Section 11 - Biological control methods

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Biological control of stored grain insect pests
Biological suppression of stored product insect pests
Insect growth regulators
Host plant and varietal resistance to post harvest insect attack
Botanical insecticides for control of storage insect pests
Entomophathogens for the control of storage pests

Biological control of stored grain insect pests

INTRODUCTION:

Biological control maybe defined as,

"a method of reducing or eliminating damage inflicted by a pest by means of a biological agent, traditionally a parasite or a predator, or by the introduction of a disease where the causal organism is specific in action."

Biological control may also be defined in a much broader sense to include,

"the manipulation of other biotic or demographic facets on the system (s) of a specific pest or pest species complex (since rarely do stored grain insects exist as a single species in an infestation) such that the reproductive processes (governing the growth of populations and their consequent abundance) and physiological processes (governing behavioural and developmental aspects) are impaired."

In combination with other applied control measures, the overall level of effectiveness (i.e. by reducing the dosages or frequency of application of the more traditional insecticides and grain fumigants) will therefore be enhanced.

Of recent development is the use of sterilization (induced chemically, or by exposure to a low-level irradiation source) which is effective for pest species where the female only mates once during its life-time (Sterile-Male Release Technique, or SMRT) or in noncontiguous areas such as islands geographically isolated from migrating reproductives. Another development is the use fo sex attractants or lures (pheromones) which can attract searching male insects to an insecticidal source, or as a disruptive or confusing agent which completely swamp the naturally produced pheromone of the female, consequently inhibiting mating. Both male and female pheromones play an integral role in the mating behaviour of stored products Phycitid moths, while Coleopteran pests such as Trogoderma inclusum and Attagenus megatoma (among others) have been shown to emit short-range pheromones that initiate the mating process. The major developmental problem for their adaptation as commercially effective control agents, is that these highly specific, volatile substances exist as complexes and not single entities, which makes their chemical elucidation and manufacture expensive.

Another "specialized" form of biological control is the use of Insect Growth Regulators (IGR's) which have been demonstrated to induce morphological aberrations; induce ovicidal effects, sterilization, and parental mortality (under some circumstances); impede metamorphosis (preventing adult emergence) and; prolongation of the immature larval stage, resulting in intermediate forms that are not reproductively viable.

Genetic manipulation (mainly by irradiation techniques) aims to displace field populations with new "artificially synthesized" populations which are more easily controlled by conventional means, or lack the characteristics which cause them to be initially regarded as economic pests.

The sequence in which genes are normally present on chromosomes can be rearranged by a series of irradiations where a new strain is developed which has full fecundity, but when mated with the existing field strain, infertile hybrids result, and the irradiated strain becomes dominant. If this strain which has been developed is highly susceptible to current control measures (such as insecticides), then the benefits become obvious. Other possible advantages are the introduction of a lethal gene i.e., susceptability to cold where a pest may be multi-voltine (several generations per year) but one generation must survive in defined winter periods, or the creation of a genotype which has the ability to be a vector for a disease pathogen removed. This last approach is seen to be advantageous for the control of tsetse flies, mosquitoes, etc., which are of extreme medical and veterinary importance and have a limited number of chromosomes, thus making manipulation less difficult. This approach has some definite attractions for stored products insect control but has not been researched in depth, possible due to the difficulty of encorporating the new genotype into the existing field population without rejection or elimination. I would also consider the use of plant extracts which have been shown to possess insecticidal properties as a "transient" form of biological control. Man has utilized plants or plant parts for a multitude of different uses since the early millenia and have become the basis of many synthetic organic processes, due to higher costs of production using the natural derivatives. The commonest plant extracts which are still widely used are natural pyrethrum extract (a mixture of pyrethrin esters, Pyrethrim I and ll and Cinerin I and ll occurring in various proportions), from Chrysanthemum cinerariaefolium. The extract has replaced the earlier use of dried flower heads (usually available as concentrated extracts of 25%; acute oral LD50 (rat) 1500 mg.kg-1) and derris dust, containing rotenone. This is extracted from the roots of the leguminous plants Derris elliptica and Lonchocarpus spp, first isolated in 1895 (acute oral LD50 (rat) is also 1500 mg.kg-1 and therefore has low mammalian toxicity, although irritating to the eyes and mucous membranes; of short persistence, and usually applied for the control of garden pests, ticks, lice and fleas). Many other plant extracts have subsequently been cited for their potential as insect control agents (such as neem powder from Azadirachta indica).

