Some laboratory observations on the rates of development, mortality and oviposition of several species of Bruchidae breeding in stored pulses

Abstract

The rate of development of six species of Bruchidae (Acanthoscelides obtectus (Say), Zabrotes subfasciatus (Boh.) and four species of Callosobruchus) attacking stored pulses was studied over a wide range of constant temperature and humidity and the rate of oviposition of five of them investigated over a range of temperatures at 70 per cent. R.H. and a range of humidities at 30°C.A hatch of the order of 80 per cent, of the eggs of A. obtectus, which are scattered among stored seeds, was recorded over the temperature range 17·5–30°C. at both 30 and 90 per cent. R.H. and over the range 15–35°C. at 70 per cent. R.H. None hatched at 40°C. The optimum conditions for rapid development of the egg are close to 30°C. and 70 per cent. R.H., where the egg hatched in just under five days. The period to hatching is increased at both lower and higher humidities as well as at lower and higher temperatures. The optimum for the rapid development of the post-oval stages is also 30°C. and between 70 and 80 per cent. R.H., in which conditions the first beetle emerged 20 days after the larvae were placed on haricot beans (Phaseolus vulgaris), the mean developmental period to adult emergence being 22·5 days, or 27·5 days including the egg stage. No beetles emerged at 35°C. and few at 15°C. Low humidity and very high humidity retarded development.The eggs of Z. subfasciatus and the species of Callosobruchus are stuck to the seeds, and data for the egg stage were not obtained separately. The optimum for the rapid development of Z. subfasciatus on haricot beans is close to 32·5°C. and 70 per cent. R.H., where the first beetle emerged in 23 days and the mean developmental period was 24·5 days. No adults emerged at 37·5 or 17·5°C. and few emerged at 20°C. or at low humidity at 35°C.The optimum conditions for rapid development of the species of Callosobruchus are 32·5°C. and 90 per cent. R.H. for C. chinensis (L.), about 30°C. and 70 per cent. R.H. for C. rhodesianus (Pic), 32·5°C. and about 70 per cent. R.H. for C. analis (F.), and 32·5°C. and 90 per cent. R.H. for C. maculatus (F.). None of the species could complete development at 37·5°C. and all four fared poorly at 35°C. Two species, C. chinensis and C. rhodesianus, were able to develop at 17·5°C. and the other two at 20°C. The performances of all the four species on dried garden peas (Pisum sativum) were poor; all did much better on cowpeas (Vigna unguiculata). The rate of development was slower on garden peas, the percentage mortality was higher and the developmental limits more constricted. In addition, the developmental period in all conditions on garden peas was so variable that the mean gave an unreliable estimate of performance and the time to the first emergence was used instead. The shortest mean developmental periods in days recorded were 22·5 for C. chinensis, 23 for C. maculatus, 25 for C. rhodesianus and 27 for C. analis. The shortest developmental period for an individual bred in garden peas was 18 days for C chinensis, 21 for C. maculatus and 26 for C. analis.The sex ratio was close to unity throughout. The developmental period of the female was slightly longer than that of the male in A. obtectus and C. chinensis, and also in C. maculatus when bred in garden peas but not when bred in cowpeas. At 30°C. on cowpeas, breeding in mass culture at a low density gave rates of development similar to those observed with isolated insects in the cases of A. obtectus, Z. subfasciatus and C. chinensis, but with C. analis and C. maculatus a large proportion of those bred in mass cultures grew much more slowly than did isolated insects.On garden peas, more than 50 per cent, of the eggs of the species of Callosobruchus grew to the free-living adult stage only in the most favourable conditions. For these species on cowpeas, and for A. obtectus and Z. subfasciatus on haricots, over 80 per cent, completed development over a wide range of conditions. Normally most of the mortality was of eggs or very young larvae, but at high humidity (80–100 per cent.) and at low (22·5°C. and below) and high temperatures (32·5°C. and above) a considerable number of individuals died after completing larval feeding, some after pupating.In all the species studied except C. analis, at emergence the female was on average heavier than the male by 0·75–1·25 mg. In Z. subfasciatus, the weight range of the two sexes at any condition rarely overlapped. Callosobruchus spp. bred from cowpeas were heavier than those reared in garden peas. The heaviest beetles were bred towards the bottom of the temperature range, at 50 and 70 per cent. R.H. The weight decreased slightly at 90 per cent. R.H., more markedly at low humidity, slightly up to 30°C. and more abruptly at higher temperatures.The rate of development of C. maculatus at 30°C. was shown to be. relatively consistent on several varieties of cowpea, the mean developmental periods on the best seven covering a range of less than 2 days (22·1 to 23·8). This mean was a further two days longer (25·7) on the two remaining varieties. Only one seed (Phaseolus aureus) from a series of ten other legumes had a mean developmental period within this range, and only from this seed and from P. mungo, brown peas and the nine varieties of cowpea were 30 or more beetles bred from 50 eggs.The lengths of the various instars of C. chinensis were determined by the dissection of samples daily, and some information on the pupae of C. rhodesianus was obtained by the use of soft X-rays.All the five species used in oviposition experiments were short-lived and had very short preoviposition periods after emergence. All laid at a maximum rate after 2 or 3 days, the rate falling quickly at high temperatures and more slowly at low ones. The highest mean number of eggs laid was 100 by C. maculatus, 75 by C. analis, 70 by A. obtectus, 50 by C. chinensis and 40 by Z. subfasciatus. C. analis and C. maculatus laid best above 30°C., and A. obtectus and Z. subfasciatus at 25°C. C. chinensis laid most eggs at 22·5°C., but the performance up to 37·5°C. was only a little inferior and varied irregularly from temperature to temperature; it was poorest at 20 and 17·5°C. The higher humidities (70–100 per cent. R.H.) were most favourable for all species at 30°C.Eggs of Z. subfasciatus laid at 15°C. did not hatch at that temperature, and likewise eggs laid at 37·5°C. did not hatch at 37·5°C. Very few eggs laid at 25°C. hatched at 15°C., but about half of a batch laid at 30°C. hatched at 37·5°C. Otherwise about 60 per cent, hatched at 20°C., nearly 80 per cent, at 35°C. and 80 per cent, or more at all intermediate temperatures. At 30°C. hatching was good at all humidities including 2 and 100 per cent. R.H.The results given in published work are compared with those obtained in this work. The value of the results in determining the pest potential of these species is discussed, and suggestions for the most profitable future lines of work are made.