3. Aspects of the ecology of prostephanus truncatus

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3.1 Survival on various substrates

It has been observed in maize stores that the wood used for building the stores is highly infested by P. truncatus .Since erecting a store incurs costs and obtaining wood is becoming increasingly difficult, the farmer endeavours to make use of his store for several years. Due to this behaviour, the question arises as to whether and how long P. truncatus cam survive on wood. If the pest cam remain alive on wood over a longer period, it would be able to bridge over the gap between storage periods. The aim of the investigation was to define the ability to survive and reproduce after survival. In addition, the most important species of wood used to build stores (see Tab. 4) were compared for their suitability as a nutritive medium for the pest.

The experiments were carried out under as natural conditions as possible. After the prescribed survival period had elapsed, maize was put into the experimental vessels to simulate filling the store. Evaluation was carried out 30 days later. In this way, survival as well as infestation of maize and reproduction of the P. truncatus still alive could be recorded.

Tab. 4: Species of wood used in the experiments to show survival (1) and reproduction (2) of Prostephanus truncatus

* The varieties of wood were classified by ODEF staff; the majority of scientific names have been taken from BÄRNER (1942).

3.1.1 Material and methods

The wood was taken out of the store whilst maize was still inside. The infested pieces of wood were sawn to a length of 6-8 cm and put into experimental vessels at random 50 beetles taken from the maize were put into each vessel. Survival test periods selected were 30, 60 and 90 days. After each of these intervals, 100 g non-infested maize grains were put into each vessel to simulate filling the store. Each variant was repeated three times. During the experiment, the temperatures in the laboratory amounted to 23-33 °C and the relative humidity to 60-98%. To measure the density of the wood, the volume was determined by water displacement (DETMERS, 1988). The experiments were evaluated 30 days After adding the maize in each case.

Determining the maximum survival period

To determine the maximum survival period on wood, naturally infested wood of a Uvaria sp. was used. According to information from the experiment described above, this wood is subject to high infestation and P. truncatus specimens settling on this show a high mortality rate. The approx. 10 pieces of wood per vessel were cut to a length of 8-10 cm and came from freshly emptied stores. To the existing natural infestation, 310 live P. truncatus were added per vessel which had come from wood in the same store. For this experiment, survival periods of 4, 5 and 6 months were chosen, and for each period, 5 repetitions were carried out. Temperatures in the laboratory amounted to 23-34 °C, the relative humidity to 25-98%.

Survival on parts of the maize plant

Possibly to be considered as media on which P. truncatus could survive are also infested maize cob cores, and where field infestation occurs, dry parts of maize stems. The cores from non-infested maize cobs were used and treated in two different ways:

1. The cores were completely stripped of grains
2. The cores were shelled as is common locally, i.e. small and incompletely matured grams remain on the core as well as whole grams which are overlooked.

The pieces of stem and the cores cut to a length of approx. 6-8 cm were deep-frozen and subsequently conditioned in the laboratory for several days under natural climatic conditions 5-7 cores or pieces of stem were put into each 1 l vessel and 100 adult specimens of P. truncatus of undetermined age and sex were added. Each variant was repeated 3 times.

In a further series, am additional investigation was carried out to determine how long P. truncatus could survive on naturally infested cores. The husks were removed from highly infested maize cobs and these were extensively stripped of boring powder and remains of grains. 6-8 cores with natural infestation and, as a variation, with the addition of so adult specimens of P. truncatus were put into each 1 l glass vessel. Survival periods of 4 and 5 months were prescribed and each variant was repeated 3 times. After the survival period ended, the cores were evaluated and the living P. truncatus were put onto non-infested maize.

3.1.2 Survival on wood

The mortality of the adult P. truncatus specimens amounted to about 60% (Tab. 5) after a trial period of 30 days on A. indica, Uvaria sp. and M. discoidea. In comparison to this, the mortality on other species reached a level of over 80%

When prolonging the survival period to 60 days, the mortality increased substantially and amounted to between 91% and 97% on almost all species of wood. Compared to the other species, A. indica with 85% and Uvaria sp. with 82% showed a lower rate of mortality. In contrast to this, the mortality on M. discoidea at 96% reached the level of the other woods. After a period of 90 days, the mortality of the adult P. truncatus amounted to over 99% on all woods tested.

