Further studies on the ability of Teretriosoma nigrescens Lewis (Col.: Histeridae) to prey on insect pests of stored products and to feed on stored food and beverage crops.

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

ABSTRACT

Of the range of stored-product insects selected, Teretriosoma nigrescens was only able to increase in numbers when feeding on populations of Dinoderus minutus, Rhyzopertha dominica and Prostephanus truncatus (Col.: Bostrichidae).

T. nigrescens does not appear to be able to breed in the absence of live prey. However, it is attracted to cereal grains, and especially to maize previously infested with P. truncatus Adult predators were observed, in the absence of live prey, to damage a few grains of wheat and maize. Compared to the damage caused by P. truncatus that caused by the predator was very slight.

These observations support others that T. nigrescens is physiologically and behaviourally well adapted to prey on P. truncatus The risks involved with the introduction of this predator into Africa appear minimal.

INTRODUCTION

From field and laboratory observations, Teretriosoma nigrescens is at present the most promising candidate for use as a biological control agent against Prostephanus truncatus in Africa. To assess the possible risks involved in the release of T. nigrescens tests were needed to see if it could cause significant damage to stored products important as staple foods and cash-crops. The ability of the predator to prey on a range of stored-product pests was also tested in order to provide information on its specificity and potential for control of indigenous pest species.

Under field conditions, T. nigrescens adults usually have a choice of commodities on which they can feed or look for prey. Some commodities may be more attractive than others. While T. nigrescens may attack a given commodity in laboratory tests ii may only do so in the field if the commodity is more attractive than others present, An investigation was undertaken to see which commodities T. nigrescens was attracted to, when given a choice.

EFFECT OF T. NIGRESCENS ON A RANGE OF INSECT PESTS OF STORED PRODUCTS

Methods

The effect of T. nigrescens on a range of storage pests was investigated (Hatch 1990 and Rees 1990). In these studies, species were selected to represent a range of feeding strategies (development of larvae inside or outside grains together with ability or inability to attack undamaged grains). Ten adult predators were added to each of three replicates containing 50 adult prey species infesting 100g of media (Lasioderma serricorne - wheatfeed and yeast 10:1; Acanthoscelides obtectus - kidney beans; Sitophilus zeamais - whole maize; Tribolium castaneum - whole-wheat flour and yeast 12:1) (Hatch 1990). Five adult predators were added to each of ten replicates of 150 g of maize infested with 40 adults of P. truncatus, Dinoderus minutus or Rhyzopertha dominica (Rees 1990). Controls, to which predators were not added, were set up in both studies, with the same number of replicates as experimental treatments. After eight weeks of incubation al 27°C and 70% r.h., the number of adult predators and prey species present in each replicate were recorded.

Results

Of the pest species offered (Table 1), T. nigrescens was only able to reduce numbers, below those initially added, of the bostrichids P. truncatus, D. minutus and R. dominica. Predator numbers increased when feeding on these species. Well-grown larvae of T. nigrescens were found in replicates containing T. castaneum suggesting that the predator had some success in feeding on this species. Populations of S. zeamais, L. serricorne and A. obtectus appeared to be unaffected by the predator.

Table 1: Effect of T. nigrescens on a range of stored product pests

Notes
1 Mean of three replicates per treatment Initial number added - 50 adult pest species, 10 T. nigrescens adults
2 Mean of ten replicates per treatment initial number added - 40 adult pest species, 5 T. nigrescens adults
* Predator larvae also found

ABILITY OF P. TRUNCATUS AND T. NIGRESCENS TO CAUSE DAMAGE AND BREED ON A RANGE OF STORED GRAINS.

Methods.

Tests were carried out to see if T. nigrescens could cause damage or breed, in the absence of prey, on a range of stored products. Results obtained were compared with the damage caused by, and breeding success of, P. truncatus on the same commodities. Twenty adult predators were added to 500 ml jars containing 50 g of an undamaged whole commodity (maize, wheat, kidney beans, cocoa or coffee), equilibrated for one week at 27°C and 70% r.h., five replicates of each commodity were set up. Twenty adult P. truncatus were added to another set of five replicates of the five commodities. After eight weeks at 27°C and 70% r.h., the grain was weighed, having first been sieved to remove dust and small fragments. The numbers of live adult insects present were also recorded. To calculate commodity weight loss on a dry weight basis, moisture content, by oven determination, was measured both at the beginning and end of the experiment.

Results.

