Release and Establishment of the Predator Teretriosoma nigrescens Lewis for the biological control of Prostephanus truncatus (Horn) in Kenya
P.H. GILES, M.G. HILL^1, F.L.O. NANG'AYO^2, G. FARRELL and G.N. KIBATA^2
Natural Resources Institute, Central Avenue, Chatham Maritime,
Kent ME4 4TB, UK
(Received 21 September, 1995; accepted 15 March, 1996)
Code Number: CS96072 Sizes of Files: Text: 45.6K Graphics: Line drawings (gif) - 6K Tables (jpg) - 250K
The first release of a biocontrol agent against a stored product pest in East Africa is described. This involved the release in 1992 of a beetle (Teretriosoma nigrescens Lewis), a meso-American predator of the Larger Grain Borer (Prostephanus truncatus (Horn)). The predator was liberated at two sites in Kenya with contrasting climatic conditions: hot, semi-arid plains, and cool, damp highlands. This paper discusses preliminary observations on the establishment and dispersal of T. nigrescens and its impact on the pest. Prostephanus truncatus numbers were reduced when T. nigrescens was present in stored maize cobs in the hot, semi-arid area but not in the cooler, damper highlands. During a period of eight months in the semi-arid site, the predator was shown to have dispersed 16 km, mainly downwind through natural shrub land. Plans for further releases and studies on the establishment and impact of the predator are discussed.
Key Words: Biological control, farm storage, Kenya, Larger Grain Borer, Prostephanus truncatus, Teretriosoma, nigrescens
La premiere publication de l'agent biocontrol contre les pestes des produits en stock dans l'Afrique de l'Est a ete decrite. Ce qui a implique la publication en 1992 de scarabee (bestiole) (Tetriosoma migrescens Lewis), un predateur meso-americain de grains (Prostephanus truncatus (Horn). le predateur a ete libere dans deux centres au Kenya dans des conditions climatiques contrastes: chaude,semi-aride dans des conditions dans les plaines, et froide dans les regions montagneuses. Ce document donne une vision sur les observations preliminaires pour etablir et distribuer le T. nigrescens et son impact sur la peste. les nombres des Prostephanus truncatus etaient reduits pendant que le T. nigrescens etait present dans l'epis de mais pendant la saison chaude, sur un terrain. Semi-arid mais pas dans une glaciere, les regions montagneuses humides. Durant la periode de huit mois sur un champ semi-aride, le predateur semble avoir une dispersion a 16 km, plus notamment a travers une terre des arbistes naturels. Les plantes sont reservees aux publications et etudes ulterieures sur l'etablissement et l'impact du predateur sont a discuter.
Mots Cles: Controle biologique, produits agricoles en stock, le Kenya, percer les grains, Prostephanus truncatus, Teretriosoma, nigrescens
The Larger Grain Borer (LGB), Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), was first recorded in Kenya, infesting maize at Taveta on the border with Tanzania (Kega and Warui, 1983). The insect had almost certainly been imported with maize from meso-America where it is indigenous (Chittenden, 1911; Back and Cotton, 1922; Giles and Leon, 1974; Hoppe, 1986). The LGB is a serious pest of stored maize in East Africa. Weight losses in LGB infested farm stores could be as high as 34% over a six-month storage period, with an average loss of 9% (Hodges et al., 1983). The indigenous pest fauna cause losses of 2-5% (De Lima, 1979).
Maize stores surveys and pheromone trapping have been used to monitor the spread of the pest in Kenya. At present it is still confined to the south east of the country, with higher numbers recorded near the towns of Taveta, Voi, Kibwezi, Loitokitok and Wundanyi, and lower numbers in Nairobi, Thika, Mwea Tebere and Mombasa. In ten years, it has spread to an area of approximately 50,000 km2 (range 40-60,000 km2). In Kenya, cassava is seldom stored long enough to be affected, but maize may be severely damaged.
The pest is currently confined largely to semi-arid areas of Kenya, where the storage period is usually less than eight months. So far, the pest has not been detected in the central highlands or the important maize surplus areas of western Kenya. The semi-arid area is mainly a grain deficit zone, and there is, therefore, little movement of locally-produced maize to other parts of Kenya to help its spread. However, maize imported and exported via Mombasa is at risk because it passes through a zone infested by P. truncatus.