Infestation of the species of wood

Infestation by P. truncatus of various species of wood was defined in the experiments investigating the mortality. For this, the average value for nine experiments per species of wood was formed. The categorising of the degree of infestation as in Table 6 constitutes an estimate and is included as an aid to orientation.

The most intensive infestation was shown by A. indica and Uvaria sp. with 49 and 39 adult P. truncatus respectively (Tab. 6). Clearly far fewer specimens, approx. 10 to 20 imagines, were found per piece of wood on the types of wood with a medium degree of infestation. Low infestation, with less than 10 beetles per piece of wood, was observed for D. guineense and F. xanthoxyloides.

Maximum survival period

After 90 days survival, there were still some P. truncatus specimens still alive, mortality amounted to a little under 100% (see Tab. 5). For this reason, another experiment was carried out to find the upper limit of survival.

Table 7 illustrates the results from the experiment described above for the survival period of 4, 5 and 6 months. Furthermore, results of the first, second and third month, as shown in Table 5, are summarized including all species of wood.

Tab. 5: Mortality (%) of Prostephanus truncatus on wood related to the period of survival and the type of wood

Tab. 6: Infestation of different species of wood by Prostephanus truncatus

After one month, mortality amounted to an average of 67.9%. It increased after two months to 86.1 %. The number of live P. truncatus was thus reduced to about 50% in comparison to the 1-month survival period. After 90 days, the mortality again clearly continued to rise and reached 99.6%.

Tab. 7: Survival of adult Prostephanus truncatus on wood (months 1-3: data from 9 species of wood (10 series of experiments); month 4-6: experiment with Uvaria sp.)

*The beetle was put onto maize and died three days later without having infested one grain.

The upper limit for survival in this experiment seems to be between 3 and 4 months. After having survived for 4 months (plus 30 days after maize was added), only one beetle was found to be alive which then died three days after the evaluation.

3.1.3 Reproduction survival on wood

After surviving for 30 days, a total of 1638 eggs were laid on the maize by the females present (Tab. 8). Prolonging the period of survival to 60 days had no effect on the number of eggs laid in total. The number of offspring after 30 and 60 days was approximately the same, as each female had laid am average of 3 eggs more after 60 days.

Further prolongation of the period to 90 days led to a fast decline in the number of progeny (by the factor of 10) despite the double number of eggs per female. In this experiment, P. truncatus was evidently no longer able to reproduce after surviving for 120 days. The epidemiologically significant limit for survival thus lies between 90 and 120 days.

Tab. 8: Reproduction of adult Prostephanus truncatus after different periods of survival on wood (summary of the results from 9 different varieties of wood; evaluation 30 days after adding maize)

The period of survival obviously had no effect on when egg-laying beg in (Tab. 8). This cam be deduced from the existence of pupae in all variants at the time of evaluation. Furthermore, the presence of pupae is an indication of rapid egg-laying within the first week of adding maize, from which can be concluded that the beetles were in good health

3.1.4 Survival on parts of maize

When infesting maize cobs, P. truncatus also bores into the cores. If highly infested, there are a great number of beetles to be found in the core. The cores of maize cobs are saved by the farmer after shelling and successively used as fuel. They thus lie on the farm for a longer period of time and constitute a possible source of infestation for newly stored maize. Highly infested cobs are given to animals as foodstuff. Thus no attention is paid to these, and they are possibly spread by the animals. If P. truncatus were able to survive for a longer time, they would also be a source of danger for the new crop.

The maize cob cores were examined in non-infested and in naturally infested forms (nat. inf. cores). A "local core" variation was introduced among the non-infested cores. This variation was to represent shelled maize as this is carried out in practice. For this reason, individual or stunted grains were left on the cob as the farmers normally do.

The naturally infested cores were treated differently from normal procedure, as secondary predator pests could have influenced the reproduction of P. truncatus For this reason, the cores were evaluated after the test period and the living specimens of P. truncatus discovered were put onto maize grains The mortality stated has been calculated according to this evaluation and, not as usual in other cases, 30 days after adding maize.

