EFFECTS OF SHIFTING TO CROWDED OR SOLITARY CONDITIONS ON PHEROMONE RELEASE AND MORPHOMETRICS OF THE DESERT LOCUST, SCHISTOCERCA-GREGARIA (FORSKAL) (ORTHOPTERA, ACRIDIDAE)

We have studied the effect of shifting the desert locust, Schistocerca gregaria (Forskal) from crowded to solitary conditions, or vice versa on the emission of the adult aggregation pheromone (as measured by re leased phenylacetonitrile) and compared this with changes in morphomet rics. Adult males of the F-0 generation resulting from shifting crowd- reared (gregarious) hoppers, fledglings or mature adults (aged 20-22 d ays after the final moult) to solitary conditions did not produce phen ylacetonitrile, similar to solitary-reared adults. Conversely, adults of the F-0 generation resulting from shifting solitary-reared (solitar ious) hoppers, fledglings or mature adults to crowded conditions produ ced pheromone at levels which were not significantly different from th ose of control adults from the crowd-reared colony. The levels of pher omone increased in the F-1 generation but decreased and leveled off in the F-2 and F-3 generations. Extreme sensitivity to crowding was demo nstrated by the fact that even pairing of one solitarious hopper with another produced F-0 adult males that produced phenylacetonitrile, alt hough in significantly lesser amounts than by males reared at a densit y of four per cage or by crowd-reared control males. In contrast, morp hometrics changes were slow and required several generations. The F/C ratio (hind-femur length to head capsule width) was more sensitive to treatment effects than the E/F ratio (fore-wing to hind-femur length), in agreement with previous findings. We conclude that pheromone titre s are a more sensitive measure than morphometrics to determine the ons et of phase change in the desert locust. Copyright (C) 1996 Elsevier S cience Ltd