|Controlling Insect Pests of Stored Products Using Insect Growth Regulators and Insecticides of Microbial Origin (NRI, 1994)|
|Section 4: Microbial control|
Utilization of other fungal species
Over 400 species of naturally occurring entomopathogenic fungi have been identified. Most do not have to be ingested by the insect to cause death. Fungal spores stick to the surface of the insect, germinate, and send out hyphae which penetrate the cuticle and invade the haemocoel. Death either occurs rapidly, due possibly to the production of complex toxic metabolites, or more slowly, due to hyphal proliferation and disruption of organs. The invading fungus then sporulates and re-enters the ambient environment to establish subsequent infections.
Entomopathogenic fungi were the first micro-organisms to be used as microbial insecticides. Only a few species have been mass produced, generally by government agencies rather than by private industries. Fungi are currently used regularly as microbial insecticides only in a limited number of countries such as Brazil, the former Soviet Union, the former Czechoslovakia, and China. Verticillium lecanii has been registered for use in the UK, and Hirsute//a thompsonii has been registered in the US (McCoy et al., 1988).
The application of Beauveria bassiana to glasshouse and field crops has been studied extensively. However, few researchers have considered its application for the control of storage pests.
Laboratory efficacy experiments
Laboratory trials were undertaken by Searle and Doberski (1984) to investigate the use of B. bassiana isolated from a soil sample against Oryzacphilus surinamensis. Humidity was found to be more critical than temperature, or inoculation rate. At 100% r.h., infection occurred rapidly within 20 days, whereas at lower humidities very little infection was observed. Tests were also carried out to determine the effect of temperatures between 7° and 25°C. The highest adult mortality occurred at 25°C and 100% r.h.; under these conditions 100% mortality had occurred within 13 days.
It was concluded that in grain stored at, or below, the recommended moisture content of 14% (70% r.h.), the fungus would be unlikely to control O. surinamensis populations.
A method of surface fermentation has been developed in the former Czechoslovakia for the mass production of two preparations of B. bassiana known as Boverol and Boverosil. In trials using Sitophilus zeamais, Oryzeaphilus surinamensis and Tribolium castaneum, Boverosil was more effective than Boverol. 7: castaneum was the least susceptible of the three stored-product pests studied (Frydocva et al., 1989).
The preparation, Boverosil, combined with the insecticide pirimiphosmethyl, has been registered in the former Czechoslovakia for the treatment of empty stores and silos against residual infestations of stored product pests.
Hluchy and Samsinakova (1989) examined the effects of a batch of Boverosil containing 50% dry fungal material and 50% amorphous silica gel (50-1 mg) against adult Sitophilus granarius and larvae of Galleria mellonella. S. granarius was the least susceptible; a dose which produced 50% mortality in G. mellonella larvae produced only 3% mortality in S. granarius adults. The LC50 appeared to be in the range of 2 x 10 8 and 5 x 108 conidia/ml.
In Iraq investigations were carried out to determine inoculation spray rates for Ephestia cautella larvae in stored dates. The results indicated that 300 000-400 000 spores/cm3 were needed to produce 96-98% mortality (Jassim eta/., 1988).
Utilization of other fungal species
Studies by Schulz and Laborius (1987) on natural fungal parasites of Prostephanus truncatus strains from Costa Rica showed that several microfungi can be isolated from dead adults. Taxonomically, all the fungal isolates belonged to the Deuteromycotina; most were species of Aspergillus and Penicillium and included the mycotoxigenic A. flavus.
Pathogenicity and virulence of spore suspensions were tested in adult P. truncatus by topical application. Virulence differed considerably between the isolates; after 4 days of incubation, mortalities varied from 13.3% to 100%. These preliminary experiments highlighted the problems involved in developing techniques using micro-organisms. The potential of fungi for the regulation of natural populations of P. truncatus remains unclear.
Toxicology Acute oral LD50 for rats, >2000 mg/kg
Metarhizium anisopliae is an entomopathogenic fungus with a worldwide distribution. It can be cultivated on both solid and liquid sterile media. An insecticide, code name BlO 1020, has been developed from a wild-type strain. It is produced by a special patented fermentation procedure in the form of pellets which are dried to granules; they have a shelf life of at least six months, particularly if stored at low temperatures. The optimum growth temperature for the fungus is 25°C. The product is very effective against Coleoptera and Lepidoptera, and has been tested against pests of ornamental crops (Reinecke et al., 1990).
Laboratory efficacy experiments
Rodrigues and Pratissoli (1990) carried out small-scale laboratory trials to evaluate the pathogenicity of Beauveria brongniartii and M. anisopliae for Sitophilus zeamais and Acanthoscelides obtectus. Adult insects were dipped in conidial suspensions (10 8 conidia/ml), returned to maize or beans, respectively, and retained at 25°-28°C and 60% r.h. B. brongniarti caused 89% mortality in adult A. obtectus within 15 days and 47% mortality in adult S. zeamais; M. anisopliae caused less than 50% mortality in either species.
