5 The significance of plant substrate for the nourishment and reproduction of Teretriosoma nigrescens

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5.1 Introduction
5.2 The ability of Teretriosoma nigrescens to survive on plant substrates
5.3 Damage and losses of plant substrates caused by Teretriosoma nigrescens
5.4 The ability of Teretriosoma nigrescens to reproduce on plant substrates
5.5 Conclusion

5.1 Introduction

Apart from the investigations on host specificity, it was also necessary to check whether T. nigrescens is able to make use of plant substrate as food. Many other open questions were directly linked with this:

- How long can the predator starve, i.e. how quickly does a T. nigrescens population break down if there is insufficient or no prey available?

- Can T. nigrescens survive a temporary lack of prey, e.g. by using plant nutritives, or can it even reproduce on plant media and thus itself become a pest and a hazard to human foods?

To find the answers to these questions T. nigrescens imagines were offered 15 different plant media (and dog biscuits containing animal substances) as potential food, and the predator's ability to survive and reproduce on these substrates was investigated. Apart from the significance of plant nourishment for the imagines, the significance of maize for the nourishment of T. nigrescens larvae was also determined.

The emphases lay on investigations using maize and coffee. Maize is one of the staple foods in Togo. Coffee is a high-grade, export product for tropical West Africa and is subsequently subject to stringent quality standards.

As T. nigrescens is associated with the pest P. truncatus it will primarily be in contact with maize. The regions in West Africa where maize is produced overlap with the regions where coffee is cultivated. If T. nigrescens is introduced as a control agent for the maize pest, it could also be confronted with coffee.

5.2 The ability of Teretriosoma nigrescens to survive on plant substrates

5.2.1 Material and method
5.2.2 Results

5.2.1 Material and method

As it was planned to release T. nigrescens in Togo, the investigations were carried out on normal trade products imported from Togo, such as maize, cassava, various species of millet, peanuts etc. To extend upon this, products were also selected for the experiments which are used at the Institute for Stored Product Protection at the BBA in Berlin to breed insect pests. In addition to wheat, sorghum, beans, maize and oat flakes, wheat bran mixed with glycerine, glucose and yeast was used for the experiments.

Unroasted, green coffee beans from five different origins (Costa Rica, Kenya, Columbia, and two regions in Brazil), a local variety of maize from Togo and Argentinian maize (trade name: "La Plata") were included in the experiments.

Since it was not known whether T. nigrescens can damage intact grains to obtain access to the nutritious endosperm, two series of tests were set up for a number of substrates. One contained a healthy, i.e. non-infested, medium for which optically undamaged maize grains and beans were chosen. The other contained a substrate on which infestation by insect pests was simulated by boring (Ø 2 mm for maize, beans and peanuts) or by coarse grinding of the medium (for wheat, sorghum, millet, rice, etc.).

The substrates were sterilised prior to beginning the experiment. For this, they were stored for one week in a deep-freeze at -19°C. This was particularly necessary for the substrates from Togo as, in some cases, they showed clear signs of infestation. This served the purpose of killing any eggs from beetles and moths, booklice and acarids but also microorganisms and fungi.

The plant media were then put into the climatic chambers used in the experiments for conditioning and kept there for at least two weeks at 27° ± 1°C and 75% ± 5% r. h.. The moisture content of the substrates was then measured using a semi-automatic moisture meter (Brabender) (Tab. 5).

Small glass bottles (Ø 3 cm, height 8 cm) closed by a piece of cotton fastened to the neck of the bottle with an elastic band were used as containers for the experiments. The bottles were filled half-full of substrate. Depending on the nature of the medium, approx. 10 g oat flakes and wheat bran, and approx. 20 g maize, beans or wheat were weighed and put in.

Controls on the one hand were starving experiments where the substrate was replaced by unsuitable materials such as glass beads (Ø 3 + 7 mm), polystyrene balls (1 cm³), gravel (Ø 3 - 5 mm) or expanded clay balls (Ø 5 - 10 mm). In this way, the agile beetles could move around without difficulty and, at the same time, find suitable refuges as resting places.

