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J Integr Plant Biol, 1962, 10 (3): -, Research Article
Studies on the Biology of Beauveria Bassiana (Bals.) Vuill. with Reference to Microbial Control of Insect Pests
C. Teng
Beauveria bassiana (Bals.) Vuill., a widespread entomogenous fungus of wide host range, has been used in many countries, with success in many cases, for the microbial control of insect pests. Since 1954, it has been employed in China for combating Grapholithe glycinivosella, Cylas formicarius and Dendrolimus spp. In the present work, attempts have been made to study the biology of B. bassiana with reference to its practical application to the control of insect pests, and the following results have been obtained: 1. Mycelial growth has been found to occur between 13–36 ℃ and to cease at 8 ℃ and 40 ℃, the optimal temperature for mycelial growth as well as for spore germination being about 24 ℃ which, according to laboratory tests with Dendrolimus punctatus, has been shown to be also conducive to infection. The optimal temperature for spore production has been found to be 30 ℃, deviating from the results of previous workers (33, 36) who claimed that the optimum for mycelial growth was the same as that for spore production. 2. Although the relative humidity, most favorable for mycelial growth and spore germination is about 100%, the spores of certain strains of B. bassiana are able to germinate at as low as 56.8%. Laboratory tests with aphids show that the percentages of mortality are higher when the relative humidity is above 80%, the degree of relative humidity most favorable for spore germination and mycelial growth being also most effective for infection. On the contrary, lower percentages of relative humidity (about 25%–50%) favor sporulation. Desiccation has detrimental effect upon spore viability. 432 hours of desiccation at (22 ± l) ℃ causes complete loss of vitality of the spores of B. bassiana. It has also been found that the spores will not germinate after 15 months when the culture medium has dried out, but will retain their viability if the moisture content of the medium is still sufficient. These results confirm the investigation of Masera, but disagree with those of Steinhaus and G6sswaldt331. 3. Light has been shown to favor the radial expansion of the mycelium, but to cause a decrease in its density. For sporulafion, light has been proved to be indispensable. Within certain limits, the amount of spore production is in direct proportion to the intensity of light. The most effective wave length for sporulation is in the blue region (under 500 mμ) of the spectrum. Diffuse light also exerts stimulative effect on spore germination. The germination percentage increases 4–5 times or more as compared with the check (without light). Spores of B. bassiana do not completely lose their germinative capacity after accumulative exposure to direct sunlight (up to (44 ± 1) ℃) for about 150 hours. Within 90 hours of actual radiation, direct sunlight seems to exert an invigorating effect as revealed by the percentages of germination as well as by the lengths of germ-tubes. This stimulative action is probably due to the ultra-violet rays in the sunlight. Further experiments have shown that spores of strain I, after being exposed to ultraviolet radiation (220y, 30w, at a distance of 30 cm) for as long as 8 hours, germinate more vigorously than those of the check. As for strain II, the mycelium is able to stand 8 hours' exposure to ultraviolet rays without apparent injury, but spores are rendered nonviable by the same duration of exposure. 4. Results of experiments indicate that, regardless of light conditions, adequate supply of oxygen encourages the growth of mycelium and the germination of spores, but hinders spore formation, causing an increase by 30%–80% in the radial expansion of mycelium and a decrease by 46%–71% in spore production, the percentages of spore germination under poorly aerated conditions as compared with that under adequate oxygen supply being 1.43% and 15.4% respectively. 5. Evidences have been obtained to show that spore germination takes place at pH 3.0–9.4 and ceases at pH 2.4 and 10.0, the optimum being about 4.4, and that vigorous mycelial growth occurs at pH 4–5. Although the pH range for sporulation is the same as that for mycelial growth, optimal pH for spore production is about 6. 