The major drawbacks to the use of fungi for insect control are thought to be their poor stability in storage situations, and their high dependence, for efficacy, on climatic conditions in agricultural situations (Kirschbaum, 1985). Commercial formulations cannot be stored at room temperature so must either be shipped fresh after manufacture or stored under refrigeration, both of which may prove difficult.
A successful rate of infection depends on spore germination. This requires optimum temperatures and relative humidities in excess of 80%. Consequently, fungi are only regarded as suitable for application in humid tropical climates and greenhouse situations. If the problems of temperature and moisture requirement necessary for conidial discharge and spore germination could be overcome, use of fungi might be a practical method for controlling insects in stored products.
(Merck Sharp & Dohme Research Laboratories)
Toxicology Acute oral LD50 for rats, >5.0 g/kg Acute dermal LD50 for rats, >2.0 g/kg
The avermectins are a mixture of natural products produced by a soil actinomycete, Streptomyces avermitilis. They are a family of macrocyclic lactones which consist primarily of four major components (A1 a, A2a, B1 a, B2a) and four homologous minor components (A1 b, A2b, B1 b, B2b). Avermectins designated as A1, A2, B1 and B2 refer to mixtures of the homologous pairs containing at least 80% of the major component.
The avermectins have nematicidal, acaricidal and insecticidal activity. Their insecticidal properties were first demonstrated in laboratory assays against Tribolium confusum. On the basis of its high intrinsic toxicity to arthropods compared to the other natural avermectins and synthetic variants, avermectin B1 was selected for crop protection purposes (Lasota and Dybas, 1991).
The selective toxicity of avermectins to specific invertebrates is believed to be due at least partially to the differential distribution of gamma-aminobutyric acid (GABAergic) neurons which, in mammals, are restricted to the central nervous system (Lasota and Dybas, 1 991 ).
Laboratory efficacy experiments
Beeman and Speirs (1984) carried out tests with avermectin B1 against a range of storage pests. It caused 100% mortality in parent adult Sitophilus oryzae, Rhyzopertha dominica and Oryzacphilus surinamensis exposed to a dose of 320 ppb on wheat. Tribolium castaneum was more tolerant; at a dose of 2.6 ppm, only 36% mortality occurred although at 160 ppb the insects appeared sluggish.
Suppression of F1 progeny was achieved at doses of 10 ppb in Sitotroga cerealella, 20 ppb in R. dominica, 160 ppb in S. oryzae and O. surinamensis, and 640 ppb in Plodia interpunctella. The half-life decay for avermectin B1 on wheat at 26.7°C and 60% r.h. was 3-6 months.
Commercial application of avermectins
Annual sales of the isolated avermectin toxin and its analogues are worth about US $ 200 million. However, sales are primarily for the control of veterinary pests and only limited use is made of the products for insect control in pre-harvest crops. Avermectin research has largely been confined to investigations using the entire organism of Streptomyces avermitilis for the production of avermectins, or using the related S. bikokenkinki, for the production of milbemycins. Attempts to synthesize new avermectin-like compounds have proved unsuccessful. However, chemical modification of the natural toxins has led to commercial success with 22,23 dihydroavermectin B1 which is used in the animal health sector (Jutsum et al., 1989).
In 1983, the high cost of avermectins and their high level of mammalian toxicity, precluded them from registration as stored grain protectants. Since then, however, their use in veterinary and public health areas has been pursued, and avermectin has been registered in the US as an outdoor control agent for the imported fire ant. In 1986, when the product Affirm was registered, technical avermectin was considered to be highly toxic to birds, fish, aquatic invertebrates and mammals. However, its use was approved for fire ant control because of the low toxicity of the bait formulation in mammalian acute toxicity trials and the rapid rate of hydrolysis which occurred. Tolerance data were not required then as the registered use did not include crop or food use.
Merck Sharp & Dohme Research Laboratories have indicated that although they are seeking to increase the range of food crops on which the use of avermectin is approved, they have no current plans to develop avermectins as grain protectants. Insufficient information is available to recommend their use as candidate protectants of durable foodstuffs at the present time.
Fungi are unlikely to be generally useful for the control of storage pests because the climatic conditions which usually prevail in storage situations are unsuitable. Temperatures are frequently far higher than the 25°C mentioned by Searle and Doberski (1984), and humidities in the commodity are generally at, or below 70%; the r.h. may be as low as 20% in some tropical climates. Overall, fungal control of storage pests may warrant further investigation but it shows no clear promise at the moment.