Other controls were where adult T. nigrescens had been fed on the eggs and larvae of P. truncatus as a known source of nourishment. For this, the predators were regularly put onto 7-day-old cultures of P. truncatus (100 adult beetles on approx. 50 g maize in 100 ml jars).

T. nigrescens multiplies on these cultures and so the beetles had to be moved every month so that the adults of T. nigrescens were not confused with their adult offspring.

These tests also served as controls for investigation into the reproduction activity of T. nigrescens on plant substrate (5.4).

For each substrate, damaged or undamaged, and for every control, 5 parallel experiments (n=5) were prepared. 10 freshly hatched T. nigrescens (at a maximum age of 14 days) were put into each jar. 50 beetles were thus required for the total of 49 series of tests. The experiments were in darkness at 27°C ± 1°C and 75% ± 5% r. h.. The experiments with the artificial substrate (starving tests) were examined once weekly when the dead insects were removed and counted. Examination of the experiments with plant substrates initially took place at weekly intervals, later every four weeks. To facilitate evaluation, all the beetles which were still alive after 9.5 months in the 5 parallel experiments were put together into a jar with approx. 30 - 40 g fresh substrate. The death of the insects was set at the middle of the period between the last and last but one examination for calculation purposes.

5.2.2 Results

Life expectancy of Teretriosoma nigrescens without nourishment

Approx. 3 months after beginning the experiment, all the specimens of T. nigrescens were dead (Tab. 6) in all the 'starving' controls. The maximum life reached by one beetle was 93 days. The average life expectancy of all 350 T. nigrescens in the 7 series of starving controls was in the region of 50.4 ± 13.5 days. The beetles kept on glass beads died first. They only lived for an average of 37.5 ± 16.9 days. The highest average life expectancy of 70.8 ± 19.9 was achieved by T. nigrescens in empty glass Petri dishes. Significant differences in the average life expectancy of T. nigrescens among the starving controls could not be determined.

Life expectancy of Teretriosoma nigrescens on plant substrates

Some beetles were still alive on most of the experiments even after 6 months of beginning the experiment (Tab. 7). Undamaged substrate generally provided a lower chance of survival than damaged substrate. All 50 T. nigrescens imagines put into the experiment with coarse maize and wheat, as well as bran and oat flakes, were still alive after six months. All T. nigrescens kept on undamaged beans and grains were dead after 12 months.

66%-100% of the beetles originally put onto the coarse, ground media, like the two varieties of maize, wheat, sorghum, cassava and oat flakes, were still alive one year after the experiment started. Some specimens of T. nigrescens were still alive after 2 years on these media. The highest life expectancy was achieved by the beetles on coarsely ground wheat. After 30 months, 13 of the 50 beetles were still alive. They achieved an average age of 870 days. The maximum age here was reached by a beetle with 3 years and 3 months.

On some plant media, including coffee and cocoa, the life expectancy of the T. nigrescens imagines was short, similar to in the starving controls. The number of beetles still alive on these substrates within the experimental period of 1 - 3 months is illustrated in Tab. 6. 3 months after beginning the experiment, all beetles on the coffee and cocoa media were dead, independent of whether the beans used were damaged or not. Thus, the life expectancy of T. nigrescens on coffee beans amounted to an average of 53.0 ± 6.5 days and on coarsely ground coffee 51.2 ± 4.6 days.

The beetles also died early on the two varieties of edible beans when these were used in an undamaged condition. After 90 days, only 20% were alive on the variety from Togo, and on the variety common on the German market, only one (=2%).

