An Experimental Demonstration of Ecological Succession in an Insect Population Breeding in Stored Wheat

Abstract

Davidson (1941), referring to damage to stored wheat in South Australia by the beetles Sitophilus granarius (L.), S. oryzae (L.) and Rhizopertha dominica Fab. and the moth Sitotroga cerealella Oliv., wrote: 'The damage to stored grain by these four species makes the material favourable food for a large number of secondary insects which are normally associated with crushed or ground cereals and cereal products. We have here an interesting example of ecological succession. Different kinds of insects find the environment of stored wheat favourable for them according to the degree of damage and spoilage which it has undergone.' Although this statement of Davidson's was a generalization based upon experience and observation rather than experiment, it nevertheless presents an accurate, if oversimplified, picture of the working of the ecosystem formed by infested, stored grain. Kiritani (1958) explained some of his results on trapping insects in a farm rice store in similar terms. The stored rice was fumigated and Kiritani found that certain secondary species, present in only very small numbers before fumigation, increased markedly afterwards. These increases he attributed to damage by rice weevils before fumigation, which rendered the food more suitable, subsequently, for other species, especially Carpophilus. Coombs & Freeman (1955) studied the insect fauna of grain residues which had accumulated in inaccessible spaces in a granary. They found that although most of the species were found in most of the samples, there was evidence that dominance by certain species was associated with the quality of the residues. Thus, when residues contained a fairly high proportion of whole grain, the weevil Sitophilus granarius or the moth Hofmannophila pseudospretella (Staint.) predominated. In residues with less whole grain and more finely-divided material there was a higher proportion of spider beetles (Ptinus spp.) and of the mealworm, Tenebrio molitor L., in the fauna. Finally, in residues of very poor quality, only the fur beetle, Attagenus pellio (L.), and the predatory window fly, Scenopinus fenestralis (L.), were found. This association of dominance by different species with residues of different quality, established by sampling at one time, may be interpreted as evidence of succession in dominance with time, because the various grades of residue represent stages in the continuous degradation of the whole grain by the insects. It is in undisturbed residues of this type that the succession described by Davidson is best observed, because the bulk of the material is small, the degree of infestation high, and the successional changes are consequently more rapid than in large bulks of stored cereals. Woodroffe (1953), in a study of the arthropod fauna of dry birds' nests, found evidence of succession among storage species in one of their natural habitats. Dry nests, in which storage insects occur, are normally situated on or in buildings or in holes in cliffs or trees, and the nest sites are used year after year, with accretion of new material and consequent confusion of the successional picture. When, however, birds are excluded