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6.1 Significance of prostephanus truncatus for the storage of maize in Togo
The accidentally introduction of Prostephanus truncatus into Togo happened presumably in 1981 (HARNISCH & KRALL, 1984). Since then, the Larger Grain Borer has spread across the whole country (RICHTER & BILIWA, 1991) and caused such damage to stored maize as had never occurred previously (cf. KRALL, 1984; PANTENIUS, 1987). This development in Togo is comparable to the situation in Tanzania where, P. truncatus after it had appeared, rapidly dispersed and established itself (HODGES et al., 1983; MAKUNDI, 1987).
Integration into the complex of pests
The field survey recording the abundance of the Larger Grain Borer by means of pheromone traps in the south of Togo showed the pest to be present throughout the year. When taking the relatively small range of the pheromone traps, i.e. 340 m (FARREL, 1990) into account, this indicates omnipresent potential to threaten the maize.
The close relationship of the Larger Grain Borer to stored maize is reflected by the course and extent of the pheromone-trap catches. The small numbers caught in the months from August to November come at the same time as the harvest and storage of maize in the long storage season. It is during the time of storage until the end of November, when the maize is infested and a population starts to build up in the store, leading to a rapid increase in numbers caught after a certain time (cf. HODGES AND MEIK, 1984). Maize is then permanently available from the end of the year until the middle of the following year, offering the pest a good nutrient substrate. This nutritive substrate is extended in the short storage period when maize is again stored A favourable factor for P. truncatus is the custom of farmers to use this maize after the harvest in the short cultivation period, and not until afterwards, to consume the maize from the long storage season which has been stored for a longer period (cf. PANTENIUS, 1987). The maize from the long storage season is, in some cases, also earmarked for sale during the period of high prices shortly before harvesting in the long cultivation period.
Investigations in individual maize stores of the long and short storage periods show how successfully the Larger Grain Borer has integrated itself in the complex of endemic pests. Where infestation occurred, P. truncatus was able to build up a substantial population in almost all stores. In the long storage period of 1988/89, an average of 7222 adult P. truncatus were found after eight months of storage in Cacaveli. In the short storage period in 1989, there were similar results with 6373 beetles being found in Gbonvé after 5 months of storage.
These two examples are, in fact, extreme values, but the pest was also successful in asserting itself against competition in other cases. Firstly, this is shown by the population density, which frequently exceeded that of S. zeamais over longer periods of storage, although this pest had been brought in from the field and thus had a competitive advantage. GOLOB and HANKS (1990) also recorded a more rapid and extensive growth of P. truncatus than Sitophilus spp. in the Tabora Region in Tanzania.
Secondly, the progression of infestation by P. truncatus in the stores by far exceeded that of S. zeamais. The ability to bore into the very hard substance itself (CHITTENDEN, 1896; DETMERS, 1988) allowed the Larger Grain Borer to settle successively on the cobs in a maize store and, in this way, to find new nutritive and breeding substrates. The difference between P. truncatus and S. zeamais lies not only in the absolute number of infested cobs at the end of the observation period, but also in the speed with which it is able to spread in the store. The great multiplication potential of P. truncatus (cf. BELL & WATTERS, 1982; NISSEN, 1989) and the efforts of the population to disperse (VERSTRAETEN & HAUBRUGE, 1987) are two factors promoting spreading.
In Togo, several factors seem to combine to support growth in P. truncatus populations. Throughout the country, the mean daily maximum temperatures in the warmest month amount to between 29 and 36 °C and in the coolest month to between 23 and 30 °C. The mean monthly temperatures in the warmest month are between 24 and 30 °C and in the coolest month between 20 and 25 °C.
The low values occur at higher altitudes of 500 to 900 m. Since day/night fluctuations are low throughout the year over extensive parts of the country, they are near to the optimum temperatures of 30-32 °C for the Larger Grain Borer during many hours of the day (cf. SHIRES, 1979; BELL & WATTERS, 1982; HAUBRUGE, 1981).
