Monitoring the Larger Grain Borer Prostephanus truncatus and its predator Teretriosoma nigrescens using pheromone baited flight traps - latest developments

Contents - Previous - Next

D.P. Rees
NRI, Chatham Maritime, United Kingdom.

INTRODUCTION

The Larger Grain Borer (LGB) Prostephanus truncatus (Horn) (Col.: Bostrichidae) is a pest of Mesoamerican origin which has recently been introduced accidentally into parts of East and West Africa. There it has become a pest of major importance of farm-stored maize and dried cassava (Dick et al. 1989, Hodges et al. 1983, Krall 1984, 1986, Golob 1988).

Insecticide-coated cardboard crevice traps baited with «Dominicalure», the synthetic aggregation pheromone of Rhyzopertha dominica (Col.: Bostrichidae), were initially used in Africa to detect P. truncatus infestations in farm-stores (Hodges 1984). A male-produced aggregation pheromone, 1-methylethyl (E)-2-methyl 2 penteonate («Trun-call 1" or T1), of P. truncatus was identified and was found to be more attractive than «Dominicalcure» (Hodges 1986, Hodges et al. 1984). Crevice-traps baited with T1 have since been widely used to monitor P. truncatus in Africa (Golob 1988). Recently, a second component of the pheromone, 1-methylethyl (E)2, (E)-4-2, 4-dimethyl-2 heptadienoate («Trun-call 2" or T2) has been isolated. Mixtures of 1:1 or 1:4 T1:T2 were found to be more attractive than T1 alone (Dendy et al. 1989a). Lures containing these synthetic pheromones are now manufactured commercially. Recently a sticky-surface crevice trap, designed initially for the detection of Tribolium spp., was found to be as effective and in many situations more versatile than card crevice-traps used previously (Rees et al. 1990 b).

In Tanzania, T1/T2 baited flight traps captured large numbers of P. truncatus in maize fields away from farm buildings (Dendy et al. 1989 b). Funnel traps similar to those of Graham (1970) and delta-type flight traps were most effective (Dendy et al. 1989 a, 1989 b). P. truncatus was detected for the first time in Ghana with T1/T2 baited flight and crevice-traps (Dick et al. 1989). The infestations located by trapping were not detected by visual inspection carried out at the same time. Pheromone-baited flight traps have recently been used in a nationwide survey of P. truncatus in Togo (Richter and Biliwa 1990).

In Costa Rica, T1 baited traps caught large numbers of the histerid Teretriosoma nigrescens (Schulz and Laborius 1986, Boeye et al 1988), a known predator and possible bio-control agent of P. truncatus (Rees 1985, 1987, 1990). in traditional on-farm maize stores in Yucatan, Mexico, both species were captured in T1/T2 baited crevice-traps. They were also detected, using flight-traps, in natural habitats away from maize or cassava (Rees et al. 1990 a). In Togo (Richter and Biliwa 1990) and Kenya (P. Giles pers. comm.) P. truncatus has also been captured in flight-traps away from maize and cassava. In detecting infestations of P. truncatus within farm and village compounds in south-east Ghana, flight traps were shown to be as effective as crevice traps (Rees et al. 1990 b).

They could be deployed more rapidly and with less disturbance to the farmer and his family than crevice-traps. The trap, by being exposed to air currents, may attract insects from a wider area than one buried within a bulk of grain. While flight-traps may not pinpoint infested stores, if P. truncatus presence is detected by them then nearby stocks of maize and cassava are likely to be at Ask.

At times when little or no maize is in store, and in market-places and transhipment centres where turnover of grain makes visual inspection difficult and the use of crevice-traps impractical, flight traps are of particular use. They can also be used to detect the insects prior to harvest and in non-crop habitats. Transects, with the deployment of a flight trap at regular intervals, could be set up along roads, allowing large areas to be rapidly checked for the presence of the insect. More detailed searches, involving farm visits and use of crevice traps, could be undertaken on the basis of results obtained.

