Oviposition behaviour of Dacus tryoni: The effects of some sugars and salts C. H. Eisemann & M. J. Rice Department of Entomology, University of Queensland, St Lucia, Brisbane, Q. 4067, Australia Keywords: Dacus tryoni, Tephritidae, Diptera, fruit flies, oviposition, egg laying, behaviour, taste receptors, chemoreceptors, stimulant, deterrent, fructose, calcium Abstract Gravid Queensland fruit flies (Dacus tryom) are stimulated by the presence of/3-D(-) fructose to lay sig- nificantly more eggs in an agar substrate. Fructose is only effective when accessible to the tarsal and/or label- lar gustatory sensilla; it greatly increases oviposition through holes in an impenetrable membrane. Threshold for the fructose effect is 4 mM, maximal response being at 50 mM and above. Sucrose and glucose are not oviposition stimulants for D. tryonL In the field situation D. tryoni probably uses fructose as a marker to locate breaks in the skin of ripe fruit, where insertion of the ovipositor is easier. The flies are deterred from ovipositing in fructose agar by the presence of molar calcium chloride, even when this is inaccessible to the tarsal and labellar gustatory sensilla. Molar sodium chloride is not inhibitory. Calcium ions apparently exert their inhibitory effect via gustatory sensilla located on the ovipositor. Introduction The Queensland fruit fly Dacus (Bactrocera) tryoni (Frogg.) is widely distributed in eastern Aus- tralia, developing in the fruits of at least 117 species of plants (May, 1963). The typical oviposition be- haviour of this species has been described by Pritchard (1969). The fly first searches the cuticular surface of the fruit, frequently dabbing the labella on the surface. Once a sufficiently stimulating area is found it attempts to pierce the skin of the fruit with the tip of its ovipositor. As soon as the ovipos- itor penetrates the skin of the fruit, the fly probes more or less vertically into the pulp before laying a group of about four eggs. The ovipositor is then withdrawn and the fly repeats the process on the same or another fruit. During this behaviour it is reasonable to assume that the fly uses input from a range of sensilla to ensure that its eggs are deposited in a suitable place for larval develop- ment; among these will be gustatory sensilla which monitor the chemical nature of the fruit. Entomol. exp. appl. 39, 61-71 (1985). 9 Dr W. Junk Publishers, Dordrecht. Printed in the Netherlands. Detailed information is available on gustatory chemoreception in relation to host plant selection in several of the phytophagous Diptera: labellar chemosensilla in Mediterranean fruit fly, cabbage root fly and others respond to water, salts and su- gars; behavioural and electrophysiological evidence shows the involvement of tarsal sensilla in detecting chemicals present on the plant surface; and the ovi- positor of some dipterans has a limited number of chemosensilla (reviewed by Schoonhoven, 1983; see also Liscia et al., 1982). The presence of receptors for glucose, fructose, sodium chloride, malic acid and quinic acid on the ovipositor of the apple mag- got fly, Rhagoletispomonella (Walsh), has recently been demonstrated electrophysiologically (Girola- mi et al., 1983). In Rhagoletis cerasi (L.), host and non-host odours have been shown to regulate the extent of egg laying (Levinson & Haisch, 1983). Host odours also play an important part in the be- haviour leading up to oviposition in D. tryoni, a subject that we plan to discuss in a separate paper. This paper describes behavioural experiments