Measurements made of the relative humidity in the south of Togo near to the coast showed a daily fluctuation range of 50% to 98% for most of the year. Only during Harmattan, in the months of December and January, was the humidity clearly lower for a number of weeks. This means that the relative humidity in this region of the country is in the favourable range for P. truncatus almost the whole year around, since the optimum is stated to be 80% (SHIRES, 1979; BELL & WATTERS, 1982; HAUBRUGE, 1987). in the other parts of the country, the relative humidity somewhat restricts the growth of populations of the Larger Grain Borer. P. truncatus however, can still complete its development cycle in a very broad range of humidity and also in grains with a water content of 85%. Thus this factor only constitutes a limited restriction (cf. SHIRES, 1979; BELL & WATTERS, 1982; REES, 1985; HAUBRUGE, 1987).
In addition, the development of the population of the Larger Grain Borer is promoted by the common method of storing cobs in Togo. This method of storage induces a substantially higher reproduction rate than when grains are stored (COWLEY et al., 1980; BELL & WATTERS, 1982; WATTERS, 1984). The opposite applies to the possible competitors (LABORIUS et al., 1985), providing P. truncatus with a double advantage.
The negative influence on the development of P. truncatus by S. zeamais and T. castaneum (HAUBRUGE & VERSTRAETEN, 1987; PANTENIUS, 1987; BÖYE, 1988; LELIVELDT, 1990) in glass jar experiments seems to have no significance in logo under practical conditions. In general, T. castaneum was recorded in the stores in a very low density. When there was infestation by P. truncatus this was also observed, although the development of T. castaneum is favourably influenced by the Larger Gram Borer (HODGES, 1984; HAUBRUGE & VERSTRAETEN, 1987).
HOWARD (1983, quoted by DICK, 1988) states that advantages would exist for S. zeamais up to a temperature of 28 °C, from 28 °C onwards, P. truncatus would then prove to be the stronger competitor. Due to the temperatures predominant in Togo, this would mean that P. truncatus would have the advantage over extensive periods during the year. Regarding the optimum temperature for S. zeamais, there are two very different statements. According to DICK (1988), this is around 25-27 °C, whereas HAINES and WEBLEY (1979) state a temperature of 30 DC.
The competition from S. zeamais in contrast to that from T. castaneum is not predation of the larvae, but in preventing P. truncatus from infesting grains. If S. zeamais has already occupied a grain with am egg, P. truncatus recognizes this and does not infest the grain or lay eggs there (HAUBRUGE & VERSTRAETEN, 1987; PANTENIUS, 1987). This mechanism seems to be so minimal in a maize store with a broad selection of food that no considerable influence cam be determined on the population development of the Larger Grain Borer.
The interaction between P. truncatus and S. zeamais evidently seems to be so strong that a certain suppression of the development in the population of P. truncatus is caused by S. zeamais on shelled maize, whereas S. zeamais is able to reproduce without restriction (HAUBRUGE & VERSTRAETEN, 1987; PANTENIUS, 1987). In the presence of S. zeamais on maize cobs, P. truncatus is able to develop just as well as in a monoculture, yet S. zeamais is clearly influenced by P. truncatus (cf. BÖYE, 1988).
Several useful insects were found in the maize stores in Togo, although X. flavipes, A. calandrae and T. elegans were the only ones to appear in higher densities on a regular basis. Surveys of the populations of useful insects in the maize stores showed no indication that useful endemic insects had adapted to the Larger Grain Borer. In general, the populations of useful insects were very small and showed no synchronization with the development in the population of the Larger Grain Borer (cf. HELBIG, 1993).
In contrast to field surveys, the jar experiments accompanying these showed a relationship of the useful endemic insects to P. truncatus. The assassin bug, X. flavipes, was able to reduce the growth in population of P. truncatus in jar experiments on maize grains as a nutritive medium. In his experiments, LeCATO (1975, quoted by HAINES, 1984) found a dependency in the efficacy of X.flavipes on freedom of movement in the substrate. In this experiment, it only existed to a certain degree among the maize grams in the jar. When maize stores are extensively infested by P. truncatus and the correspondingly high amounts of boring powder occur, freedom of movement is definitely strongly restricted for the predator This could be the reason for the non-existence of synchronization with the population density of P. truncatus.
The efficacy of the bug is presumably restricted by the way the larvae of P. truncatus live. These live inside the grain and are surrounded by boring powder making access for the predator difficult as his limbs are not adapted to this. This is presumably also likely regarding the relationship with S. zeamais. The larvae develop inside a grain with am intact shell and are protected from the predator in this way. Another factor under practical conditions is that X. flavipes, finds sufficient other nutrition in the maize stores as it is known to be a very effective antagonist of T. castaneum, O. surinamensis and Ephestia cautella (LeCATO et al., 1977).
