Effects of Temperature and Culture Media on Vegetative Growth of an Entomopathogenic Fungus Isaria sp. (Hypocreales: Clavicipitaceae) Naturally Affecting the Whitefly, Bemisia tabaci in Texas

Abstract

The effects of temperature and mycological media on mycelial growth and estimates of spore production of an indigenous entomopathogenic fungus, Isaria sp., found during natural epizootics on whiteflies in the Lower Rio Grande Valley of Texas, were investigated. The radial growth (mm/day) of Isaria sp. as a function of temperature fits a linear model; with faster growth on Sabouraud dextrose agar with yeast extract, SDAY slopes (0.23) than on Sabouraud maltose agar, SMA slopes (0.14) from 20 to 30 degrees C, with an optimal temperature of 30 degrees C (SDAY: 4.1 mm, SMA: 3.1 mm). Moderate growth occurred at 25 degrees C (SDAY: 3.4 mm, SMA: 2.7 mm). Growth was lowest at 20 degrees C (SDAY: 1.9 mm, SMA: 1.8 mm). No fungal growth was observed at 35 degrees C and 40 degrees C. However, when Isaria sp. was exposed to 35 degrees C for the first 7 days, it could recover and grow when transferred to 25 degrees C (SDAY: 3.5 mm, SMA: 2.8 mm). No recovery or growth occurred after transfer from 40 degrees C to 25 degrees C. The average conidial production on SDAY after 20 days incubation at 25 degrees C and a photoperiod of 14:10 h light: dark was 1.2 x 10(8) conidia/cm(2) with 100% spore viability. When compared on SDAY at 25 degrees C, the radial growth rate of I. javanica ex type CBS 134.22 (5.1 mm/day) was greater than seven Isaria isolates including Isaria sp.; but maximum growth rates were similar among all related Isaria isolates (90-97%). The Isaria sp. fungus tolerates high temperatures (35 degrees C), suggesting that it is naturally selected for the subtropical semi-arid environment, where it could serve as an important natural control agent of the sweet potato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) biotype B, one of the most invasive and economically damaging insects to agriculture.