4 Host specificity of the predator Teretriosoma nigrescens

Contents - Previous - Next

4.1 Host specificity of Teretriosoma nigrescens within Coleoptera storage pests
4.2 Host specificity of Teretriosoma nigrescens among species of Lepidoptera storage pests

4.1 Host specificity of Teretriosoma nigrescens within Coleoptera storage pests

4.1.1 Introduction
4.1.2 Material and methods
4.1.3 Results
4.1.4 Conclusion

4.1.1 Introduction

T. nigrescens is simultaneously confronted with other post-harvest pest beetles in a maize granary. These investigations are to provide answers to the following complex of questions:

- Can T. nigrescens make use of individuals of other Coleoptera in addition to P. truncatus and thus restrict the growth of other pest populations?

- Is T. nigrescens dependent on P. truncatus as a host or can the predator also reproduce on the populations of other Coleoptera?

- What effect does T. nigrescens have on a Coleoptera population offered to the predator in a mixed culture including P. truncatus as a host?

- Does T. nigrescens restrict the growth of a P. truncatus population when a second Coleoptera is present?

- Can T. nigrescens reproduce in a population of P. truncatus mixed with another Coleoptera?

4.1.2 Material and methods

For these investigations, mainly species of beetle were selected which occur alongside P. truncatus in Central America (Costa Rica) and in Africa (Togo and Tanzania) as pests on maize (PANTENIUS, 1987: BÖYE, 1988; LELIVELDT, 1990; HENCKES, 1992).

Apart from primary pests in the family of wood-borers (Bostrichidae), weevils (snout beetles) (Curculionidae) and saw-toothed beetles (Cucujidae), the emphasis was also placed on six species of tenebrionid beetles (Tenebrionidae) which are secondary pests. Table 2 lists the 18 species of Coleoptera (plus a subspecies) with which the experiments on the host specificity of T. nigrescens were carried out.

Table 2: Hostspecifity of Teretriosoma nigrescens (Col.: Histeridae); List of tested Coleoptera and Lepidoptera pest species

Five cultures of pests (A. diaperinus, D. porcellus, C. dimidiatus, L. oryzae and T. confusum) taken from wild specimens captured in 1989 and imported from Togo were used for these investigations. For the three last named species of beetle, a wild culture from Togo and a laboratory culture were used in the experiments as comparisons. All other insects came from cultures which have been raised for several years at the Institute for Stored Product Protection at the BBA in Berlin.

4.1.2.1 Host specificity of Teretriosoma nigrescens in monocultures

The optimum temperature for breeding the selected host insects was between 32°C for P. truncatus and 20°C for some species of moth. A mean temperature of 27°C and a r. h. of 75% were chosen for all experiments. These conditions approximately correspond to the humid-warm climate in Togo and, at the same time, allow a comparison with work already published.

The duration for the experiments was set at 8 weeks. A development cycle of T. nigrescens is concluded during this period and under these conditions.

The F₁ generation of some pests grew up already in 4 - 5 weeks in this climate. For this reason, preliminary experiments determined the suitable number of host insects which should be used per jar to prevent overpopulation in the tests during the experimental period and thus avoiding fungus development on the cultures.

The exact age of the host specimens used was unknown. The beetles were taken from cultures where the F₁ generation had just hatched.

Irrespective of the number of host insects, 10 T. nigrescens imagines per experiment were used. With a constant predator-prey ratio of 1:10, for example, there would only have been 5 T. nigrescens in many experiments, and even only 2 predator imagines used in some. With this low number there would have been a relatively high risk of only having male specimens in the experiment. The results on the ability of T. nigrescens to reproduce on the host populations provided could then have been distorted.

Glass jars with a capacity of 300 ml (Ø 7 cm) were used in the experiments. Steel gauze was soldered into the plastic screw lids.

As T. nigrescens is not able to walk along smooth glass walls, the radius of action in the experiments was restricted for the predator by the volume of nutrient medium for the host insect. Since finding prey organisms in a smaller area is more simple than in a larger one, the substrates were not weighed, but the jars were all filled approximately half-full of nutritive media. Thus, approx. 150 g maize, wheat or sorghum, 100 g oat flakes or cassava or SO g wheat bran (mixed with glucose, glycerine and yeast) were used per experiment.

Mostly 100 or 50 imagines were put into 10 jars per species of pest. After one week of incubation in the climatic chamber at 27 ± 1°C and 75% ± 5% r. h. and without light, 10 young (0-14-day-old) T. nigrescens imagines were put into half the number of experiments. At this point, there were already eggs and larvae of the hosts in the tests. The remaining 5 experiments without the influence of the predator served as controls (Fig. 9B). Normally, 1,000 or 500 hosts and SO predators were thus required per series of experiments.

