2 Infestation and losses in farm-level maize storage

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Maize stores at several locations and during three periods of storage were examined for infestation occurring and the damage and loss resulting. In addition to the pest, a survey was also made of the useful fauna. This summary provides excerpts from the results. The complete findings of the investigation cam be read in HELBIG (1993).

2.1 Material and methods

2.1.1 Survey area

The investigations were carried out in Southern Togo in the "Region Maritime". Venue for the work was the central office of the "Direction de la Protection des Végétaux" (DPV) in the Cacaveli district of the capital, Lomé.

The Republic of Togo is a country in West Africa. The total area of the state, which is a narrow strip of land stretching from the north to the south between 6° and 11° north and 0° and 2° east, amounts to 56,785 km². The coastline measures 53 km, the distance from north to south, approx. 550 km. The neighbouring countries are Benin to the east, Ghana to the west and Burkina Faso to the north (ANONYMOUS, 1991).

Togo has a tropical climate with 2 rainy seasons in the south (March to June; September to October) and 1 rainy season in the north (April to July). The coastal zone is relatively dry with approx. 900 mm mean annual precipitation. The central part of the country is quite damp with 1500 mm mean annual precipitation. The dry north has a mean annual precipitation of about 1200 mm. The mean monthly air temperature in Lomé amounts to 26.5 °C (minimum 24.5 °C; maximum 28.0 °C) (ANONYMOUS, 1991).

2.1.2 Analysis of infestation losses

During the course of the storage period, samples of 100 maize cobs were taken from farm-level granaries and analysed. The samples of maize cobs were examined for the presence of pest insects and useful insects as well as for evidence of loss. Maize cobs infested by fungi or showing damage caused by rodents were excluded from the investigation.

Sampling and evaluation of the samples

When taking samples from the stores, the maize stores, the straw roofs were removed allowing free access to the whole of the upper store area. Samples were taken according to the method described by PANTENIUS (1987). The complete 100-cob sample was kept in a cotton sack and gassed as quickly as possible using hydrogen phosphide to conserve the status of infestation and loss shown by the samples.

The most important element of the evaluations is the single-cob analysis which was used for each individual cob to provide the complete data on the degree of loss and infestation. For this, the number of damaged grains on the cob were counted. Completely destroyed grams were also counted using the insertion points on the actual core. The damaged and undamaged grains were separated, sieved using 3.15 mm and 0.4 mm mesh and weighed. The weight of gram recorded was the fraction left on the 3.15 mm sieve. After sieving, the damaged and undamaged grains were counted using a gram-counter. The fraction passing through the 0.4 mm sieve was referred to as boring powder. All insects, gram and cob fragments, and other waste remained on the 0.4 mm sieve. The insects were sorted out of this mixture, defined in number and species and recorded separately for each maize cob.

All fractions, undamaged and damaged grams, fragments, insects and the boring powder were kept separately. After all cobs in one sampling lot had been treated, the gram and boring powder fractions of the whole sample were weighed, the undamaged grains counted and the fraction of fragments again examined for insects

The single-cob analysis was carried out using random sub-sample of the 100-cob sample. The remaining cobs were treated according to a method based on the Sample-Weight-Method (PANTENIUS, 1987). See HELBIG (1993) for a description of this method and the findings determined.

Analysis of infestation nom ally comprised classifying and counting the species of insects found Classifications were made using the keys and descriptions of LESNE (1924), VRYDAGH (1955), WEIDNER (1982), DOBIE et al. (1984) and HALSTEAD (1986). Unknown species were sent to CAB International (British Museum) to be defined. Infestation by Lepidoptera was only recorded qualitatively, the species were not defined. Sitophilus spp. (Coleoptera, Curculionidae), very significant primary pests in the field of storage, were defined using random specimens. Here, only S. zeamais could be determined and is thus quoted subsequently. Unless otherwise stated, only adult insects were counted. An exception was made with Xylocoris flavipes (Heteroptera, Anthocoridae), for which adults and larvae were both counted.