In jar experiments, a certain effect of A. calandrae was determined in monocultures as well as in populations with S. zeamais. BÖYE (1988) discovered that A. calandrae parasitised the larvae of P. truncatus offered to it in individual grains. He carried out experiments to test the development of the population consisting of A. calandrae and T. elegans on loose grains and on husked maize cobs. BÖYE (1988) discovered am influence of the Hymenoptera on the population development of the Larger Grain Borer in both cases. Despite these findings, there is not likely to be any great effect under practical conditions in a maize cob as the wasp will find it extremely difficult to move between the grains and husks with such large quantities of boring powder.
T. elegans did not affect the population development of the Larger Gram Borer in the jar experiment. In contrast, the relationship with S. zeamais was confirmed.
WILLIAMS AND FLOYD (1971) showed that T. elegans was only able to parasitise the larvae of S. zeamais which were lying on the surface of the grain. They assumed that this Hymenoptera is too small, i.e. the ovipositor is too short, to be able to pierce a grain deeply.
For T. elegans, SHARIFI (1972) discovered a preference for older larval instars. This could also be related to the length of the ovipositor since older larvae are closer to the surface of the grain than young ones. The length of the ovipositor could also be of significance in the parasitising of P. truncatus since the larvae can rarely be found on the surface of the gram.
The results lead to the conclusion that none of the three antagonists has a significant effect on P. truncatus. All three useful insects are known to be antagonists of other storage pests in the community with P. truncatus in maize stores (COTTON, 1923; VAN DEN ASSEM & KUENEN, 1958; WILLIAMS & FLOYD, 1971; SHARIFI, 1972; LeCATO et al., 1977; ARBOGAST, 1985; LELIVELDT, 1990). Parasitising, i.e. predation, cam in fact occur to a small extent but is likely to be more coincidental as none of the useful insects is adapted to the environment created by P. truncatus .Somewhat better results could possibly be expected in grain storage as this environment can be considered more favourable for the useful insects
Survival during periods when the food supply is inadequate is an important phase under the epidemiological aspect of a pest. The tendency and ability of P. truncatus to bore into wood and the difficulties this involves for control were recognized relatively early by CHITTENDEN (1896, 1911). Extensive experiments to determine the possible survival period on wood produced a maximum period of approx. 3 months (BÖYE, 1988; DETMERS, 1988). P. truncatus which had bored into the wood survived for a considerably longer time than ones kept without substrate.
These results were confirmed by this investigation. A survival period of 3 months was determined for adult male and female insects. Having survived, the individuals were still able to infest maize and reproduce. A survival period of 4 months was only recorded for one individual which was subsequently no longer able to bore into a maize grain.
The farm-level maize stores are normally constructed of wood which is infested by P. truncatus .Considering the various degrees of infestation according to types of wood, the danger of passing on infestation from one storage period to the next can be reduced by selecting a relatively resistant type of wood. Good storage hygiene, as practiced by some farmers, involves changing the infested wood at least, or ideally, building a new store after each storage period. Another strategy could be to treat the wood with insecticides.
The problem of survival becomes more critical with the ability of P. truncatus to reproduce and multiply on dry wood as was first determined in this investigation. Multiplication was observed on M. esculenta and P. regia (HELBIG et al., 1990), and reproduction on S.tragacantha. The ability to reproduce on wood can have considerable effects on the epidemiology of the pest if dry wood of a suitable species is available to provide the beetles with a suitable environment during the non-storage period.
In relation to time during survival on wood, a considerable mortality occurred for the adult P. truncatus individuals and survival was restricted to an upper limit of about 3 months. These limits do not apply to reproduction and there are always adult P. truncatus present which can infest the newly stored maize. The ability of P.truncatus to reproduce on wood also explains why the beetles have been caught in the natural environment, far distant from human settlements and maize stores (REES et al., 1990; RICHTER & BILIWA, 1991). In addition, reproduction on wood cam be the reason for the beetles having been caught throughout the year even outside the maize storage periods in this investigation.
Damage and loss
The good conditions for development of P. truncatus combined with the absence of limiting factors led to very strong growth of the population in the maize stores. This growth resulted in very high damage and losses during maize storage far surpassing the level of that caused by other storage pests.
