Part 1.2

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INTEGRATED CONTROL OF MAIZE STORE PESTS WITH SPECIAL REFERENCE TO THE LARGER GRAIN BORER: EXPERIMENTAL STUDIES IN BENIN AND TOGO

C. ADDA¹, C. BORGEMEISTER^1,2, A. BILIWA³ & M. ABOE⁴

1	International Institute of Tropical Agriculture (IITA), Benin
2	Institut für Pflanzenkrankheiten, University of Hannover, Germany
3	National Plant Protection Agency, Lomé, Togo
4	Regional Action Centres for Rural Development (CARDER), Lokossa, Benin

Introduction

The loss recorded in the stores of small farmers in the aftermath of the introduction of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) in Africa was very high, exceeding 20% of harvest after six months (Hodges et al., 1983; Keil, 1988; Pantenius, 1988; Borgemeister et al., 1994). Several control methods were put in hand to remedy this critical situation. In this regard, the application of binary insecticides (combining organophosphates and pyrethroids) was proposed and tested successfully (Golob, 1988; Biliwa et al., 1987; Fandohan et al., 1992). But in some countries like Benin, a low adoption rate was observed, possibly because of the socio-economic situation and the difficulties encountered in the distribution of the said products. (Adda, 1991; Agbaka, 1996). Rather what was noted was the indiscriminate use of banned products or cotton pesticides by the farmers. Given this situation, the desire to avoid the risks of pollution and toxicity created by such practices in the rural sector stimulated research into biological control (Böye et al., 1988). The predator, Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) was identified in Central America (Haines, 1981; Böye, 1988) and its effectiveness was promptly demonstrated (Rees, 1985). The research on the peculiarities of the predator carried out both in the laboratory (Pöschko, 1993) and in rural stores (Camara, 1996) led to the conclusion that although the predator feeds on other preys, it far prefers P. truncatus.

To complement the planned biological control which in reality only targets one of the major maize store pests, particularly under the conditions prevailing in Southern Benin and Togo, the Larger Grain Borer project of the International Institute of Tropical Agriculture (IITA) identified a control approach, based on the results obtained from different experiments earlier carried out in the project. The use of local varieties in combination with timely harvest of maize, the sorting of cobs before storage, and general sanitation of the storage space are considered to be good options for the on-farm preservation of maize (Borgemeister et al., 1994; Meikle et al., 1998; IITA unpublished data). In order to better appreciate the storage system thus proposed by IITA, it was necessary to compare it with other existing systems, i.e. traditional practices of the farmers and the recommendations of the national extension services (represented here by the Service de Protection des Végétaux [SPV]), and to allow the different stakeholders, especially the farmers, to participate so that they also can arrive at their own conclusions which will then prevail. It must be noted that the varieties planted, the storage unit used, the storage method adopted, the extent of infestation by the insects, especially P. truncatus, the agricultural practices of the farmers, etc. are so varied that indigenous maize storage and preservation systems differ from one region to another, which justified the decision to carry out the trial in different agro-ecological zones of Benin and Togo, two West African countries seriously threatened by the Larger Grain Borer.

Materials and Methods

The studies were carried out in places that are most threatened by P. truncatus, and always started with a survey of the maintenance method and questionnaires, with a view to acquiring information on indigenous agricultural practices. Based on the information gathered, a storage system referred to as the ‘Farmers’ system’ is compared with the ‘IITA system’ and the ‘SPV system’ which originated from the SPV. The farmers in the selected locations participate actively in the work to be done, from establishment and farm maintenance to maize harvesting and storage, and eventually in the participatory evaluation of stocks in the last months of storage. The same applies to the SPV Officers of the countries concerned. The maize is grown on plots of land (3 to 4 ha), negotiated from the farmers who have previously been sensitised on the study being carried out. The farms are fertilised at a rate of 150 kg of NPK (15-15-15) per hectare.