To control stored product pests, farmers were advised by Government agencies to mix insecticide with shelled grain and use residual sprays in stores (Muhihu and Kibata, 1985). Synthetic insecticide dusts containing 2% malathion or 1% pirimiphos-methyl were commonly used for protecting stored grain in Kenya. In 1991, 42% of farmers surveyed were reported to be using insecticide (Stabrawa, 1993). Neither of these compounds by themselves is very effective against P. truncatus. Since 1992 a dust containing 1.6% pirimiphos-methyl and 0.3% permethrin, applied at 50 g per 90 kg shelled grain, has been promoted for use by farmers. This formulation is very effective against all stored product pests.
Various non-chemical measures are practised for aiding grain conservation on the farm, such as store hygiene, cob segregation, sun-drying, smoking and seed selection, though none of these are as efficacious as insecticide use. Another means of reducing losses caused by P. truncatus may be through biological control with a New World beetle, Teretriosoma nigrescens Lewis (Coleoptera: Histeridae), commonly found as a predator of P. truncatus in Mexico. Teretriosoma nigrescens can cause substantial reductions in pest numbers when confined with P. truncatus in laboratory cultures (Rees, 1985; Rees, 1991). The predator was first released in Togo in 1991 (Markham and Herren, 1990; Boeye et al., 1992; Mutlu, 1994). Prostephanus truncatus is a resident in the natural environment, feeding and breeding on wood (Giles and Kibata, 1990; Nang'ayo et.al, 1993; Nang'ayo and Hill, unpublished observations). Reduction of P. truncatus populations by T. nigrescens in this situation may result in less frequent cross-infestation to stores and therefore give an overall reduction in losses (Hodges, 1994). However, where pesticides are used on stored maize, the predator will have no effect because it is even more susceptible to these compounds than the pest itself (Golob et al., 1990). Histerid beetles (family Histeridae) have been rarely used as biological control agents. To date there has been no clearly documented case of an histerid biological control agent being successful in effecting reductions in the density of its prey. Clausen (1978) recorded eleven species in the genus Hister introduced in the early and middle parts of this century to various islands in the Pacific and the Caribbean, USA, Australia and Mauritius, for control of muscid flies. Some species established but no significant control was recorded. The best known example of an histerid used for biological control is Plaesius javanus (Erichs.) for control of the banana weevil Cosmopolites sordidus (Germ.)(Coleoptera: Curculionidae) (Simmons, 1935; Clausen, 1978). This predator was introduced from Java to Australia and several islands in the Pacific and Caribbean. While its impact has never been rigorously investigated, there is circumstantial evidence to suggest that it has had a significant impact upon pest populations in Fiji (Simmons, 1935). All of these examples are of soil-dwelling histerids attacking prey living in the soil or in banana corms at or below ground level. In the present case, the histerid T. nigrescens is specifically adapted for pursuing its host through tunnels in wood and other substrates (Hinton, 1945).
Biological control using T. nigrescens has the advantage of low cost and sustainability. On the farm it would be expected to slow the rate of growth of P. truncatus populations, where the farmer does not use insecticide, although if late in arrival its effects may be minimal. At the end of the storage period T. nigrescens may reduce P. truncatus populations in woody material and maize residues in the granary fabric or environment close to the store. Even without the pest the predator can survive for about a year (Rees, 1990).
Extensive safety testing of the predator in the laboratory in the UK (Murphy and Cross, 1992) and in Kenya (IIBC, unpublished) showed that T. nigrescens would not consume beneficial insects or damage stored grains or animal feed. Permission was therefore given by Kenyan authorities for mass rearing and release at two contrasting sites. In its area of origin in Mexico, studies in farm stores have shown that while T. nigrescens is frequently present (Boeye, 1988), its numerical response to increasing prey density is poor, and it is unlikely to effect good control of LGB populations (Rios lbarra et al., 1992; Markham et al., 1994b). Teretriosoma nigrescens was first released in Africa in Togo in 1991 (Helbig, 1995; Mutlu, 1994). Preliminary observations of its effects on LGB populations in stores in Togo show that its establishment led to a reduction in LGB populations in stores of up to 80% with a 34% reduction in grain losses after 8 months storage (Mutlu, 1994). The predator was released in Benin in 1992 and in addition quickly spread east across the border from Togo (Markham et al., 1994a). lt has established well and is spreading rapidly (Markham et al., 1994a; R.H. Markham and C. Borgemeister pers. comm.)
In 1992, T. nigrescens was first released in Kenya. This paper presents primary observations of its establishment, dispersal, and impact on the LGB.