6. From the data secured, it seems evident that B. bassiana is non-specific in nitrogen requirement, making good to very good use of both organic and inorganic sources of nitrogen. Among the inorganic sources, nitrate nitrogen is more utilizable than ammonium nitrogen. Urea is very effective for sporulation, especially at high concentrations. The beneficial effect of organic nitrogen on spore production is obvious in the presence of light but not in the dark. B. bassiana is capable of utilizing maltose, sucrose, raffinose, glycerin, soluble starch as well as glucose, among which sucrose is most favorable for sporulation. Among the carbohydrates tested, lactose, inulin and sorbitol rank next in utilizability; xylose, rhamnose, cellulose or lactic acid are poor carbon sources for B. bassiana so far as spore production is concerned. It is interesting to find that B. bassiana generally causes the final pH to rise to 7.8–8.0 in those culture media containing the above-mentioned poor sources of carbon, and to lower to 5.0–5.2 in the media containing suitable sources of carbon. Within certain limits, the amount of spore production is in direct proportion to the concentration of glucose (1%–5%) in the culture medium, but mycelial growth bears no such relations. By adding 0.2 mg% of manganese or of iron to the culture medium, an increase in spore production by 150% and 20% respectively can be obtained. In tap or distilled water, the germination percentage of spores is always very low and the time required for germination much delayed. In 0.5% sugar solution or in decoctions of leaves of various higher plants, or by adjusting the pH of distilled water to 4.4, the spores germinate much more readily and the percentage of germination becomes much higher. 7. The spores of B. bassiana have been demonstrated to be highly resistant to various insecticides and fungicides. They have been found to be viable after being treated with the following: <1/500 1605 (46.6% emulsion), <0.5% dipterex (50% alcoholic solution), <0.5% DDT (5%), <1.5% 666 (6%), <0.05% nicotine (redistilled), <1/400 lead arsenate, 1/400 rotenone (2.5%), and <0.25% ceresan. Besides, at lower concentrations, 1605 (1/12000) and dipterex (0.01%–0.3%) apparently stimulate spore germination. Tests have been made with the outcome indicating that the volatile substances or gases emitted by tissues (leaves or flowers) of various kinds of higher plants (woody and herbaceous) are not only non-injurious to the spores, but appear to bring forth beneficial influence on spore germination, causing an increase in germination by 30%–90%. It has also been observed that the spores of B. bassiana remain viable in solutions of sucrose of 2.0 molarity or of sodium chloride of 1.5 normality, under a temperature of 25℃. The vitality of spores differs according to their age. Spores from 20-day-old cultures give a germinating percentage not to exceed 2.5%, while spores from 3-9-15 months old cultures yield 8%–9% germination. 8. Results of the present investigation likewise indicate that the spores of B. bassiana are highly resistant to both low and high temperatures. They are able to germinate normally after being subjected to –21 ℃ for 400 hours; and within 300 hours of exposure, this low temperature promotes the growth of germ-tubes. This stimulative effect seems more evident under humid conditions. Spores of B. bassiana do not completely lose their viability after being exposed to 80 ℃ up to 2 hours, but become incapable of germination after being subjected to 100 ℃, 90 ℃, and 86 ℃ for 5, 20 and 40 minutes respectively. This disagrees with the result obtained by Headlee who states that spores “may be exposed for five hours to as high as 209 ˚F (98.3 ℃) without injuring the germination”. The discrepancy may be due to the difference in the experimental methods employed. Under high relative humidity (95%), the spores are killed by 144 hours exposure to 40 ℃, while under dry air conditions, they lose their viability only after a prolonged exposure of about 200 hours to the same temperature. 9. Having analyzed the results so far acquired, it is not difficult to find that mycelial growth and sporulation are two different developmental stages. Environmental conditions most suitable for sporulation differ from those for mycelial growth, but conditions suitable for spore germination are usually similar to the latter. For examples, the optimal temperature for spore formation is 30 ℃, and that for mycelial growth and spore germination is about 24 ℃; low relative humidity (25%–50%)favorable to spore production is deleterious for mycelial growth or spore germination; adequate oxygen supply is essential for spore germination and mycelial growth, but retards spore production; the most effective pH for sporulation is about 6, but that for mycelial growth or spore germination is 4–5. A thorough understanding regarding the requirements of the different developmental stages is helpful in devising means for propagating the fungus. 10. The high adaptability and high resistance of B. bassiana to adverse conditions account for its universal distribution, wide host range and the practicability of employing it as an agent in the microbial control of insect pests.
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