On completely bare cores, without any grains, P. truncatus was able to survive for 60 days (Tab. 9) and also produced offspring after survival (Tab. 10). Survival was not possible for any longer in this variation Mortality amounted to 100% after 90 and 120 days, and no more offspring could be found.

The presence of some grains allowed P. truncatus to survive for a longer period. After 4 months, there were still living beetles in the "local cores,' variant (Tab. 9) These beetles were also sufficiently fit after this period to infest the maize grams added and to reproduce (Tab. 10).

The cores with natural infestations only tested over 4 and 5 months since survival over a shorter period could be assumed. In both variants with natural infestation, the adult P. truncatus were able to survive for a period of 5 months (Tab. 9). Mortality after this long test period amounted to almost 100%. In the variant "nat. inf cores + 50 P. t. ", reproduction of P. truncatus was also evident after 5 months (Tab. 10).

Tab. 9: Mortality (%) ) of Prostephanus truncatus on various parts of maize plants in relation to the period of survival

*cores, local i.e. during shelling a number of single and stunted grains were left on the core
**P. truncatus reproduced on the stems which is why all imagines found were taken into consideration in calculations and not only the ones initially added.
***cores with natural infestation
*'** cores with natural infestation to which 50 adult P. truncatus were added.

The maize stems take up a special position in this experiment because P. truncatus was found to reproduce on them. The mortality rates stated refer to all adult beetles found and not only to the ones initially added The number of offspring includes only those produced on maize.

Despite the ability to reproduce on maize stems, the mortality of P. truncatus was very high from the second month (Tab. 9). After 4 months, a mortality rate of 99.78/. was determined, corresponding to the level of the other substrates. But also the low number of P. truncatus surviving was sufficient to produce offspring and thus to lay the basis for creating a new population (Tab. 10).

Tab. 10: Reproduction of Prostephanus truncatus on maize related to the survival period on various parts of maize plants (average sum of the pupae, larvae and eggs per vessel)

*, **, ***, **** = legend see table 9

3.2 Reproduction on various substrates

In several investigations, (BÖYE, 1988; REES et al., 1990, RICHTER & BILIWA, 1991) P. truncatus has been found in the wild far from human settlements and maize stores by using pheromone traps (Trunc-call I + II). From this observation, it can be probably assumed that the pest can also adequately nourish itself and possibly reproduce on media other than stored maize and cassava chips.

Various tests with different species of wood have been carried out which have shown that only am extension of life has occurred in the sense of survival, but in which no full nourishment or even reproduction could be determined (DETMERS, 1988). This aspect has again been taken up for this reason. Some species of wood used in building stores and other species of Togo flora were taken into consideration here (see Tab. 4). in addition, various pans of the maize plant were also included and maize grains served as a medium for comparison.

Material and methods

For each of the three repetitions, 4-6 pieces of each type of wood (see also Chapter 3.1, Tab. 4) were sawn to a length of 5-7 cm and slightly split at the ends. The cores of the maize cobs and the maize stems were prepared as described in Chapter 3.1. 50 adult P. truncatus of undefined age were put into each 1 l vessel. A variant with 300 g maize grains was included as a comparison. The evaluation carried out after 60 days recorded the living and dead stages of the Larger Grain Borer.

Results

P. truncatus was able to develop completely from the egg to the imago stage and then multiply (Tab. 11) on three species of wood and on the dry stems of the maize plant. Multiplication of the beetle was observed on M. esculenta and P. regia. Reproduction was possible on S. tragacantha and the maize stems although the brief experimental period allows no statements regarding multiplication.

The most favourable nutritive medium for the Larger Grain Borer was obviously M. esculenta since there was an average of 645 offspring per vessel forming the greatest number of offspring. The number of offspring on P. regia was comparatively low, amounting to an average of only 172 per vessel. 46 and 45 offspring respectively were counted on 5. tragacantha and on the maize stems. The number of adult offspring on these media was lower than that of the beetles added initially.

No juvenile stages of P. truncatus were found on any other species of wood tested. The females presumably laid no eggs on these types of wood. The mortality of the adult P. truncatus on these varieties of wood was generally so high that they cam be assumed unsuitable as a substrate inducing reproduction.

Tab. 11: Number of offspring of Prostephanus truncatus on different species of wood and on maize after 60 days

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