MATERIALS AND METHODS
Rearing of T. nigrescens for release. Teretriosoma nigrescens was reared on P. truncatus in maize in 2 litre glass jars, with vented lids, in a constant environment room set at 26 C and 70% relative humidity. Seven hundred grammes of maize were put into each jar, to which were added 200 unsexed and unaged adult P. truncatus. Two weeks later, 20 unsexed and unaged adult T. nigrescens were added to the jars; these were removed two weeks later. Three weeks after this a further 150 P. truncatus adults were added to the jars. Five weeks later the adult T. nigrescens were collected, followed by a second collection after a further three weeks. The average production of adult T. nigrescens was 185 (SE+/-5.4, n=41), with a mean of 77 at the first collection and 108 at the second. This represents a ninefold increase in the population in successive generations. Once separated out, adult T. nigrescens were stored in the constant environment room in jars containing crushed maize and pieces of filter paper. They may live for over a year under these conditions.
Release of T. nigrescens. Releases were made in May 1992 at two villages: Makueni in Kibwezi Division, 170 km SE of Nairobi in the semi-arid zone, and Mgange in Wundanyi Division, 130 km SSE of Makueni, in the Taita Hills. The physical characteristics of the two sites are shown in Table 1. Temperature and relative humidity data were collected from thermohygrographs positioned in the shade in ventilated stores. The climate and habitat at Makueni are typical of the shrub land plains in the P. truncatus- infested area, whereas the climate of Wundanyi is similar to that in the intensive maize-growing areas in the central and western highlands.
Farmers' stores in each release area and 10-11 others selected in the periphery, less than 2.5km away, were visited regularly to obtain information on storage practice, and grain samples. In addition, three 5 litre capacity open-topped bait tins, containing uninfested shelled maize, were left in each store for infestation monitoring. Analysis of these samples and bait tins was carried out to determine whether the predator was present and assess its impact on LGB, if any.
Monitoring the effect of T. nigrescens in stored maize and its spread from the release cribs
Sampling experimental cribs. At eight-week intervals all the mesh bags were removed from the crib and one cob was taken at random from each for analysis. The twelve new cobs and the eight old cobs were kept in separate sample bags for removal to the laboratory. The mesh bags were then returned to their original positions.
Sampling continued until February-March 1993 when all the cribs were emptied, after 40 weeks at Mgange and 44 weeks at Makueni. Both the cobs and the dust that had accumulated at the base of the cribs were sieved and examined for T. nigrescens immatures and adults. Insects were also recovered during shelling of the cobs and sieving maize from the moisture content bags.
Sampling farm stores. Samples of 20 cobs or 2 kg of shelled grain were removed at random from farmers' stores at eight-week intervals until stocks were exhausted. The numbers of farm stores sampled decreased steadily from 30 (Makueni) and 27 (Mgange) to eight at 24 weeks. Only five farmers in each area reported using synthetic insecticide in their stores. Bait tins were removed and examined at 8, 16 and 24 weeks.
Analysis of grain samples. All insects (LGB and other species) were removed from cobs or from shelled grain by screening with a 4.75 mm round hole sieve. Insects were identified, counted and recorded on a shelled grain weight basis. Three sub-samples of grain were examined for perforations caused by insects. The numbers of damaged and undamaged grains were counted and weighed to calculate % insect-bored grains and % weight loss. Insect-perforated grains in half (5-20 g) of one sub-sample were carefully dissected for internal infestation. Moisture content (mc) was determined using a Dickey-John capacitance meter, calibrated against the standard oven method. Mean moisture contents at Mgange were 15.3% compared with only 11.3% at Makueni, over the period 8- 24 weeks. At both sites the mc fell over time, at Mgange from 16.3% to 14.3% and Makueni from 11.7% to 10.9%.
Monitoring predator spread with pheromone traps. Teretriosoma nigrescens is attracted to the pheromone of P. truncatus (Schultz and Laborius, 1987) so that traps baited with synthetic pheromone are ideal tools for monitoring the predator. The spread of T. nigrescens from the release area into the surrounding environment was monitored using delta traps baited with pheromone, dispersed from a vial containing 2 mg of Trunc-call 1 and 2 (Dendy et al., 1989) at a ratio of 1:1 by weight. Traps and lures were supplied by AgriSense-BCS Ltd, UK. Monitoring began two weeks after field release of the predators and traps were set at each of the eight cardinal and sub-cardinal compass points, initially at a distance of 1 km from the release sites. They were replaced at two-weekly intervals and a record kept of the numbers of T. nigrescens and P. truncatus captured. At Makueni the ring of traps was moved to a radius of 2 km in January 1993, and finally to a distance of 4 km in April 1993.