Calculating damage and loss

Two methods were used to calculate the dry-weight losses. the Count-and-Weight-Method (CWM) and the Sample-Weight-Method. This abridged version only includes the results of the CWM. The advantage of the CWM is the use of am inner standard as a reference (PANTENIUS, 1987; SALUNKHE et al., 1985). This makes am estimation of loss possible for any point in time during the period of storage and makes measuring moisture superfluous.

Apart from losses, the damage caused to maize cobs due to infestation by a pest was recorded. The damaged grains are defined as a percentage of the total number of grains. The formulae used to calculate the losses and the damage can be seen in PANTENIUS (1987, 1988) or HELBIG (1993).

The damage data referring to the total sample was extended upon by am analysis of the damage structure. For this, the damage to the individual cob was calculated. This cam give am impression of the intensity of damage the farmer is faced with when he opens the maize husks and looks at the cobs.

The analysis of the damage structure was aided by allocation into categories. The following categories were formed:

In the glass jar experiments, the loss was calculated from the difference between the initial and final weights taking the moisture content into consideration. The procedure for samples corresponded to the method described. To record the damage precisely, samples were taken from the maize in each case, and used to define the number of grains per weight unit. In this way, an average number of grains per glass jar could be calculated. This procedure was necessary, because in experiments with P. truncatus, the number of grams cam no longer be seen at the end of the experiment.

Statistical evaluation

The statistical evaluation of the data was carried out using the SPSS program package (Statistics Program System for the Social Sciences) on a PC (BEUTEL & SCHUBÖ, 1983; BROSIUS, 1988). if the assumption of nom al distribution seemed justified, models of variance analysis were used. Variance stabilising measures employed were the following transformations (SACHS, 1978):

for insect populations: x'= 1g10 (x);
for percentages: x'= arcsin with p = x / 100.

Comparison of the simple mean value was made using the t-test, the comparison of multiple mean values was made using the Tukey-HSD test (SACHS, 1978; LÄUTER & PINCUS, 1989). Significantly varying mean values are marked by different letters in the illustration of the results. Where nom al distribution could not be assumed, the Mann-Whitney test for the ordinal scale was used (LORENZ, 1988). An error probability of 5% was applied in the three tests.

2.1.3 Survey of farm-level stores

In Southern Togo, maize is normally stored in traditional stores of the "Ebli-va" type in which the cobs are stored with the husks. This investigation was carried out on storage without any kind of insecticide in order to record natural infestation and useful fauna.

Long storage period 1988/89

The objective of the survey during this storage period was to record the status of infestation and loss at sites with and without infestation by P. truncatus, Four sites were selected of which two (Cacaveli and Avéta), according to the information available, showed regular infestation by P. truncatus, and the two others (Davié and Kovié/Mission-Tové) were free of infestation. In Davié and Cacaveli, four stores were erected on the DPV station, respectively. 250 kg of a local variety of yellow-grained maize was taken from one field and stored in each store. Apart from these stores, samples were taken from a total of 8 farm stores at two locations (Avéta and Kovié-Mission-Tové) (cf. Table l). Sampling began during the period from the end of August to the beginning of September 1988 and continued over a period of eight months.

The second and third samples were taken at 6-week intervals, after which intervals amounted to 4 weeks. For the first and second samples, a sub-sample of 30 cobs was examined according to single-cob analysis. Beginning with the third sampling, the sub-sample was increased to 50 cobs. The remaining cobs in the 100-cob sample were evaluated using the sample/weight method.

Short storage period 1989 and 1990

During the short storage period in 1989 surveys were made of four stores in four different places (Tab. l). The maize for all four stores, a local, white-grained variety, was bought centrally from one field (DPV) and placed at the disposal of the farmers for storing. Approx. 250 kg maize was stored in each store. Samples were taken at 4-week intervals.

