3.3 Interaction with useful endemic insects

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During investigations of useful insect populations in the maize stores, X. flavipes, A. calandrae and T. elegans were the only species regularly occurring in larger numbers (see Chapter 2). A. calandrae and T. elegans are known to be cosmopolitan and have already been described in their relationship to P. truncatus (BÖYE, 1988). No investigations concerning the relationship between X. flavipes to P. truncatus are so far known. For this reason, the predator was to be tested initially to discover its acceptance of P. truncatus larvae as food.

In this investigation, the larvae of P. truncatus were parasitised by X. flavipes from which cam be concluded that this predator has am antagonistic potential for the Larger Grain Borer (see HELBIG, 1993). Subsequently, all three antagonists were examined for their interaction with P. truncatus on its typical nutritive substrate.

Material and methods

The influence of endemic enemies on the development of P. truncatus populations was investigated in a monoculture of the pest and a mixed culture with S. zeamais. Apart from the populations of the pest and the useful insect, the loss parameters were also recorded.

20 pairs of adult P. truncatus were put onto 250 g shelled maize of a local, yellow-grained variety in 1 l glasses. The Hymenoptera were provided with nourishment in the form of drops of honey solution on the wire-gauze lid. The time for the experiment amounted to 8 weeks and every variant was repeated three times. After a preliminary period of 7 days, the useful insects were put into the experimental vessels. The parasitoid A. calandrae was added at intervals of one week in lots of 5 pairs. In the experiment with the assassin bug X. flavipes, 10 individuals, larvae and imagines were added once only. The following climatic data was measured in the laboratory: temperature 24-34 °C, relative humidity 25-99%.

The experiment with the parasitoid T. elegans in a monoculture of P. truncatus was carried out at a later date since the first breeding of T. elegans did not produce enough specimens. 10 imagines were put into each vessel at weekly intervals. During the experiment, the temperature in the laboratory was 21-33 °C and the relative humidity 30-99%. Evaluation was carried out according to the procedure described under 2.1.2.

For the mixed experiment, 30 adult S. zeamais of unknown sex and 15 pairs of adult P. truncatus were put onto 300 g maize grains. The number of Hymenoptera remained the same, with X flavipes, 20 specimens were added per vessel. The temperature amounted to 23-31 °C, the relative humidity 55.99%.

Results

Monoculture

The development of the P. truncatus was clearly affected by the presence of the other insects (Tab. 12). in the variants with A. calandrae and X. flavipes, P. truncatus with 220 and 312 adults respectively after 8 weeks, had clearly developed less than in the control with 737 beetles after 8 weeks. The best population development was recorded for the pest with T. elegans where am average of 1113 beetles was recorded after 8 weeks. Since the variant with T. elegans was not carried out at the same time as the others (see above), more favourable environmental conditions or a better fitness of the beetles could have led to this distinctly higher rate of multiplication. Under the aspect of the efficacy of the useful insect, the results cam only be interpreted to mean that there was evidently no negative influence exerted by these on the population development of P. truncatus.

Due to the varying developments in the pest population when various useful insects were involved, there were also differences in loss parameters among the varying variants. The damage to the maize grains was reduced in the presence of A. calandrae at 59.9% and X. flavipes at 67.2% in comparison to the control (Tab. 12). The dry-weight loss and the production of boring powder were significantly lower than in the control with X flavipes as well as with A. calandrae. Losses amounted to 10.3% in the presence of A. calandrae, and 17.9% in the presence of X. flavipes. In contrast, losses amounted to 35.8% in the control.

Tab. 12: Population development of Prostephanus truncatus with damage, loss and the production of boring powder on maize in the presence of endemic antagonists after 8 weeks

Mixed population

The isolated investigation between P. truncatus and its antagonists was to provide initial orientation. In farm-level storage, however, P. truncatus never occurs as a single pest. Apart from this, the useful insects are already known as antagonists of S. zeamais and other storage pests (LeCATO et al., 1977; ARBOGAST, 1985). In one further experiment the interaction between the antagonist and P. truncatus in the presence of S. zeamais was to be investigated.

