5. Aspects of the ecology of teretriosoma nigrescens

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Prior to release of the predator T. nigrescens in Togo, a number of aspects of its ecology in the new environment were to be investigated on request of the national institutions in charge. A substantial element of these investigations was the relationship of the predator to plant-based foodstuffs. In addition, the interaction of T. nigrescens with two species of Bostrichidae were investigated.

5.1 Behaviour of teretriosoma nigrescens when faced with plant-based stored produce

The reaction of the predator when faced with plant-based stored produce was investigated in closed and open jars. This exposed T. nigrescens to two very different situations. In the open vessels, the predator was able to choose between using the plant substrates as food or leaving the vessels. In the closed vessels there was no choice. This situation was to force the beetles to undertake survival strategy in order to allow an estimate of the possible damage potential.

5.1.1 Material and methods

Attraction of various stored produce for T. nigrescens

The stored produce, except for the maize and the coffee beans, was bought at a local market The maize came from an existing stock of a local, yellow-grained variety, the coffee was a sample of the "Robusta" variety from OPAT. Whole and ground grains and beans were used for the experiments; in some cases a drill was used to bore holes in the product.

The stored products were put onto plastic plates with a hole in the bottom measuring 1.5 to 2 mm in diameter to allow the beetles access to the media (in some cases without a hole). The vessels were evaluated every 24 hours The stored product was sieved using sieves with 2.0 and 0.4 mm mesh The fractions were then examined visually. The beetles found were recorded according to their species and number and then released again on the bottom of the cage. The stored produce was returned to the plates. 3 repetitions were made for each produce and type of plate. The experimental period lasted 4 days resulting in a total of 24 observations per product.

In the second series of experiments, the plates with holes were used. The cocoa kernels used in the experiments had been fermented and dried beforehand according to local methods.

Time spent by adult T. nigrescens on various types of stored produce

The stored produce tested was identical to that stated above. 10 adult T. nigrescens were put onto the products in each vessel. The neutral medium used as a comparison was screwed-up newspaper. The following types of vessel were used:

1. A plastic plate with 4 small holes in the side wall
2. A small basket, approx. 6 cm high. The bottom of the basket was lined with a piece of newspaper.
3. A round cardboard plate of 22 cm diameter.

Evaluation was carried out at intervals of 24 hours when the substrates were sieved with 0.4 and 2.0 mm sieves. The fragments were subsequently examined visually. The number of T. nigrescens found were counted and all other beetles discovered were removed from the products, which were then returned to the vessels. The remaining T. nigrescens were put back on the products. If the nutritive medium became wet, the experiment was eliminated. A new experiment was then set up with the number of beetles from the previous day. T. nigrescens which were dead or did not look healthy were also substituted.

Damage to stored produce

In each case, so g of the nutritive medium and 20 adult T. nigrescens were put unto the 200 ml jars. Used as nutritive media were maize grains (variety: local, yellow), peanuts, cassava chips, coffee beans and cocoa beans. Evaluation was carried out after 4, 8 and 12 weeks. The losses were calculated according to the method described in Chapter 2 1.2.

5.1.2 Attraction of various types of stored produce

The attraction of the various products for T. nigrescens was tested in two series of experiments. For this, the produce was offered either whole or coarsely ground in open vessels. These vessels were set up in the wire-gauze cage (see chapter 4.1.1.) and were thus accessible for a large number of T. nigrescens.

In both series, the number of the T. nigrescens found on the individual products was very low (Tab. 22). The largest number on any of the products was found on the peanuts with a hole in them. Three T. nigrescens were found in 24 observations. There was not a single predator on many of the substrates although the number of observations was very high.

In comparison to the total number of beetles found on the individual substrates, T. nigrescens rarely comes into appearance (Tab. 22). The relative frequency of T. nigrescens rarely exceeded 2%, the highest value being 4.7% recorded on coarsely ground cocoa, a nutritive medium rather untypical for this spectrum of beetles.

Tab. 22: Migration of Teretriosoma nigrescens into the open vessels with varying stored produce (g = coarsely ground; the number of individuals in total and relative to the number of all beetles of all species found)

Total number = sum from 24 observations (2 types of plate, 3 repeats, 4 days of observation)
** Total number = sum from 12 observations (3 repeats, 4 days of observation)

5.1.3 Time spent on various stored products

The time adult T. nigrescens spent on various products was tested by adding beetles to open vessels. The experiment was carried out in two series inside the wire-gauze cage. The results of the second series are illustrated.

