The ecology of a natural population of Queensland fruit fly, Dacus tryoni II. The distribution of eggs and its relation to behaviour

Abstract

The oviposition of Queensland fruit fly is described and the distribution of eggs within and between fruits is analysed. Olfactory and visual cues direct females to fruit. A brief exploratory phase then follows after which the female applies the ventral surface of her head to areas of fruit surface and then moves forward to probe these same areas with her ovipositor. The female lays her eggs into the fruit either after piercing the cuticle with her ovipositor or through existing holes such as previous oviposition holes or holes made by codling moth larvae. As many as 70% of the eggs laid into hard, shiny fruits were laid into existing holes, but in softer fruits with protuberances over the surface less than 20% were laid in this way. The difference is related to a backward component in the push by the ovipositor, leading to difficulty in piercing hard, smooth unbroken skin. Ovipositions are further grouped on single fruits owing to the favourability of certain areas in relation to such physical factors as wind and illumination. In the laboratory it was not possible under any circumstances to achieve a uniform dispersion of ovipositions when a succession of single females was given the choice of a number of oviposition sites. Contagious distributions were the rule, although randomness of oviposition was exhibited in certain experimental designs. In the field, under natural and experimental conditions, oviposition holes were generally contagiously distributed between fruit. The data offer no support to the hypothesis that oviposition by D. tryoni is affected, in a limiting way, by previous acts of oviposition by other females. An alternative hypothesis for egg limitation at high density, based on aggressive interactions between females on fruit, is suggested.