, the small strain distributing its eggs better than random, the large strain tending to overcrowd its eggs. The evidence suggests that this is due to the larger weevils’ greater preference for large grains. By correlating the results with those from other sources it is nevertheless concluded that both strains tend to avoid laying in grains already containing life. The importance of this factor in oviposition behaviour is discussed.