Tab. 1: Location and general data for the stores investigated in the individual storage periods

50 cobs were evaluated using single-cob analysis, the other 50 cobs were treated according to the sample/weight method (see Chapter 2.1.2),

During the short storage period in 1990, a total of four stores were erected under practical conditions in two places, Davié and Gbonvé three of which were on farms with the other on the experimental land of the DPV (Tab. 1).

The maize, a local, yellow-grained variety, was bought from farmer in Gbonvé The first sample was taken from the stores immediately after storage and the following samples then at 4-week intervals. Other methods correspond to those of the short storage season 1989.

2.2 Long storage season 1988/89

The survey of infestation by pests and the losses in farm-level storage of maize were carried out in the long storage season 1988/89 at four locations with 4 stores in each case. In Cacaveli, and in Davié traditional stores were set up on the experimental premises of the DPV by those experimenting. In Kovié/Mission-Tové and Avéta, surveys were made of stores erected by farmers.

2.2.1 Pest and useful insect populations

Infestation by the non-native pest P. truncatus, occurred in the long storage season 1988/89 in Cacaveli and in Kovié In Kovié only one store was infested by P. truncatus, in which the population density remained at a very low level throughout the storage period. In contrast, the population of P. truncatus, at Cacaveli increased rapidly and after 8 months, reached a density of 7222 beetles per 100 cobs (Fig. 1). No infestation by P. truncatus was recorded at Davié or Avéta.

Apart from the Larger Grain Borer, the population dynamics of S. zeamais were observed. This snout beetle is a cosmopolitan primary pest on stored maize and can cause extensive damage. Its abundance was recorded additionally as it firstly, cam contribute to clarifying the losses occurring and secondly, as it is significant in view of its interaction with P. truncatus,

Fig. 1: Infestation by primary pests in a maize store in the long storage season 1988/89 at Cacaveli (mean values from 4 repetitions)

There was a low abundance of S. zeamais in most stores which reached am average maximum of 1500 - 2000 beetles per 100 cobs (cf. Fig. 1 and 2). Also at the Cacaveli location (Fig. 1), the population of S. zeamais was able to establish itself well and reached a density comparable to the other locations despite the keen competition from P. truncatus.

Infestation of the maize cobs by Lepidoptera amounted to am average of approx. 40-60% (Fig. 1 and 2) at the Cacaveli, Davié and Kovié/Mission-Tové) locations The high values for Cacaveli at the end of the storage period presumably result from problems in recording. Where there is great damage caused by P. truncatus, damage due to Lepidoptera cam no longer be clearly seen. infestation by Lepidoptera was far lower at Avéta where it amounted to approx. 20-25%.

Fig. 2: Infestation by primary pests in a maize store in the long storage season 1988/89 at Davié (mean values from 4 repetitions)

Secondary pests

In the long storage season 1988/89, the secondary pests occurring regularly were Cathartus quadricollis (Coleoptera, Silvanidae), Tribolium castaneum, Palorus subdepressus (Coleoptera, Tenebrionidae), Cryptolestes spp. (Coleoptera, Cucujidae) and Carpophilus spp. (Coleoptera, Nitidulidae). The species C. pusillus and C. ferrugineus were determined for Cryptolestes spp . In Carpophilus spp., mainly C. dimidiatus but also C. obsoletus occurred. In addition, Palorus depressus, Oryzaephilus surinamensis (Coleoptera, Silvanidae) and Typhea stercorea (Coleoptera, Mycetophagidae) occurred in low numbers in individual stores.

Populations of useful insects

The survey of the useful insect populations was also carried out using the 100cob sampling method. The specimens inside the maize cobs were recorded according to species and number. Only adults were recorded for the Hymenoptera and larvae and imagines for the Heteroptera.