T. nigrescens was unable to breed on any of the commodities tested. However, in feeding, predator adults excavated holes in a few grains of maize and wheat (Table 2). In some replicates, almost all the T. nigrescens present were found within one or two damaged grains. In contrast, P. truncatus bred on maize, and very slowly on wheat, causing a greater percentage weight loss to commodities than the predator. Burrowing and probable feeding by adult P. truncatus also caused weight loss in cocoa beans. These observations on the feeding behaviour of P. truncatus are similar to those made by Shires ( 1977).

Table 2. Final number of insects and percentage weight loss caused by T. nigrescens or P. truncatus on a range of commodities after eight weeks at 27°C and 70% r.h

Key:
1 - Five replicates per treatment initial numbers added - 20 adults.
* - some evidence of feeding damage observed
* * - mature larvae also present

ATTRACTION OF T. NIGRESCENS TO A RANGE OF STORED GRAINS

Methods

T. nigrescens adults were exposed to combinations of stored products to see to which they were most attracted. Polythene boxes (30 x 21 x 13 cm) were filled with 1.51 of «Hortag». This inert material, of particle size 5 mm, provided a grain-like matrix through which the insects could move rapidly. Buried at opposite ends of the box were two tetrahedral bags, 6 cm high, made from plastic mesh with apertures of 2 mm. Bags were filled with 40 g of either Hortag or a commodity (Table 3). Three replicates of each combination were set up. Thirty adult T. nigrescens were added to each box. After 24hrs at 27°C and 70% r.h., the number of insects present in each bag was recorded (Table 3).

Results

In the absence of other commodities T. nigrescens appeared strongly attracted to maize and cassava and, to a lesser extent, wheat (Table 3). They appeared especially attracted to damaged and holed grains in which the soft endosperm was accessible. The predator was not attracted to either cocoa or kidney beans. When given a choice between maize and another commodity, more insects were found in the bag containing maize. Disinfested maize, previously infested with P. truncatus was found to be more attractive than uninfested maize.

Table 3 Choice chamber test: mean number* of T. nigrescens in each bag after 24 hrs at 27°C and 70 % r. h.

* Mean of three replicates; number of adult T. nigrescens per replicate = 30
** Disinfested maize taken from a culture of P. truncatus.

DISCUSSION

From the range of insects tested T. nigrescens was only an effective predator of those pests which bore into grain producing flour-filled tunnels, i.e. the Bostrichidae. In Africa, Dinoderus spp. are established pests of construction materials, such as bamboo, and sometimes maize and cassava (Haines 1981). As well as any effect on P. truncatus any reduction in pest status of Dinoderus spp. following introduction of T. nigrescens would also be of benefit to African farmers. If the use of P. truncatus as prey for mass-rearing of T. nigrescens for release is impractical then another storage bostrichid could be used instead. Other bostrichids, and possibly species of families with similar habits, may also be preyed upon. The diameter of tunnels produced by prey, critical in allowing T. nigrescens access to them, may play an important part in determining if a given species is at risk from attack (Rees 1990). It is difficult to explain why T. nigrescens appeared not to prey on L serricorne with its readily accessible larvae of almost identical size and form to those of P. truncatus Further observations are needed to confirm that T. nigrescens does not attack L serricorne or other anobiids.

T. nigrescens appeared unable to attack the juvenile stages of S. zeamais and A. obtectus. These develop concealed within grains and are inaccessible to the predator. These observations support studies on whole maize cobs infested with S. zeamais and P. truncatus in which numbers of the bostrichid were significantly reduced by the predator leaving those of S. zeamais unaffected (Leliveldt and Laborius 1990, Rees 1987). Even if, upon release, T. nigrescens is effective at controlling P. truncatus serious losses caused by Sitophilus spp. are likely to continue.

A few T. nigrescens larvae appear to have completed all or part of their development preying on T. castaneum. On grain infested with P. truncatus juvenile stages of T. castaneum may only be preyed upon on a casual basis by T. nigrescens. Tribolium larvae, relatively well sclerotized and capable of rapid movement, are probably more difficult to catch and kill than sedentary, soft-bodied, bostrichid larvae. It is not known if T. nigrescens would be attracted to infestations of Tribolium if P. truncatus was not present.

