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BIOLOGICAL CONTROL OF THE LARGER GRAIN BORER, PROSTEPHANUS TRUNCATUS (HORN) (COLEOPTERA, BOSTRICHIDAE) IN GUINEA

T.H. BALDE

Division of Plant Protection (DPV), Conakry, Republic of Guinea


Introduction

Maize is a very important product both for human consumption and chicken feed in the Republic of Guinea. It is sometimes used as fodder for cattle. Available data show that the cultivation of maize is a major occupation in Middle and Upper Guinea. According to the preliminary findings of a survey carried out in 1995, production is estimated at 107,700 tons in an area of 104,300 hectares, and an average yield of 1.03 ton/ha, which is much lower than world standard which is fixed at 3.7 tons/ha.

In Middle Guinea, maize is very common in the highlands as a semi-intensive crop that is often inter-cropped with tubers and vegetables. In Upper Guinea, it is cultivated out-of-season in the lowlands, while during the rainy season, it is inter-cropped with rice on hillsides. In Lower Guinea, it is cultivated in the lowlands. The cultivation of maize is not very popular in the forest areas of Guinea.

Originally from America, the maize cultivated in Guinea is infested by many pests, the most important of which are: Fusarium spp., Ustilago maydis, Helminthosporium spp., Alternaria spp., Puccinia maydis, Cladiosporium spp., weeds, nematodes, rodents, birds and insects (Zonocerus variegatus, stem borers and store pests, etc.).

In an effort to ensure the regular availability of maize all year round, producers of maize look for ways of storing it. However, the quality of the maize deteriorates in storage as a result of pest infestation, the most common of which are microorganisms, rodents, mites and insects.

The Larger Grain Borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), is a recently introduced store pest in Guinea. Since its appearance in Middle Guinea in the 80s, the Larger Grain Borer identified by the Plant Protection Service and Laboratory has become the most virulent pest of maize stores and dried tuber chips stocks in the rural environment.

The investigations recently carried out in Middle Guinea recorded an average of 20% loss of dry matter, and even total loss of stock in extreme cases, after 6 months in storage. In the absence of appropriate insecticides to control these maize store pests, the measures applied by the farmers to reduce damage in the affected areas are limited to traditional methods (application of wood ash, peels of dried citrus, pepper powder, etc.). Consequently, classical biological control would appear to be the better option in solving the problem posed by the larger grain borer.

In order to develop and carry out a sound and sustainable control of store pests in cassava, cowpeas, maize, banana and other food crops, Guinea, through its Biological Control Programme (PGLB), is enjoying the support of the International Institute of Tropical Agriculture (IITA). The Larger Grain Borer Department, created within the PGLB in 1992, first concentrated on a survey of the distribution of the pest. Recent results obtained in south-west Benin and in Kenya showed that the exotic predator, Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) has effectively reduced the infestation of the larger grain borer (C. Borgemeister, IITA, pers. com.). In view of these encouraging results, the Larger Grain Borer Department of the PGLB established in January 1996 a unit order to mass rear the predator in the Plant Protection laboratory in Kindia. More recently, some pre-release surveys of the predator were carried out in the most infested areas by the PGLB in collaboration with IITA. The first releases of T. nigrescens took place in August 1997.

Achievements

A lot of activities were carried out in pursuit of the integrated control of the larger grain borer.

Distribution of the Larger Grain Borer in Guinea

The different surveys carried out on the distribution of the larger grain borer since the discovery of its primary range, Timbi-Touni in the District of Pita, facilitated the coverage of the different provinces of Guinea, three of which are border townships: Koundara (Termessč), Gaoual (Foulamory) and Forécariah (Pamalape) sharing a border with the Republics of Senegal, Guinea Bissau and Sierra Leone respectively, as well as two ports: Conakry and Kamsar. The results obtained from the trappings show a widespread infestation in all the Fouta region, the headquarters of its primary range. In Lower Guinea, out of the five localities investigated, the pest was only found in Télimélé which shares a border with Middle Guinea, and in Kindia, precisely at the National Seed Centre which provides most of the seeds used by various projects and farmers groups.

