Oviposition behaviour as influenced by the oviposition deterring pheromone in the large white butterfly, Pieris brassicae

Abstract

This thesis deals with a detailed analysis of egglaying behaviour of adult females of the Large White Butterfly, Pieris brassicae, and the way this behaviour is influenced by the oviposition deterring pheromone (ODP) in order to investigate the prospects for field application of this pheromone in cabbage pest control.The study begins with a short introduction on the role of oviposition behaviour in the relationship between herbivorous insects and plants (Chapter 1).In Chapter 2 the ultimate effect of the ODP on egglaying behaviour of Pieris brassicae was demonstrated. In a choice situation with control leaves and ODP treated cabbage leaves females performed a strong preference to lay their eggs on the control leaves. The pheromone, which is associated with the eggs, appeared to be water- and methanol-soluble and could be stored for more than three years without loosing activity. In addition we found some evidence that eggs may contain large amounts of pheromone. The existence of this oviposition deterring pheromone in P. brassicae might contribute to a regular distribution of eggs over available hostplants.In Chapter 3, a detailed description is given of egglaying behaviour of P. brassicae in the absence of the pheromone. This description formed the ethological framework on which further investigations of the behavioural effects of the ODP were based. In addition to this ethogram, the various sensory cues which may be involved in oviposition behaviour are discussed.Perception of the oviposition deterring pheromone by P. brassicae females was studied in some detail in Chapter 4. Electrophysiological experiments indicated that females possess sense cells specifically sensitive to the ODP in their fore-tarsal taste hairs. In morphological studies, by means of the scanning electron microscope, and by electrophysiological recordings we established the presence of contactchemoreceptors on the ovipositor of P. brassicae females. Stimulations with ODP solutions indicated that to some extent these taste hairs might be involved in ODP perception. Responses to hostplant chemicals revealed that hostplant recognition is not mediated by sensory input from these hairs.Chapters 5 and 6 are concerned with an experimental analysis of female preoviposition behaviour in order to determine the detailed behavioural effects of the ODP and the chemoreceptors females actually employ in ODP perception. The experimental set-up was identical in both chapters with single females offered a choice between a control and an ODP treated leaf. The behaviour of females was recorded by means of a keyboard system and afterwards analysed for several parameters, including the sequence of behavioural acts.In Chapter 5 the results with untreated females are described. Our observations indicated that the ODP induces only minor changes in pre- alighting behaviour of these females, suggesting that the pheromone is not very volatile. Most obvious behavioural differences between the two leaves were observed after landing, which indicate that tarsal contactchemoreceptors probably mediate the behavioural response to ODP. In addition, females were observed to perform a reduced tendency to stay on or around the treated leaf. This observation is congruent with the idea that the ODP induces P. brassicae females to disperse.In Chapter 6, these experiments were extended to six groups of females with various sensory ablations. When all six tarsi, whether or not in combination with the antennae, were inactivated (by HCl treatment), oviposition was almost completely suppressed and females did not discriminate anymore between the two types of leaves. In females with intact chemoreceptors on at least one pair of tarsi egglaying activity was about normal and also discrimination was found to occur. Behavioural evidence was presented that females may employ antenna] olfactory hairs, fore-tarsal taste hairs, as well as taste hairs on midand hind-tarsi to perceive the pheromone. Chemoreceptors on the ovipositor were not found to be involved in ODP perception. Finally, it was observed that females did not respond to the ODP anymore once the first egg had been laid.In Chapter 7, the first field tests with the ODP are described. In control experiments without pheromone, the plants chosen for oviposition by P. brassicae females were distributed rather evenly over the cage. When the majority of plants was sprayed with ODP, oviposition attempts were mainly observed on plants along the sides of the cage. This strongly suggests that the ODP indeed evokes females to disperse from the field. The presence of the pheromone did not result, however, in a significant decrease of oviposition on treated plants. This might have been due to our experimental conditions.In Chapter 8, behavioural responses of Pieris rapae butterflies to the ODP of P. brassicae are investigated. ODP treated cabbage leaves remained deterrent to ovipositing P. rapae females for at least 8 days. A conspecific ODP in P. rapae could not be demonstrated. It is concluded that this interspecific effect of the ODP enhances the prospects for field application of this pheromone in cabbage pest control