Table 5: The ability of Teretriosoma nigrescens to survive on plant materials and the losses due to the predator

50 imagines/experiment; evaluation after 6 months at 27°C and 75% r.h.;
UD: undamaged T: from Togo
D: damaged
-: not done

Table 6: The ability of Teretriosoma nigrescens to survive on various plant substrates

50 imagines/experiment; evaluation after 1, 2 and 3 months at 27°C and 75% r.h.
UD: undamaged
T: from Togo
D:damaged
-: all beetles were dead (1),(2),(3): replicates

Table 7: The ability of Teretriosoma nigrescens to survive on variouse plant substrates

50 imagines/experiment; evaluation after 6-30 months at 27°C and 75% r.h.;
UD: undamaged
*: n=40
T: from Togo
D: damaged
**:n=30
#: experiment stopped
P.t.: P.truncatus
-: all beetles were dead (1),(2),(3): replicates
(10): 10 alife beetles were removed to use them for another experiment (3.4)

5.3 Damage and losses of plant substrates caused by Teretriosoma nigrescens

5.3.1 Material and method
5.3.2 Results

5.3.1 Material and method

In order to be able to make statements on the possible damage caused by T. nigrescens, the substrate in the experiments (5.2) was optically examined for traces of eating after a period of 6 months.

In analysing the losses, the existing meal from eating was sieved, weighed and related to the fresh weight of the media initially put in. The ground substrate was sieved into the glass bottles before being weighed to remove the meal occurring during grinding so that this did not go into the calculations as meal occurring during eating.

To extend upon this, a second experiment on loss and damage was set up: in each of 5 parallel experiments 100 adult T. nigrescens were put on approx. 150 g shelled maize in 300 ml jars. Loss analysis was carried out after 6 months as described. The damage was defined as the number of damaged grains related to the total number of grains.

5.3.2 Results

Apart from a few exceptions the beetles were able to bore or gnaw into the substrates offered. Thus, six months after the beginning of the experiment, there was a small quantity of meal from eating on the bottom of the vessels used in the experiment. Tab. 5 shows a summary of the data from loss analysis. For uninfested substrate, this amounted to between 0.05% and 0.10% in comparison to the quantity of substrate used. The proportion of the weight of eating meal in the infested substrate was a little higher at between 0.3 % and 1.5%.

No traces of boring by T. nigrescens could be observed on coffee or cocoa beans. Similarly, the common, white, edible beans with their smooth, hard surface did not offer the insects any points where they could attack the beans either.

There were clear signs of eating only on individual seeds in the other tests with whole grains and beans. During the course of time, these seeds were completely hollowed out. During the search for T. nigrescens which had survived in the experiments, up to 8 specimens of the beetles were discovered concealed in one of these hollowed-out grains or in one bean.

In the investigation carried out additionally, 4.75% ± 0.56% of the average of 564.2 ± 7.4 grains weighed were damaged after six months. Of the total number of 500 T. nigrescens introduced, 474 (94.8 ± 2.7/experiment) were still alive. The weight loss of maize caused by the boring meal of T. nigrescens amounted to around 0.32% ± 0.01%.

5.4 The ability of Teretriosoma nigrescens to reproduce on plant substrates

5.4.1 Material and method
5.4.2 Results

5.4.1 Material and method

The experiments described under 5.2 provided the basis for this investigation. Random samples were inspected with a binocular microscope for the eggs and larvae of T. nigrescens during the monthly examinations. The experiments described, with P. truncatus as a host, served as controls.

5.4.2 Results

Reproductive activity of T. nigrescens on the plant nutritives without hosts could not be proven. Neither eggs, nor live nor dead larvae of T. nigrescens could be observed in the experiments during the whole experimental period of 3 years.

Reproduction of the predator took place in the controls. When the insects were moved to new P. truncatus cultures, an average of 5 - 6 T. nigrescens larvae of varying ages were found in the experiments. Reproduction could also be observed with one-year-old beetles with P. truncatus as a host (3.2).

5.5 Conclusion

In contrast to initial assumptions, T. nigrescens is not purely a predator but can also make use of plant food. This is illustrated by the unexpectedly high life expectancy of the imagines on plant media in comparison to the starved controls.