During the long storage period of 1988/89 and the short storage period in 1989, damage due to infestation by P. truncatus amounted to around 60-65% during the observation periods (8 and 6 months of storage). At this level, they were about twice as high as the damage in comparative stores where damage normally amounted to 30-35%.
The dry-weight losses during infestation by P. truncatus were even higher than the damage. On infestation by P. truncatus, they amounted to 36.5% after 8 months in the long storage period in 1988/89 and to 26.5% after 6 months in the short storage period. In contrast, in the stores without infestation by P. truncatus losses measured in the long storage period 1988/89 were 9-14% and in the short storage period in 1989, were 6-12%. Thus losses after infestation by the Larger Grain Borer were more than twice as high as without infestation. In the short storage period of 1990, the level of damage and loss was lower on the whole. Nevertheless, the effect of infestation by P. truncatus was clearly reflected in the loss parameters
A distinct increase in damage and loss on infestation by P. truncatus was also recorded in other investigations. In Tanzania after 3 to 6 months of storage there were losses of 30-35% in some stores (HODGES et al., 1983 A; GOLOB, 1984). PANTENIUS (1987) recorded 30% after 6 months in Togo after infestation by P. truncatus compared to 7.1% without infestation by P. truncatus FANDOHAN et al. (1992) also reported losses Of over 30% after 5 months of storage in Benin. This very large damage potential of P. truncatus has also been confirmed in numerous jar experiments. (DEMIANYK & SINHA, 1987,1988; SUBRAMANYAM et al., 1987),
In comparison to the high damage and losses in the regions to which P. truncatus has been carried, they are apparently far lower as a rule in its native region. BÖYE (1988) recorded damage of 11-32% and losses of 3-14% after infestation by P. truncatus and 6-7 months of storage. Other investigations show that higher losses can occur in Central America. HOPPE (1986) and GILES & LEON (1974) determined losses of up to 40% in the Honduras and in Nicaragua. These, however, seem to be exceptions. In this respect, the fact that no specimens of T nigrescens were found in the samples by GILES & LEON (1974) must be taken into consideration.
During investigations by HOPPE (1986), T. nigrescens only occurred in two of the infested stores. Thus, this is evidently the same phenomenon as observed in Africa. High losses occurred due to the absence of limiting factors.
The additional investigations carried out regarding the progression of infestation, the infestation-loss ratio and the structure of damage caused highlight the particular role of P. truncatus in the complex of pests. When infestation occurred at am early stage, the Larger Grain Borer was able to spread to about 80% of the maize cobs in the store. The damage and loss development in the stores was very closely correlated to the population development of the pest. The regression factor was normally higher for P. truncatus than for S. zeamais (cf. HELBIG, 1993). The structure of damage was clearly influenced by infestation by P. truncatus. With high infestation, more than 75% of the grains were damaged on 66% of the cobs.
In addition to the significance of the Larger Grain Borer on the stored product maize, it also has to be considered a significant pest on cassava chips. Although cassava did not prove to be such a favourable nutritive medium as maize (NYAKUNGA, 1982), very high losses have been recorded for it. HODGES et al. (1985) found dry-weight losses amounting to 74% for fermented cassava chips and 52% for unfermented ones after a storage period of 4 months. In this investigation, a jar experiment showed 80% dry-weight loss after 8 weeks in comparison to 29% for D. bifoveolatus which is considered to be a significant pest.
When the extent of cassava chips produced and stored is considered, the significance of this loss potential becomes clear. In the "Region des Plateaux"', "Region de la Kara" and "Region Centrale" in Central and Northern Togo, substantial proportions of the cassava produced are processed into cassava chips. In the "Region des Plateaux", approx. 50% of all farmers make more than 50% of their cassava harvest into chips (AYEVA, undated). in the regions "de la Kara" and "Centrale", more than 75% of cassava harvested is made into chips by 95% of farmers (LAMBONI, undated).
The majority of cassava chips is stored which can last from a few weeks up to a year. The stored produce is not normally treated with insecticides although the insects are the greatest problem according to farmers (AYEVA, undated; LAMBONI, undated).
The cassava chips are thus am excellent nutritive medium for P. truncatus. Significant here are not only the losses of cassava itself but also the role of the chips as am alternative source of nutrition to maize, allowing the population of the Larger Grain Borer to continually multiply.