T. nigrescens appears unable to breed without live prey. Damage caused by adult feeding was very slight in comparison to that produced by P. truncatus Being apparently so closely associated with P. truncatus in the field (Boeye 1990, Leliveldt and Laborius 1990, Rees et al. 1990), the chance that the predator may be found on commodities on which the pest is not normally found appears minimal. Predator adults appear most attracted to damaged grains. Under conditions of farm storage such grain is almost invariably present and may be attacked by T. nigrescens in preference to intact grains. When live food is present, T. nigrescens will be even less inclined to feed on grain material. However, its ability to feed on grain fragments etc., should permit its survival through periods when prey is unavailable.

The risk that introduction of T. nigrescens may pose to stored commodities appears minimal, and the potential benefits from its reduction of the impact of P. truncatus may be considerable. T. nigrescens therefore remains the most promising candidate as a bio-control agent for use against P. truncatus in Africa.

REFERENCES

Boeye, J. (1990) Ecological aspects of Prostephanus truncatus (Horn) (Col.: Bostrichidae) in Costa Rica. pp 73-86 in Markham, R.H., and Herren, H.R. (Eds.) Biological control of the Larger Grain Borer. Proceedings of an IITA/FAO Coordination meeting, Cotonou, Republic of Benin, 2-3 June 1989, IITA, Ibadan, Nigeria.

Haines, C.P. (1981) insects and arachnids from stored products: a report on specimens received by the Tropical Stored Products Centre 1973-77. Tropical Products institute Report no. L54, 73pp.

Hatch, R. (1990) Report on a year industrial placement in the Pest Biology Section of the Storage Department, Natural Resources Institute (NRI), Chatham, Kent. NRI unpublished report.

Leliveldt, B. and Laborius, G.-A. (1990) Effectiveness and specificity of the antagonist Teretriosoma nigrescens Lewis (Col.: Histeridae) on the Larger Grain Borer Prostephanus truncatus (Horn) (Col.: Bostrichidae). pp 87-102 in Markham, R.H., and Herren, H.R. (Eds.) Biological control of the Larger Grain Borer. Proceedings of an IITA/FAO Coordination meeting, Cotonou, Republic of Benin, 2-3 June 1989, IITA, Ibadan, Nigeria.

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. 23.191-195.

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) 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, 153-165.

Shires, S.W. (1977) Ability of Prostephanus truncatus (Horn) (Col.: Bostrichidae) to damage and breed on several stored food commodities. J. Stored Prod. Res. 13. 205-208.

The IIBC LGB screening programme for beneficial insects: Summary of results

S.T. Murphy and A.E. Cross
IIBC, Ascot, United Kingdom

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Note: Paper with full results is being submitted to the Journal of Stored Products Research

MATERIALS AND METHODS

In the screening programme for beneficial insects, two basic types of experiment were conducted (at 27°C and 70% r.h.) for all of the insects screened, apart from coccinellids (details of the screening procedure used for these will be given later). in the first experiment, the ability of T. nigrescens adults and larvae to damage and feed on various stages of the test insect was investigated relative to its ability to feed on LGB larvae. In each experiment feeding trials were conducted in small ventilated clear plastic boxes (13 x 8 x 6 cm) containing one of the following prey types: the test insect in isolation, the LGB in isolation, and the test insect and LGB presented together. Each prey type was exposed to either two adults or one larva of T. nigrescens for a period of time that varied according to the species and stage of test insect. Only one larva of T. nigrescens was used per box because this stage is highly cannibalistic. Approximately three replicates of each prey type were set up and the numbers of the test insect placed per box varied according to the species being tested; however, five LGB larvae were used per box in all cases. In each experiment, each prey type was set up without the predator in a parallel set of boxes to measure natural mortality during the course of the experiment.

In the second experiment, the olfactory attraction of T. nigrescens adults to particular stages of the test insects was investigated. This experiment was conducted in an olfactometer based on a design described by Vet et al. (1983). To measure the attraction response, a particular stage of test insect was compared either with a chemically inert control or with LGB infested maize grain. In each comparison in the olfactometer, 200 adult T. nigrescens were tested.

RESULTS AND CONCLUSIONS

SILKWORM MOTH (Bombyx mori (L.))

In the feeding experiment, silkworm eggs, second instar larvae and cocoons were exposed separately or in combination with LGB larvae to T. nigrescens for seven days. Adults were not tested in view of their large size and mobility compared with the predator. The numbers of the silkworm immature stages presented in individual replicates of the experiment were four, six, two and two respectively. All these stages of the silkworm were presented in the experimental boxes on leaves of their food plant, white mulberry (Morus alba L.) which was replaced daily. In all trials, no damage or feeding was observed on any stage of the silkworm. In contrast, the LGB larvae were eaten by both stages of the predator when presented in isolation and together with the silkworm. Similar results to these have been obtained by the GTZ programme in Germany (A. Laborius, pers. comm.).