The following sections will cover in depth the more traditional biological control agents, such as hostplant resistance of stored cereal grain to post-harvest insect attack; parasites and predators; and entomopathogens (Bacteria, viruses, fungi, protozoa and nematodes).

Biological suppression of stored product insect pests

Adelaide C. Quiniones

Introduction

Biological suppression of stored-product insect pests is becoming recognized as an acceptable strategy. copple and Mertins (1977), described "biological insect pest suppression" as man's ues of living organisms or their products to reduce populations of pest insects. Topics here will concentrate on methods of manipulation (used singly or in combination) of parasitoids, predators, pathogens and other living organisms, their environment, or their natural products for the purpose of protecting stored commodities.

In recent years, studies of biotic agents and their products e.g., pheromones, have led to consideration of their potential for the suppression of stored-product insects. This marks a considerable change in thinking. Previously, all organisms were considered unacceptable contaminants of stored products; but now the presence of biological control agents is considered less harmful than residues of chemical pesticides. In addition, insect pests of stored produts are developing resistance, the cost of pesticides and their application is rising rapildly, and there is growing awareness of woild food shortages. All these factors have made it necessary to find new strategies for managing insect pests of stored - products.

Pheromones

Pheromones seem to have the promise for early practical use in manipulating stored product insects, and they have many advantages. They are active in minute quantities and over considerable distances, they are safe, and they are often effective most immediately. They also are highly specific. Pheromones can be used. (a) for surveillance and detection of an infestation and (b) for control of infection, sometime by communication disruption or mass trapping with lures and devices that will attract and kill the insects.

Surveillance and detection of infestation. In one study conducted in Milwaukee, Wisconsin, the pheromones of Trogoderma and the black carpet beetle, Attagenus megatoma (F), were used either singly or in combination to bait traps, and the traps were then placed in three warehouse and grain elevator locations. The traps treated with pheromones caught significantly more target insects than untreated traps; a previously undetected population of Trogoderma variable B. was discovered; and the seasonal emergence of A. megatoma was observed and charted oven a two-year periold. The primary component of the Trogoderma pheromone, 14 - methyl 8 - hexadecenal is now used routinely by the Animal and Plant Health Inspection Service (APHIS), U.S. Department of Agriculture, in the major U.S. port facilities where large numbers of mostly T. variabile are caught.

In other trapping studies conducted in California military warehouses, the location and efficiency of similar traps received particular attention. Traps near the exterior walls accounted for the most of the captured Trogoderma, and the pheromone - baited traps caught significantly more Trogoderma than unbaited traps. Dichlorvos was the killing agent in these traps. (Burkholder, W. E.).

Wheat germ oil is another material that is an attractant for Trogoderma larvae, and several attractive components of the oil have been identified. As a result, a combination of wheat germ oil, and Trogoderma sex hormone is now being used in the APHIS Trogoderma - trapping program for Khapra beetle detection.

Male lesser grain borers, Rhyzopertha dominica (F.), produce an aggregation pheromone that attracts both sexes. The pheromne is responsible for the characterestic "whitish" color of grain infested with these borers. When the synthesized pheromone (Williams et al, 1981) was tested in a warehouse in Texas, baited traps were effective in monitoring populations for as long as five weeks. Thus, perforated grain - probe traps baited with this pheromone might provide a means of suppressing populations of the grain borers that develop deep within grain bins.

The pheromones of other grain-infesting beetles and weevils are being isolated, identified, and developed and will probably be available within a few years. Those for Tribolium spp., Sitophilis spp., Stegobium paniceum (L) (drugstore beetle) and Lasioderma serricorne (F) (cigarette beetle) could be especially useful. In addition pheromones are available for monitoring some of the moth species that are of concern in warehouses. For example, several species of stored-product Pyralidae respond to the synthesized sex pheromone (Z, E) - 9, 12-tetradecadien -1 - 01 acetate (Brady et al 1971; Kuwahara et al 1971).

Control of infestation. The use of pheromones for control depends on the efficacy of traps in capturing the attracted populations of placing them to an area where other suppression methods can affect them. Also, mating disruption with pheromones may produce some suppression, although this technique most often has greatest impact when it is used in combination with other techniques.

