Experiments on the relative abundance of two sibling species of grain weevils.

Abstract

The small "strain" and the large "strain" of Calandra oryzae L. are sibling species. The small "strain" is common in stored wheat and rare in stored maize. The reverse is true for the large "strain." A series of four experiments showed how wheat favoured the small "strain" and how maize favoured the large "strain." Given a choice of wheat and maize the small "strain" and the large "strain" laid most of their eggs in wheat but the proportion was larger for the small "strain" as compared with the large "strain." When the insects were reared for several generations in wheat they laid more of their eggs in wheat. Likewise when reared in maize they laid more of their eggs in maize. But this "host conditioning" was not sufficient to prevent them from laying many eggs in the "wrong" grain. The innate capacity for increase of the small "strain" was greater than that of the large "strain" in wheat but in maize the large "strain" had a greater innate capacity for increase than the small "strain." In crowded cultures wheat again favoured the small "strain" by permitting greater maximum populations as compared with the large "strain." Maize favoured the large "strain" in this respect. When the two "strains" occurred together in crowded cultures one always drove the other out. The small "strain" was the successful one in wheat and the large "strain" was the successful one in maize. Although these four series of experiments illustrate ways in which wheat favours the small "strain" and maize favours the large "strain" they do not, in themselves alone, account for the segregation of the two "strains" in stored grain.