The majority of adult T. nigrescens left during the first two days (Fig 23, 24 and 25). The proportion of beetles found on most of the stored goods had declined to less than 30% after 2 days.

Fig. 23: Time spent by Teretriosoma nigrescens (T.n.) on the various substrates after artificial introduction (relative frequency of beetles found, as a mean of all types of vessel)

Fig. 24: Time spent by Teretriosoma nigrescens (T.n.) on the various substrates after artificial introduction (relative frequency of beetles found, as a mean of all types of vessel)

Fig. 25: Time spent by Teretriosoma nigrescens (T.n.) on the various substrates after artificial introduction (relative frequency of beetles found, as a mean of all types of vessel)

The beetles left more quickly than the newspaper. There were more beetles left only on maize, sorghum, coffee beans and coarsely ground coffee beams. On the third day, the proportion of T. nigrescens found on the substrates had, however, also fallen to a level comparable to the control. The products evidently had no special attraction for the beetles. In total, an average of 82% of T. nigrescens had left after 3 days, and after 5 days, this already amounted to 93%. No tendency could be observed for a larger number of adult T. nigrescens to remain on any of the substrates for a longer period of time.

5.1.4 Damage to various stored products

The relationship of T. nigrescens to various kinds of stored plant produce was also tested in a vessel in which the beetle was not able to escape. The vessels were closed and the predator had to survive in this environment. The purpose was to see if T. nigrescens would turn into a pest on the stored produce in such an extreme situation.

Macroscopically, the damage caused to maize and coffee beans was hardly visible. It was not until examination with a stereo microscope that slight damage to the fruits could be observed. No measurable losses occurred with these two storage products (Tab. 23). Very small damage was determined to the cocoa beams and peanuts and a minimal loss was observed which rose only negligibly during the course of the experiment.

Tab. 23: Damage and loss caused by Teretriosoma nigrescens to various stored products in closed vessels after artificial introduction

The damage could not be identified definitely macroscopically but only using a stereo microscope

These are individual of fruits which T. nigrescens had bored bored. In general, the insects all remained in this one fruit giving the impression that they had only bored into these to find a refuge.

The same phenomenon occurred with the cassava chips. However, the loss here was somewhat greater, presumably due to the soft consistency of the chips. The loss here was also chiefly due to the boring activities and not to feeding, as a comparatively large quantity of boring powder was found.

A high mortality rate for the predator was recorded after 2 months on the nutritive media cocoa beans, coffee beans and maize (Tab. 24). After 3 months, the mortality for all three stored products amounted to 100%. T. nigrescens was able to survive better on the cassava chips and on the peanuts. After 2 months, the mortality rate recorded was 5%, after 3 months it was in the region of 13.3% and 20%.

Tab. 24: Mortality rates of Teretriosoma nigrescens on various stored products in closed vessels after artificial introduction

5.2 Influence on some species of bostrichidae

Due to its kairomone effect (BÖYE et al, 1992), T. nigrescens should be seen as a relatively specific predator. Despite this, there is a possibility of an effect on other species living in the environment of P. truncatus. Since extensive experiments on this matter have already been carried out (PÖSCHKO, 1993), only 2 significant Bostrichidae, D. bifoveolatus and Rhyzopertha dominica (Coleoptera, Bostrichidae) were included in this investigation.

Material and methods

The interaction of T. nigrescens with other species of Bostrichidae have been investigated to define how far the predator is able to reproduce and how the population of the Bostrichidae is able to develop. 100 adult specimens of R. dominica and of D. bifaveolatus were put onto 150 g nutritive substrate in 1 l glass jars. The substrates used were sorghum for R. dominica and cassava chips for D. bifoveolatus. After 7 days, 15 adult specimens of T. nigrescens were added. The experiment was carried out with 3 repeats and lasted 8 weeks. For the evaluations, the number of adult beetles and the losses were recorded (see Chapter 2.1.2).