In view of the results of the feeding experiment, the attraction experiment was confined to checking the olfactory response of T. nigrescens to the cocoon, the most important stage for the silkworm industry. However, in all comparisons in the olfactometer, the cocoons were not found to be attractive to T. nigrescens.

It can be concluded, therefore, from these investigations that T. nigrescens is unlikely to attack silkworms in the field.

HONEY BEE (Apis mellifera L.)

For the first experiment, feeding trials were conducted on: honey bee eggs, presented to T. nigrescens in honey comb for two days; late instar larvae, presented to T. nigrescens in honey comb for three days; and pupae, sealed in honey comb cells and presented to T. nigrescens for 14 days. Like silkworm moths, adult bees were not tested in view of their large size and mobility compared with the predator. The numbers of the immature stages presented either in isolation or in combination with LGB larvae in individual replicates were, 20-30, 48 and 8-16 respectively.

When the honey bee stages were presented in isolation, only the egg stage was eaten by the predator; small numbers of this stage were eaten by both adults and larvae of T. nigrescens. During the course of the experiment, these two predator stages ate respectively in 24 hours an average of 5.0% and 1.1% of the eggs presented However, when the eggs were presented together with LGB larvae, the latter were always taken in preference to the eggs. In the bee larvae and pupae feeding trials, T. nigrescens larvae did burrow into the wax cells of the honey comb in some replicates when these stages were presented in isolation. This burrowing did sometimes damage the larvae and pupae in the cells but there was no indication that once in the cells the predator fed on these honey bee stages. Again, however, when these stages were presented in combination with LGB larvae, the predators only attacked the LGB larvae. Similar results to those described here have been obtained by the Federal Bavarian Institute of Beekeeping, Erlangen, Germany, under the GTZ programme (A. Laborius, pens. comm.).

In the second experiment, the attraction of T. nigrescens to bee larvae contained in honey comb was tested. When the bee larvae were paired with the inert control in the olfactometer, there was no significant difference between the two test substances. However, when the larvae were paired with LBG-infested maize, the bee larvae proved to he more attractive than the maize. This preference for the bee larvae may have been due to the presence of honey in the comb.

Despite this result, the experiments on the attraction of T. nigrescens to honey combs containing bees were not continued in the light of other results obtained on bees by the Institute of Apiculture, University of Frankfurt, Oberursel, F.R. Germany (A. Laborius, pers. comm.).

These results clearly indicated that even if T. nigrescens were attracted to a beehive for some reason (as indicated by the IIBC attraction experiment) there are a number of very effective barriers which would stop the predator causing damage to the honey comb and bee brood. Thus, predation of the honey bee by T. nigrescens in the field can he considered to be extremely unlikely.

HYMENOPTEROUS PARASITOIDS

Screening tests were conducted on the wasp Anisopteromalus calandrae (Howard) (Pteromalidae) which is an important parasitoid of several stored product beetle pests including the LGB (Boeye et al 1988), Sitophilus spp. (Okamoto 1971 a, b; 1972). The life history of this wasp on Callosobruchus maculatus (F.), which attacks cowpeas (Vigna unguiculata (Walp.)) and other legumes, is as follows (Garcia Saez 1988). After paralyzing a host larva within a bean, a single egg is laid by the female on or near the host. Several hosts may he attacked in this way within the same bean. Eggs hatch in 24 hours and larvae teed for 810 days before pupating within the bean.

In view of the known close association between A. calandrae and the LGB, no attraction experiment with T. nigrescens was conducted during the IIBC screening programme. Larvae and young pupae of A. calandrae, reared on C. maculatus, were extracted from their host and mixtures of stages were exposed to T. nigrescens either in isolation or in combination with LGB larvae. Three A. calandrae larvae/pupae were used per replicate and the period of exposure to the predator was two days.

When presented in isolation, a small number of A. calandrae larvae and pupae were eaten by T. nigrescens larvae; during the whole period of the experiment, an average of 26.4% of the A. calandrae stages were eaten. In the choice trial, however, where the predator larvae were presented with LGB larvae and A. calandrae larvae/pupae, the predators only took the LGB larvae. Adult predators showed no interest in the wasps either in isolation or in combination with the LGB. Similar results to those just described were obtained by IIBC during screening studies on the braconid Heterospilus prosopidus Viereck, which is another polyphagous parasitoid of stored product beetle pests. This latter wasp does not, however, attack the LGB. It is concluded, therefore, that in a natural situation where wasp larvae and pupae would be present with their host, foraging T. nigrescens larvae would attack the LGB and not the young stages of the wasps. Thus, in grain storage cribs in the field, the predator would almost certainty pose no threat to parasitoid wasps.

