ISOLATION AND CHARACTERIZATION OF ENTOMOPATHOGENIC ...

2.4 ENVIRONMENT INFLUENCE ON NATURAL
PATHOGENICITY
Environmental factors which influence the virulence of entomopathogens must be
considered for the successful development of the fungus as a biocontrol agent. Of all the
ecofactors that influence epizootic of a mycopathogen, none is more critical for sporulation,
germination and invasion of the host than high humidity (>90% RH) (Getzin, 1961; Allen et al.,
1971). Pathogenesis occurs at much lower ambient values (Ferron, 1978; Ramoska, 1982)
probably because of high humidity in the microclimate at the insect cuticle. It is certain,
however, that the external sporulation never occurs on the killed insect, if the relative humidity
is too low.
The rapidity of mycelial development and therefore, the rapidity of the evolution of
infection depends on temperature. In general, optimum values fall between 20°C and 30°C
(for example, 23°C for Beauveria brongiartii, 24°C for Entomophthora obscura, 25°C for
Beauveria bassiana and Nomuraea rileyi and 27°C-28°C for Metarhizium anisopliae) with
limits between 5° and 35°C. Temperatures lower than the optima distinctly retard the
development of mycosis without necessarily affecting the total mortality (Ferron, 1978).
Atmospheric temperature between 20°C and 30°C did not limit the growth and
sporulation of N. rileyi. On the other hand, since the fungus will not grow below 15°C and
sporulate above 30°C, long periods at these extreme limit both the initiation and development
of epizootics (Ignoffo et al., 1976). The spore germination and colony growth of the C-3
isolate of Verticillium lecanii had similar temperature optima. Conidia germinated more rapidly
between 20°C and 25°C. Both germination and growth declined steeply above 25°C and
ceased above 30°C (Burges, 1981). Gillespie and Crawford (1986) found that the infectious
conidia of three isolates of M. anisopliae did not germinate at 93 per cent RH and that
germination was inhibited significantly below 98 per cent RH. They reported similar results in
the isolates of Verticillium, Beauveria and Paecilomycos, suggesting that the requirement for
high humidity was fundamental to these fungi and strains with reduced dependence on
humidity were unlikely to be found. Drummond et al. (1987), however, found that some
isolates of Verticillium lecanii could tolerate lower humidities.
Hywel-Jones and Gillespie (1990) examined spore germination of M. anisopliae and
B. bassiana at 20-30°C and found higher germination levels (95% in 10-14 h) in M. anisopliae
as compared to B. bassiana (95% in 14-15 h).
The optimum temperature for the development of the fungus is not necessarily the
same for the development of the disease. However, the influence of temperature on the host
insect must be taken into consideration, since very short periods between moults resulting
from a high temperature may reduce, for example, the duration of the instar to an extent that
penetration of the fungus through the integrument is impeded.
Survival of M. anisopliae applied to soil depend on a range of physical and biological
factors such as soil moisture, temperature and other soil microbes (Lingg and
Donalson, 1981). The inoculum of N. rileyi when exposed to direct sunlight on the upper leaf
surface of cabbage reduced the half-life of spores to 3.6 h (Fargues et al., 1983).
Rath (1992) found that the distribution and abundance of M. anisopliae and B.
bassianas strains varied with rainfall, soil type and pH. However, there was no obvious
relationship between temperature and the distribution of M. anispliae strains. However, the
strains could be separated on the basis of the temperature range over which they germinated
(McCammon and Rath, 1994).
M. anisopliae DATF-001 spores were able to germinate on Czapeck-dox agar at all
temperature from 2 to 25°C. Rath et al. (1995) tested the third instar larvae of Adoryphrous
couloni with a concentration of 4.1 x 10 6 spores/g at different temperature. DATF-001 was
pathogenic at all the temperatures and the LT50 values ranges from 36.1 days at 15°C to
188.9 days at 5°C.