The effect of mode of exposure to Beauveria bassiana on conidia acquisition and host mortality of Colorado potato beetle, Leptinotarsa decemlineata

The effects of the mode of exposure of second instar Colorado potato beetle s to Beauveria bassiana on conidia acquisition and resulting mortality were investigated in laboratory studies. Larvae sprayed directly with a B, bass iana condial suspension, larvae exposed to B, bassiana-treated foliage, and larvae both sprayed and exposed to treated foliage experienced 76, 34, and 77% mortality, respectively. The total number of conidia and the proportio n of germinating conidia were measured over time for four sections of the i nsect body: the ventral surface of the head (consisting mostly of ventral m outh parts), the ventral abdominal surface, the dorsal abdominal surface, a nd the legs. From observations at 24 and 36 h posttreatment, mean totals of 161.1 conidia per insect were found on sprayed larvae, 256.1 conidia on la rvae exposed only to treated foliage, and 408.3 conidia on larvae both spra yed and exposed to treated foliage, On sprayed larvae, the majority of coni dia were found on the dorsal abdominal surface, whereas conidia were predom inantly found in the ventral abdominal surface and mouth parts on larvae ex posed to treated foliage, Between 24 and 36 h postinoculation the percentag e of conidia germinating on sprayed larvae increased slightly from 80 to 84 %), On the treated foliage, the percentage of germinated conidia on larvae increased from 35% at 24 h to 50% at 36 h posttreatment, Conidia germinatio n on sprayed larvae on treated foliage was 65% at 24 h and 75% at 36 h post treatment, It is likely that the gradual acquisition of conidia derived fro m the continuous exposure to B. bassiana inoculum on the foliar surface was responsible for the increase in germination over time on larvae exposed to treated foliage, The density and germination of conidia were observed 0, 4 , 8, 12, 16, 20, and 24 h after being sprayed with or dipped in conidia sus pensions or exposing insects to contaminated foliage, Conidia germinated tw ice as fast on sprayed insects as with any other treatment within the first 12 h, This faster germination may be due to the pressure of the sprayer en hancing conidial lodging on cuticular surfaces. (C) 2001 Academic Press.