After am experimental period of 8 weeks, a varying number of adult P. truncatus had developed in the different experiments although the differences were not significant (Tab. 13). The lowest number of adult P. truncatus at 96 was recorded with A. calandrae, followed by X .flavipes at 203. The control and the variant with T. elegans both showed similar development of the pest population with almost 250 beetles.

The number of S. zeamais was reduced with A. calandrae and T. elegans but no significant differences occurred (Tab. 13). Due to the development of the pest population, a weak influence of A. calandrae on P. truncatus and S. zeamais can be deduced. X. flavipes only reduced the population of P. truncatus; T. elegans only had am effect on S. zeamais.

The development of the population of useful insects was generally better in the presence of S. zeamais than with P. truncatus as the sole host. S. zeamais was of major significance to T. elegans in particular, a parasitising of P. truncatus by this Hymenoptera evidently did not occur at all. With A. calandrae, the overall development not only was better, but the number of live adults was clearly higher in comparison to the experiment on a monoculture.

Tab. 13: Population development of Prostephanus truncatus and Sitophilus zeamais in a mixed population in the presence of endemic antagonists on maize after 8 weeks

The small differences in population development among the variants are also reflected in the loss parameters. There were no significant differences determined in the damage or in the production of boring powder (Tab 14). Nevertheless, it has to be stated that the comparatively low level of boring powder production determined for A. calandrae at 13.7 g corresponded to the lower number of adult P. truncatus (Tab. 13). The lowest losses recorded were for A. calandrae at 170%. The three other experiments showed a very similar level of loss at approx. 23-24%.

Tab. 14: Influence of endemic antagonists on damage, loss and the production of boring powder in a mixed population of Prostephanus truncatus and Sitophilus zeamais on maize after 8 weeks.

3.4 Losses of various varieties of maize

In Togo, the varieties of maize mainly cultivated are local. The use of hybrids is still rare. One reason presumably lies in the method of storage (storage of cobs) since the very thin and open husks of the hybrid varieties only inadequately protect the grains. Insects thus have easy access to the grains. In some regions, the farmers prefer yellow-grained, local varieties as these are particularly resistant to infestation by pests. The yellow-grained types are preferred although white-grained varieties are liked for their flavour.

The following experiment is to examine whether this widespread opinion also applies to the pest P. truncatus .A hybrid variety, "Ikenne", was included in the experiment. The medium used in the experiments was maize cobs in their husks.

Material and methods

Cobs which externally non-infested were deep-frozen, conditioned for some days after defrosting and then put into 21 pass vessels (approx. 600-700 g per glass). 25 pairs of adult P. truncatus were put into each vessel. The following varieties were used: a local, white-grained variety; a local, yellow-grained variety; the hybrid Ikenne (very kindly provided by IRAT). The experimental period amounted to 4 and 8 weeks and three repetitions of each experiment were made.

The experiment was carried out using maize cobs whose husks were intact and unopened. Any damage or loss existing at the beginning of the experiment could thus not be determined or recorded. For this reason, a random sample of 24 cobs was taken from each variety as a standard and the damage and loss of these were subtracted from the respective results. The method of loss analysis used was the CWM (see Chapter 2.1.2).

Results

The yellow, local variety showed the least damage after 4 weeks with 5.88/. and after 8 weeks with 24.6% (Tab. 15). The differences from the other varieties were in some cases quite substantial, but in no case significant. The hybrid variety showed the greatest damage at both points in time with very little difference from the: white local variety.

Tab. 15: Damage and loss caused by Prostephanus truncatus to maize cobs after 4 and 8 weeks depending on the variety

The varieties showed the same order for loss as for damage (Tab. 15). By far the lowest losses were recorded for the yellow, local variety. After 8 weeks, losses amounted to 14% The white, local variety 31.66% after 8 weeks showed slightly less loss than the hybrid with 34.99%. A significant difference was not determined in any case.

The production of boring powder, similar to the damage, shows considerable differences among the different varieties, which were, however, only significant in one case (Tab. 16). This was after 8 weeks with a difference of 74. I g between the yellow, local variety and the hybrid. The white, local variety at 84.0 g boring powder was between the other two.

The population of P. truncatus developed best of all on the hybrid maize (Tab. 16). With am average of 1254 beetles it was 86.1% greater in comparison to the yellow-grained maize. The development of the P. truncatus population on the white, local variety with 987 beetles was also better than on the yellow-grained maize.

