Results of test recommended at the Cotonou coordination meeting

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The significance of plant materials for nourishment and development of the predator Teretriosoma nigrescens
Recherches concernant l'attraction exercée par diverses denrées stockées sur le Teretriosoma nigrescens Lewis (Col.:Histeridae) prédateur du Prostephanus truncatus (Horn) (Col.:Bostrichidae)
The honeybee (Apis mellifera L.(Hym.: Apidae)) as a potential host of Teretriosoma nigrescens Lewis (Col.: Histeridae), predator of the Larger Grain Borer
The silkworm (Bombyx mori L. (Lep.: Bombycidae) as a potential host of Teretriosoma nigrescens Lewis (Col.: Histeridae), predator of the Larger Grain Borer
Ability of Teretriosoma nigrescens to survive and breed on stored product pests other than Prostephanus truncatus
Further studies on the ability of Teretriosoma nigrescens Lewis (Col.: Histeridae) to prey on insect pests of stored products and to feed on stored food and beverage crops.
The IIBC LGB screening programme for beneficial insects: Summary of results
Discussion - second session

The significance of plant materials for nourishment and development of the predator Teretriosoma nigrescens

M. Pöschko, G.-A. Laborius and F.A. Schulz
BBA Berlin, GTZ Hamburg and TU Berlin, F.R.Germany.

ABSTRACT

Survival and breeding ability of Teretriosoma nigrescens Lewis were tested on 16 different plant materials.

T. nigrescens is able to feed on a range of plant foodstuffs. When feeding on plant commodities with a high carbohydrate content like maize, cassava or wheat the beetle's lifespan can exceed 20 months. The weight losses of the plant materials caused by T. nigrescens are very low.

T. nigrescens is not able to breed on plant materials. After feeding on plant food for 9.5 months and even after 16 months, T. nigrescens is still able to proliferate on its prey Prostephanus truncatus (Horn).

INTRODUCTION

The predator Teretriosoma nigrescens (Col.: Histeridae) is the most effective antagonist of the maize pest Prostephanus truncatus (Col.: Bostrichidae). Both insect species are common in Mesoamerica (Rees 1985; Boeye 1988). This is presumed to be one of the reasons why control measures against P. truncatus are not generally necessary in this area. Since its introduction in the early 1980's, the pest P. truncatus has become established in Africa, notably in Togo and Tanzania (Harnisch and Krall 1984; Laborius et al 1985). Under optimal climatic conditions and in the absence of natural enemies the populations of P. truncatus increased and spread very quickly. Today, P. truncatus is one of the most important storage pests in East and West Africa. P. truncatus causes losses on maize and dried cassava of a magnitude which was previously unknown in these areas (Pantenius 1987).

It is under consideration to release the predator T. nigrescens for biological control of P. truncatus in Togo. Numerous investigations have been done to ensure that the predator will not itself become a pest.

1. ABILITY OF TERETRIOSOMA NIGRESCENS TO SURVIVE AND BREED ON PLANT MATERIALS

MATERIAL AND METHODS

The commodities tested were obtained from a traditional market in Togo (T) or different sources in Berlin (FRG). Raw coffee seeds from five distinct coffee producing countries (Kenya, Zaire, Costa Rica, Colombia and two different regions of Brazil) were used. Some of the plant materials were studied either undamaged (whole grains) or damaged (mechanically broken grains). The damaged grains were used to simulate a previous pest infestation and to make it easier for the predator to reach the internal part of the kernels.

The commodities were equilibrated for 4 weeks at 27°C ± 1°C and 75% ± 5% r.h. All tests were carried out under these climatic conditions. About 20g of each commodity were put in glass tubes (80 mm long, diam. 40 mm). Ten newly emerged (1-14 days) adults of T. nigrescens were added to each tube. All experiments were carried out with five replicates. Therefore a total of 50 adult beetles were used in each trial.

Two types of checks were included: T. nigrescens was kept either without any food on 4 different unconsumable artificial materials (glass beads, gravel, vermiculite and clay balls (used for hydroculture)) and in empty glass dishes, or with its natural prey P. truncatus as food. In the latter case T. nigrescens was transferred mensually on to a 7-day-old culture of 100 adult P. truncatus with about 50 g of maize grains. The samples were inspected every week when the beetles were kept on artificial materials. Tests using natural plant commodities were checked once every month. Dead insects were recorded and removed.

After six months the weight losses of the commodities caused by T. nigrescens were investigated. This was done by sieving off the flour in the samples and comparing its actual with the initial weight of the plant materials.

In the present study the following questions were studied:

- determination of the lifespan of T. nigrescens without any food,
- ability of T. nigrescens to feed and survive on plant commodities in the absence of any prey,
- ability of T. nigrescens to breed on plant materials only.