All nutritive media on which T. nigrescens was able to survive for a long time are marked by a high content of carbohydrates (maize 65%, oat flakes 62%, wheat 60%, cassava 32%). Apart from various sugars, starch is the main substance contained (SOUCI et al, 1986). A starch test using iodine potassium iodide solution on the intestines and excrement of the insects was able to show that adult T. nigrescens had taken up and digested maize and thus starch.

After investigating the content of the intestines of T. nigrescens, it could be assumed that about half the beetles had eaten maize before they were taken out of the experiment. If they had excreted the substrate taken up without digesting it, the majority of the balls of excrement would have been discoloured by the iodine potassium iodide solution.

With a fats content of around 50% and a protein of approx. 20%, the carbohydrate content of peanuts and almonds with about 10% plays a minor role. The insects did, in fact, live longer on these media in comparison to the starved controls, but did not reach such a great age as on the substrates which are rich in carbohydrates.

Animal proteins as the main content of dried dog food, could evidently not be made use of by T. nigrescens as nourishment.

Costa Rica, the native region of P. truncatus and T. nigrescens, like West Africa, is a significant coffee-growing region. Infestation of coffee and the appearance of T. nigrescens in the plantations associated with this, is unknown.

In laboratory experiments P. truncatus was able to cause damage to coffee, but the beetles died within 2 months without reproducing (SHIRES, 1977). T. nigrescens in contrast, even under extreme laboratory conditions, caused no damage to coffee or cocoa beans since it did not accept this substrate as food. Consequently the beetles quickly starved to death.

The results of a "Choise Chamber Test" carried out by REES (1990) extends on these findings. This experiment on choice investigated the affinity of T. nigrescens to various plant substrates. In comparison to the controls no greater affinity of T. nigrescens to coffee or cocoa could be proven.

The suspicion that insecticide residues could have been the reason for the rapid death of T. nigrescens imagines on the coffee samples was refuted by a biological testing process involving the vinegar/fruit fly Drosophila melanogaster as a test species and by chemical residue analyses carried out by the GTZ-Pesticide Residue Analysis Laboratory in Darmstadt.

Caffeine present in coffee beans with 0.8% - 1.8% is presumably the reason for the rapid death of the T. nigrescens imagines. NATHANSON (1984) carried out experiments on the insecticide effect of caffeine and other methyl xanthines. He used ground coffee-beans as stomach insecticide as well as synthetically produced methyl xanthine. Depending on the concentration used and on the species of insect, he was able to observe effects restricting eating and growth, an influence on the fertility of the insects and highly toxic effects which led to the death of the insects within a short time. Methyl xanthines are also present in cocoa beans and in tea leaves amongst others. This also explains the short life expectancy of the T. nigrescens imagines on cocoa beans although, like e.g. peanuts, they have a starch content of approx. 10%.

Although T. nigrescens is able to make use of plant substrate as food, the fear that it could become a pest insect on stored foods itself is unfounded. The losses in plant media caused by T. nigrescens were low enough to neglect. The beetle is morphologically hardly able to gnaw or bore into intact grains. The strong mandibles are more suitable for catching and dismembering prey organisms (Fig. ISA, B + C). This explains the low loss of undamaged grains and beans observed and the longer life expectancy of the beetles on the mechanically damaged substrates. If the beetles do not succeed in penetrating the hard casing of the seeds, they die relatively early. Consequently, there are still many specimens in the experiments with infested material which have free access to the soft, nutritive endosperm and thus produce more boring meal.

Several beetles could be frequently observed closely crowded together in a completely hollowed-out grain of maize. Since there was no nourishment left in the grain, it can be assumed that the beetles were experiencing a kind of rest phase in this aggregate. In this way, the otherwise very agile beetles create a suitable microclimate for themselves in which they conserve energy and thus survive for a long time.

However, a refuge alone without the chance to take in nourishment now and again, is not sufficient to guarantee survival for long. This is proven by the starving controls with gravel, expanded clay balls, polystyrene and glass beads.

