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2.1 The pest Prostephanus
truncatus (Horn) (Coleoptera:
Bostrichidae)
2.2 The predator Teretriosoma
nigrescens Lewis (Coleoptera:
Histeridae)
2.1 The pest Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae)
2.1.1 The spread of Prostephanus
truncatus
2.1.2 Damage and losses caused by
Prostephanus truncatus in
Africa
2.1.3 Ways of controlling Prostephanus
truncatus by chemical means
2.1.4 Ways of controlling Prostephanus
truncatus biologically
2.1.1 The spread of Prostephanus truncatus
Intensive dealing in foodstuffs, in particular wheat, has led to storage pests spreading almost across the world. P. truncatus is a recent example.
Originally native to tropical Central and northern South America, it presumably spread from Mexico after 1971 to East Africa (MUSHI, 1984; NISSEN et al., 1991). Not until a considerable population density had been reached, was it identified in Tanzania in 1981 (ANONYMOUS 1981; DUNSTAN & MAGAZINI 1981). In the meantime, the beetle has crossed the borders into the neighbouring countries of Kenya (KEGA & WARUI 1983), Burundi and Rwanda (LABORIUS 1988).
At the beginning of 1984 P. truncatus was discovered in a completely different region of Africa - in Togo. A survey among farmers afflicted led to the assumption that it had first appeared in West Africa in 1981 (KRALL 1984; HARNISCH & KRALL 1984). Today, the pest populations have already spread into the neighbouring states of Benin (DICK et al., 1989), Ghana (KRALL & FAVI, 1986) and Guinea (KALIVOGUI & MUCK, 1990).
Transport of foodstuffs from surplus regions into deficit regions across the African continent and the exchange of goods in traditional marketplaces promote the rapid propagation of this pest which is also able to fly (RICHTER & BILIWA, 1991).
2.1.2 Damage and losses caused by Prostephanus truncatus in Africa
P. truncatus is a pest with a high damage potential. The beetle infests maize cobs shortly before as well as after harvesting. Investigations in one village in Tanzania showed 20% of the stored produce to already have been infested shortly after harvesting. At the end of the harvesting period (after 5 -6 months) 80% of the granaries were infested by P. truncatus (HODGES, 1984). After being stored for 3 - 6 months, the maize cobs in storage showed a weight loss of up to 34%. The average weight loss was in the region of 9% (HODGES et al., 1983).
Extensive damage was also recorded in Togo. In some maize stores there were holes bored by P. truncatus in 100% of the cobs after 9 months in storage (KRALL, 1984). According to PANTENIUS (1988), an average weight loss of 30.2% occurred in maize stores after 6 months. Prior to the appearance of P. truncatus, the average overall loss in this region was 7.1%. The cobs had been damaged so badly by P. truncatus that the remaining corn substance was no longer suitable for human consumption, and would even not be eaten by cattle.
If the larger grain borer were to spread throughout all maize-growing areas in Kenya, projections show that an estimated 10% (100,000 t) of all stored maize (1 million tonnes) would be lost annually. This would be enough to feed over 100,000 people for one year and corresponds to an economic loss of over DM 30 million per year (LABORIUS et al., 1985).
P. truncatus also causes extensive damage to dried cassava roots (maniac). In one test store in Tanzania, losses of up to 70% in fermented and 50% in unfermented cassava were recorded after only 4 months of storage (HODGES et al., 1985).
2.1.3 Ways of controlling Prostephanus truncatus by chemical means
The use of insecticide to control P. truncatus is made very difficult by the local method of storing maize. The cobs with husks are normally stored under the roofs of houses or openly on wooden frames so that they can dry.
Contact and/or stomach poison is spread on inert powder materials or as liquids. They do not penetrate the substrate treated and can only provide protection on the surface of the stored produce. P. truncatus individuals already inside the cob before application are thus able to escape effective control.
In 1981/82, initial experiments were carried out in Tanzania using the insecticides available there. These proved to be extensively ineffective (GOLOB 1984; GOLOB et al., 1983). Later however, all insecticides belonging to the phosphoric acid-esters proved hardly suitable to control P. truncatus. In contrast, several long-term experiments using pyrethroids, like permethrin and deltamethrin on shelled maize grains, showed good results (LABORIUS et al., 1985; LABORIUS 1988). To provide additional protection from other pests, a pyrethroid was recommended in combination with a phosphorous acid-ester compound (pirimiphosmethyl or chlorpirimiphos-methyl) (GOLOB, 1988).
