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The entomopathogenic muscardine fungus, Nomuraea rileyiwas
isolated from local population of the corn earworm larvae,
Helicoverpa armigera (Hubner), and regarded as an
indigenousentomopathogen. Pathological studies were conducted
todetermine its inoculation method, storage period, effects
oftemperature, relative humidity, photoperiod, culture
substrateson its pathogenicity and sporulation. Biocontrol
potential of N.rileyi for corn earworm on sweet corn field was
also investigated. The spraying method is the most effective
in assaying pathogenicity of this fungus to corn earworm larvae.
Mortality has decreased 50% when the larvae were reared on
artificial diet than on natural foodafter the larvae inoculated
with fungal spores. There was no reduction inmortality while
the larvae were reared on diet without disinfectant within48 hr
after inoculation. The effect of temperature on pathogenicity
of N.rileyi to H. armigera was tested by spraying 4th-instar
larvae at aconcentration of 5x106 conidia/ml at temperatures of
10, 15, 20, 25, and30℃. N. rileyi was more virulent at 20℃
than 30℃, and with a lowLT50 value at all tested temperatures.
No significant difference wasfound in pathogenicity from 15 to
30℃ when the 5th-instar larvae wereinjected with a dosage of
1,000 conidia/larva. An assay was conductedwith various
temperature alternations between 20℃ and otherinoculation
temperatures. The proper temperature was ranged from 15to 30℃.
However the optimal temperatures for fungal develop-ment
indiseased larvae were 20 and 25℃. The germination rate at 18
hr aftertreatment was higher at 20 and 25℃ than that at 30 and
35℃. Higherlevel of conidia germination occurred when they
incubated on cuticlewash out than they incubated on SMA+Y
medium. The LC50 values of various instars were determined by
using spraying application method. The values of LC50 decreased
withincreasing larval instars. Body surface of larvae is
positively correlatedto the instars (r= 0.99) and the number of
conidia fall on the cuticleis also positive related stage (r=
0.986). The mortality of neonate 4th-instar larvae were 2.6
fold hegher than that of larvae at quiescent stage,the present
data have indicating that corn earworm larvae could escape
fromfungal infection by molting. The 5th-instar larvae have a
longerdevelopmental time and heavy spore loading were more
susceptible to infectionthan other instars. The highest
larval mortality occurred at 48 hr after incubation under 100%
RH, but no significant difference was found among
treatmentsSporulation on cold dried cadavers were ranging from
50 to 100% RHwith maximal rates between 95 and 100% RH. The
time required forsporulation at 100% RH was 2 days earlier than
at 95% RH. Mortalities caused by incubating 5th instar under
variousphotoperiod regimes had no significant difference. But
the LT50 valueswere shorter when incubated in the light rather
than in the dark.Incubation of cadavers under 24L and 12L had a
22-fold increase inconidial production than under full dark.
High humidity and more than12 hr illumination is then necessary
for conidial production on cadavers.Yeast extracts were more
effective in conidial germination than other nutrients. Yeast
extracts at 0.1 and 1% could increase 13.2~18.8%more larval
mortalities than the control, whereas maltose and saccharoseat
0.01% had 15.5~16.4% more than the control. Pig liver as
culturesubstract had the highest spore production, i.e. 1.9x109
conidia/g, amongeight solid-state culture materials tested. The
pathogenicity of conidiaproduced by various culture materials
was assayed with 1x106 conidia/mlon 4th-instar larvae. Job''s
tear, mung bean, rice, and pig liver showedhigh larval
mortalities. Pig liver is thus recommended as a good andcheaper
material for mass production of N. rileyi spores. The 4th-
instar larvae were susceptible, causing 90.5~100% mortalitywhen
conidia were applied to 4 crops at 107 conidia/ml. Among them,
only application to chrysanthemum leaves resulted in lower
mortality thanother crops. The 5-instar larvae had the highest
mortality (94.6%) onthe soil with 20% water content, while lower
mortality (41.7%) was foundon 10% water content as sprayed the
soil surface with 108 conidia/ml ofthe conidial suspension.
Five fungicides, eight insecticides and nine herbicides commonly
used in corn fields were evaluated for their inhibition on
survival of N. rileyi by the paper disk test. Zineb and
propineb showed the highestinhibition of conidial germination
than other fungicides. However noinhibitory effect on
germination was found in insecticides and herbicidestested
except chloropyrifos. Application of N. rileyi conidial
suspension to the newly hatchedlarvae had better insecticidal
effect than 40.46% carbofuran (EC) at 800fold dilution. In
December, the individual corn ear injection by twosaveinterval
was better than the spraying as measured the marketable ears
anddegree of larval injury. Therefore, it is suggested that N.
rileyi has thepotential to be a biocontrol agent for corn
earworm.
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