Other experiment sites and each type of storage system selected is represented in a completely randomised design by three identical granaries with a minimum of 500 kg of maize (in husks). In order to evaluate stocks scientifically every month, and over a storage period of eight months, these samples are used to estimate the damage and loss using the counting and weighing method (Harris & Lindbald, 1978; Boxall, 1986); the moisture content of grains is measured by using the standard oven-drying technique (ISO, 1980), and the population density of insects is determined after carefully sifting the grains. The method used to gather the data being inadequate or inaccessible to the farmers, participatory evaluation of the granaries is usually carried out between the 6th and 8th month in storage, at a time when P. truncatus infestation is very high. This participatory evaluation involves taking three samples (healthy, moderately attacked, or severely attacked) of 100 cobs from each store, which are then weighed and shelled, and the weight of the grains is recorded according to category. Finally, the grains are sorted out and the insects are systematically collected. The percentage of healthy cobs per sample allows the farmer to determine the best storage system for stock protection.

Experiment in Benin

In Benin, the studies were carried out in the southern part of the Mono region (more precisely Dogbo situated 125 km north-west of Cotonou) characterised by a Southern Guinea Savannah climate, with a hot and humid weather, two rainy seasons and two dry seasons, and where P. truncatus made its appearance about ten years ago; and Banikoara (in the Borgou region, north-west of Benin, about 800 km from Cotonou) located in the Sudan Savannah region with a hot and dry climate, one rainy season and one dry season where P. truncatus was first reported about 4 years ago. The maize varieties used are:

^*	the local variety Gbobe (very common in South-west Benin) which is a floury early-maturing variety (90 days) but has a relatively low yield (less than 2 t per ha);
^*	the improved DMR variety (Downy Mildew Resistant) is a semi-flint variety with a short cycle (90 days) made available to farmers by national extension officers in the rural area. This variety has an average yield of 2.5 t per ha;
^*	the improved TZSR-W variety (Tropical Zea mays Streak Resistant White) is a flint variety with a long cycle (120 days) and a yield that can reach or exceed 4 t per ha.

The trial at Dogbo involved three treatments, replicated three times. These treatments are as follows:

^*	IITA system = Gbogbe local variety (in husks), early harvest and sorting of cobs for visual damage (i.e., damaged cobs were discarded for storage);
^*	SPV system = DMR variety (in husks), early harvest and application of Sofagrain (i.e. the binary insecticide, currently recommended for treating maize in store by SPV);
^*	Farmer's System = Gbogbe variety (in husks), late harvest (no sorting and no Sofagrain application);

The trial at Banikoara involved four treatments, replicated three times. These treatments are as follows:

^*	IITA system = Gbogbe local variety (in husks), early harvest and sorting of cobs for visual damage (i.e., damaged cobs were discarded for storage);
^*	SPV system = DMR variety (in husks), early harvest and application of Sofagrain (i.e. the binary insecticide, currently recommended for treating maize in store by SPV);
^*	Farmer's System = TZSR-W variety (in husks), late harvest (no sorting and no Sofagrain application);
^*	SPV/2 system = TZSR-W variety, late harvest, the cobs were dehusked, application of Sofagrain.

Experiment in Togo

In Togo, the experiment was carried out in Atakpamé (about 175 km north of Lomé) in the plateau region characterised by the Sub-Guinea Savannah climate, where P. truncatus was identified as the most virulent maize store pest. The survey of farmers in this region revealed a general practice of storing maize with other crops (beans, sorghum, etc.), thereby forcing most of the farmers to harvest their maize early. The varieties of maize used during the test are:

^*	the improved NH1 which is a flint variety with a long cycle (120 days); it has a relatively high yield (3 to 4 t par ha);
^*	the improved Ikenne variety which is a semi-flint variety with a short cycle (90 days) and high yield (3 to 4 t per ha);
^*	a local floury variety with a short cycle (90 days), with a relatively low yield (less than 1.5 t per ha).

The experiment was replicated three times using the following six treatments:

^*	System I = NHI improved variety (in husks), early harvest and sorting of cobs for visual damage;
^*	System II = Ikenne improved variety (in husks), early harvest and without any protection method (farmers’ system);
^*	System III = local variety (in husks), early harvest and sorting of cobs for visual damage (IITA system);
^*	System IV = Ikenne variety (in husks), early harvest and sorting of cobs for visual damage;
^*	System V = Ikenne variety (in husks), early harvest and application of Sofagrain (SPV/1 system);
^*	System VI = Ikenne variety (stored as grains after 3 months), early harvest and application of Sofagrain (SPV/2 system).