A further source of information on dispersal of T. nigrescens came from two LGB pheromone trap transects, established since May 1991 at the Kenya Agricultural Research Institute (KARI) National Range Research Station (NRRS), Kiboko. These transects were about 15-30 km northwest of the release site, set to collect data on P. truncatus phenology and abundance in relation to vegetation (F.L. Nang'ayo and M.G. Hill, unpublished data).
Infestation of new cobs. Eight weeks after the predator release at Makueni, cobs in the predator release cribs had fewer adult P. truncatus than the controls, and were of better grain quality. However, except for numbers of insects other than LGB, the observed differences were not statistically significant (Table 2). The same comparisons at 16 weeks revealed a similar pattern, but by this time the differences were significant (Table 2). It seems likely that the predator was affecting P. truncatus populations by then.
Infestation of old cobs. In the old cobs T. nigrescens did not provide any significant reduction in infestation or damage at either site. At Makueni, in contrast to the situation in the new cobs, total infestation by P. truncatus increased in the old cobs, from 16 to 24 weeks' storage, by 11 % in the control cribs and 52% in the release cribs.
At Makueni, after 16 and 24 weeks' storage, there were significant differences in the numbers of other stored product insects. Greater numbers of Gnatocerus maxillosus (Fab.) (Coleoptera: Tenebrionidae) and fewer Sitophilus spp. (mainly S. zeamais Motsch. (Coleoptera: Curculionidae) were found in control cribs compared with the release cribs. Tribolium castaneum (Herbst)(Coleoptera:Tenebrionidae), Rhyzopertha dominica (F.)(Coleoptera: Bostrichidae) and nitidulid beetles (mainly Carpophilus spp.) were also found.
Final sampling of experimental cribs. At the time of emptying in February-March 1993, after 40 weeks at Mgange and 44 weeks at Makueni, each crib contained 300 cobs. The mean total weight of cobs was 29kg at Mgange and 21kg at Makueni, the difference reflecting the greater damage at the latter site. The numbers of T. nigrescens recovered from the cribs at the end of the release period are given in Table 3. There were evidently more T. nigrescens adults in the Makueni cribs than at Mgange. There were many more T. nigrescens in Makueni control cribs than in Mgange and many more of the control cribs were infested at the former site. In fact, T. nigrescens had spread to all the control cribs at Makueni, whereas in the Taita Hills less than a quarter contained T. nigrescens. There were more dead adults in the Makueni release cribs than in Mgange cribs (50% compared with 20-22%). More immatures were found at Mgange than at Makueni (10.5 and 0.5 per crib, respectively, in the basal dust and cob sievings).
A mean total of 178 adult T. nigrescens was recovered from each of the Makueni release cribs, and 64 from Mgange, compared with the original release of 250 crib^-1 nine or ten months before. In addition, of the total T. nigrescens individuals found in the basal dust and sievings, 0.8% were immatures (mainly large larvae and pupae) at Makueni and 33.9% at Mgange. It is likely that more were concealed in the grain.
Sampling farm stores. Teretriosoma nigrescens was not recorded in any of the 59 farmers' stores or 58 bait tins in and around the release areas during 24 weeks' storage.
Predator spread monitored with pheromone traps. Five months after release at Makueni, in September 1992 (Table 4), the first T. nigrescens adult was caught in the ring of traps at l km from the release area. By the end of December 1992, six of the eight traps had caught the predator and the traps were moved to a distance of 2 km. By March 1993, seven of the eight traps had caught the predator at 2 km and the traps were moved to a distance of 4 km. By June 1993, the traps set SW and NW had both caught T. nigrescens adults at a distance of 4km from the release sites. A total of 81 T. nigrescens had been caught in the traps by June 1993, with a noticeable peak occurring in January and February, lagging slightly behind a comparable peak in total P. truncatus catches (Fig. 1). The high trap catches of P. truncatus in the period November to February coincided with the onset of the short rains. There is a strong correlation between the onset of the short rains and a sharp increase in adult P. truncatus catches in pheromone traps in Kenya (Nang'ayo et al., 1993). From these preliminary data it seems that numbers of adult predators in the traps may also increase at this time.
