1. Plants leave legacy effects in the soil they grow in, which can drive important vegetation processes, including productivity, community dynamics and species turnover. Plants at the same time... Show more1. Plants leave legacy effects in the soil they grow in, which can drive important vegetation processes, including productivity, community dynamics and species turnover. Plants at the same time also face continuous pressure posed by insect herbivores. Given the intimate interactions between plants and herbivores in ecosystems, plant identity and herbivory are likely to interactively shape soil legacies. However, the mechanisms that drive such legacy effects on future generations of plants and associated herbivores are little known.2. In a greenhouse study, we exposed 10 common grasses and non-leguminous forbs individually to insect herbivory by two closely related noctuid caterpillars, Mamestra brassicae and Trichoplusia ni (Lepidoptera: Noctuidae) or kept them free of herbivores. We then used the soil legacies created by these plant individuals to grow a plant community composed of all 10 plant species in each soil and exposed these plant communities to M. brassicae. We measured conditioning plant biomass, soil respiration and chemistry of the conditioned soils, as well as individual plant, plant community and herbivore biomass responses.3. At the end of the conditioning phase, soils with herbivore legacies had higher soil respiration, but only significantly so for M. brassicae. Herbivore legacies had minimal impacts on community productivity. However, path models reveal that herbivore-induced soil legacies affected responding herbivores through changes in plant community shoot: root ratios. Soil legacy effect patterns differed between functional groups. We found strong plant species and functional group-specific effects on soil respiration parameters, which in turn led to plant community shifts in grass: forb biomass ratios. Soil legacies were negative for the growth of plants of the same functional group.4. Synthesis. We show that insect herbivory, plant species and their functional groups, all incur soil microbial responses that lead to subtle (herbivory) or strong (plants and their functional group) effects in response plant communities and associated polyphagous herbivores. Hence, even though typically ignored, our study emphasizes that legacies of previous insect herbivory in the soil can influence current soil–plant–insect community interactions. Show less
In this thesis, the role of plant-mediated soil legacy effects in shaping aboveground plant-insect interactions was investigated. This work shows that soil legacy effects on plant-insect... Show moreIn this thesis, the role of plant-mediated soil legacy effects in shaping aboveground plant-insect interactions was investigated. This work shows that soil legacy effects on plant-insect interactions are a common phenomenon in nature, both in individual responding plant species and their associated herbivores, as well as in responding plant communities and associated herbivores. This thesis elucidates two potential mechanisms how soils can influence aboveground insects. First, soils can alter the way plants defend them selves in terms of secondary chemistry and phytohormonal defenses. Second, soils can also directly impact aboveground insects by changing the insect microbiome. This work shows that a large part of the insect microbiome is taken up directly from the soil microbiome, and leads to exciting new research directions on the longterm and evolutionary implications of soils for aboveground insects. Show less
Crops may escape from the field and establish feral populations. Crops may also cross with wild plants. Both are points of concern for Genetically Modified crops. We study this problem for the crop... Show moreCrops may escape from the field and establish feral populations. Crops may also cross with wild plants. Both are points of concern for Genetically Modified crops. We study this problem for the crop oilseed rape (Brassica napus) and its wild relative B. rapa. Hybrids between the two species perform well in the lab, even under conditions of water stress. Seedlings of crop and hybrids are, however, vulnerable to attack by slugs. By excluding slugs from field plots we showed that slug herbivory causes a strong reduction of seedling survival. Slugs avoid seedlings with high glucosinolate (GS) content. Modern cultivars of B. napus are low in GS content, while wild B. rapa has a high GS content. By applying GS to leaf discs we showed that the aliphatic GS progoitrin repels the slugs, whereas the indole GS glucobrassicin had no effect, even at elevated concentrations. We reanalysed large two studies in which the slug Arion lusitanicus was presented with leaves of different plant species. Slugs preferred plant with low Dry Matter Content (high water content) and shade-intolerant plant with low Specific Leaf Area. These correlations provide useful leads for further experimental research. Show less
As first step the impact of herbivory and abiotic factors on population dynamics of Arabidopsis thaliana were studied. Ceutorhynchus atomus and C. contractus were identified as the major insect... Show moreAs first step the impact of herbivory and abiotic factors on population dynamics of Arabidopsis thaliana were studied. Ceutorhynchus atomus and C. contractus were identified as the major insect herbivores on A. thaliana population, reducing seed production by more than 40%. Mortality from February to April was identified as a key- factor. Next, variation in morphological and life history trait and sutability of A. thaliana as food for weevils for A. thaliana population growing in the dune and inland habitat were studied. These variatioan were genetically and environmentally based. Plants at its home site had the highest fitness, which demonstrates adaptation to local environment. To examine the cause of differences in plant defense we compared herbivory on fruits with data on glucosinolate concentration of seeds, collected in the field. Fruit damage by adult weevils and their larvae was not correlated with field concentration of glucosinolates. We conclude that other factors might also be involved in defense against herbivory by the weevils. Finally, to test if glucosinolates evolve as a contrasting selection pressure by different herbivores we look if the generalist herbivore Spodoptera exigua and the specialist herbivores Plutella xylostella were differently affected by chemical compounds in the leaves. No significant correlations were found between larval weight of P. xylostella and glucosinolates in the leaves but glucosinolates negatively correlated with larval weight of S. exigua. We suggest that generalist herbivores may play a major role in this process. Show less