Occurring regularly at all locations were the species X. flavipes, Theocolax elegans, Anisopteromalus calandrae (Hymenoptera, Pteromalidae),Cephalonomia waterstoni and Holepyris sylvanidis (Hymenoptera, Bethylidae). Abundance of Hymenoptera was generally low and exceeded 10 specimens per 100 cobs in only isolated cases. In contrast to these useful insects, the assassin bug, X. flavipes occurred in greater numbers.

2.2.2 Damage and loss situation

Clearly the greatest damage in this storage period was caused in Cacaveli (Fig. 3). After 8 months of storage, the damage amounted to 62% in comparison to 47°% in Avéta After 7 months) and around 30% in Davié (Fig. 4) and Kovié/Mission-Tové. As the damage in Cacaveli at the beginning of storage was just as great as at the other three locations, the qualitative and quantitative composition of the spectrum of insects cam be seen as the reason for the very differing results in the extent of the damage.

The losses also showed considerable difference between individual locations Whilst the losses at the point of going into storage amounted to approx. 1% at all locations, there were considerable differences in the subsequent period. After 8 months, losses in Cacaveli amounted to 36.5% as compared to below 20% at Avéta and Kovié.Mission-Tové. Davié with 9%, shows the lowest losses after 8 months of storage.

The production of boring powder, an indication of high infestation by 1' truncatus was, at am average of 666 g per 100 cobs, almost three times as high as in Avéta Comparatively little boring powder was observed at the two other locations in Davié and Kovié/Mission-Tové. with a maximum of 100 g.

Fig. 3: Damage and loss situation in the maize store at Cacaveli in the long storage season in 1988/89 (mean value A from repetitions)

Fig. 4: Damage and loss situation in the maize store at Davié in the long storage season in 1988/89 (mean value from 4 repetitions)

2.3 Short storage period in 1989

In the short storage period in 1989, four stores at four locations, Gbonvé, Davié, Avéta and Mission-Tové were included in the survey of the infestation and loss situation. The maize was put into store at the beginning of January, 1988, the storage period amounted to 6 months. The maize for all four stores came from one field and was given to the farmers to store. Considering the maize came from one source, a uniform initial infestation by pests cam be assumed for the cobs going into storage. Differences in the infestation and loss analysis can thus be directly allocated to storage.

2.3.1 Pest and useful insect populations

In the short storage period in 1989, infestation by P. truncatus was observed at the Gbonvé, and Davié, locations. In Avéta and in Mission-Tové no P. truncatus were observed in the maize stores examined.

In Gbonvé, (Fig. 5), early infestation led to a very intense development in the population of P. truncatus within the 6 months of storage. After a strong increase in abundance from the 3rd month, 6373 beetles per 100 cobs were counted After 5 months.

Infestation by P. truncatus evidently occurred far later in Davié, The first beetles were found after four months. The abundance of the pest at the end of the survey period was thus correspondingly low. Nevertheless, it still has to be mentioned that, with 1008 beetles per 100 cobs in the sixth storage month, quite a large population had developed in two months.

S. zeamais occurred at all locations and was able to establish itself well. At Gbonvé, and Davié, where P. truncatus also occurred, the development of the S. zeamais population was relatively weak. After six months, 1404 adult S. zeamais were found at Davié, The number of beetles After four months in Gbonvé, was 1800 although this increased only minimally to 2021 per 100 cobs by the sixth month of storage.

Fig. 5: Infestation by primary pests in the maize store at Gbonvé in the short storage season in 1989

Fig. 6: Infestation by primary pests in the maize store at Mission-Tové in the short storage season in 1989

At Avéta and at Mission-Tové (Fig. 6), the populations of S. zeamais evidently had better scope for development than in Gbonvé and Davié. Both locations showed comparatively strong growth of pest populations. After a storage period of six months, there were 2835 beetles per 100 cobs in Avéta and 3121 in Mission-Tové.