After T. nigrescens had been inserted, the tests were put into a climatic chamber at 27°C ± 1°C and 75% ± 5% r. h. without light for 8 weeks. The tests were sieved to separate the beetles from larger particles of substrate and from the boring meal (mesh 2.0 mm and 1.0 mm or 0.7 mm) (Fig. 9C). They were then spread on a white tray. The T. nigrescens imagines showing running activity and the larvae of the predator could now be easily gathered up out of the experiments and counted.

From the known number of T. nigrescens imagines and the number of predator imagines now found, it could easily be determined whether reproduction (number of progeny / number of parent insects inserted < 1) or multiplication (number of progeny / number of parent insects inserted > 1) of T. nigrescens on the species of insect provided had taken place or not.

In preliminary experiments, only isolated eggs of T. nigrescens could be observed in the experiments. The fine-meshed sieve used to separate the eggs from the boring meal quickly became blocked. The search for the few pupae of T. nigrescens, which were frequently concealed inside the maize grains and did not fall out when being sieved, was particularly difficult and time-consuming. For this reason, counting the eggs and pupae of the predator in the experiments was dispensed with.

The evaluation of all experimental material took several days and so the tests which had been sieved and the host insects, after T. nigrescens imagines had been taken out, were put into small Petri dishes and frozen for 24 h at -18°C.

In this way, all experiments could be kept at a development stage of 2 months. This avoided any beetles hatching whilst evaluation was being carried out and facilitated counting the otherwise agile species which, in some cases, could fly.

The tests with the beetles were then spread out on a white tray whose base (30 x 20 cm) had been divided into strips approx. 2 cm wide using a fine felt pen (Fig. 9D). The number of beetles could be quickly determined with these lines and a counter. Only completely preserved beetles were counted. A comparison of the number of host imagines which had developed a) under the influence of T. nigrescens, and b) in the control, clearly showed whether the predator had accepted the species of beetle offered to him as food, or not.

At least two test series were carried out in this way on different substrates for each species of host. The food for the host insects was firstly the substrate normally used by the Institute for Stored Product Protection to breed the species of pests. As T. nigrescens is mainly to be found with P. truncatus in maize stores, maize was also used as a test substrate. In the experiments with secondary pests which are reliant on the primary pests providing access to the meal in the substrate grains, approximately half the maize grains were replaced by coarse maize meal.

Experiments on cassava were additionally set up with 9 species of pest. For this, the dried roots from Togo were broken up into pieces of about 1 - 2 cm³.

Statistical evaluation of the results was carried out using the t-test described under 3.3.2.

4.1.2.2 Host specificity of Teretriosoma nigrescens in mixed cultures with Prostephanus truncatus

T. nigrescens was offered 100 imagines of one species of beetle to be tested as a host with 100 adult P. truncatus as a known host. The substrate selected for all hosts was maize.

As there were 200 adult hosts in each of these mixed cultures, 200 P. truncatus imagines were also used for the control (n=5).

The experiments were set up and evaluated as in the method described under 4.1.2.1.

4.1.3 Results

T. nigrescens progeny could only be regularly found in 5 of the total experiments with 19 species of Coleoptera pests (including one subspecies) on various substrates. These include both species of Bostrichidae, R. dominica and D. porcellus and S. oryzae from the family of Curculionidae, the saw-toothed grain beetle O. surinamensis and T. stercorea (Appendix: Fig. 12 A, B + C). On all other beetle populations, there was either no or only a very low degree of multiplication of T. nigrescens.

A significant growth-restricting effect could only be proven for O. surinamensis and O. mercator populations, in single cases in the populations of R. dominica, D. porcellus, the tenebrionid beetle L. oryzae and the species of dermestid beetle, T. granarium. (Appendix: Tab. 3; Fig. 10 A, B + C). The growth of the populations of all 13 other Coleoptera tested remained uninfluenced by T. nigrescens.

In mixed cultures, where a species of pest beetle was kept with P. truncatus on maize, there was normally a high degree of multiplication of T. nigrescens (Appendix: Fig. 13).

In addition to the populations of P. truncatus T. nigrescens also significantly suppressed the growth of other species of pest which remained uninfluenced by the predator in monocultures on maize (Appendix: Tab. 3; Fig. 11 A + B). These included D. porcellus, S. granarius granarius and S. oryzae, the tenebrionid beetles, A. diaperings, L. oryzae, P. ratzeburgi, T. castaneum and the two strains of T. confusum, the saw-toothed beetle C. pusillus and the laboratory strain of the Nitidulida C. dimidiatus.

Important details and particular findings are commented on in the following discussion of the results.

Contents - Previous - Next