Knowledge of the distance over which P. truncatus is attracted to pheromone-baited flight traps will allow recommendations to be made on optimum flight trap density. It may also help in the search for the actual origin of the captured insects. Use at optimum density is important to minimize cost of traps used. If traps are placed too close together insects may become confused by a multiplicity of pheromone sources, leading to a reduction in catch.

OBSERVATIONS OF THE DISTANCE OVER WHICH P. TRUNCATUS IS ATTRACTED TO A PHEROMONE-BAITED FLIGHT TRAP.

Methods

Two investigations have recently been undertaken in Mexico to determine over what distance P. truncatus can be attracted to pheromone-baited flight traps. Adult insects, marked with a fluorescent dye, were released at a central point in a small array of pheromone-baited delta flight traps, each of which was positioned at a known direction and distance from the release point. When recaptured, insects from the release point could be identified by the presence of the coloured dye. Adult P. truncatus were released and traps placed in a recently harvested maize field and an abandoned henequen (Agave fourcroydes) plantation in Yucatan state, south-east Mexico (Rees et al 1990 a, 1990 b). in a similar study (Farrell 1990) P. truncatus were released in a field of newly planted sorghum in Guanauato state, central Mexico. Traps in both studies were baited with synthetic T1/T2 aggregation pheromone.

Rees et al. (1990) dusted, for each release, 500 adult P. truncatus with a red, yellow or green fluorescent dye. Marked insects were added to 40g of disinfested insect-damaged maize held in a 90 x 130 mm bag made of plastic mesh (hole size 3 mm). The bag was placed between two plastic plates, 150 mm dia., held 50 mm apart by tour wire clips: this protected the insects from rain and direct sun. The whole assembly was secured to a concrete block, 400 mm above ground level, at the release site. Flight traps were hung 1.5 m above the ground roughly north, south, east and west (where possible) from the release point. Traps were examined after three days.

Farrell (1990) placed between 800 and 3,000 marked insects at the release point which consisted of a clay tray of unspecified diameter, placed on the ground protected by a wire cage of 1 cm wire mesh to give protection from birds and covered by a plastic tray against sun and rain. No food was provided for the insects. Up to 12 traps, where possible, were hung at 30° intervals around the release point at 0.5 m above the ground from cane tripods. Traps were examined, for marked insects, daily for three days. In both studies, following insect recapture, the experiment was repeated with a progressively larger distance between release point and traps: 10 to 250 m, Rees et al. (1990) and 10 to 400 m, Farrell (1990).

Results

In Yucatan, marked P. truncatus were caught in both situations at all distances (10 to 250 m) tested. Some observations suggested that most insects took several days to arrive at the traps. The proportion of marked insects recaptured declined with the distance they had to fly. Traps placed upwind of the release point caught most insects (Table 1). in Guanauato, marked insects were captured up to 340 m from the release point but none were caught in traps at 400 m distance. The percentage of insects recaptured was apparently unaffected by distance (Table 1). Almost all insects caught arrived in the first 24hrs. Insects flew strongly upwind to pheromone sources over 20 m, less so at 50 m, and in an apparently random manner over longer distances. In both studies, unmarked, wild P. truncatus and T. nigrescens were captured even at sites at which no maize had been grown nearby.

THE EFFECTIVENESS OF DIFFERENT FLIGHT TRAP DESIGN IN CATCHING P. TRUNCATUS AND T. NIGRESCENS

Methods

A comparison was made by Key and Tigar (1990) between the effectiveness of four designs of flight traps in monitoring wild populations of P. truncatus and T. nigrescens. Delta flight traps (ex. AgriSense BCS Ltd. ) were used in three configurations: (1) folded into a tunnel as recommended by the manufacturer; (2) suspended as a flat sheet with the sticky surface exposed; (3) as (2) but with two sheets back-to-back, thus having sticky surfaces on both sides. When hung, (2) and (3) were stabilised by the attachment of half a house-brick. A funnel trap made from locally obtained materials was also used. This consisted of a funnel made from an acetate sheet, folded to form a mouth 4-5 mm diameter inserted into a collection pot 8.5 x 11 cm. Four examples of each trap were hung in a randomly selected position, 1 m from the ground in bushes and trees. Traps were each baited with a vial of T1/T2 synthetic pheromone. Traps made from a delta-trap base were examined and replaced daily for four days. Funnel traps were examined after four days. The entire test was repeated four times.