Secondary pests

In the short storage period in 1989, the spectrum of species corresponded to that of the long storage period in 1988/89. The most important pests again were C. quadricollis, T. castaneum, P. subdepressus, Cryptolestes spp. and Carpophilus spp. In addition, isolated specimens of P. depressus and O. surinamensis were observed.

Populations of useful insects

When surveying the populations of useful insects in the short storage period in 1989, the same spectrum of species was determined as in the long storage period in 1988/89. Hymenoptera occurring in the maize stores were A. calandrae, T. elegans, C. waterstoni and H. sylvanidis plus the Heteroptera X .flavipes.

2.3.2 Damage and loss situation

The monitoring of damage to and loss of stored maize showed similar results for the locations Davié. Avéta and Mission-Tové Starting from a level of 4-8% when being put into store, the damage gradually increased (% infested grains) during storage and, after 6 months, reached approx. 28-37% (cf. Fig. 8). Losses during this period amounted to approx. 5-11%.

Completely different results were obtained in the maize store in Gbonvé (Fig. 7). After a rapid increase in damage from the third month of storage onwards, 65.4% of the maize grains were damaged after six months

Fig. 7: Damage and loss situation in the maize store at Gbonvé in the short storage season in 1989

Fig. 8: Damage and loss situation in the maize store at Mission-Tové in the short storage season in 1989

The loss at this point amounted to 26.5%. Thus the damage in Gbonvé after 6 months was about twice as high as at the other locations; losses were at least 2.3 times as high.

A very great difference between the locations was also determined for the production of boring powder. At all three locations with a comparatively low degree of damage and loss, i.e. Davié, Avéta and Mission-Tové between 78 and 109 g boring powder per 100 cobs was measured after 6 months of storage (cf. Fig. 8). in Gbonvé this level was clearly exceeded already after 4 months of storage with 258 g boring powder (Fig. 7). After six months, 795 g boring powder was in the 100-cob sample at Gbonvé

2.4 Short storage period 1990

In the short storage period in 1990, the loss situation and pest infestation were investigated in four maize stores at two locations, Gbonvé (I and II) and Davié, (I and II ). The maize was put into store at the beginning of January, 1990 and stored for 6 months. Used was a local, yellow-grained variety from one field.

2.4.1 Pest and useful insect populations

In the short storage period, the maize stores showed a relatively low degree of infestation by primary coleopteran pests. Infestation by Lepidoptera, in contrast, was comparatively high and was, on average, in the region of 80% damaged cobs (cf. Fig. 9 and 10).

The non-native pest, P. truncatus occurred in three stores during this period of storage, in Gbonvé-I (Fig. 9), in Gbonvé-II and in Davié-II. Adult specimens of P. truncatus were already observed in the samples one month after going into storage. Despite this early infestation of the stores, the development of the Larger Grain Borer population remained very weak over a period of three months. Its abundance did not rapidly increase until the fifth month of storage. Nevertheless, the populations of P. truncatus with 517 beetles in Gbonvé-I and 738 beetles in Gbonvé-II, remained comparatively small until the end of the storage period. In Davié-II, the population developed far better and, at this point, reached a density of 1797 adult P. truncatus

Fig. 9: Infestation by primary pests in a maize store in the short storage season in 1990 at Gbonvé-I

Fig. 10: Infestation by primary pests in the maize store in the short storage season 1990 at Davié-l

S. zeamais was already present in all stores with an average of 69 beetles per 100 cobs when the maize was stored. However, the population development of this pest was also weak in general. In three stores, in Davié-I with 471 beetles (Fig. 10), in Gbonvé-I with 758 beetles (Fig. 9) and in Davié-II with 767 beetles per 100 cobs, the population of S. zeamais remained comparatively small until the end of the storage period. After relatively rapid growth in Gbonvé-II, the population maximum of 1326 adult S. zeamais was found after four months of storage. Subsequently, the density of the pest declined. Only in this case a result comparable to the other storage periods was found.