This investigation and results already published show the significance of P. truncatus in Togo, the new environment it has settled in. The confluence of many favourable environmental factors and conditions promote strong growth of the population of the Larger Grain Borer in maize stores. With a high density of the pest, the farmers suffer very high losses endangering the staple diet of the whole family.
This investigation was carried out in individual stores in a small region. The situation in these stores combined with general phenomena enable a certain estimation of the problem to be made. The actual significance of P. truncatus, in relation to all maize stores in the country cannot be assessed from this investigation. In contrast to the similarly very significant pest S. zeamais, which occurred in all stores and whose population development showed no great differences among various stores and storage periods, the infestation of P. truncatus varied greatly.
Considering infestation of P. truncatus and the losses resulting from this, it mainly becomes significant whether infestation takes place at all. infestation did not occur in all stores on a regular basis, and additionally, the proportion of infested maize of the total amount of maize in the country is not known. Apart from this, the time of infestation is very important. Since P. truncatus is evidently not brought in from the fields (cf. HELBIG, 1993), the point in time of infestation, i.e. the remaining period of storage, is a major factor in the total extent of loss.
The reason behind P. truncatus occurring only at various points could be a result of it only recently being introduced into Togo. This is contradicted to am extent by the results of am investigation by RICHTER and BILIWA (1991) who were able to provide evidence of P. truncatus existing over the whole area using pheromone traps (Trunc-call. I+II).
Another reason could be attraction due to the aggregation pheromone. When first males have discovered a maize store, other individuals are also attracted to it. The stores surrounding this then remain non-infested. The significance of wood under this aspect has not yet been clarified.
P. truncatus is a pest which has been carried to Togo and has evidently found good conditions there for its development, and is not restricted by any antagonists. In such a case, the classic biological control procedure is to introduce an antagonist from the native home of the pest (HAINES, 1984; KRIEG & FRANZ, 1989). HAINES (1984), however, refers to this method as being relatively insignificant since the useful insects are normally also imported with the pest.
In the case of P. truncatus the classic method was still used since there were neither evidence nor any signs of an antagonist having spread here. The search for a specific and effective antagonist in the native home of the pest led to the predator T. nigrescens (BÖYE, 1988). The kairomone effect makes it possible for the useful insect to find the pest in all environments (BÖYE, 1988; REES et al., 1990). Where the achievements of T. nigrescens as a predator are concerned, it has turned out to be an effective antagonist which cam reduce the population of P. truncatus to a great extent. (REES, 1985, 1987; BÖYE, 1988; LELIVELDT & LABORIUS, 1990).
Prior to being released in Togo, the population development and efficacy of T. nigrescens were examined under near-to-practical conditions in a quarantine cage. Apart from this, jar experiments were carried out to define its efficacy on various types of nutritive media and in various prey-predator ratios.
The population dynamics of T. nigrescens
The climatic conditions in the south of Togo evidently had no negative effect on the development of the population of T. nigrescens. Continual growth of the population was observed under near-to-practical conditions in the wire-gauze cage Extensive reproduction and rapid development of the useful insect were recorded in jar experiments, also in the wire-gauze cage.
The number of adult offspring produced by T. nigrescens was very high in the wire-gauze cage as well as in the jar experiments, also indicating favourable development conditions. In the investigations of BÖYE (1988), the rate of multiplication for T nigrescens was considerably lower. This good rate of multiplication was possible although both experiments were carried out with a very close prey-predator ratio which cam reduce the rate of multiplication for T. nigrescens and cam also prompt cannibalism (LELIVELDT & LABORIUS, 1990).
REES (1985, 1987) and LELIVELDT (1990) also determined a comparatively low rate of multiplication for T. nigrescens. REES (1985, 1987) carried out his investigations at 27 °C and 70% relative humidity, LELIVELDT (1990) chose 30 °C and 75% rel. humidity and recorded better multiplication of the predator than REES (1985, 1987). in the experiments carried out here, the rate of multiplication recorded was higher than that of LELIVELDT (1990). This indicates optimum conditions for T. nigrescens in a warm, humid climate. The tendency approaches statements by HINTON (1945) who reported that many Histeridae only reproduce in a humidity of at least 90-95% On the whole, the results lead to the conclusion that the development conditions for T. nigrescens are very favourable in the south of Togo.