Fumigating the stored produce using methyl bromide respiratory poison (CH3Br) or hydrogen phosphide (PH3) is effective (HAASHEM & REICHMUTH, 1989; DETMERS, 1990, 1993) but requires a method which is far too extensive and unsuitable for practical use in stores on a small-farm level.
2.1.4 Ways of controlling Prostephanus truncatus biologically
In its native home, P. truncatus only occasionally causes considerable damage, whilst in Africa it has become a real threat to staple foods (KEIL, 1988; PANTENIUS, 1987, 1988; LABORIUS, 1990). Studies carried out to investigate this phenomenon produced initial bases for a concept on the biological control of P. truncatus (SCHULZ & LABORIUS, 1987; LABORIUS, 1988).
This biological control makes use of natural agents (viruses, bacteria, fungi), parasitoids or predators to combat organisms causing damage. As opposed to the disadvantages involved in chemical control:
- generation of resistance
- no long-term efficacy
- relatively unspecific effect
- additional pollutant for the environment
- risks during handling, biological control has many benefits.
The interaction of two biological systems occurs here with variable features reducing the risk of resistance to a minimum (OTTO, 1992).
The economical effect of many biological measures lies in their long-term effect. Successes are mainly a result of the import of useful arthropods (FRANZ & KRIEG, 1982).
Disadvantages include a long "initial phase" before a suitable method can be developed, and no 100% immediate efficacy.
The spectrum of microorganisms tested for P. truncatus produced only a narrow range of potential for a control programme. Of 80 bacteria strains isolated in the immediate environment of the beetle or in the insect itself, only 2 strains had a pathogenic effect on the pest.
Somewhat more successful was the search for pathogenic fungi isolated in dead insects which produced a fungus (Metarrhiziurn anisoplia) highly virulent to P. truncatus. There are still some unanswered methodical questions restricting direct application of the fungus, since it must be remembered that some varieties of fungus not only can become a hazard to insects but also to warm-blooded animals including man (SCHULZ & LABORIUS, 1987; BURDE, 1988).
Eight protozoans were discovered to be parasites of P. truncatus in Central America and Africa. However, but for one exception, they showed only a low rate of infection (LIPA & WOHLGEMUTH, 1986; LELIVELDT, 1990). Laboratory experiments using spores of Nosema sp. and Mattesia sp. did, in fact, lead to increased mortality in the P. truncatus populations, yet showed no similar results in experiments simulating actual conditions, which could have been developed into successful control methods (HENNING et al., 1992; HENNING, 1993).
Hymenopterous wasps (Pteromalidae) Anisoptera calandrae and Chaetospila elegans were observed to be biological opponents of P. truncatus in Costa Rica (BÖYE, 1988, 1990). Despite some successes in laboratory experiments, no further investigations were carried out as the breeding of the parasitoids turned out to be difficult and both species were already native to Africa (LELIVELDT, 1990; MARKHAM et al., 1991).
Described as the main, immediate antagonists of P. truncatus in Costa Rica were the predatory hister beetle T. nigrescens and a predatory bug (Calliodis sp.). Neither of these predators had so far been recorded on the African continent. Experiments showed that, in contrast to T. nigrescens, the predatory bug was only able to effectively reduce the number of eggs and larvae of P. truncatus there was a high population density (BÖYE, 1988; (BÖYE, et al., 1988). In more detailed investigations comparing the efficacy of the two predators, T. nigrescens seemed to be the most suited as a biological control agent for P. truncatus (BÖYE, 1990; LELIVELDT & LABORIUS, 1990).
2.2 The predator Teretriosoma nigrescens Lewis (Coleoptera: Histeridae)
2.2.1. The morphology, biology and ecology of Teretriosoma
nigrescens
2.2.2 Biological control of Prostephanus truncatus
using Teretriosoma nigrescens
2.2.1. The morphology, biology and ecology of Teretriosoma nigrescens
Belonging to the family of hister beetles (Histeridae), of which T. nigrescens is also a member, are over 3,700 species measuring only a few millimetres in size. The T. nigrescens imagines are also only about 2.3 mm in size (BÖYE, 1988; LELIVELDT, 1990). Approximately 80 species of hister beetle are native to Central Europe (JACOBS & PENNER, 1988). HINTON (1945) was only able to identify 14 species throughout the world in conjunction with stored produce. Evidence of T. nigrescens could so far only be found in Central America (Costa Rica) and in Mexico (BÖYE, 1988; REES et al., 1990). The first description of T. nigrescens was made by LEWIS (1891).