Manipulation and Suppression of Stored - Product Insects with Pheromones and Entomopathogens

One innonative strategy involves the use of pheromones as lures in devices that contain insect pathogens, a technique that is particularly successful against insect parts in stored products. Their habitats provide ideal conditions for inducing disease epizootics, especially when the pathogen have desiccation - resistant spores. Even small habitats with localized populations can contain highly concentrated populations of insects that have excellent potential for the creation of epizootics. Of course, a pathogen would not kill the insects immediately. Rather, an infected, spore-ladened insect returns to its natural habitat and infects others of its kind. It is an especially promising method for long-term control of insect pests of stored products when the insects can be exposed to the pathogen by using an effective pheromone baited device.

The combinations of pheromone with pathogens or biotic agents for suppression of dermestid beetles (Coleoptera: Dermestidae) was first proposed by Burkholder. The effect was to suppress population of Trogoderma glabrum Herbst. In this trial, the sex pheromone of the beetles (14 - methyl - 8 hexadecenal) was combined with a protozoon pathogen, Mattesoa trogodermae Canning (Neogregarininda: Ophoryocystidae). Because failure to bring the insect into contact with the pathogen would doom the strategy, the following conditions were established. The adult males used as the test population were situated downwing from the sites where the pheromone plus spores was available; adult males that come to lure were redistributed among emerging females and mated; and dead adults were available as food for offspring.

In this model system, subsequent generations of T. glabrum were substantially suppressed by a single introduction of M. trogodermal spores into high density (32 adults/m²) population of adult males. The treated populations increased only fourfold in the first post treatment generation and fell belwo pre-treatment levels in the second generation. Meanwhile, the control had a 24 - fold increase in the first generation and 100 - fold increase in the second generation. (The low-density treated populations (2 adults/m²) and also the low density controls increased 12-fold during the first generation). Results demonstrated that with the dense population, a 48-hours exposure of the pheromone plus spores was sufficient to achieve distribution of effective doses within a radius of 1.25 m around the sites of the treatment and the attracted males attempted copulation with the pheromone source, which increased spore transfer. Spore transfer to the subsequent generations, however, was mainly the result of larvae that ingested either dead, contaminated adults, or larval food the adults had contaminated by contact. Although the maximal distance of the spore transfer was limited by the size of the experimental arenas (2 x 2 m), it is likely that in a natural environment, the pathogen would be dispersed substantially further, since flight by contaminated adults is possible.

Burges and Hurst (1977) noted sudden and spectacular mortality of stage moths (Plodia interpunetella) in maize storage facilities in Kenya when the insects were exposed to bacillus thuringiensis Berliner, although mortality of similar moths exposed in the same way in laboratory jars was only progressive. As a result, they suggested that cadavers of larvae are the most potent source of infective materials because they contain many spores and crystals of B. thuringiensis, adults, frass or eggs and are capable of rapidly killing larvae that feed on them. In fact, when Burges and Hurst spread B. thuringiensis spores and crystals in the surface to the grain in the jars, rather than applying the material to one point source on the surface, significantly more spores and crystals were required to start an epizootic. Therefore, the presence of infected adult moths in newly harvested and untreated grain could produce epizootic of this pathogen, but epizootics may also arise because infected larvae immigrate into the storage area from adjacent stored grain, residues of food from local farms, transport vehicles, or terminal stores or bags contaminated with freass and insect bodies.

Since the severe mortality of moth larvae caused by B thuringiensis in the Kenya study (Burges & Hurst 1977) was enhanced by high concentrations of spores and crystals in the larval cadavers, it might be possible to mimic nature by providing a high concentration of B. thuringiensis spores and crystals in a sumulated larval cadaver (SLC). Since some laboratory studies indicate the existence of promising attractants or feeding stimulants for insects larvae, these compounds could be combined with the pheromones that attract the adults. Such baits might be formulated with pathogen or small paper clips (to simulate a larval cadaver) or other devices and placed in grain. In this way an epizootic might follow the introduction of a few SLCs, and the initial rapid kill of feeding larvae would provide actual cadavers thereby accelerating the suppression of the insect population.