Efficacy of T. nigrescens
The abundance of T. nigrescens rose over the whole period in the maize stores in the experiment under near-to-practical conditions. One beetle consumes approx. 1.1 to 5 larvae or 6 eggs of the Larger Grain Borer, and one larva of the predator approx. 3.5 pest larvae per day (REES, 1985; BÖYE, 1988; LELIVELDT & LABORIUS, 1990). The high predatory achievement of the individual combined with the abundance of T. nigrescens thus produced a high suppression potential for the control of P. truncatus.
The population development of the pest was correspondingly low in the maize stores. 2 months after introduction of T. nigrescens to the stores, a reduction in the number of adult specimens of P. truncatus in comparison to the control. During the time after this, the population of the Larger Grain Borer could only multiply negligibly and amounted to 760 beetles after an experimental period of 9 months in contrast, a gradation of the population was observed in the stores without antagonists which reached a maximum density of 4086 P. truncatus At the end of the observation period, the pest population in the presence of the predator had been reduced by 80%.
Comparable investigations under similar experimental conditions are not available. Therefor, jar experiments over shorter periods of time are used for comparison. In several experiments with monocultures of P. truncatus the reduction in pest population in the presence of the predator ranged from 74% to 89 .5% after 14 to 16 weeks (REES, 1987; BÖYE, 1988; LELIVELDT, 1990).
In this experiment, the efficacy of the predator was comparatively good although the population of P. truncatus was able to develop over a longer period without being disturbed (cf. Chapter 4.1 1). If the predator is introduced at a later point, efficacy can be reduced REES (1987) discovered a lower rate of reduction in the number of pests when the useful insect was introduced at a later point (73.8% compared to 89.5%). The same effects were observed by LELIVELDT (1990) in a number of experiments, but not in others. According to this, the effect could be a result of this factor, but it does not occur on a regular basis.
Due to the very good adaptability of T. nigrescens to varying densities of prey (LELIVELDT, 1990), the point of introducing the predator is only relevant for short experimental periods. The high rate of multiplication with an abundant food supply allowed the predator to achieve a high degree of efficacy after only a brief period.
The good efficacy of T. nigrescens can also be due to the relatively close prey-predator ratio in the stores According to estimations of the ratio using the samples, it amounted to 3.6:1 one month after beginning the experiment and decreased to a value of 1:1.2 during the course of storage. This ratio is very close compared to a survey in the natural environment in Costa Rica, where an average ratio of 75:1 was recorded (BÖYE, 1988).
When comparing the density of the pest in Togo and in Costa Rica, a relation to the prey-predator ratio can be observed. In Costa Rica, a maximum density of 1617 P. truncatus was recorded per cob for a comparable storage period (BÖYE, 1988), although the maximum in the cage was 11 beetles.
Due to the fact that the reproduction rate of T. nigrescens was reduced not only in own jar experiments (cf. HELBIG; 1993) but also recorded by others (REES, 1985; LELIVELDT, 1990) in a close prey-predator ratio, the extensive reproduction in the wire-gauze cage could indicate the use of alternative hosts. In general, it is possible for T nigrescens to be a predator of other prey (LELIVELDT, 1990; REES, 1991, 1992; PÖSCHKO et al., 1992 A), although statements on the possible hosts are contradictory. This experiment determined that T. nigrescens had no effect on the population development of S. zeamais, corresponding to the results of other experiments (LELIVELDT, 1990; PÖSCHKO et al., 1992 A). No difference in the population development of Carpophilus spp., C. quadricollis and O. surinamensis was discovered in any of the variants either. In the case of T. castaneum, Cryptolestes spp. and P. subdepressus, the abundance was reduced in the presence of T. nigrescens.
An estimation of this phenomenon is difficult due to the contradictory results which have been published (cf. LELIVELDT, 1990; PÖSCHKO et al., 1992 A; REES, 1992). An indirect effect on the population development could be possible in a mixed population of secondary pests. When the Larger Grain Borer is less abundant less boring powder is produced making environmental conditions for the secondary pests less favourable.
The remarkable suppressive effect of T. nigrescens on population growth of the Larger Gram Borer also affected the loss parameters. The damage after 9 months, in comparison to the control, was reduced by 41.6%, and losses fell by 46.5%. The influence on the production of boring powder was even greater than for these values. Whilst the damage and losses are also caused by other pests alongside P. truncatus and belonging to the pest complex, the boring powder is produced by P. truncatus to a great extent. This is why the difference between the variants could be recognized a little earlier and was distinctly higher at the end of the observation period at 58.2%.