Table 1: Summary of observations of the distance over which adult P. truncatus can be attracted to a T1/T2 baited flight-trap.

Key
1. Initial number minus those remaining at release point at end of test. Number of P. truncatus released each time - Rees et al. (1990) 500 adults, Farrell (1990) 800-3000 adults.
2. Mean temperature over release period: 31°C day, 23°C night.
3. Mean temperature over release period: 25°C day, 17°C night.
* Traps left in place four days
** Traps and release points stolen

Results

Delta traps, used as recommended by the manufacturer, caught less insects than the other designs, however most insects were captured in funnel traps (Table 2). insects attracted to the pheromone source were also found amongst vegetation close to all trap types.

Table 2: Numbers of insects captured in different flight-trap design.

DISCUSSION

The density of flight-traps needed to provide good coverage appears to be low. If P. truncatus can be attracted to a trap from 250-350 m, the area of ground from which it may have originated is al least one to two hectares. This area may be much larger if frequent changes in wind direction occur during the time traps are in place. In particular density of vegetation and availability of food, amongst many factors, may account for the differences observed in the recapture of P. truncatus in the two studies. It may have been easier for insects at the Guanauato site to cross more rapidly and successfully an open cultivated field than through, as in Yucatan, dense vegetation several metres high across undulating ground. Insects released without food may have been more disposed to disperse from the release point Climatic variations are also likely to affect trap catches. Farrell (1990) demonstrated that catches of wild P. truncatus were adversely affected by rainfall. In contrast those of T. nigrescens were unaffected.

When choosing the best trap design, as well as effectiveness, consideration must be given to what is practical in terms of supply and use under field conditions. For example, traps with exposed sticky surfaces are likely to become entangled in the vegetation in which they are hung. If alternative designs do prove to be more effective there remains the cost of their manufacture.

Funnel traps, unlike sticky traps, do not kill captured insects. Such traps can be used to obtain live samples of both species for scientific investigations, often more quickly than by searching stores. Such traps could be used to monitor the spread of T. nigrescens upon its release. Insects captured could he released after identification to avoid the risks of «trapping out» the dispersing predator. By Using such traps it may be possible to monitor predator populations without adversely affecting their rate of establishment.

Observations of the distance P. truncatus can fly in a few days will have implications for the action needed to prevent its spread across international frontiers. If it can fly several hundred metres in such a short time it may he able to travel several kilometres over its life-span of several months. This, coupled with an apparent ability to live in non-crop habitats (Rees et al. 1990 a, Helbig et al. 1990), may mean it can cross such barriers unaided, even if total control of the trade in infestable commodities was attempted.

REFERENCES

Boeye J., Burde S., Keil H., Laborius G.-A. and Schulz F.A. (1988) The possibilities for biologically integrated control of the Larger Grain Borer (Prostephanus truncatus (Horn)) in Africa. pp 110-140 in Proceedings of the workshop on the containment and control of the Larger Grain Borer, Arusha, Tanzania, 16-21 May 1988. FAO, Rome. Report 2

Dendy, J., Dobie, P., Saidi, J.A., Smith, J. L. and Uronu, B. (1989 a), Trapping the Larger Grain Borer Prostephanus truncatus in maize fields using synthetic pheromones. Entomol exp. appl.

Dendy, J., Dobie, P., Saidi, J. A. and Sherman, C. (1989b), The design of traps for monitoring the presence of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) in maize fields. J. Stored Prod. Res. 25,187-191.

Dick, K. M., Rees, D. P., Lay, K. K. and Ofosu, A. (1989) Occurence of Larger Grain Borer, Prostephanus truncatus (Horn), in Ghana. FAO Plant Protection Bulletin 37, 132.

Golob, P., (1988) Current status of the Larger Grain Borer Prostephanus truncatus (Horn) in Africa. Insect Sci. Appplic. 6, 737-745.

