Rhynchophorus ferrugineus (Olivier) (Coleoptera: Dryophthoridae) is an economically important pest of palm trees in the subtropics. Beauveria bassiana (Balsamo-Crivelli) Vuillemin (Hypocreales: Clavicipitaceae), has been shown to be pathogenic against R. ferrugineus in laboratory and field studies. However, because they remain inside the trunks until adulthood, the slowing of feeding and increases in mortality of internally feeding R. ferrugineus larvae over time after B. bassiana treatment has not been established. To explore the potential of acoustic methods to assess treatment effects, sound impulses produced by untreated, 10(4)-, and 10(6)-conidia ml(-1) B. bassiana-treated larvae in palms were recorded for 23 d, after which the palms were dissected and the larvae examined. Analyses were performed to identify trains of impulses with characteristic patterns (bursts) produced frequently by moving and feeding larvae but only rarely (3-8% of the larval rate) by interfering background noise or tree vibrations. The rates of bursts, the counts of larval impulses per burst, and the rates of impulses in bursts decreased significantly over time in both B. bassiana treatments but not in the control. This supports a hypothesis that larvae had briefer movement and feeding bouts as they became weaker after infection, which reduced the counts of larval impulses per burst, the rates of bursts, and the rates of impulses in bursts. There is considerable potential for use of acoustic methods as tools for nondestructive assessment of effects of biological control treatments against internally feeding insect pests.