Results and Discussions

The studies carried out in the Mono region south of Benin revealed that the presence of P. truncatus fluctuated slightly during the 1995-96 storage period, suggesting the strong presence of T. nigrescens whose population has increased tremendously in the stores. There was no difference in the loss recorded in the three systems until the sixth month when the loss recorded was lower than 8% (Fig. 1). This result was much less unsettling than the previously recorded 20% loss in traditional granaries in the same period, for the same duration in storage, and in the same region (Adda, 1991). This low loss record was confirmed during the 1996-1997 storage period, where up till the eighth month in storage, losses, even in the "Farmers system", were less than 10% (Fig 2). The low incidence of P. truncatus in the granaries treated with Sofagrain did not stop the latter from recording high losses, a fact that must therefore be attributed to Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) which was probably present in many cobs before Sofagrain powder was used.

Fig.1.

Loss pattern during the IPM trial in Dogbo, southern Benin, main storage season 1995-1996 (for details of the treatments see materials and methods section).

In Banikoara, northern Benin, the fluctuation of insects was generally very low, thereby explaining the low loss rate (less than 0.5%) obtained in six months of storage in all the storage systems. At this time, the population of P. truncatus was almost zero in all the granaries, and it was only in the eighth month of storage that a considerable number of P. truncatus was observed, but only in the ‘Farmers’ system’ (Fig. 3), followed by a slightly higher loss rate of as much as 2.5%, but which is still insignificant when compared to the situation in the Mono Region for the past few years.

According to the work done in Atakpamé (Togo) during the 1996-1997 storage period, the incidence (low at the time) of P. truncatus was noted between the fourth and the seventh month in storage (less than 15 P. truncatus per 1000 g of maize) only in System I (including the NH1 variety) and System II (‘Farmers’ system’).

Fig.2.

Loss pattern during the IPM trial in Dogbo, southern Benin, main storage season 1996-1997 in the ‘Farmers’ System’ treatment (for details of the treatment see materials and methods section).

Fig.3.

Grain Borer infestation during the IPM trial in Banikoara, northern Benin, 1995-1996 storage season (for details of the treatments see materials and methods section).

However, the population increased considerably in the eighth month of storage, exceeding a density of 700 P. truncatus per 1000 g of maize. The losses, which were then relatively low (barely over 7% in any of the storage systems until the 7th month) increased considerably, peaking at 20% especially in the ‘Farmers’ system’ (Fig 4). In storage systems III, IV, V, and VI where P. truncatus fluctuated very little up to the 8th month in storage, losses were maintained at a level lower than 8%.

Fig.4.

Loss pattern during the IPM trial in Atakpamé, southern Togo, main storage season 1996-1997 (for details of the treatments see materials and methods section).

These last results confirm the findings in the same year in the Mono region of Benin (see above). The participatory evaluation of the granaries at the end of the storage period (Fig 5) enabled the farmers in particular to observe that the granaries installed according to the ‘IITA System’ (System III) or the ‘SPV system’ (System V) recorded the lowest percentage of cobs infested by insects, which makes it obvious that they are the most effective storage system.

Conclusions

The work carried out in rural stores both in Benin and Togo between 1995 and 1997 made it possible for us to have an idea of the progress being made in the biological control of the Larger Grain Borer by its natural enemy, T. nigrescens. The development of an integrated storage management system can be easily realised on the condition that it is clearly specified for an agro-economical zone that has already been tested. When compared to the indigenous system, the integrated control system as defined by the Larger Grain Borer project, and to which other techniques like the use of local preservation products can be added, clearly facilitates the preservation of a larger amount of stored products at less cost, especially as it relates to long-standing traditional storage practices.

Fig.5.

IPM trial Atakpamé, southern Togo, main storage season 1996-1997: Participatory evaluation of the percentage of infested cobs after eight months of storage (for details of the treatments see materials and methods section).

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