Results

The population development of D. bifoveolatus was clearly reduced by T. nigrescens (Tab. 25). Whilst there were an average of 546 adult D. bifoveolatus in the control after 8 weeks, the variant with the predator contained only 45 and, at this figure, even less than at the beginning of the experiment. T. nigrescens was able to reproduce on the host, D. bifoveolatus. After 8 weeks, the 15 adult T. nigrescens which had been added, had produced 4.3 beetles and 27.8 larvae as offspring.

In the case of R. dominica, the development of the population was slightly but not significantly reduced by the presence of the predator (Tab. 25). it is thus not clear whether the reduced number of adult R. dominica was due to the effect of the predator, or was caused by chance. The predator did not reproduce during the experimental period. The only larva found during the evaluation died some days later (after evaluation it was replaced on R. dominica).

Tab. 25: Population development of Dinoderus bifoveolatus (D.b.), Rhyzopertha dominica (Rd.) and the predator Teretriosoma nigrescens (T.n.) after 8 weeks

* The larvae died three days after evaluation

This very varied development of the population in the different variants was clearly reflected in the results of the loss analysis (Tab. 26). With D. bifoveolatus as the host, there was a strong reduction in loss when T. nigrescens was added. Whilst the loss in the control amounted to 41. 1%, it fell to 19.5% in the presence of the predator. Analogous to the development of the population, a slight and insignificant difference was determined in the case of R. dominica between the two variants.

Tab. 26: Loss and the production of boring powder by Dinoderus bifoveolatus (D. b.) and Rhyzopertha dominica (R. d.) in relation to the presence of Teretriosoma nigrescens (T.n.) after 8 weeks

5.3 Release of teretriosoma nigrescens

T. nigrescens was released in Togo in the village of Tsagba, located east of Notse, near to the Benin border. Fig. 26 shows the observation stores entered on a plan of the village and numbered. 2 circles have been drawn around the point of release with a radius of 100 m. and 200 m. At the time of release, 11.45 a.m. there was a slight wind (0-1 m/s) from north-east. At 1.30 p.m., the wind came from the south at the same speed.

Material and methods

A very large maize store in Tsagba, strongly infested by P. truncatus, was emptied and the maize was sorted according to the intensity of infestation. 10 small stores (approx. 300 kg) were set up in Tsagba and a comparable village with this maize Whilst building the stores, care was taken to put approximately same amount of infested maize into each store. 10 stores were set up in another village to serve as a parallel control for the infestation and loss situation.

Fig.26: Plan of the village of Tsagba with the numbered stores for observation and the release point

In Tsagba, the stores were completely covered by a gauze material (Pig. 27) as a kind of frame tent (length 2 m x width 2 m x height 2.5 m). The T. nigrescens flying to this point could thus be identified on the gauze by the observer and collected. The release point was located in the centre of the village. The stores were distributed as evenly as possible around this point.

4000 adult specimens of T. nigrescens were released on 29.01.91 at 11.45 a.m. The beetles were sieved out of their transport substrate maize and put into a clay bowl set in the shade of a tree. By permanently observing the stores, the T. nigrescens flying here could be precisely recorded. The beetles found were collected, counted and put under the gauze at the end of the day. The intensity of observation was continued until the end of the day of release and also on the morning and afternoon of the following day. Observations were subsequently not carried out so intensively and ended three days after release.

Results

The first specimens of T. nigrescens were found on stores no. 9 and no. 4, 20 minutes after release. The greatest number of T. nigrescens totaling 53 was found on the stores on the day of release (Tab. 27). Of the 53 beetles, 34 were found on stores no. 9 and no. 1 which were located to the north and very close to the point of release. The least number of T. nigrescens was found on stores 5, 6 and 8, located at a distance of 140-170 m south and east of the point of release.

The number of T. nigrescens found clearly declined on the following days. Whilst 16 beetles were found on the second day of observation, on the third and fourth days, only 3 and 6 were recorded. As on the first day, the majority of T. nigrescens were located on stores 9, 1 and 4 in the proximity of the point of release. In total, 78 of the approx. 4000 T. nigrescens released, corresponding to 1.95%, were discovered on the four days of observation.

Tab. 27: Teretriosoma nigrescens caught on the gauze of the observation stores after release in Tsagba on 29.01.91

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