As T. nigrescens is associated with the primary pest P. truncatus it will chiefly come into contact with substrate which has already been damaged. However, it can be assumed that T. nigrescens would prefer P. truncatus as food rather than the plant substrate.

Under conditions similar to life in the wild, no affinity of T. nigrescens to plant substrates (maize, sorghum, millet, beans, rice, cassava, coffee, cocoa and peanuts) could be proven in Togo (HELBIG, 1993; HELBIG et al, 1992 a, d). The investigations were carried out in a cage containing maize infested by P. truncatus The T. nigrescens imagines, in the choice test of REES (1990) mentioned earlier, preferred maize samples from P. truncatus cultures in comparison to uninfested maize samples.

Reproduction of T. nigrescens could not be observed on plant media without the presence of P. truncatus This could be because only adult T. nigrescens can take in and digest plant substrate whereas the larvae are dependent on animal food. The results on the ability of T. nigrescens larvae to survive on maize meal also speak in favour of them living purely as predators. The egg-larvae quickly starved on maize meal. A control where the larvae were fed on prey organisms was not set up, but a similar one was carried out by LELIVELDT (1990). The larvae were in small Petri dishes sprinkled with maize grains from P. truncatus cultures. The empty spaces were filled with maize meal. Living larvae of P. truncatus were put in daily to secure nourishment of the T. nigrescens larvae. In this way they developed into adult beetles.

Some of the L₂ put into the maize meal were presumably not quite mature. They nevertheless entered the pupating stage, presumably due to the lack of suitable food. Thus, apart from freshly hatched T. nigrescens imagines of normal size, there were also a number of smaller specimens.

T. nigrescens females, i.e. imagines, which had not mated and which had also never eaten specimens of P. truncatus be bred in this way. This could be interesting for future experiments.

From the head capsules removed from the bodies of the larvae found in the experiments on maize meal, can be concluded that cannibalism had not only taken place among the L, but also among the L₂ of T. nigrescens. BÖYE (1988) also observed this phenomenon during a high population density. Since it is also known that imagines eat their brood (LELIVELDT & LABORIUS, 1990), the eggs laid on the plant substrate are presumably eaten by the beetles themselves due to a lack of prey. The fact that there were no head capsules of egg-larvae which had starved in the experiments with adult T. nigrescens on plant substrate supports this theory.

The fact that T. nigrescens can also make use of plant nourishment makes successful control of the pest, P. truncatus probable. Normally, the population densities in a predator-prey relationship undergo great fluctuations (KRIEG & FRANZ, 1989). If there is sufficient prey available, the predator population rises due to the increased supply of food. A high density of predators leads to rapid decimation of the prey. This, again, leads to a reduction in the density of the predator due to lack of food, which again, causes a greater increase and spreading of the prey organisms, to close the cycle.

This cyclical oscillation in intervals in the density of predators and prey is interrupted by the ability of T. nigrescens to also be able to make use of plant nourishment. If only a few specimens of the host exist, the size of the population of T. nigrescens stagnates for a longer period. The beetles are able to feed on plant substrate to prevent a sudden collapse in the population density. Even if no prey organisms can be found over several months, a renewed mass multiplication of P. truncatus will be faced with a large number of predators without any long delay in time. In addition, this increases the probability that T. nigrescens will be able to successfully establish itself after its release in Togo.

In studying all these results, it should not be forgotten that the experiments with T. nigrescens on plant substrate in the laboratory meant an extreme situation for the predator. E.g. the density of beetles selected for the experiments was approx. 10 times higher than findings in the wild. BÖYE (1988) found an average of only 2 - 3 adult T. nigrescens per maize cob in Costa Rica (50 g - 70 g maize) which was infested by P. truncatus The otherwise very agile beetle was not able to escape the prescribed situation to find sources of food it preferred. As the reproduction of the insects did not take place on plant substrate, it can be assumed that they primarily live as predators. Since only this method of nourishment serves to preserve the species by reproduction.

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