Mass rearing of the predator T. nigrescens

In November 1995, the Guinea Biological Control Program (PGLB) requested for specimens of T. nigrescens as well as a starter culture of this predator from IITA. Since January 1996, the predator has been mass reared in the national Plant Protection laboratory at Kindia. From 300 adult T. nigrescens, 9000 active insects were obtained before the first releases in August 1996. This result shows that it is possible to achieve mass production of T. nigrescens in order to ensure adequate provision of the predator for subsequent releases.

Pre-Release Surveys

As a prelude to the introduction of the natural enemy of the larger grain borer, two surveys were jointly carried out in the Labé region between March and April of 1996 and 1997 by the PGLB and the Larger Grain Borer Project of IITA-Benin. The objectives of these surveys were to:

to closely monitor the present state of the incidence of P. truncatus in the Labé region, through the use of pheromone traps, and especially through thorough inspection of traditional maize stores;

to prepare the T. nigrescens release campaign after identifying the most appropriate zone for such an operation;

to collect pre-release data related among others to the damage and losses recorded on the maize and to the population density of insects, especially Pt.

In 1996, a total of 30 traps were distributed (then withdrawn at the end of a week) in a radius of 50 km around the central part of Labé, along the major road axis to the West, East, North and South (see Tab. 2 for the data on trappings and releases of T. nigrescens in the Labé region). In 1997, 27 pheromone traps were laid, following the same axis as in 1996, and 33 granaries were surveyed for maize stock samples as against only 26 in 1996. In 1996, the survey carried out in traditional stores focused only on the maize that had already been shelled, winnowed and preserved in bags where P. truncatus and other store insect pests were multiplying rapidly. Samples of 500 grams of maize grain were removed from each store after sifting out the sample in order to separate healthy grains from those already infested. In 1997, 500 grams of maize were removed from 30 available cobs, generally in the form of seeds to estimate damage and loss. The percentage of damage was calculated by multiplying the number of infested grains by hundred and by dividing it by the total number of grains.

It must be noted that before the 1996 sampling, many of the infested grains or those reduced to powder had already been eliminated by cleaning of the maize by the farmers. The survey zone was divided into: Labé North, Labé South, Labé East, Labé West and Central Labé in order to ensure a better interpretation of results (see Tab. 1 and 2). Tab. 1 shows that P. truncatus population fluctuation in 1997 was slightly lower than that of 1996.

Tab.1.
Result of pheromone traps set in the Labé region of Guinea
(period of trapping March and April 1996 and 1997, respectively).

1996

No. of P. truncatus in the traps

Zones

S

traps/zone

0

1-50

51-150

>150


North
South
West
East
Central
Total

5
4
9
9
2
29

0
0
0
0
0
0

1
1
1
4
0
7

3
3
4
5
0
15

1
0
4
0
2
7


1997

North
South
West
East
Central
Total

5
6
7
7
2
27

0
0
0
0
0
0

2
5
6
7
0
20

3
1
1
0
1
6

0
0
0
0
1
1


Although the insect was present in all the areas surveyed, almost 75% of the traps (20 out of 27) captured relatively low numbers (less than 5 P. truncatus per trap). In 1996, only 27% of the traps (7 traps out of 26) felt in this category. The only trap with more than 150 P. truncatus was set-up on the Labé market. This confirms that maize trading centres are active transmission channels for the pest, and therefore aid its propagation.

The northern and western zones of the Labé region are the most threatened zones (Tab. 2). While 100% and 80% of surveyed stores were infested by P. truncatus in the western and northern zones of Labé, respectively, around 43% and 50% of the stores were affected in the southern and eastern zones, respectively. The average percentage of loss recorded in the western zone of Labé is about twice that recorded in the other zones. An exceptional maximum loss of about 27% was recorded in the Western zone.

The rather reduced pest complex was dominated by P. truncatus, which is responsible for the greater part of the damage observed. Among coleopteran, the following were identified in numerical order: P. truncatus, Tribolium castaneum, Sitophilus spp. and Oryzaephilus spp. (the latter has become very scarce.

In some stores considerable damage was caused by Sitotroga cerealella and occasionally by Plodia interpunctella. The maize grains had low moisture contents, which probably explains the weak presence of Sitophilus spp. and the absence of S. cerealella in some areas. It should be noted that the before mentioned insect densities are clearly less than in reality, because before bagging the maize, the farmers had shelled and winnowed the commodity and thereby allowing a great number of insects to escape.

