Pergamon PII: SOO22-1910(96)00015-7 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ J. Insect Physiol. Vol. zyxwvutsrqponmlkjihgfedcba 42, No. 8, 111-776, 1996 pp. Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-1910/96 $15.00 + 0.00 Effects of Shifting to Crowded or Solitary Conditions on Pheromone Release and Morphometrics of the Desert Locust, Schistocerca gregaria (Forsk%l) (Orthoptera: Acrididae) AROP LEEK DENG,* BALDWYN TORTO,*$ AHMED HASSANALI,* E. E. ALIT Received 19 July 1995; revised 5 December 1995 We have studied the effect of shifting the desert locust, Schistocerca greguriu (Forskil) from crowded to solitary conditions, or vice versa on the emission of the adult aggregation phero- mone (as measured by released phenylacetonitrile) and compared this with changes in mor- phometrics. Adult males of the F0 generation resulting from shifting crowd-reared (gregarious) hoppers, fledglings or mature adults (aged 20-22 days after the final moult) to solitary conditions did not produce phenylacetonitrile, similar to solitary-reared adults. Con- versely, adults of the F, generation resulting from shifting solitary-reared (solitarious) hop- pers, fledglings or mature adults to crowded conditions produced pheromone at levels which were not significantly different from those of control adults from the crowd-reared colony. The levels of pheromone increased in the F, generation but decreased and leveled off in the F2 and F, generations. Extreme sensitivity to crowding was demonstrated by the fact that even pairing of one solitarious hopper with another produced F, adult males that produced phenylacetonitrile, although in significantly lesser amounts than by males reared at a density of four per cage or by crowd-reared control males. In contrast, morphometrics 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 titres are a more sensitive measure than morphometrics to determine the onset of phase change in the desert locust. Copyright 0 1996 Elsevier Science Ltd Phase transformation Pheromone production Morphometrics Schistocerca gregaria INTRODUCTION Two phases of locusts have been distinguished: solitaria and gregaria (Uvarov, 1921). The phase status of the desert locust, Schistocerca gregaria (Forskal) has com- monly been characterised on the basis of morphometrics (Dirsh, 1951, 1953; Gunn and Hunter-Jones, 1952; Hunter-Jones, 1958; Stower et al., 1960; Symmons, *International Centre of Insect Physiology and Ecology (ICIPE), P.O. Box 30772, Nairobi, Kenya. TDept. of Crop Protection, Faculty of Agriculture, University of Khar- toum, P.O. Box 32, Shambat, Sudan. fTo whom all correspondence should be addressed. 1969) colour (Gunn and Hunter-Jones, 1952; Nickerson, 1956; Hunter-Jones, 1958; Ellis and Carlisle, 1961), behaviour (Ellis, 1959; Ellis and Pearce, 1962; Gillett 1972, 1973, 1988; Roessingh et al., 1993; Roessingh and Simpson, 1994; Islam et al., 1994) reproductive charac- teristics such as the number of ovarioles, eggs per egg- pod and pods per female (Hunter-Jones, 1958; Injeyan and Tobe, 1981), and endocrine balance, especially in relation to juvenile hormone (Ellis and Carlisle, 1961; Ellis and Novak, 1971; Wilson and Morgan, 1978; review by Pener, 1991). Solitarious desert locusts are non-communal. The nymphal stages are often characterised by a green or straw colour and the adults are greyish brown. Typical 771