Also, McGaughey (1976) reported that B. thuringiensis prevented infestations of Indian meal moths, P. interpunctella, and almond moths, Ephestia cautella (Walker), in stored corn and wheat when cat 120 mg of a formulation of B. thuringiensis was applied to 1 kg. of grain. Treatment of the surface layer (to a depth of 100 mm) was more effective than treatments at depths of 33 or 67 mm and as effective as treatment of the entire grain mass. The formulation was less effective in controlling the Angoumois grain moth, Sitotroga cerealella (Olivier), since doses that gave complete control of the Indian meal moth and the almond moth reduced emergence of adult Angoumois grain moths by only about one-third. In addition, Kinsinger and McGaughey (1976) reported that viability of applied B. thuringiensis and granulosis virus (GV) was only slightly reduced one year after treatment of wheat in a farm grain bin, and believed that with proper timing of application, either pathogen could protect the grain from Indian meal moth for one year. They also suggested that residual activity of the materials would extend protection even longer.

An SLC formulated with an attractant bait would lure the young feeding stages that are particularly susceptable to the pathogen. As a result, the larave would theoretically die before inflicting extensive damage on the stored grain. Ideally, such an SLC should be attractive both physically and chemically; it should contain a feeding stimulant to insure ingestion, and it should contain enough pathogen to be lethal. It could consist of pieces of corrugated paper, paper straws, or a a natural material (such as wheat straw) that are coated with the pathogen and the attractant. The ideal system should be a laminated structure made of safe and biodegrable materials. Adjuvants or stickers of the type curently used in the pesticide industry might be useful in binding the pathogen to the attractant. Distribution of the SLCs in the wall or cracks of empty bins, under conveyors, or other areas where residual populations exist would also enhance population suppression.

Another method of effectively distributing a pathogen among stored-product insects would be to provide a pheromone - baited or light-baited device with an open reservoir containing a pathogen such as B. thuringiensis. The insects attracted by the pheromone or light would become dusted with the pathogen and would distribute it within the insect population and habitat.

Beneficial insects

The use of beneficial insects to control pest insects in stored products has not received extensive study because there are problems with the introduction of insects into a food product and because control is prompt. Studies have demonstrated effective suppression, however. Perhaps the most promising use of beneficial insects would be with commodities such as seed corn, peanuts, and other unprocessed materials that may already contain predators and parasitoids.

Predators. The predacious bug Xylocoris flavipes (Reuter) and several other anthocorid bugs of the sub-family Lyctocorinae that frequently occure as predators in storage ecosystems appear to be promising agents for suppression of both Coleoptera and Lepidoptera in stored products, since they prey on most stages of many of these species. Also, the predator has a high capacity to increase its numbers so as to reduce the population when prey is scarce. The difficulty is that X. flavipes. although it is effective against many unprotected insects, is incapable of penetrating hard materials like seeds, and therefore it is ineffective against weevils that infest grain and pulse. Nevetheless, its role in an integrated suppression program for certain commodities warrants further study.

Parasitoids. Two common parasitoids that in stored products are Bracon hebetor Say (Braconidae) and Venturia canescens (Gravenhorst, Ichneumonidae).

Takahashi (1973) studied the suppression and regulation of E. cautella populations by the action of V. canescens. He noted that the adults attack host larvae on the food surface and do not penetrate the food surface and do not penetrate the food surface appreciably. Bracon hebetor can attack host larvae in deep layer of food, but the population disappears. Takahaski therefore suggested that procedures should be developed to maintain the population level of the parasitoid. The parasitoids could be attracted by either a food (Nectar) source or by their pheromones.

Laeluis pedatus (Say) (Bethylidae) is another parasitoid associated with dermestid larvae. Burkholder, and Carlson (1978) showed that Anthrenus flaviceps Le Conte (furniture Carpet beetle) possesses a supra - anal organ that serves as a defense mechanism against this parasitoid, but that several other species of Anthrenus do not and thus can serve as hosts. Another bethylid, Cephalonmia tarsalis (Ashmead), a parasitoid of Oryzaephilus surinamensis L. (sawtoothed grain beetle), was studied as a possible means of supressing the host insect.

Finally, there are several promising pteromid (Hymenoptera; Pteromolidae) parasitoids of grain and pulse weevils. Anisopteromalus colandrae (Howard) has long been known to be of considerable economic importance in the control of grain weevils and a number of other stored-product pests. Lariophagus distinguendus (Forst) and Choetospila elegans Westwood are other cosmopolitan parasitoids of grain weevils.