Tab.2.
Summary of the data on P. truncatus trap catches and releases* of T. nigrescens
in the Labé region of Guinea in 1997.

Trap sites

Sub-districts

Districts

Zones

No. P. truncatus


Central Pita
Kokoulo
Daradhepole
Lengui Petel
Bouroundji
Central Laafou
Koreyghani
Bourouwal Korbe
Central Lélouma
Central Popodara
Antenne
Market
Toury
Kourahoe Lingue
Sarekali
Sireya
Central Dionfo
Lappi
Balafoya
Moukijigue
Samaya
Djibery
Djaberepili
Wenduseguinta
Telire
Madina Maleri
Gosso

Central Pita
Central Pita
Daralabé
Popodara
Popodara
Laafou
Koura Mangui
Korbe
Central Lélouma
Popodara
Central Labé
Central Labé
Tountouroun
Tountouroun
Tountouroun
Kalan
Dionfon
Tangaly
Tangaly
Koin
Kankalabé
Kankalabé
Bodié
Kebaly
Bourouwal
Sannou
Sannou

Pita
Pita
Labé
Labé
Labé
Lélouma
Lélouma
Lélouma
Lélouma
Labé
Labé
Labé
Labé
Labé
Labé
Labé
Labé
Tougoué
Tougoué
Tougoué
Dalaba
Dalaba
Dalaba
Dalaba
Pita
Labé
Labé

South
South
South
West
West
West
West
West
West
West
Central
Central
North
North
North
East
East
East
East
East
East
East
South
South
South
North
North

57
8
16
39
19

47
10
28
65
34
114
287
37
57
91
44
2
47
28
13
21
2
34
21
103
29


*

Release sites of T. nigrescens in the Lélouma district were Laafou, Diountou Missidé, and Central Lélouma
and in the Labé district Sarékaly, Daleen, Hafia, and Central Labé.

Some investigations carried out show that P. truncatus exists in abundance, posing a serious threat for maize produced and stored in the Labé region. The lack of appropriate control measures and the absence of indigenous natural enemies for P. truncatus make the release of its exotic natural enemy T. nigrescens imperative.

First Releases of T. nigrescens

The first release of T. nigrescens, the natural enemy of the larger grain borer, was done in eight sites at a rate of 1,000 individuals per site. The sites were: Popodara, Laafou, Diountou-Missidč, Central Lélouma, Sarékali, Daleen, Hafia and Central Labé. Other releases have been scheduled in all the zones of the regions threatened by the pest.

Conclusions

The larger grain borer, discovered in 1987 in Pita Region, is today present in all the localities of the Labé region and in some maritime zones of Guinea. Maize is one of the most valuable food crops for a large part of the population in the concerned zones. Because of the serious nature of the damage caused by P. truncatus to stored maize stores, a control strategy must be put in place, based on sensitising farmers to modify some of their traditional maize production and storage practices (for example early harvesting, monitoring of the drying process, sanitation of the storage environment, etc.), while awaiting the successful establishment of T. nigrescens for effective control of the pest. Furthermore, as a result of the rapid spread of the larger grain borer, it is necessary that the Kankan and N'Zérékoré regions, so far considered free of the pest, are checked again, given the closeness of the affected regions and the time lapse since the first survey.

 

References

1.

Final Report of the Guinean Biological Control Programme (PGLB) Conakry, 1996.

2.

Larger Grain Borer (Prostephanus truncatus) in Guinea. Report of the activities of the Larger Grain Borer Division of the PGLB, 1996.

3.

Project proposals for the integrated control of the Larger Grain Borer (Prostephanus truncatus), stem borers and maize diseases in the Republic of Guinea, August 1996.

4

Mission Report - The Incidence of Prostephanus truncatus and a brief highlight on maize production and preservation in the Labé region. Fouta District - Republic of Guinea, 31st March - 10th April 1996.

5

Agricultural Development Policy circular No. 2 Volume 1. Executive Summary, March 1997.

6

Mission Report - Evaluation of the dispersal of Prostephanus truncatus and the level of infestation and depreciation of maize stock before releasing the natural enemy, Teretriosoma nigrescens in the Labé region. Fouta District - Republic of Guinea, 23rd March - 3rd April.

 

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