Indications of the type of paddy grain most commonly infested by Sitophilus sasakii (Tak.) and Rhyzopertha dominica (F.) have been obtained from the examination of infested samples of paddy grown in Trinidad, W.I., and in British Guiana.Small-scale experiments with sound mature paddy have shown that S.sasakii is unable to feed and breed in a grain with an intact husk, even when the moisture content is high. It is also probable that R. dominica finds it extremely difficult to attack and infest such grains.The rapid multiplication of S. sasakii in paddy is dependent on there being a high proportion of grains with badly damaged husks. Infestation can occur in grains with a slight separation of the lemma and palea, or with slightly split husks, but the adult that develops is often unable to emerge from such grains.
The results of trials with some synthetic pyrethroids, especially cypermethrin and permethrin, against pests of cotton, various fruits, vegetables and brassicas are summarised. These compounds and cypermethrin in particular have given good results at low dosages against many insect species on these crops. Cypermethrin shows promise against ectoparasites attacking sheep and cattle and also is being evaluated for various non‐agricultural outlets.
Most of the work on the bionomics of the lesser grain borer, Rhyzopertha dominica (F.), has been done with wheat as the food material and it has often been assumed that the pest would behave and develop in the same way when it infested other grains, e.g., paddy and rice. This assumption does not, however, take into account the different characteristics of the paddy and wheat grain and the effect that these differences might have on the biology of R. dominica.It might be expected that the most marked effect would be on feeding and consequently on oviposition. Breese (1960) has shown that, if the husk of the paddy grain is sound and intact, the adult of R. dominica is usually unable to feed, but even if the husk is damaged or defective it might be expected from the work of Birch (1945) that its mere presence or possibly the harder texture of the rice kernel could reduce feeding and depress the oviposition rate. Birch found that, in wheat, eggs were laid at about one-eighth of the normal rate when the females did not have access to damaged grains for food. This observation was however made in preliminary experiments which are not described in detail. Similarly Crombie (1942) merely stated that adults of R. dominica are unable to attack intact wheat grains, and in all his oviposition experiments he made two oblique scalpel cuts in each grain, a practice also followed by Birch. Gay & Eatcliffe (1941) refer to the "proven ability (demonstrated daily in breeding cultures) of R. dominica to attack and increase in undamaged wheat." In view of the uncertain evidence on which these contradictory statements appear to be based, it seemed advisable to re-examine the effect of whole wheat on oviposition before making any comparisons of the oviposition in wheat and other grains. Methods.Most oviposition and feeding studies were made using the brick slabs described by Breese (op. cit.), in the cells of which individual females could be confined with a single grain. Experiments were conducted at 29-4°C. and 75 per cent. E.H., and the slabs and grain were fully equilibrated with these conditions. Unless otherwise stated, the females used were from stock cultures kept at 29-4°C. and 75 per cent. E.H. on bruised Canadian durum wheat. They were between two and three weeks old at the beginning of an experiment and only females which had shown a good laying rate and had fed actively for at least six days previously were selected. Such females are referred to as ' guaranteed layers.'Oviposition on cut and on whole wheat.
Nausibius clavicornis (Kug.) is a cosmopolitan Cucujid that has frequently been recorded from sugar. It is commonly found in this commodity in Trinidad, where it has not been collected from any other.The adult and other stages are described, and an account is given of the life-history on Haydak's formula at 85°F. and 75 per cent. R.H. Development has also been studied on two types of unrefined sugar.Copulation can take place within one day of the emergence of either sex and the female may begin laying within four days of copulation. The average number of eggs laid was 275 and the overall viability was 85 per cent. The average incubation period was just under five days.The larvae of both sexes may undergo only five moults in development, but the more usual number for females is six. When larvae are disturbed during development, both sexes tend to have six larval instars. Severe disturbance may induce a seventh moult. The mean development period from oviposition to the emergence of the adult was 30 days when there were five larval instars; when there were six, development took about two days longer. Larval mortality on Haydak's formula was low.Ovipositing females lived for up to 163 days under controlled conditions, but males tended to outlive females by about 50 days.On unrefined sugars (e.g., raw or yellow-crystal sugar), adults lived for almost as long (73:99) as on Haydak's formula, but oviposition was greatly reduced and the viability of the eggs was much lower. Larval mortality was high, especially in the first instar, and the total development period was greatly increased.It is unlikely that N. clavicornis could multiply rapidly in raw or unrefined sugar, and any deterioration in stored sugar directly attributable to it would probably be small in comparison with that caused by other factors.
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