The loss data can only be compared to the practical surveys carried out by BÖYE (1988) with some restrictions. Very differing infestation and loss developments occurred in Costa Rica in the different stores. There was only a comparable situation regarding the infestation by P. truncatus in test-farm B where the 1st storage period was more in agreement. After 7 months, BÖYE recorded damage of 31.8% and losses of 13.6% in this store. These values are almost identical to the values in the wire-gauze cage after 7 months. As already stated above, BÖYE (1988) recorded a higher abundance of P. truncatus in the stores. In contrast to this, the substantially greater density of S. zeamais in the cage and the 4 months of storage prior to the beginning of the experiment should also be taken into consideration.
In general, it can be concluded from the results in Togo that an efficacy of T. nigrescens regarding the suppression of P. truncatus existed under near-to-practical conditions which is comparable to findings in Costa Rica (BÖYE, 1988). The results of jar experiments specifically set up for comparing Togo and Costa Rica confirm the results of the near-to-practical experiment. Apart from this, these are also in line with the findings of other experiments (cf. REES, 1985; LELIVELDT, 1990).
When using the predator T. nigrescens to biologically control the Larger Grain Borer, not only maize storage but also other habitats for the pest have to be considered. HELBIG (1993) showed in investigations that effective suppression of the population development of P. truncatus could evidently also be possible on cassava chips and on wood from M. esculenta. It is thus hoped that the predator will pursue P. truncatus if it goes in search of other sources of food. To contain the populations in the natural environment outside maize storage would substantially reduce the pest's potential for infestation.
Controlling P. truncatus with the predator T. nigrescens is of similar significance for the storage of cassava chips as for maize. Every year, approx. 120,000 to 150,000 t cassava are processed into cassava chips in Togo (ADAM, 1988). Due to the high loss potential of P. truncatus on this product (cf. Chapter 3.5), farmers could suffer huge losses. As insecticides are normally not used (LAMBONI, undated; AYEVA, undated), the implementation of biological control can make a substantial contribution to reducing losses caused by the infestation of P. truncatus.
Risks involved in releasing T.nigrescens
Importing am exotic insect a strange fauna to control a pest which has spread there always involves a risk. Here, this risk has two aspects. First, there is no guarantee of the useful insect being effective. The largest potential loss would, in this case, be the investment of resources prior to release and in the initial phase up to the point when it is discovered that there is no effect.
The second and far greater risk is a possible undesired side-effect of the useful insect in its new habitat. The predator could turn into a pest attacking plants or plant products, or other species of animal as a target organism Possible hosts could be insects used for economic purposes, e.g. the honeybee or silkworm. Apart from this, parasitising other insects of the endemic fauna is undesired as this could destroy the ecological balance.
There is a relatively low risk of T. nigrescens not having an effect. The population of the predator was able to develop well under the climatic conditions in the wire-gauze cage. The climate in central and northern parts of Togo are not greatly different from the conditions in its native home, practically eliminating this as a serious restrictive factor. The first findings in the natural environment in Togo support this assumption (RICHTER et al., 1992).
T. nigrescens is able to survive for long periods without its actual prey (PÖSCHKO et al., 1992 A; REES, 1992). This ability to survive for a long period is due to the insect making use of plant materials to some extent and to its ability to feed on other host organisms (LELIVELDT, 1990; REES, 1991, 1992; PÖSCHKO et al., 1992 A and B). Apart from being able to survive for long periods, oligophagous useful insects benefit by being able to make use of other prey as an alternative when their main prey is in short supply (KRIEG & FRANZ, 1989). This allows the predator to retain its higher density and quickly adapt to changes in the abundance of the target organism.
In jar experiments, T. nigrescens adapted its reproduction rate to the density of its prey (cf. also REES, 1985; LELIVELDT, 1990). This adaptation allows the predator to retain a high degree of efficacy even when it is not in great abundance as can occur during extensive migration of the population. On the other hand, this mechanism also ensures a certain population density for the prey which is significant in preserving the population of the predator.
Another factor affecting efficacy is the method of detecting the prey. The kairomone effect (BÖYE et al., 1992) provides the predator with a very effective mechanism (HELBIG et al., 1992) of finding its prey in any habitat. Apart from this, the kairomone effect gives the predator high specificity, also promoting efficacy. On the whole, T.nigrescens is a hardy and highly effective predator, with a risk which can be estimated as low.
