The Larger Grain Borer(LGB), Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae)1 is unusual amongst major destructive storage pests in having a discrete, definable distribution. Whilst many pests of stored products have been distributed by trade and become cosmopolitan worldwide, LGB has only recently become a serious pest outside its presumed area of origin. Early records in the literature refer mainly to LGB infestations of maize, dried roots and wooden artifacts, imported to the United States and Europe from Mexico and Central America. Furthermore, LGB attracted widespread international attention in 1981 with the discovery of an extensive and destructive outbreak by this pest in farm-stored maize in Tanzania.
¹ See below for taxonomic description.
LGB occurrence in Africa apparently resulted from two separate introductions reported first from Tanzania, and another from Togo. A more recently outbreak in Guinea may represent a third discrete introduction, but details have yet to be confirmed. From Tanzania, LGB has spread into Kenya, where it currently affects only the southeastern parts of the country, Burundi, where infestation is widespread but apparently of low intensity, and into northern Malawi. From Togo, the pest has spread into Benin, Ghana, and more recently, specimens have been collected from Burkina Faso, in the area bordering Togo and from the capital Ouagadougou, and in Nigeria. From its known distribution in the countries of eastern Africa it is likely that Rwanda, Uganda and Zaire are potentially affected (?), whilst Mozambique and Zambia are at high risk.
Impact of LGB in traditional farming systems:
LGB is capable of infesting maize before harvest. In Africa (unlike Mexico) field infestations are difficult to detect though newly harvested cobs may be heavily damaged. Harvesting as early as possible will reduce preharvest problems. However, in some areas of Africa where LGB occurs, farmers traditionally leave their crop in the field for many months before harvesting. This is clone to allow adequate drying of the cobs, and so that other farm operations, such as harvesting cotton or tobacco, can take place.
Losses in maize left in store for 6 months or more have been found to exceed 35% of its weight when infested with LGB. However, the real losses sustained by individual households on farms are likely to be less than this because the quantity of food diminishes as grain is removed for consumption. Spot observations on maize after about 6 months storage in villages in western Tanzania showed losses averaged 9%, and it was estimated that by the end of the storage season losses would exceed 15%. This compares to average losses in this part of Africa, when LGB is absent, of less than 1% during the first 6 months post harvest (during the season) and less than 5% for the whole year. Initial indications from Togo are that losses in cassava infested with LGB are much greater; 73% has been lost over one year.
Although shelled maize can be heavily damaged by LGB, the beetle is more successful on maize stored on-the-cob. The female requires a stabilized surface in order to lay eggs, this it obtains in cob maize, as the grains are firmly cemented to the core. Similarly the weight in a 100 kg bag of grain also provides sufficient stability to allow effective development. Many farmers store maize on-the-cob and this tradition predisposes towards heavy LGB damage. The current control strategy in East Africa necessitates shelling both to provide a less than optimal environment for the insect, and to enable the efficient application of insecticide. Shelling, therefore, is a move away from tradition and has required the development of a new extension package for farmers, including the introduction of alternative storage containers. There is still scope to develop methods of protecting stored produce which are more compatible with traditional farming practices. An ongoing Overseas Development administration financed, Natural Resources Institute (ODA/NRI) project with the Kenyan Agricultural Research Institute (KARI) is evaluating the efficacy of treating cobmaize with a range of organophosphorus/pyrethroid emulsifiable concentrate (ec) formulations, and so far, results have been promising. (See Technical Section 3.4, these Guidelines).
LGB has been shown to have very wide biological tolerances for development. These wide ecological tolerances have since been demonstrated in studies by NRI in Mexico which have shown that LGB can occur from sea level to some 2500m altitude, in habitats ranging from semi-desert to high rain forest. The use of commercially available pheromone-baited flight traps specific to LGB has also revealed that it is much more widely and continuously distributed than previously suspected, when assessment was based solely on sampling of stored commodities alone. LGB has now been recorded in high altitude areas of Burundi (> 1800m) away from the high risk areas near the common border with TANZANIA, and there are recent indications (that now confirm) it may already be established in RWANDA.
Some records of LGB from pheromone-baited flight traps indicate that it can occur far from known sites maize production and storage. Moreover, longer-term studies of flight activity suggest that substantial numbers of adults can be caught in areas of natural vegetation throughout the year, apparently unrelated to the maize harvest and storage cycle. This is currently under evaluation by the Plant Protection Service (PPS) in BURUNDI, through support by an ongoing UNDP/FAO project. LGB is therefore widely distributed in Meso-America2 not only in the store and in maize fields, but also in many other environments including forest, grassland and in plantation crops. There is evidence that this diversity of habitat is beginning to be reflected in Africa. The beetle has been found in hardwood forests and in game reserves in East Africa. Expansion into alternative hosts will make control more difficult and may also enable agricultural crops to be infested more easily.