Tab. 16: Production of boring powder and population development of Prostephanus truncatus on maize cobs after 4 and 8 weeks depending on the variety

3.5 Losses for cassava chips

Due to its spreading to the north of Togo, P. truncatus also became significant as a pest on cassava chips. For this reason, the loss potential of P. truncatus on this stored product was subjected to a comparative investigation. The losses occurring on infestation by Dinoderus bifoveolatus (Coleoptera, Bostrichidae) were used as a comparison. D. bifoveolatus is among the most significant pests on cassava chips and causes loss, as does P. truncatus due to its boring activity.

Material and methods

100 adult beetles were put onto 300 g cassava chips in 1 l glass vessels. The cassava chips were examined for infestation prior to being used, deep-frozen and finally conditioned under laboratory conditions. Evaluation according to the method stated in Chapter 2.1.2 was carried out after am experimental period of 8 weeks.

Results

After an experimental period of 8 weeks, the Larger Gram Borer had caused damage amounting to 79.9% (Tab. 17). Losses were thus 63.7% greater than for D. bifoveolatus. These high losses were mainly caused by the cassava chips being turned into boring powder. With 209.8 g, 69.9% of the nutrient media initially inserted was turned into boring powder by P. truncatus.

During the experimental period, the population of P. truncatus showed clearly better development than that of D. bifoveolatus. These results are firstly evidence of cassava chips being suitable as nutritive and breeding substrate for the pest. Secondly, this high rate of multiplication is a further reason for the high degree of damage potential of the Larger Grain Borer.

Tab. 17: Losses and the production of boring powder caused by Prostephanus truncatus and Dinoderus bifoveolatus to cassava chips and their population development after 8 weeks

3.6 Pheromone trapping in the field

Since no infestation by P. truncatus has yet been discovered on fields in Togo (MATCHIKO, 1988), newly stored maize in good, hygienic conditions is presumably infested by beetles present in the wild outside the field. It is also possible that the insects temporarily stay in the wild when spreading from infested to non-infested stores during the storage period. Observations of the Larger Grain Borer in the wild were carried out over a period of 43 weeks (April 1990 to January 1991) using pheromone traps (Trunc-call I + II).

Material and methods

Monitoring was carried out using pheromone baited delta-flight-traps, a construction designed by the DPV and GTZ. They were made from a piece of transparent plastic (DIN A4) folded into an equilateral triangle and fixed together at the top. The bottom was covered with adhesive tape and the outer openings were closed over half their length. A piece of corrugated cardboard treated with Deltamethrin and firmly holding a pheromone capsule with the synthetic pheromone "Trunc-call I + II", was put into the traps. The traps were replaced and evaluated at weakly intervals

The traps were set up at the following locations

Results

The numbers of insects caught over the year showed characteristics common to all locations. After an initial increase at the beginning of the period of observation, the numbers remained on a higher level until the end of August (33rd to 35th week). in the following weeks, from the beginning of September to the end of November, clearly fewer specimens of P. truncatus were found in the traps. The figures again rose rapidly within a few weeks towards the end of the year.

The number of P. truncatus caught at the various locations deviated greatly. Fig. 15 shows the locations with greater catches. The highest numbers were caught in Davié where up to 1000 specimens of P. truncatus were caught per week. Catches at both other locations varied more or less considerably from the 13th to the 33rd week between approx. 100 and 400 P. truncatus per week. During the period from the beginning of September (35th/36th week) to the end of November (47th/48th week), the time of lower catches, the number of P. truncatus captured amounted to between 10 and 70.

At the three locations with fewer catches (Fig. 16), a maximum number of 88 P. truncatus per week was caught from April to July, although the figures were mostly far below these. The rapid increase of specimens caught at the end of the year and beginning of the following year is striking: in Gbonvé 593 P. truncatus were caught in one week. However, figures in this period also rose above the level for the whole year in Adétikopé.

Fig. 15: Numbers of Prostephanus truncatus caught in pheromone traps at three locations in the field

Fig. 16: Numbers of Prostephanus truncatus caught in pheromone traps at three locations in the field

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