This investigation also involved examining some undesired risks. The behaviour of T. nigrescens on stored plant-based produce and the possible damage to these were also investigated.
According to HINTON (1945), the larvae and imagines of Histeridae are carnivorous. It can be assumed in this respect, that plant substrates are not a sufficient nutritive medium for T. nigrescens. This was confirmed in experiments here examining the behaviour of T.nigrescens on various stored produce. There was no distinct attraction of the predator to any of the stored products. When adult specimens were introduced to these stored products, they remained on them so briefly that this experiment also showed that no interest in the stored products existed.
In no-choice jar experiments, T. nigrescens consumed the products offered to only a slight extent which was minimal and probably served to preserve survival. Reproduction of the predator was not observed in any case. The same results were obtained in the investigations by PÖSCHKO et. al (1992 B) and REES (1992). it can be assumed from the results obtained here that T. nigrescens will not develop into a pest attacking plant materials.
The study of the behaviour of T. nigrescens in association with useful insects involved the economically relevant species Apis mellifera L. and Bombyx mori L. plus the useful 1 insects. A. calandrae, Heterospilus prosopidus Viereck and the Coccinellidae. The experiments gave no indication of a predatory relationship between T. nigrescens and the species mentioned (LABORIUS, 1992; LABORIUS & MAUTZ, 1992;MURPHY & CROSS, 1992).
In addition, various tests have been carried on the host specificity of T. nigrescens. Investigations by the author showed reproduction of the predator and a distinct reduction in the development of the host population in am experiment with D. bifoveolatus. Reproduction was also recorded with simultaneous effect on the prey population with Sitophilus oryzae, R. dominica and Dinoderus minutus (LELIVELDT, 1990; PÖSCHKO et al., 1992 A; REES, 1992). The relationship is still not clear in the case of T. castaneum REES (1987) has shown that reproduction takes place on this host, although LELIVELDT (1991) and PÖSCHKO et al. (1992 A) were unable to record any reproduction.
T. nigrescens was unable to reproduce in experiments with Lasioderma serriocorne, Acanthoscelides obtectus, S. zeamais, Tenebroides mauritanicus, Trogoderma granarium, O. surinamensis, C dimidiatus, S. granarius and C. ferrugineus (LELIVELDT, 1990; PÖSCHKO et al., 1992 A; REES, 1992) as species of host. Due to the findings of these studies, an influence on the population development of the species tested can be neither proven nor eliminated. In no-choice experiments with the moths Plodia interpunctella, Ephestia kuehniella and E. elutella, eggs were eaten to an insignificant degree (PÖSCHKO et al., 1992 A). As WAAGE (1990) pointed out, these kinds of experiments are always conservative as the artificial situation created normally does not occur in the natural environment. Findings resulting from no-choice experiments in closed jars without any alternative nourishment constitute a potential situation, the relevance of which is unknown for practical conditions.
Experiments with mixed populations have so far only been carried out for S. zeamais and T. castaneum. The effect of T. nigrescens on P. truncatus was not influenced by the presence of the other species in these experiments (REES, 1987; LELIVELDT, 1990). This provides evidence for the specificity of the predator. The population development of T. castaneum was influenced by the presence of T. nigrescens in a mixed culture (REES, 1987; LELIVELDT, 1990). However, as already stated, it is still not clear whether this is a direct or indirect effect with secondary pests. REES (1987) found no negative effect of the predator on the population development of S. zeamais, although LELIVELDT (1990) determined less offspring of S. zeamais.
As this result conflicts with the effect in the monoculture, it is assumed that T. nigrescens gains access to the eggs and larvae of S. zeamais due to the boring activities of P. truncatus. HAUBRUGE (1987) contradicts this, however, as he observed that P. truncatus does not infest grams already occupied by S. zeamais.
Numerous tests carried out in view of the planned release of T. nigrescens in Togo were to provide information on possible, undesired side-effects of the predator. Studies showed no indication of negative effects stemming from this useful insect it can be assumed, according to results so far, that no damage to man, the environment or agriculture will be incurred if T. nigrescens is released among foreign fauna. There have been no signs of any negative influences of any Histeridae which have so far been released. Throughout the world 17 species in 81 programs have been imported to different countries to date without any undesired consequences being reported (WAAGE, 1990). Thus, the introduction of T. nigrescens to Togo was feasible.