2 Meso America = Mexico and Central America
Given its strong dispersal capacity and prevalence in non-maize habitats, quarantine restrictions such as bans on the movement of produce, are likely to be of only limited value, as well as being difficult and expensive to enforce. The main value of improving phytosanitary standards should he seen in delaying the long-distance spread of Larger Grain Borer via the international grain trade. Regular monitoring via the establishment of networks of pheromone-baited flight traps, combined with periodic surveillance of stores, would, however, seem to be of particular value. Timely warning of Larger Grain Borer infestation in a particular area would alert extension services and farmers to the potentiality of high food losses, and allow them to take the necessary precautions to minimize these losses if they should subsequently occur.
Most of the LGB-infested areas in Africa are marginal maize producers where farmers store for relatively short periods, and only for their own consumption. In the future, the pest will likely spread to areas where maize is of more commercial value, and where large quantities are stored for long periods. The advent of market liberalization will remove barriers to private trade, causing much more flexible movement of produce both within and between countries. Inevitably those countries which are not infested with LGB will think in terms of placing restrictions on grain imports from countries that are infested. Such restrictions will further hamper marketing opportunities for farmers and traders. It is also likely that movement within countries may also he restricted to prevent grain passing from infested areas into, or through, uninfested areas. Additionally, farmers who might wish to store for long periods in order to take advantage of higher prices prevalent at the later stages of the storage season, may find difficulty in doing so because of the severity of LGB infestations. Although it is presently the marginal and deficit farmers who are affected all farmers will inevitably be forced to introduce protective measures to counter the LGB problem.
The species of the Bostrichidae are principally wood-borers but some will breed in stored products and a few are important pests. The general form of the adult Bostrichidae, illustrated in Plate 13a, is rather uniform and typical of boring beetles. The body is cylindrical and the head ventral to the prothorax so that it is not visible from above. Characteristically, the pronotum has rasp-like teeth, hooks or horns, the antennae are straight, i.e. not elbowed, with a loose 3- or 4-segmented club (Plate 13a). At the posterior of the beetle the elytra are usually somewhat flattened and slope more or less steeply downwards: this sloping region is called the declivity. The elytral declivity is frequently decorated with ridges, tubercles or hooks, all of which are useful recognition features. The tarsi are all five-segmented (the first is criptic).
Key
1. - Posterior tarsus always shorter than tibia. Anterior region of pronotum with large hooks or horns on the pronotum or elytra. Length 2.5-4.5 mm. (Principally storage species) - Dinoderinae - 2- Posterior tarsus never shorter than tibia. Pronotum or elytra frequently decorated with large hooks or horns. Length exceeding 4.5 mm. (Mainly wood-boring species, but some pests of dried root crops.) - Bostrichinae
2. - Posterior half of pronotum with flattened tubercles (Plate 1.1b); elytral declivity gently convex (Plate 1.1c) and all elytral hairs distinctly curved. General appearance as in Plate 1.1a. (Common pest throughout warm and tropical regions; infesting various commodities, especially whole cereals) - Rhyzopertha dominica (Fabricius)
- Posterior half of pronotum without flattened tubercles but with punctures (Plate 1.1d and f); declivity strongly convex or flattened and steep (Plate 1. 1e and g) and elytral hairs all erect or only erect on apical one-third of elytra - 3
3. - Elytral declivity steeply sloping but without ridges or ornamentation. Posterior region of pronotum often with two more-or-less distinct shallow depressions (Plate 1. Id). Punctures of elytra irregular, not arranged in rows. (Common in tropics; frequent pest of felled bamboo, but occasionally infest maize and dried cassava.) - Dinoderus sp.
- Elytral declivity flattened and with pronounced curved ventro-lateral ridges forming a semicircle when viewed from behind (Plate 1.1h). Posterior of pronotum without shallow depressions (Plant 13f). Punctures of elytra arranged in longitudinal rows. (found in South and Central America, East Africa and West Africa as an important or serious pest of maize and cassava.) - Prostephanus truncatus (Horn)
Plate 1.1. Insects: Coleoptera - Bostrichidae
(a) - Rhyzopertha dominica - dorsal view (life size 2.0-3.0 mm).
(b) - Rhyzopertha dominica - dorsal view of prothorax, showing flattened tubercles on posterior half.
(c) - Rhyzopertha dominica - lateral view, showing gently convex declivity of elytra.
(d) - Dinoderus minutus - dorsal view of prothorax, showing punctured posterior half and two foveae (arrowed).
(e) - Dinoderus minutus - lateral view, showing strongly convex declivity of elytra (life size 2.5-3.5 mm).
(f) - Prostephanus truncatus - dorsal view of prothorax.
(g) - Prostephanus truncatus - lateral view, showing flattened declivity of elytra (life size 3.0 4.5 mm)
(h) - Prostephanus truncatus - posterolateral view, showing curved carinae (arrowed) at apex of elytral declivity.
(illustrations not all to same scale).
