Studies on some Factors that Influence Phase Dynamics of the Desert Locust, Schistocera gragaria (Forskal) (Orthoptera: Acrididae)

Abstract

The rate and the degree of reversible change in the phase characteristics of the desert locust, Schistocerca gregaria (Forskal), on crowding and uncrowding have been compared by two new parameters, aggregation pheromone titre (as measured by released phenylacetonitrile) and haemolymph pigment composition (as measured by absorbance ratio at 460 and 680nm) with colour and morphometrics. Changes in the four parameters followed different patterns. Adults of the F0 generation resulting from uncrowding crowd-reared (gregarious) nymphs, fledglings or mature adults were grey or dull yellow and did not produce phenylacetonitrile similar to solitary-reared adults. Conversely, adults of the F0 generation which emerged from crowding solitary-reared (solitarious) nymphs, fledglings or mature adults were yellow in colour and produced pheromone levels which were not significantly different from those of the control adults from crowd-reared colony. The levels of pheromone increased in the F1 generation relative to those of the control but decreased and levelled off in the F2 and F3 generations. Extreme sensitivity to crowding effects was demonstrated by the fact that adult males of the F0 generation resulting from crowding solitarious hoppers in groups of only two per cage produced phenylacetonitrile, although in significantly lesser amounts compared to those from normal experimental crowding condition (four/cage) or from crowd-reared control insects. Haemolymph pigment composition, on the other hand, gave a contrasting trend. It changed rapidly in nymphs, but slowly and erratically in adults both resulting from uncrowding or crowding at hopper stage. Significant shifts were recorded in both situations in nymphs within the F0 generation and in the adults by the end of the F2 and F3 generations of solitarisation and gregarisation, respectively. In contrast to pheromone emission and haemolymph pigment composition, morphometrics changes were slowest, significant shifts taking several generations. The F/C (hind-femur length to head capsule width) ratio was more sensitive to treatment effects than E/F (fore-wing to hind-femur length) ratio in agreement with previous findings. The primer effects of the volatiles emitted by live gregarious nymphs (first, third to fifth and fifth instars and their faeces), on solitarious nymphs and fledglings did not follow the same pattern. The volatiles from live gregarious first instar nymphs and faeces had no significant effect on haemolymph pigment composition but was associated with slight increase in the weight of test solitarious first instar nymphs and their development compared to control. In contrast, the volatiles from live gregarious third to fifth instar and their faeces induced formation of black patterns (melanization) on the bodies of first instar solitarious nymphs, changed their background colour from green to pinkish or pale green and arrested their development which was followed by death within three to four weeks as second or early third instar nymphs. On the other hand, this pheromone system had no notable effects on haemolymph pigment composition, weight and developmental time of solitarious second instar nymphs but had a slight effect on their colouration. Likewise, the colour, pheromone emission and haemolymph pigment composition of adults resulting from exposure of solitarious fledglings to the pheromone were not affected. In summation, this study has shown that: (a) the different phase characters change at different rates during uncrowding and crowding of the desert locust. Of the four parameters monitored, the emission of adult aggregation pheromone and integumental colour are more sensitive measures of the onset of phase change in adult desert locust. At the nymphal stages, integumental colour and haemolymph pigment composition are more suitable for monitoring phase change; (b) chemical communication (pheromones) in the desert locust appears to play an important primer role in phase change. The releaser/primer pheromone produced by nymphal gregarious locusts is stage- dependent and that of the late instars significantly affect the development of first instars of solitarious locusts.