RESULTS

Without any food T. nigrescens was able to survive for a maximum of 3 months. In Table 1 the average lifespan of the predator on 5 different unconsumable materials is given. The combined average lifespan of the predator without any food was 46.5 + 12.2 days.

TABLE 1: Average lifespan of 50 adult T. nigrescens on unconsumable materials (starvation set-ups)

On most of the plant materials T. nigrescens was able to survive for more than six months (Table 2). In the trial with P. truncatus as food for the predator, 47 of the original 50 T. nigrescens were still alive after half a year. The weight losses of the plant materials due to T. nigrescens were generally lower than 1%. The losses caused on previously damaged materials were higher than on undamaged commodities.

Under natural conditions 2-3 adults of T. nigrescens were found in one maize cob (on average about 1 40g of maize) infested with P. truncatus (Böye 1988). in the present laboratory studies 50 adult T. nigrescens were kept on 100g of maize. So the losses due to the predator under natural conditions would be negligible.

On previously damaged commodities a much higher proportion of beetles survived for half a year than on undamaged materials (Table 3). On some of the damaged commodities such as maize, sorghum, wheat, cassava as well as rolled oats more than 80% of T. nigrescens were alive 12 months after setting up these experiments. This survival rate was no different from the control where the predator was fed on its prey P. truncatus Even after 20 months being kept on plant materials some adults of T. nigrescens were still alive.

TABLE 2: Number of adult T. nigrescens still alive after being kept on different plant materials for 6 months and the losses due to the predator after this time.
(5 replications, 10 adults/replication = 50 beetles in total)
(UD = undamaged; D = damaged; T = from Togo; * 6x5 replications )

TABLE 3: Number of dead T. nigrescens (in total 50 beetles/trial) on undamaged and damaged commodities after 6 months (T = from Togo)

The plant materials which did not allow the survival of any beetle for more than 6 months are given in Table 4. On cocoa beans the average lifespan of T. nigrescens is comparable to its lifespan on unconsumable artificial materials. The lifespan of the predator on raw coffee seeds and on undamaged Phaseolus beans is similar to those kept in empty glass dishes.

TABLE 4: Average lifespan of 50 adult T. nigrescens on different plant commodities
(on coffee n = 6 x 50 = 300)
(UD = undamaged; D = damaged; T = from Togo)

2. ABILITY OF T. NIGRESCENS TO BREED ON P. TRUNCATUS AFTER BEING KEPT ON PLANT MATERIALS FOR DIFFERENT TIMES

MATERIAL AND METHODS

T. nigrescens is not able to breed on plant materials. Even after one year no eggs or larvae of the predator were found in any sample. Kept on cultures of its natural prey P. truncatus however, T. nigrescens can still breed after 12 months. The purpose of this trial was to ascertain whether T. nigrescens may temporarily or permanently lose is breeding capability if feeding on plant materials. After 9.5 months 10 T. nigrescens were removed from each of a set of trials using damaged plant materials and were transferred into P. truncatus cultures (100 adult P. truncatus were put on 150g maize grains in glass jars for 1week, so that eggs and larvae of the prey were present to feed the predator). As a comparison newly hatched adults of T. nigrescens kept on its prey were also transferred into new P. truncatus cultures.

RESULTS

The number of progeny in the samples was counted after 8 weeks (Table 5). A comparison of the breeding capability of T. nigrescens originating from the two different sources is given in Table 6.

After feeding on plant materials for 9.5 months T. nigrescens was stall able to breed on its prey P. truncatus Compared to the control sample the breeding ability of T. nigrescens kept on plant materials was lower but the predator was still able to slow down the rate of pest population increase.

The same trial was repeated with T. nigrescens that had been kept on plant food for 16 months. The predator still kept its ability to proliferate on its prey. After a period of 3 months on P. truncatus cultures 30.7 ± 12.2 progeny (larvae and adults) of T. nigrescens and 158.8 ± 74.5 adults of P. truncatus were counted (n=6).

TABLE 5: Ability of T. nigrescens to multiply on P. truncatus cultures after being kept for 9.5 months on several damaged plant commodities (T = from Togo, 8 weeks after set-up)

TABLE 6: Number of T. nigrescens progeny (larvae and adults) produced on P. truncatus cultures after being kept for 9.5 months on plant materials (n=11) and the number of adult P. truncatus after 8 weeks without the influence of the predator (n=5) (T.N.= T. nigrescens, P.T.= P. truncatus)

3. THE ABILITY OF T. NIGRESCENS TO DIGEST CARBOHYDRATE

As demonstrated T. nigrescens is able to survive on several plant commodities for longer than 20 months. This indicates that the beetle is able to feed on plant materials. All these commodities contain a high percentage of carbohydrate (about 60%). The following tests were carried out to show whether or not T. nigrescens is able to digest starch.

