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Abstract
The infectivity of 4 isolates of Metarhizium anisopliae to puparia of Ceratitis capitata
treated as late third-instar larvae in unsterilized soil was investigated in the laboratory
under controlled temperature and moisture. At 20-30 degrees C, mortality in puparia
was highest at water potential of -0.1 and -0.01 mega Pascal (MPa) and lowest at water
potential of -0.0055 and -0.0035 MPa in all the isolates. In wetter soil however,
isolates ICIPE 20 and 60 caused significantly higher mortality than ICIPE 18 and 69.
The survival of conidia in drier soil (-0.1 MPa) was not adversely affected at all
temperatures. However, in wet soil (-0.0035 MPa) there was drastic reduction in colony
counts in ICIPE 18 and 69 at 25 and 30 degrees C but conidial density in ICIPE 20
and 60 remained at the initial level at 14 days after inoculation at all temperatures.
When ICIPE 20 was evaluated against three other fruit fly species (Ceratitis cosyra,
Ceratitis rosa, and Ceratitis fasciventris), significant reduction in adult emergence
and higher pupal mortality occurred in C. cosyra and C. fasciventris than in C. rosa
at a combination of 15 and 20 degrees C and -0.1 and -0.0035 MPa. However, at higher
temperature and the same moisture level, the isolates were equally pathogenic across
the 3 species. It is probable that in addition to pathogen cycling and multiplication
from dead infected insects in the soil, a balance between microbial degradation and
replenishment of inoculum of virulent isolates occur through fluctuations in, and
intricate interactions between temperature and moisture levels. This study is indicative
of the potential of using isolate ICIPE 20 for soil inoculation against pupariating
third-instar larva of fruit flies, thus providing a novel alternative to chemical
soil application.