A striking feature of the Histeridae is their especially hard chitin shell which is mostly a shiny black. Only the forewings may bear some reddish marks. The hard forewings (elytra) do not completely cover the abdomen but are stunted and leave two of the abdomen segments uncovered. The antennae of the Histeridae are jointed and thicken up towards the tip like a club. The tibia of the first pair of legs is toothed and is the feature used to define the species.
Hister beetles are predators. They hunt insects which have their broods on dung, carcasses and the rotting remains of vegetation (JACOBS & PENNER, 1988). According to CHINERY (1973), the imagines presumably live on carcasses and rotting vegetable substrate. Most species of Histeridae can reproduce on several species of host. A minority have become dependent on a few or even on only one species of host (HINTON, 1945) due to their habitat or natural selection. Some strikingly flat species, for example, have become specialized in living on species of boring beetles under bark, others live in birds' nests on the ground where they feed on mites, flea or fly larvae. Many foreign species live on ants and termites.
According to investigations so far, T. nigrescens is a polyphagous predator with a distinct preference for P. truncatus as a host. Its ability to reproduce could also be proven on two species of Dinoderus, on Sitophilus oryzae, Rhizopertha dominica and Tribolium castaneum, however comparative experiments confirmed that P. truncatus is the host preferred by T. nigrescens (REES, 1987, 1991; LELIVELDT, 1990; PÖSCHKO et al., 1992 b).
Histeridae imagines can develop to 2 - 3 years of age and reproduce several times (JACOBS & PENNER, 1988). This also seems to apply to T. nigrescens. During long-term experiments, the T. nigrescens imagines reached an age of over 20 months. They were still able to reproduce after 16.5 months PÖSCHKO et al, 1992 a).
Initial investigations into the development cycle of T. nigrescens were carried out by REES (1985) who used P. truncatus populations on maize as host cultures for the predator. It took T. nigrescens 8 weeks (56 days) to develop from the egg stage to the imago at 26°C and 70% r. h.. LELIVELDT (1990) observed a cycle of less than 50 days for T. nigrescens at 30°C and 75% r. h..
The relatively large eggs (1.1 mm x 0.5 mm) were laid directly in the substrate individually. The larvae (2 - 3 mm long) hatched at 26°C after about 7 days (REES, 1985). At 30°C, one female specimen of T. nigrescens laid an average of 1.5 ± 0.9 eggs daily. The maximum amounted to 3 within 24 hours. The egg stage here lasted about 6 days, the larva stage about 26 days all together (LELIVELDT, 1990).
According to HINTON (1945), all species of Histeridae have three larva stages. REES (1985) and LELIVELDT (1990) were able to prove that there are only two larva stages, basing their studies on measuring the diameter of the head capsule of T. nigrescens larvae. The skin (cuticula) of the longish-shaped Histeridae larvae is normally milky white. Only the head and the dorsal area (tergite) of the thorax are brownish and have a firm exoskeleton (HINTON, 1945). The head of T. nigrescens larvae is flattened and bears crescent-shaped mandibles (REES, 1985). Histeridae larvae are predators according to HINTON (1945). They pupate in a hollow in the earth whose walls are secured by secretion emitted from the anus of the larva. REES (1985) observed that the larvae ready to pupate at 1.0 - 1.2 cm in length used their mandibles to extend their pupating chamber inside a maize grain which had already been damaged. The pupating stage took 17 - 18 days at 30°C (LELIVELDT & LABORIUS, 1990).
An interesting pattern of behaviour on the part of the Histeridae which was also practiced by T. nigrescens, remains to be noted here. When in danger, the insects fall into a kind of cramped paralysis, referred to as thanatosis.
The extremities, like legs and feelers are concealed in grooves on the underpart of the body. The head can also be almost completely drawn in to the fore part of the prothorax (Fig. 1; view through REM [raster electron microscope]). This paralysis makes the beetle uninteresting for many of its predatory enemies (GUNTHER et al., 1983).
