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- The complex foraging strategy of the specialised gallfly Urophora cardui (Diptera: Tephritidae) for host plants (Cirsium arvense, Asteraceae) (2004)
- Nearly all herbivorous insects in terrestrial ecosystems depend on plants for their survival and reproduction. They dominate terrestrial ecosystems due to species and individual abundance. Interactions between these two groups are thus of a high significance for the analysis and the understanding of complex interactions in terrestrial ecosystems. Foraging for host plants by herbivorous insects is of a central importance. In the present thesis the foraging strategy of the specialised gall fly Urophora cardui on the creeping thistle, Cirsium arvense, was investigated as an example for the foraging of a specialised herbivorous insect. Males and females of U. cardui use the larval host plant as rendezvous place. The males establish territories on the plant, which they defend against conspecifics. The females lay eggs into axillary buds in order to initiate gall development. Male and female body size, measured as weight at eclosure or capsule width, was not correlated with male respectively female longevity. Males lived shorter than females. Capsule width of males and females was not significantly different, while weight at eclosure and fresh weight at death was. Females weighed more, which may be due to their higher need of energy during adult life. Both sexes lost body weight during life. All behaviours, which are performed by males and females on the host plant were defined, recorded and analysed. Females spent most of the time on resting, probing axillary buds, running on the plant and grooming. Males spent most of the time on copulation and patrolling their territory. The behaviour of both sexes was highly variable between individuals. Concerning the movement pattern on an already chosen host plant, females concentrate on the upper parts of the host plant. They were mainly occupied with extensive probing of various axillary buds, which occurred in a suitable developmental stage at the top of the plant. In contrast males patrolled the whole plant, although only the upper leaves became marked. If they encountered another male threatening and fighting were inevitable. Fights lasted several hours interrupted by threatening periods. Mating of males and females usually followed oviposition. Neither females nor males accepted modified host plants or models of thistles. Their behaviour on modified thistles was reduced mainly to running around the plant and grooming. These results indicate a rigid host plant template using the input of several senses, the flies always recognise models and modified plants as a non-host plant. Both sexes were able to discriminate host plants and non-host plants from a distance of 0.8-2m. The time male and female flies needed until they selected one of the host plants in a particular host plant stand depended on the number of non-host plants, host plants and the number of suitable hosts. The decision-time became shorter, if there were not too many suitable host plants. This may be due to decreasing sampling time of the host plants present. U. cardui females did not prefer plants of a certain height. In contrast the branching level, which indicates the number of axillary buds, and the number of flower buds played a significant role during foraging for host plants. Plants with an intermediate branching level were preferred, while those with many flower buds were avoided. Males of U. cardui were able to select their territorial plant on olfactory cues or on visual cues likewise. In contrast, emales were not able to recognise their host plant on olfactorial cues alone. But, if male-marked and unmarked host plants were available they significantly preferred the marked thistles. The differentiation between marked and unmarked C. arvense was made according to olfactorial cues, since plants did not differ in their height, nor in their branching level, nor in the number of flower buds. Thus, female selection of larval host plants depended on male choice for territories. This result is remarkable, especially in evolutionary terms, since males select the larval host for the offspring of their predecessor. Interestingly males preferred plants marked by conspecifics also. On the tip of the females’ ovipositor there is a receptor field with several morphological different receptor types. Mainly they seem to have mechanoreceptive as well as chemosensory functions. These receptors enable the females to measure and analyse the inner structure of the plant tissue at the axillary bud. The width of the apical meristem of the axillary buds was proven to influence female choice of the oviposition site. Axillary buds with an apical meristem-diameter above 0.62mm had a higher probability to became chosen by the U. cardui females. The clutch size was adjusted to the diameter of the apical meristem, indicating, that females were able to estimate the quality of the respective axillary bud influencing larval performance. These results were summarized in a model of the foraging strategy of U. cardui.