Adult beetles were kept on damaged maize for 4 weeks. The guts of 20 T. nigrescens were dissected and treated for 5 minutes with a diluted iodine/potassium iodide solution (IPI). IPI turns the colour of starch from white to dark blue. Afterwards the guts were transferred to lactic acid (60%) for 5 minutes to bleach the tissues. Following the preparation of the guts they were examined under a microscope,

In about 50% of the dissected guts of T. nigrescens dark blue coloured maize flour pieces could be detected.

To investigate whether or not T. nigrescens digests the carbohydrate found in the gut, the frass of the beetles was examined.

Thirty adult T. nigrescens, kept on damaged maize for 6 weeks, were washed with warm water to remove the maize flour from their bodies. After 48 hours incubation in a small glass dish IPI was added to the frass on the bottom of the dish. None of the frass heaps turned dark blue. This indicates that T. nigrescens is able to digest the carbohydrate which had previously been detected in the gut.

DISCUSSION

The current experiments show that the predator T. nigrescens is able to feed on plant materials. Nevertheless the fear that it could become a pest of stored products is unfounded.

T. nigrescens has a low capability for damaging whole grains. Only in a few cases was it able to break the hard and smooth seed coat. On maize for example, only about 5% of the grains were damaged after half a year. Consequently, the beetles died quite quickly of starvation. On the other hand, it was easy for the beetles to feed on damaged grains. Therefore T. nigrescens lived longer on broken grains. In comparison to the losses due to P. truncatus on maize and cassava the losses caused by T. nigrescens were negligible. Under practical conditions the large amount of flour produced by P. truncatus is available to the predator to feed on.

As demonstrated, T. nigrescens is able to digest starch. Therefore the predator survived for a longer time on substrates with a high content of carbohydrate and died more quickly on, for example, cocoa beans and coffee seeds.

It is known that T. nigrescens is very closely associated with the pest P. truncatus. Therefore it will occur mainly with infested and damaged foodstuffs. Under natural conditions, which are different from laboratory trials, T. nigrescens can choose between its prey and the plant food. Since T. nigrescens is not able to breed on plant materials P. truncatus would be the preferred food. Only under these circumstances will the beetle be able to survive and reproduce.

The ability of T. nigrescens to survive without prey for a long period is very useful with regard to the planned biological control strategy for P. truncatus in Africa. Normally, the predator population decreases quickly when the prey is no longer available. In this case the number of T. nigrescens remains stable for a long time even in the absence of any prey. If the population of P. truncatus increases a high number of predators will still be present to attack the pest immediately.

Pöschko et al. (1992) demonstrated an influence of the substrate on the breeding ability of T. nigrescens on RHIZOPERTHA DOMINICA populations. Relating this finding with the present data it can be seen that plant materials must have an important role in the survival and development of the predator. Altogether, T. nigrescens seems to be a very significant biocontrol agent of P. truncatus such that chemical control measures could be reduced or in particular cases even rendered unnecessary.

REFERENCES

Böye, J. (1988) Autökologische Untersuchungen zum Verhalten des Großen Kornbohrers Prostephanus truncatus (Horn) (Col.: Bostrichidae) in Costa Rica. Dissertation, Institut für Phytopathologie der Christian-Albrechts-Universität zu Kiel, 195 pp.

Harnisch, R. and Krall, S. (1984) Further distribution of the Larger Grain Borer in Africa. FAO Plant Prot. Bull. 32 (3),113114.

Laborius, G.-A., Leliveldt, B. and Keil, H. (1985) Der Große Kornbohrer, Prostephanus truncatus (Horn). Ein neuer Vorratsschädling in Afrika. Der praktische Schädlingsbekämpfer 37 (9), 179-186.

Pantenius, C. (1987) Verlustanalyse in kleinbäuerlichen Maislagerungssystemen der Tropen, dargestellt am Beispiel von Togo. Dissertation, Institut für Phytopathologie der Christian-Albrechts-Universität zu Kiel, 249 pp.

Pöschko M., Laborius, G.-A. and Schulz, F.A. (1992) Ability of Teretriosoma nigrescens to survive and breed on stored product pests other than Prostephanus truncatus This publication.

Rees, D.P. (1985) Life history of Teretriosoma nigrescens Lewis (Col.: Histeridae) and its ability to supress populations of Prostephanus truncatus (Horn) (Col.: Bostrichidae). J. Stored Prod. Res. 21, 115-118.

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