2.2.2 Biological control of Prostephanus truncatus using Teretriosoma nigrescens
T. nigrescens is well adapted to the habitat and habits of P. truncatus. Due to its body having almost the same dimensions as the pest, T. nigrescens imagines can pursue P. truncatus without difficulty into the bore-holes. The forelegs with their broad, zig-zag tibia are used for digging and to push away the dust in the tunnels. The agile, longish-shaped larvae are also able to move well in the tunnels to reach the pest's brood.
Chiefly however, T. nigrescens is able to search specifically for its prey. The aggregation pheromone secreted by the Bostrichida, P. truncatus, to attract mates and for determination of the sex, can also be perceived by T. nigrescens (BÖYE, et al., 1992 c; SCHULZ & LABORIUS, 1987; REES, 1990; REES et al., 1990). During investigations in natural conditions in Mexico (REES 1990) and in maize stores in Costa Rica (BÖYE, 1988) T. nigrescens could only be observed in connection with P. truncatus.
In laboratory experiments on the predatory behaviour of the beetle, evidence was found to confirm that not only the larvae of T. nigrescens but also the imagines are able to make use of the eggs and larvae of P. truncatus. According to LELIVELDT (1990), one T. nigrescens imago eats an average of 5.7 eggs or 4.9 larvae of P. truncatus per day. REES (1985) defined a value of 1.7 P. truncatus larvae on average. Over the same period, the predator's larvae ate up to 3.5 P. truncatus larvae. According to this, one T. nigrescens larva requires around 60 P. truncatus larvae up to the point when its development into an imago is complete. According to (BÖYE, (1988), one T. nigrescens imago kills an average of 1.1 P. truncatus larvae in 24 hours and the predator's larvae, an average of 4.3 pests.
Further experiments investigating the question of whether T. nigrescens is able to suppress the growth of a P. truncatus population provided promising results: REES (1985) placed T. nigrescens imagines and P. truncatus adults in a ratio of 1:10 on shelled maize in container experiments. Whilst the number of adult P. truncatus was reduced after 8 weeks in the presence of T. nigrescens, the number of pests increased to ten times the original number in the control experiments without the predator.
Apart from maize, containment of P. truncatus could also be achieved by T. nigrescens on other substrates PÖSCHKO et al., 1992 b). After 8 weeks, a reduction in the number of P. truncatus imagines of 51% on sorghum and 27% on wheat could be observed as compared to the control tests without T. nigrescens (on maize 78%).
LELIVELDT (1990) was able to determine an increase in the predatory ability of T. nigrescens at 30°C in comparison to 26°C (with P. truncatus cultures on maize).
In laboratory experiments in Costa Rica (BÖYE, 1988) on shelled maize a decline of 87% was achieved by the predator in the P. truncatus population after 110 days as compared to the control test. In a corresponding test using maize cobs, the population of P. truncatus was reduced by 72%. By using T. nigrescens the loss of shelled maize and cob maize could be reduced by 76% and 62% respectively. The damage to the grain and the cobs caused by P. truncatus could be reduced by 47% and 21 % respectively.
Good results were achieved in Togo in small stores of maize under semi-practical conditions within the storage period of 9 months (HELBIG, 1993; HELBIG et al., 1992, b, c). T. nigrescens reduced the number of P. truncatus imagines by 46.5% and the damage to the grain by around 42% in comparison to the untreated control. The population of T. nigrescens increased within the experimental period from 67 to 876 specimens per 100 cobs.
After field experiments in Southern Togo, which were carried out with a view to developing a suitable method for releasing T. nigrescens (BÖYE, et al., 1992, a), the Histerida was released in January, 1991 in Togo (BÖYE, & FISCHER, 1993). After one year, the useful insect was already found between 2 and 10 km from the point of release (BÖYE, et al., 1992, b). Eight months after its release, lower losses were suffered in the village where T. nigrescens was present in maize stores (approx. 20%) than in the control village without the predator (approx. 36%).
The population of the pest, at 1,340 imagines/100 cobs, in stores where T. nigrescens was present, was clearly less dense than in the control stores where 3,198 P. truncatus imagines/100 cobs were counted (RICHTER et al., 1992).