Sixteen T. nigrescens were captured in the pheromone traps at the KARI NRRS (Table 4). The first catch was in January 1993, some nine months after the release, showing that by this time the predator had dispersed at least 16 km from the release site at Makueni, far beyond the ring of pheromone traps. In contrast, by June 1993 only one T. nigrescens had been caught in the delta traps set at Mgange.
Although the new cobs in T. nigrescens release cribs at Makueni were heavily infested by several insect species, fewer P. truncatus were found than in the controls. The reduced number of P. truncatus was not accompanied by a reduction in grain damage, apparently because of the presence of many other stored product insects. By contrast, at Mgange T. nigrescens caused no measurable reduction in P. truncatus numbers, even in the release cribs. In laboratory studies, under controlled conditions of 27 C and 70% RH, Rees (1985 and 1987) found that 60 P. truncatus larvae were consumed during the complete development of each T. nigrescens larva. No published information is available on the effect of lower temperatures on the predator, but it is likely that in Mgange, at a mean temperature of only 17.7 C, T. nigrescens may develop only slowly. The higher grain moisture content at Mgange may also have had some influence. Studies in Mexico suggest that the predator is more limited by cold than its prey, since T. nigrescens was more abundant in warm, moist coastal areas (Tigar et al., 1994). In studies with a single laboratory strain of P. truncatus, 17.7 C was close to the limit for development of the predator (Bell and Watters, 1982).
At both Makueni and Mgange, T. nigrescens was not found in the farm maize stores a few metres from the release cribs. Therefore, it is difficult at present to gauge what effect the predator has on P. truncatus populations in stores. There is evidence that G. maxillosus numbers were reduced in the presence of T. nigrescens though direct predation has not been observed. Reproduction by T. nigrescens has been reported in the laboratory on other stored product insects including other bostrichids (Rees, 1991), T. castaneum (Rees, 1987) and Sitophilus oryzae (L.)(Coleoptera: Curculionidae) (Puschko et al., 1992). Predation of other pest species is of direct economic benefit. There is now strong evidence that the predator is well established around Makueni, as nearly 100 adult T. nigrescens have been detected at distances of up to 16 km from the release site. It is too early to determine whether the predator has established at Mgange. However, it should be noted that in the case of the histerid predator Plaesius javanus introduced into Australia for control of banana weevil in the 1920s, the predator was recorded for several years before finally dying out (Wilson, 1960). Thus, it may be necessary to reserve judgement on the establishment of the predator until its breeding and dispersal is confirmed over several seasons and in a range of habitats. The pheromone monitoring sites at the KARI NRRS will continue monitoring the further spread and impact of the predator on P. truncatus. Extensive pre-release data already collected should allow comparison of P. truncatus seasonal abundance before and after the introduction of T. nigrescens.
The fact that this study did not detect significant reductions in pest populations in the presence of the predator gives no indication of the likely future benefits of its establishing as a biological control agent of LGB in East Africa. Indeed it is virtually impossible to measure correctly the impact of a classical biological control introduction directly following its first release. The abundance and impact of the predator is very much influenced by, amongst other factors, the starting ratios of the prey and predator (Rees, 1991; Pschko, 1994; Hill and Nang'ayo, unpublished data) and ambient temperatures (Tigar et al., 1994). However, the results presented here, and those obtained by others in population dynamics studies in West Africa (Rios lbarra et al.,1992; Markham et al., 1994b; Puschko, 1994 and meso-America; Markham et al., 1994a; Helbig, 1995), would tend to support the conclusion that the predator is likely to be most beneficial in reducing pest populations breeding in natural environments outside of maize stores (Markham et al., 1994a). In that case, the biological control will be completely integrated and compatible with chemical control being practised within farmers stores.
Further monitoring will continue at Makueni and Mgange. To gain more information on the climatic factors that affect the establishment and spread of the predator, there are plans to release it in six agro-climatic zones between Wundanyi and Voi in Eastern Province. Post release monitoring and impact assessment will also be carried out.
The invaluable help of laboratory and field assistants in Kiboko, Nairobi, Muguga, Makueni and Mgange is acknowledged. The work was undertaken within the framework of the UK/Kenya Larger Grain Borer Research Project (KARI/NRI), with financial support from Britain's Overseas Development Administration. This paper is published with permission of the Director, Kenya Agricultural Research Institute. Permission to release the predator was given by the Kenya Technical Committee on the Importation of Exotics. Thanks are due to Dr Rick Hodges for constructive criticism of the draft manuscript.
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Copyright 1996 The African Crop Science Society
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