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7.1 Biology of Teretriosoma
nigrescens
7.2 Host specificity of Teretriosoma
nigrescens
The predator T. nigrescens was released in Africa (Togo) in January 1991 as a biological control agent for P. truncatus, The following results of this investigation into the biology and host specificity of T. nigrescens have also been a contribution to justifying the introduction of the Histerida:
7.1 Biology of Teretriosoma nigrescens
• Preparation of the genitalia of 1,000 T. nigrescens imagines showed a ratio in sexes of 1:1.
• One T. nigrescens female in P. truncatus cultures was able to produce 107 F1 offspring in its reproductive phase of a maximum of about 10 months.
• With increasing age, the T. nigrescens females produce fewer offspring on average. The maximum rate observed (at 0 - 6-week-old imagines) was in the region of 0.84 ± 0.19 F1 individuals per female per day.
• T. nigrescens females die earlier than males in P. truncatus cultures.
• T. nigrescens imagines live a maximum of 3 months without food.
• T. nigrescens imagines can feed on plant substrates with a high content of starch, such as maize, wheat, sorghum and cassava. A starch test using an iodine potassium iodide solution on the intestines and excrement of the insects was able to prove that adult T. nigrescens had taken in maize and digested it.
• Adult T. nigrescens can reach an average age of 1 - 2 years on plant substrate containing starch. The greatest life expectancy of T. nigrescens imagines at 870.0 ± 177.5 days could be observed on ground wheat. The oldest imago reached an age of 3 years and 3 months.
• The damage and losses of plant substrates which are caused by T. nigrescens imagines are so negligent that they can be ignored. The beetles are morphologically hardly capable of gnawing into intact grains of substrate.
• The larvae of T. nigrescens live only as predators. When there is a shortage of food cannibalism occurs among T. nigrescens larvae.
• Reproduction of T. nigrescens did not take place on plant substrates without the host. This is a reversible reproduction pause. T. nigrescens imagines kept on plant substrate for 16 months were again ready to reproduce after being put onto P. truncatus, cultures.
• Apart from the species of host, maize evidently has a great significance as a substrate in the development of T. nigrescens.
7.2 Host specificity of Teretriosoma nigrescens
• Multiplication of T. nigrescens only took place in P. truncatus, populations on maize within the experimental period of 8 weeks in pure host cultures (number of parent specimens used / number of F1 individuals > 1).
• Offspring of T. nigrescens could also be observed in the cultures on all substrates on which P. truncatus reproduced (maize, cassava, sorghum and wheat).
• The growth of a T. nigrescens population is dependent on the number of prey organisms in P. truncatus, cultures.
• T. nigrescens significantly restricted the growth of P. truncatus populations on all substrates which the pest was found to breed upon.
• In mixed cultures, where a species of storage pest beetle was kept with P. truncatus, a strong reproduction rate of T. nigrescens occurred.
• The aggregation pheromone of P. truncatus has a kairomone effect on T. nigrescens and thus has an attractive and phagostimulant effect on the predator.
• In addition to the P. truncatus populations, T. nigrescens significantly suppressed the growth of populations of other species of pest in mixed cultures which were unaffected by the predator in monocultures on maize (D. porcellus, S. granarius granarius, S. oryzae, A. diaperinus, L. oryzae, P. ratzeburgi, T. castaneum, both strains of T. confusum, C. pusillus and the laboratory strain of C. dimidiatus).
• In cultures where P. truncatus had been killed, only slight reproduction activity of T. nigrescens could be observed.
• In monocultures of D. porcellus, R. dominica, S. oryzae, O. surinamensis and T. stercorea, a small number of T. nigrescens offspring could be observed on a regular basis. In the populations of the 14 other Coleoptera species (including one subspecies) tested as hosts only isolated or no offspring of T. nigrescens could be determined.
• A significant growth-restricting effect by T. nigrescens on the host populations in monocultures could only be proven regularly in 0. surinamensis and 0. mercator populations, in isolated cases in cultures of R. dominica, D. porcellus, L. oryzae and T. granarium. The growth of the populations of all other 13 Coleoptera species (including one subspecies) remained unaffected by T. nigrescens.
• Although T. nigrescens eats the eggs of storage pest species of Lepidoptera offered to it in small Petri dishes, the predator has no effect on the development of moth populations on substrate.
According to the results presented, T. nigrescens is closely associated with P. truncatus, as a host. The fear that it could become a threat to endemic insects which do not occur in connection with P. truncatus, in granaries is unfounded.
The data collected on the biology of T. nigrescens show that rapid establishment of the Histerida, and connected with this, success as a biological control agent for P. truncatus, in Africa, can be expected.