Studies on Stenobracon deesae (Cam.), a parasite of certain Lepidopterous borers of graminaceous crops in India

Abstract

Experiments were conducted to investigate oviposition and host selection by Stenobracon deesae (Cam.), a Braconid parasite of certain Lepidoperous borers of sugar-cane, maize and sorghum in India. The fecundity and sex-ratio of this parasite and the duration of its life-cycle were also studied. Chilo zonellus (Swinh.) and Corcyra cephalonica (Staint.) were used as hosts. The former is one of the principal hosts in the field, but the latter is not normally a host of this parasite. The femal Stenobracon normally avoids laying eggs on hosts that are already parasitised by its own kind when unparasitised hosts are available; but in the absence of healthy hosts or when these are too crowded, parasitised hosts are attacked by it. At a temperature of 26°C. and relative humidity of 75 per cent., the life-cycle of the parasite was completed, on an average, in 23.3 days, but under laboratory conditions at Delhi it took 14.1 days in June-July and 43.7 days in November-December. The average duration of adult life of females in June-February, mainly in ordinary laboratory conditions, was 35.7 days, and single females lived as long as 122 and 128 days. The adult male lived for 42.7 days on the average. In the laboratory, the female lays about 24 eggs on an average, but only about one-third of them yield adults. Males predominate. The sex-ratio appears to vary according to the host; among adults reared on Chilo larvae, 28.8 per cent. were females, but among others reared on Corcyra larvae, only 6.2 per cent. were females. The phenomena of discrimination between parasitised and unparasitised hosts and of the change of sex-ratio with change of host are discussed. It is suggested that the ovipositor enables the parasite to recognise the host and that the stimuli received by the ovipositor from the host influence the functioning of the spermatheca which in turn controls fertilisation and finally the sex of the progeny.