Secondary pests

The secondary pests occurring in the short storage season in 1990 as well as in the other storage periods, were C. quadricollis, T. castaneum, Carpophilus spp., Cryptolestes spp. and P. subdepressus. No other species were recorded in addition to these. The degree of infestation by secondary pests was very low in general. Only in individual cases or at the end of the storage period, a slight increase in the density of pests was observed.

Populations of useful insects

Only very low densities of the most important useful insects could be recorded for the short storage period in 1990. There were frequently times when there were no specimens of some species in the samples. The spectrum of species again included X. flavipes, T. elegans, A. calandrae, C. waterstoni and H. sylvanidis.

2.4.2 Damage and loss situation

In comparison to the other storage periods, the level of damage and loss in the short storage period in 1990 was generally very low. The greatest damage occurred in Gbonvé-I with 19.8% (Fig. 11). in Gbonvé-II and Davié-II, the damage recorded was of approximately the same level, whilst in Davié-I (Fig. 12), damage amounted to only 13%.

Losses remained at their initial level of 1-2% in all stores during the first 3 months but then gradually increased After six months of storage, the highest losses were recorded in Gbonvé-I at 11.7%. The losses in Gbonvé-II and Davié-II were just under 10% at this point, whilst only 5% was recorded in Davié-I.

The production of boring powder was also comparatively low in the short storage season of 1990. The highest figures were found after 6 months in Davié-II and Gbonvé-I with 143.7 g and 119.3 g respectively. Extremely little boring powder occurred in Davié-I with a maximum of 15.8 g. A striking fact was the very rapid increase in boring powder in Gbonvé-I (Fig. 11), Gbonvé-II and Davié-II in the last two to three months of storage. Since there was also a rapid increase in the population density of the Larger Grain Borer over the same period, the boring activities of this pest cam be seen as the cause of the boring powder, and consequently also of the increase in damage and losses.

Fig. 11: Damage and loss situation in the maize store at Gbonvé-l in the short storage season in 1990

2.5 Progression of infestation in the maize store

When storing maize as cobs with the husks, investigation of infestation by pests also has to take their distribution on individual cobs into account if infestation, even with a very high population density, were limited to only a few cobs, the loss for the farmer would presumably be lower than if the same population were evenly distributed over all the cobs.

In defining the extent of the primary pests P. truncatus and S. zeamais in the maize store, only those cobs containing at least 3 beetles and one infested grain were defined as being infested. This definition could possibly lead to the infestation being underestimated, but is should prevent overestimation due to specimens occurring "by chance". in each case, the same number of cobs was included in the evaluation as in the loss analysis according to the count and weight method (Chapter 2.13 ). Only the results for the long storage period in 1988/89 are given here.

Fig. 12: Damage and loss situation in the maize store at Davié-l in the short storage season in 1990

Infestation by P. truncatus in Cacaveli in the long storage period in 1988/89 was significant (Fig. 13). in Kovié-Mission-Tové, one of the 4 stores examined showed a low degree of infestation which, when forming averages, leads to a mean level of infestation of almost zero.

P. truncatus settled in the first cobs within the first 6 weeks in Cacaveli (Fig. 13). In the following 1.5 months, infestations became so intense that after 3 months of storage 35% of the cobs were already infested. In this phase, the main reason for infestation is presumably due to immigration and not to offspring being produced in the store. In the following months of storage, the proportion of cobs infested by P. truncatus continued to increase until it amounted to 81% after 8 months.

Fig. 13: Dispersal of Prostephanus truncatus in maize stores in the long storage period in 1988/89 (proportion (%) of cobs with at least 3 beetles and one damaged grain; average from 4 repetitions)

S. zeamais had entered the store from the field and had infested up to 8% of the cobs at the time of going into storage (Fig. 14). S.zeamais spread considerably during the first three months of storage. The proportion of cobs infested rose to values of between 27% in Davié, and 55% in Kovié-Mission-Tové. The subsequent spread of S. zeamais up to the end of the storage period was considerably more reserved, and, in some stores, even stagnated. After 8 months of storage, 40% to 67% of the cobs were infested by S.zeamais depending on the location. The high degree of infestation in Avéta is mainly due to one store which was inside the farmer's house and contained am above-average population of the pest.

