Submitted to: Mycopathologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/17/2008
Publication Date: 1/6/2009
Citation: Cabanillas, H.E., Jones, W.A. 2009. Effects of temperature and culture media on vegetative growth of an entomopathogenic fungus Isaria sp. (Hympcreales: Clavicipitaceae) naturally affecting the whitefly, Bemisia tabaci in Texas. Mycopathologia. 167:263-271. Interpretive Summary: The silverleaf whitefly is considered one of the most damaging whitefly pest species in economically important crops worldwide. Fungi, that attack insect pests, can be effective in controlling whitefly and serve as alternatives to broad-spectrum chemical pesticides, especially due to concern about chemicals residues in crops, and hazards to the environment and human health. An indigenous insect fungus species was recently discovered killing whiteflies naturally in the Lower Rio Grande Valley of Texas. However, the practical use of these fungi depends on our knowledge of its temperature limits and its growth on culture media. We studied the influence of temperature and culture media on the growth of this fungus. We found that this fungus grows better in Sabouraud dextrose agar with yeast extract than on Sabouraud maltose agar. Maximum growth was achieved at 30 ºC; moderate growth occurred at 25 ºC; and lowest growth resulted at 20 ºC. This fungus was able to produce high number of spores in these culture media, which is an important feature for its easy mass production in common media. These findings will be immediately useful in the development of an effective commercial product for management of the silverleaf whitefly. The discovery of this fungus naturally adapted to high temperatures, along with these findings, show its potential as a biological control agent of the silverleaf whitefly especially for the subtropical semi-arid environment.
Technical Abstract: The effects of temperature and mycological media on mycelial growth and estimates of spore production of the newly discovered entomopathogenic fungus Isaria poprawskii sp. nov. were investigated. The general response of daily radial growth of I. poprawskii as a function of temperature fits a linear model on SDAY or SMA for 20 ' T ' 30 ºC but a curvilinear model for 20 ' T ' 35 ºC. At 25 ºC for 14 days, vegetative growth rates of I. poprawskii were greater on Sabouraud dextrose agar with yeast extract (SDAY) than on Sabouraud maltose agar (SMA). Maximum mycelial growth rates of I. poprawskii, as determined by colony diameters (mm/day), occurred at 30 ºC (SDAY: 4.1 mm, SMA: 3.1mm). Moderate growth occurred at 25 ºC (SDAY: 3.4 mm, SMA: 2.7 mm). Growth was lowest at 20 ºC (SDAY: 1.9 mm, SMA: 1.8 mm). When this fungus was exposed to 35ºC for the first 7 days, its growth was inhibited, but it could recover and grow when transferred to 25 ºC (SDAY: 3.5 mm, SMA: 2.8 mm). No growth occurred at 40 ºC; and no recovery or growth occurred transferred to 25 ºC. The average spore production on SDAY after 20 days incubation at 25 ºC and a photoperiod of 14:10 h light: dark was 1.2 x 108 conidia/ cm2 with 100% spores’ viability. When compared on SDAY at 25ºC, the absolute growth rate of I. javanica ex type CBS 134.22 (5.1 mm) was greater than the other Isaria isolates; but relative growth rates were similar among all selected Isaria isolates. I. poprawskii tolerates high temperatures (35ºC), suggesting that it is naturally selected for the subtropical semi-arid environment where it serves as an important natural control agent of the silverleaf whitefly, Bemisia argentifolii (Hemiptera: Aleyrodidae), one of the most invasive and economically damaging insects to agriculture.