Faculty of Agricultural Sciences, University of Hohenheim, 70599 Stuttgart, GERMANY.
Adults of the codling moth, Cydia pomonella L. (Lepidoptera, Tortricidae), live on host plant surfaces, differing considerably in their structural, chemical, and physicochemical characteristics according to host plant species, cultivar, plant organ (e.g. upper/lower leaf sides, fruits), phenological stage, environmental conditions, and orchard management practices. This large, variable world provided by plant surfaces can profoundly affect directly and indirectly many aspects of insect–plant interactions, such as attachment, locomotion, oviposition site selection, egg adhesion, and also survival of adults and their offsprings. Despite their importance, little attention has been given to the structural and wetting properties of the codling moth’s host plant surface and their effect on insect–plant interactions of this important pest. Therefore, studies in this thesis were undertaken to investigate the effect of structural and physicochemical characteristics of the substrate on two main codling moth-plant interactions: (1) the attachment ability of adults, and (2) the adhesion of their eggs.
The first part of this thesis was performed to (1) analyze tarsal morphology of male and female C. pomonella to know more about their pretarsal attachment devices, and (2) to investigate their attachment ability on a variety of smooth and rough substrates, using a centrifugal force device. On all smooth artificial substrates tested, both sexes of C. pomonella adults achieved excellent attachment ability, by means of their smooth, flexible and well developed arolia. Hydrophobicity of the substrate had no considerable effect on friction forces. Cydia pomonella females showed a very good attachment ability to the smooth Plexiglas® substrate in both positions: horizontally (friction forces were 11 times their own body weight), and vertically (adhesion forces were 7 times their own body weight). Thus, it can be concluded that the attachment system of C. pomonella is rather robust against physicochemical properties of the substrate and is able to achieve a very good attachment on vertical and horizontal plant surfaces.
Results on the epoxy resin substrates (six substrates made of polymerized Spurr resin, differing only in surface asperity size: 0 μm, 0.3 μm, 1 μm, 3 μm, 9 μm, and 12 μm) revealed that the attachment ability of both sexes was significantly affected by surface roughness. Maximal friction force was measured on the smooth substrate whereas minimal friction force was assessed on microrough substrates with 0.3 μm and 1.0 μm size of asperities. On the remaining rough substrates, friction forces were significantly higher but still lower than on the smooth substrate. It is worth to note that both sexes generated similar friction forces on the same substrate (for all smooth or rough substrates tested), in spite of the considerable difference in their body mass, suggesting that both sexes attach effectively to variable rough plant surfaces in their habitat. However, since smooth surfaces have been reported previously to be the most favorable substrates for ovipositing females of C. pomonella, it is possible that they use their attachment system to sense the substrate texture and prefer those substrates to which their arolia attach the best.
A better survival of the codling moth offspring is assumed to be ensured by the selection of suitable oviposition sites by females, as well as by a proper adhesion of deposited eggs to these sites. In apple orchards, eggs of the first generation of the codling moth are laid on leaf surfaces in the vicinity to small fruits, later in the growing season, most eggs are deposited directly on fruits. In the second part of this thesis, egg adhesion of the codling moth to different leaf and fruit surfaces of the domestic apple, Malus domestica Borkh., was investigated by measuring the pull-off force required to detach the eggs from the plant surface. Morphological and physicochemical properties (wettability, free surface energy) of the tested plant surfaces were analyzed to evaluate their role in egg adhesion. Furthermore, eggs and their adhesives covering leaf or fruit surfaces were visualized. Eggs on the smooth upper leaf sides of the tested cultivars were easily detached, requiring similar pull-off forces (total average of 6.0 mN). Up to 2-3 times stronger pull-off forces had to be applied to detach eggs from the trichome-covered lower leaf side (total average of 17.1 mN), and these forces differed significantly between cultivars, owing mainly to different trichome covered areas. Whereas on the waxy fruit surface of all apple cultivars tested, eggs were very tightly adhered, and required 4-10 fold stronger pull-off forces than those previously measured on upper and lower leaf surfaces of the identical apple cultivars (total average of 63.9 mN). Cydia pomonella eggs adhered stronger on the upper and middle fruit sections of all cultivars tested, than on the lower section.
The influence of plant surface properties on egg adhesion, as well as the mechanisms used by the moth to overcome the presumable anti-adhesive properties of apple fruit surfaces, were discussed. Additionally, the results were debated in the context of the oviposition site selection, female attachment, as well as offspring survival of the first, second and third generations of the codling moth.