Fig. 14: Dispersal of Sitophilus zeamais in maize stores in the long storage period in 1988/89 (proportion (%) of cobs with at least 3 beetles and one damaged grain; average from 4 repetations)

2.6 Structure of the damage to the maize cobs

The damage and loss situation occurring In farm-level storage is normally defined using the values "damage" and "dry-weight loss" (see Chapter 2.1.2). These parameters are stated as average values for a random sample from a store. They give an impression of the extent of losses the farmer faces. In addition, their standardisation is an excellent aid in scientific work and allows comparisons between different stores and different investigations. However, the significance of these values in the losses relevant to the farmers is limited. Here, an attempt was made to illustrate the damage by analysing its structure to provide a more realistic picture.

Whilst shelling the maize, the farmer decides on the use of the maize by looking at the individual cobs. If the cob shows extensive damage, it is used for foodstuff (cf. also PANTENIUS, 1987). With medium damage, some of the grains are sorted out and, with hardly any damage, all the grains are used for human consumption (personal observation).

On the basis of the farmer's procedure, the cobs were classified into categories (see Chapter 2.1.2). This kind of classification can give an idea of the damage from the viewpoint of the farmer and of the use to which individual cobs are put. In addition, an analysis of the damage structure can give an impression of the influence of primary pests on the quality of the damage.

Tables 3 and 4 are examples of the damage structure in Cacaveli and Davié, in the long storage season in 1988/89. The results of the other stores and storage periods can be found in HELBIG (1993). Cacaveli is a location with a high degree of infestation by P. truncatus Davié, showed no infestation by this pest (see Chapter 2.2.1). The maize in the four stores at each location was of the same origin and, consequently, showed the same damage structure when stored.

A change in the structure of the damage was evident in Cacaveli after only 1.5 months (Tab. 2). Cobs in categories 4 and 5 occurred with a frequency of 4%. This pattern continued in the following months. The proportion of cobs showing damage of over 75% grew from month to month. After 8 months of storage 75% of grains on 66% of cobs were damaged.

The striking fact is the strong concentration on the extreme values. 90% of cobs in every storage period were allocated to categories 1, 2 and 5. Only 10% fell into categories 3 and 4. This shows the enormous damage potential of P. truncatus. When a cob is infested, damage of over 75% is caused within a short time. It can be assumed, regarding the damage, "hat from one month to the next, cobs in categories I and 2 move into categories 3 and 4. At the same time, cobs in 3 and 4 enter category 5.

In Davié, where there was no infestation by P. truncatus, the analysis of the damage structure showed a completely different picture (Tab. 3). The increase of damage occurred far more slowly in respect to the total number of infested cobs as well as the damage level on the single cob. The number of non-infested cobs remained at the same level throughout the total storage period.

Where cobs were infested when being stored, the damage increased. The proportion of cobs in category 2 slowly fell whilst this increased in category 5. There were far more cobs in the middle categories 3 and 4 than in Cacaveli. After 8 months of storage, about 20% of the cobs showed damage of more than 75%. In contrast to this, about 60% of the cobs showed no or little damage.

Tab. 2: Development of the damage structure during storage of maize cobs when infested by Prostephanus truncatus in the long storage period in 1988/89 at Cacaveli (proportion (%) of 120 (0 and 1.5), i.e. 200 cobs per month in the damage categories)

Tab. 3: Development of the damage structure during storage of maize cobs when not infested by Prostephanus truncatus in the long storage period in 1988/89 at Davié (proportion (%) of 120 (0 and 1.5), i.e. 200 cobs per month in the damage categories)

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