Rust-red flour beetle, Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), is a major pest of stored products worldwide. Its local significance as a pest of stored grains will likely be influenced by the relative performance of the beetles on the grains that are commonly stored in the area and the relative rates at which these insects infest those resources. Infestation of cotton seed storages by T. castaneum is also well known in Australia, but no information regarding the suitability of cotton seed as a resource for these beetles is available.
I therefore quantified the performance of laboratory-cultured (~160 generations) and field collected (1st generation) populations of T. castaneum across their life cycle on different structural forms of wheat (Triticum aestivum L.), sorghum (Sorghum bicolour L.) and cotton seed (Gossypium hirsutum L.). The highest numbers of individuals developed successfully on wheat and sorghum (flour and kibbled seeds) while on cotton seed all neonate larvae of the laboratory-cultured population died before pupation. Some of the field population larvae did, however, develop on cotton flour, but their number was significantly lower in comparison to the numbers that developed on wheat and sorghum. Field population females had a consistently higher fecundity (~78% more eggs) than the laboratory population females. These results suggest that field populations of T. castaneum should be used in experiments.
In spite of the unsuitability of cotton seed as a food resource, the infestation of cotton storages by T. castaneum is commonly reported. Tribolium species are thought to have evolved from inhabitants of decaying tree logs and to be capable of utilising fungi. I used an olfactometer to test whether T. castaneum responds preferentially to the volatiles that emanate from the fungi associated with cotton seed over those that emanate from cereals. Pairwise comparisons of attractancy between cotton seed, wheat and sorghum (seed form and as volatiles with water- and ethanol-based extracts), were conducted. Beetles were attracted more strongly to linted cotton seeds contaminated with fungi than to wheat and sorghum. Further tests revealed that it is the fungi on the lint that produces the active volatiles, because the beetles did not respond to sterilised cotton lint. Tests with five different fungal cultures were variable across the cultures, with only one of them being significantly attractive to the beetles.
The response of T. castaneum to the three resources was therefore tested in a series of experiments in the laboratory, glasshouse cages and the field. Results from the tests in relatively small arenas in the laboratory and glasshouse revealed that the response of T. castaneum adults to food resources was twice as strong when walking as when flying. A clear preference for linted cotton seeds was evident in walking T. castaneum, especially in small-scale arenas in the laboratory, where at least 60% of beetles released preferred linted cotton seeds over wheat or sorghum. Similarly, their attraction, when walking (in 1 m3 cages), was five times stronger to linted cotton seed than to the conventional grain resources of this species. However, this pattern was not consistent in the field over a 20 m range and the beetles did not show a preference to any particular test resource. These results suggest a focus on walking beetles in trapping studies for population estimations and the use of active volatiles from linted cotton seed for development of effective food-based trapping lures.
Finally, in large scale field experiments around grain storages and across the agricultural landscape beetles dispersing around bulk grain storages were caught using traps baited with wheat, sorghum or cotton seed (with lint stubble). The beetles demonstrated a significantly stronger attraction to fungus infected cotton seed. No significant difference in their infestation potential (offspring per female) in any resource was observed. Beetles caught in grain traps deployed across the landscape indicated that these beetles can detect and infest grain even far from other stored grain (>1 km). These findings suggest that stored cotton seed in its raw form, with lint, should be considered a significant resource for T. castaneum, but the rearing results on cotton seed itself as a resource suggest that the insects are feeding primarily on fungi and this warrants further tests.
This thesis confirms a strong ecological association of T. castaneum with fungi, at least with those strains found on cotton seed. Cotton seed stored in bulk is therefore an important resource for T. castaneum and it will affect their dispersal in relation to bulk grain storage. Despite being recognised as a serious grain pest, the results suggest that T. castaneum is primarily adapted to locating and feeding on fungi and perhaps specific types of fungi and this insight opens up the possibility of exploiting this particular aspect of their ecology for management purposes.