The interaction of Environmental and Genetic Factors in determining the Weight of Grain Weevils, Calandra granaria (L.) (Col., Curculionidae)

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Abstract
Summary.: Cultures which are deep (up to 11‐0 cm.), more densely inhabited by larvae (up to 150 life per 100 grains), produce heavier weevils. The central part or the bottom half of a culture produces heavier weevils than the top part. Cultures left undisturbed produce heavier weevils than those which are disturbed earlier. The weevils from nearly all cultures exhibit a weight‐cycle. The first ones to emerge are light; those that emerge at the time of maximum emergence are heaviest and those which emerge last are lightest. On the whole, conditions which produce heavier weevils induce a more marked weight‐cycle. Nearly all the effects described in (1) and (2) must be ascribed to the influence of the metabolic products which accumulate to a greater extent in deep and crowded cultures. Part of the weight‐cycle, however, is due to genetic factors, so that it may be observed in very shallow cultures, A weight above the mean is correlated with a shorter developmental period and both characters are inherited. There is no evidence that reduction of weight in overcrowded cultures or at the end of the emergence period is due to competition of larvae within the grain. Larger grains produce heavier weevils, even though the whole of the grain is not utilized by the larva. Deeper, more heavily infested cultures become hotter, damper and accumulate more CO2. Some evidence was obtained that accumulated CO2 reduces the weight of weevils. None was found for the view that water affects weight. The physiological basis for the increase in weight in deeper, more crowded cultures is not known. There appear to be two principal genetic weight types in Calandra granaria. When cultured in 3times1‐in. tubes, these have mean weights of 1–90 and 2–46 mg. In the small line a subsidiary type of mean weight 2–07 mg. could later be selected out. In both lines weevils whose mean weight was more than 5 per cent, below the mean of the line produced very few offspring. Selection for larger size in the large line was ineffective, partly because of differential fecundity. There was some evidence for the occurrence of a gene producing premature death of the adult weevils in the last two inbred generations. Weevils increase in weight by about 5 per cent, in about the first third of their lives. On the whole, lighter weevils and those with a shorter developmental period tend to put on more weight, though not enough to obscure any initial weight‐cycle. It is suggested that population studies should pay more attention to the quality as well as to the numbers of the insects produced.
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