COCCINELLID BEETLES

In most regions of the world, coccinellid beetles (Coleoptera: Coccinellidae) are important predators, both in the adult and larval stage, of aphids and other homopteran pests that attack agricultural and forestry crops (Clausen 1940). Thus, these predators play an important role in keeping populations of these plant-feeding pests under control. Because of the nature of their hosts, most of the life histories of the majority of coccinellids are spent on plants: the immature stages are confined to their hosts plant food and the adults spend much of their time searching the plants for their prey. Thus, because of this characteristic, the IIBC screening experiment for coccinellids was aimed at determining whether or not foraging T. nigrescens adults can successfully search on plants. The bean plant (Phaseolus vulgaris) was chosen for the experiment because this is commonly grown in Africa and is attacked by several homopteran pests.

The experiment was designed as follows. An individual bean plant, approximately 10 cm high, with 34 leaves was covered with a clear cylindrical ventilated cage that completely covered the plant to the base of its stem. Five adult T. nigrescens were placed on the bottom of the cage next to the stem of the plant and their searching behaviour and position on the plant was then recorded at five-minute intervals for 80 minutes. This procedure was replicated 10 times and all observations were made at 27°C, 70% r.h..

In eight of the replicates, all of the beetles spent all of the experiment time trying to burrow into the base of the cage. No attempts were made to climb or fly onto the plants. Two beetles in each of the remaining two replicates attempted to climb the plants but fell off within 5-10 minutes of starting the exercise. Movement on the plants was extremely slow and the beetles appeared to have great difficulty in obtaining a grip on the plant stems and leaves. No "searching behaviour" was noticed when the beetles were climbing. The remaining beetles in these replicates tried to burrow into the base.

It seems clear from these observations that T. nigrescens is not adapted to searching on bean plants because of its apparent inability to grip the plant surface. Furthermore, when released into a cage containing a plant, there is a strong tendency for the beetle to burrow rather than climb. Because of the nature of these observations it seems unlikely that T. nigrescens would behave differently on other crop species and it is also equally unlikely that T. nigrescens would be able to attack the immature stages of coccinellids if present on plants. In view of the results of this investigation, further experiment to test the feeding behaviour of T. nigrescens on coccinellids were not considered necessary.

REFERENCES

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

Clausen, C.P. (1940) Entomophagous insects. London; Mc Graw Hill Publishing Company, 688 pp.

Garcia Saez, C. (1988) Sex ratio and fitness in a parasitic wasp. PhD Thesis, imperial College, University of London, UK. 180 pp.

Okamoto, K. (1971 a) The reproduction curve of a host in a host-parasite interacting system and parasite free system. Japanese Journal of Ecology 21, 197-203.

Okamoto, K. (1971 b) The synchronization of the life cycles between Callosobruchus maculatus (L.) and its parasite Anisopteromalus calandrae (Howard). Japanese Journal of Ecology 20, 233-237.

Okamoto, K. (1972) The synchronization of the life cycles between Callosobruchus chinensis (L.) and its parasite Anisopteromalus calandrae (Howard). II. The relationship between the development of the parasite and "he development stage of the host. Japanese Journal of Ecology 22, 238-244

Discussion - second session

Queries were raised as to why, apparently, T. nigrescens controls different species of Sitophilus to different degrees. It was felt that this was more a function of how the predator had been cultured rather than a real effect. In any case, probably due to the inaccessibility of the Sitophilus larvae, the controlling effect is never very great.

It was re-iterated that T. nigrescens shows some cannibalism at high population densities.

It was reported that no work had been done on the effect/impact of T. nigrescens on solitary bees.

Despite the fact that all the tests prescribed at Cotonou had been completed some delegates had misgivings about the release of the predator. It was pointed out that the Inter-African Phytosanitary Council of the OAU was satisfied that any release would be environmentally sate and that it was impossible to pre-determine efficacy (as opposed to safety) of the predator without releasing it. The meeting was reminded that it was only empowered to give recommendations concerning the release and give guidelines for the future. Whether the predator was released or not was ultimately the responsibility of the country concerned.

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