Insect Growth Regulatiors

Chemicals that have juvenile hormone activity (IGRs) have been studied in a closed environment and have been found relatively successful against several stored-product moths and beetles (Leshiavo 1976; McGregor and Kramer 1975; Strong and Diekman 1973). For example, Methoprene and hydropene applied at a rate of 20 mg/kg to diets prevented emergence of pupae in Tribolium castaneum (Herbs") and substantially reduced emergence of pupae in Tribolium confusum Jacquelin du Val (Leschiavo 1976). Methoprene at 1 mg/kg or higher prevented emergence of adult (Oryzaephilus mercator (Fauvel) and O. surinamensis in treated rolled oats or cornmeal; and hydropene at 10 and 20 mg/kg in wheat almost completely inhibited production of adult progeny by Sitophilis granarius (L.). Incresasing concentrations of both compounds reduced the populations of adult progeny of Sitophilus oryzae (L.), but not enough to provide useful control.

Likewise, Walker and Bowers (1970) demonstrated that eggs of Lasioderma serricorne (F.), a cosmopolitan pest of stored products, were sensitive to IGRs. Thus, pey suggested that methoprene maybe useful in controlling L. serricorne on package commodities.

IGRs could be added to attractant impregnated baits instead of directly to food. The insects might then cease to develop or behavior may be disrupted and reproduction impaired.

Conclusions

Biological suppression procedures can be an important part of pest management programs for stored-product insects. Whit th increased availability of pheromones, pathogens, IGRs, parasitoids and other suppression methods, systems of biological suppression that make use to innovative manipulation are increasingly promising. These approaches should be developed further and evaluated for efficacy, cost, and safety.

References

Brady, V.E; J.H. Tumlinson lll; R.B. Brownlee; and R.M. Silverstein. 1971. Sex stimulant and attractant in the Indian meal moth and in the almond moth, Science 171:802.

Burkholder, W.E. and R.J. Dickie. 1966. Evidence of sex pheromones in females of several species of Drmastidae J. Econ. Entomology 59: 540-43.

Huffaker C.B. and P.S. Messenger, 1976. Theny and Practice of Biological Control. Academic Press, Inc. New York.

Loshiavo, S.R. 1976. Effect of the synthetic regulators Methoprene and hydropene on survival, development or reproduction of six species of stored-product insects. J. Econ. Entomol. 60: 395-99.

McGaughey, W.H. 1975. A granulosis virus for Indian meal moth control in stored wheat and corn. J. Econ. Etomology 68: 346-48.

_________, 1976. Bacillus thuringiensis for controlling three species of moths in stored grain. Can. Entomol. 108: 105-12.

McGauhey, W. H. 1978. Moth control in stored grain: Efficacy of Bacillus thuringiensis on corn and method of evaluation using small bins. J. Econ. Entomol. 71(5): 835-839.

McGregor, H.E and K.J. Kramer. 1975. Activity of insect growth regulators, hydropene and methoprene, on wheat and corn against several stored grain insects. J. Econ. Entomol. 68: 66870.

Papavizas, G.C. 1980. Biological control in Crop Production Allanheld, Osmum & Co. Publisher, Inc. New Jersey.

Williams, P., and T.G. Amos. 1974. Some effects of synthetic juvenile hormones and hormone analogues on Tribolium castaneum. Aust. J. Zool. 22: 147-53.

Table 1. Predators and parasites of stored product insect pests in tropical and subtropical storage

GPr = general predator; not particularly adapted to storage environment, usually highly polyphagus, often found only in small numbers and usually not significant in the natural control of pests.
SPr = storage predator; adapted to storage environment although some species are also found in other habitats, usually oligophagus and sometimes found in large numbers.
SPa = storage parasite; adopted to the storage environment, although some species are also found in other habitats, usually oligophagus.

(sometimes with a very harrow host-range) and sometimes found in large numbers.

* - occasional or rare species where they occur, maybe abundant
** - infrequent
***- common
*** - very common
- These species are not obligate predators; they are also secavengers or secondary pests on certain commodities, in some situations, therefore, they are economically undesirable rather than beneficial.

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