Graham, W M., (1970) Warehouse ecology studies of bagged maize in Kenya - 1. The distribution of adult Ephestia (Cadra) cautella (Walker) (Lepidoptera: Phycitidae). J. Stored Prod. Res. 6, 147-155.

Helbig, J., Detmers, H.B., Laborius, G.-A. and Schulz, F.A. (1990) Investigations of the capability of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) to develop on different kinds of wood. in Proceedings of the 5th International Conference on Stored-Product Protection Bordeaux, France, Sept. 9- 14, 1990. in press.

Hodges, R.J. (1984) Field ecology and monitoring of Prostephanus truncatus (Horn). pp 32-48 in Proceedings of GASGA Workshop on the Larger Grain Borer Prostephanus truncatus 24-25 February 1983, TPI Slough, Eschborn 1984.

Hodges, R.J. (1986) The biology and control of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) - a destructive storage pest with an increasing range. J. Stored Prod. Res. 22, 1-14.

Hodges, R.J., Dunstan, W.R., Magazini, I. and Golob, P. (1983) An outbreak of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) in East Africa. Prot. Ecology 5, 183-194.

Hodges, R.J., Cork, A. and Hall, D.R. (1984) Aggregation pheromones for monitoring the greater grain borer Prostephanus truncatus. pp 255-260 in British Crop Protection Conference Pests and Diseases, Brighton, Nov. 1984.

Key, J. and Tigar, B. (1990) An examination of the relative effectiveness of different trap designs against Prostephanus truncatus and Teretriosoma nigrescens. in Larger Grain Borer Control Programme, Irapuato, Mexico. Quarterly report No. 3. September 1990. NRI unpublished report.

Krall, S. (1984) A new threat to farm-level maize storage in West Africa: - Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). Trop. Stored Prod. Inf. 50, 26-31.

Krall, S. (1986) Further distribution of the larger grain borer (Prostephanus truncatus) in West Africa. FAO Plant Prot. Bull. 34, 213-214.

Rees D.P. (1985) Life history of Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) and its ability to suppress populations of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). J. Stored Prod. Res. 21, 115-118.

Rees D.P. (1987) Laboratory studies on predation by Teretriosoma nigrescens Lewis (Col.: Histeridae) on Prostephanus truncatus (Horn) (Col.: Bostrichidae) infesting maize cobs in the presence of other maize pests. J. Stored Prod. Res. 23, 191195.

Rees, D.P. (1990) The effect of T. nigrescens on three species of storage Bostrichidae infesting shelled maize. J. Stored Prod. Res. (in press).

Rees, D.P., Rodriguez, R. and Herrera F.J. (1990 a) Observations of the ecology of Teretriosoma nigrescens Lewis (Col.: Histeridae) and its prey Prostephanus truncatus (Horn) (Col.: Bostrichidae) in the Yucatan peninsula, Mexico. Trop. Sci. 30, 153165.

Rees, D.P., Rodriguez, R. Herrera, F.J. and Ofusu A. (1990 b) Advances in monitoring Prostephanus truncatus (Horn) (Col.: Bostrichidae) and Teretriosoma nigrescens Lewis (Col.: Histeridae) populations. in Proceedings of the 5th International Conference on Stored-Product Protection. Bordeaux, France, Sept. 9-14, 1990. in press.

Richter, J., and Biliwa, A. (1990) Evaluation de la répartition du Prostephanus truncatus (Horn) (Col.: Bostrichidae) par utilisation de pièges à pheromone sur le territoire du Togo. in Proceedings of the 5th International Conference on Stored Product Protection. Bordeaux, France, Sept. 9-14, 1990. in press.

Schulz, F.A. and Laborius, G.-A. (1986) Strategy for bio-integrated control of Prostephanus truncatus (Horn) (Col.: Bostrichidae). pp 497-503 in E. Donahaye and S. Navarro (Eds.) Proc. Int. 4th Work. Conf. Stored-Product Protection, Tel Aviv, Israel, Sept. 1986.

IIBC/NRI/KARI collaborative project on biological control of Larger Grain Borer

M.J.W. Cock
IIBC, Nairobi, Kenya

Summary

1. The IIBC/NRI/KARI plans to implement biological control of LGB in Kenya as one component of a NRI/KARI IPM research project.

2. We accept that TN is safe for release in Africa as a biological control agent and the operational plan put foward here is based upon the assumption that Kenya agrees to its introduction and release.

3. One focus of the programme is the role of the natural environment in the dynamics of LGB and TN. Where does LGB breed in the natural environment? What is the relative importance for LGB infestation of stores of a) Carrying infested cobs from the field into the store b) Colonisation of the stores by immigration from other stores c) the field or d) the natural environment.

4. Because pesticides are already widely used to protect stored food in Kenya we cannot assume TN will be effective in stores.
Therefore we need to be able to assess its impact on the rate at which stores are colonised by reduction of reservoirs of LGB in the natural environment, field crops and non - pesticide protected stores. Obviously farmers that do not protect their stores with pesticides will also benefit from the action of TN in the store.

5. The objectives of the planned programme can be set out as follows:

Overall objective: To reduce the pest status of LGB in Kenya by the introduction of TN.

Component objectives:

a) Assess the role of the natural environment in LGB population dynamics,
b) Survey (with NRI/KARI project staff) for indigenous natural enemies and assess their importance,
c) Study the population dynamics of LGB outside of the field and store system (i.e. natural environment),
d) Study the dispersal patterns of LGB in the natural environment
e) introduce, release and assess impact of TN
f) if TN is effective, arrange monitoring and distribution programme.

6. The work programme is planned as follows:

a) identify suitable experimental sites in the natural environment, as well as field and store sites.
b) Survey for indigenous natural enemies and determine their Importance.
c) Determine the breeding sites of LGB, their prevalence and importance as a reservoir for LGB.
d) Conduct studies on dispersal of LGB from reservoir to maize fields and to storage and vice versa to compliment studies on the field-store interaction by NRI/NARL.
e) Conduct pre-release studies on the population dynamics of LGB at prospective TN release sites. The biocontrol studies would focus in particular on the population dynamics in the natural environment and thus be complementary to those of NARL/NRI in the maize fields and stores.
f) Arrange necessary clearance and permits and import TN into quarantine at Muguga.
g) Quarantine TN at Muguga and undertake any further testing required in Kenya which cannot be done in the UK. h) Adapt and develop as necessary suitable mass rearing techniques for TN.
i) Obtain permission to release TN.
j) Culture TN in sufficient numbers for release.
k) Release TN for establishment in LGB infested areas, including natural environment.
l) Monitor establishment and spread of TN.
m) Monitor dispersal of TN between the natural environment, maize fields and stores.
n) Assess impact of TN on LGB in natural environment and, together with NRI/NARL staff, in the field and store.

7. The release sites planned (which of course have to be within the present restricted area of LGB distribution) are:

Kiboko: the site of the main IPM programme field base; an area of marginal maize production with erratic rainfall.

Taita Hills (Wundanyi): a higher and wetter area between Kiboko and the coast, which is comparable with the main upland maize producing area of Kenya.

8. Details of the monitoring and assessment methods are currently being finalised and will take into e consideration the deliberations of the present meeting.

Discussion - fourth session

It was acknowledged that all the quarantine testing of T. nigrescens at the BBA, Berlin had been scrupulously done. The predators had been collected in Costa Rica in 1988 and only brought to Togo (via Berlin) in November 1989. This was prior to the completion of all the safety tests prescribed at Cotonou and as such attracted some criticism.

Participants were reminded that countries wishing to release biological control agents were only bound to inform all neighbouring countries of their in tent and were not obliged to wait for their approval. It was generally agreed that monitoring of the release and subsequent spread were essential components of any release programme and more work would need to he done developing methodologies, prior to release, to ensure maximal data collection. This would be demanding of both personnel and finances. In the same way if T. nigrescens were to be subsequently reared on a large scale for widespread release then this would also be costly it was agreed to approach suitable donor agencies for the necessary funds.

Contents - Previous - Next