Plant defence in a tritrophic context : chemical and behavioural analysis of the interactions between spider mites, predatory mites and various plant species

The spider mite Tetranychus urticae Koch is a serious pest in field crops, glasshouse vegetables and fruit crops. It is a generalist herbivore with several hundreds of host plant species. Phytoseiulus persimilis Athias-Henriot is one of its natural enemies. Investigations of the tritrophic system of plant, T. urticae and P. persimilis will contribute to a better knowledge about the direct and indirect defence defensive strategies of plant species.Host plant acceptance by the spider mite T. urticae , as a measure of the plant's direct defence, was investigated for eleven plant species. The degree to which the spider mites accepted a plant was expected to depend on differences in nutritive and toxic constituents among plant species. At the level of plant species, a large variation in the degree of acceptance by T. urticae was found. Except for ginkgo ( Ginkgo biloba ) most plants were accepted or well accepted by the spider mites. At the level of plant family, four plant species from the Fabaceae were compared to four plant species from the Solanaceae. It was shown that all species from the Fabaceae were accepted by the spider mites for feeding, while plant species from the Solanaceae varied in spider mite acceptance from well accepted (tobacco: Nicotiana tabacum ) to poorly accepted (sweet pepper: Capsicum annuum ).Some of the plant species that had already been investigated with respect to spider mite acceptance were tested for their degree of indirect defence. After spider mite-infestation the plants attracted the predatory mite P. persimilis . The results showed that plants from all species significantly attracted the predatory mites when infested by spider mites. Experience with the spider mite-infested leaves of the investigated plant species did not affect the predatory mite's choice. Based on the results that spider mites did not survive on ginkgo leaves, these leaves were treated with jasmonic acid to induce a mimic of a spider mite-induced volatile blend. The predatory mites were slightly attracted to the induced volatile blend of jasmonic acid treated leaves. In summary, plants do invest in indirect defence after being attacked by spider mites, even when some plants have already a strong direct defence.Subsequently , it was investigated to what degree spider mite-infestation of plants from all species resulted in the emission of novel compounds that were not emitted by undamaged or mechanically damaged plants of the same species. Therefore, the volatiles emitted by T. urticae -infested leaves were analysed and compared to volatiles emitted by clean and mechanically damaged leaves of the same plant species. Almost all of the investigated plant species produced novel compounds that dominated the volatile blend after spider mite infestation, such as methyl salicylate, terpenes, oximes or nitriles. However, spider mite-infested eggplant and tobacco emitted only a few novel compounds and in small amounts. Methyl salicylate was found as dominant compound in six of the investigated plant species and as a less dominant compound in two plant species. However, it was concluded that methyl salicylate alone does not necessarily indicate spider mite-damage of the plant.In the introduction (Chapter 1) a hypothesis was postulated that plant species with a weak direct defence would invest in the production of novel compounds after spider mite-infestation, in contrast to plant species that possessed a strong direct defence. However, although plant species that have a weak direct defence can use indirect defence to defend themselves, they do not always emit novel compounds. At the level of plant family qualitative differences in volatile blends from spider mite-infested leaves compared to mechanically-damaged leaves were more prominently found in the Fabaceae than in the Solanaceae.A fractionation method was developed for identification of the biologically active compounds in mixtures of volatile compounds (or volatile mixtures), which is more selective and efficient than conventional techniques such as comparison of volatile profiles or the use of synthetic mixtures. With this method bioactive compounds that mediate interactions within and among species can be determined more quickly. First, separation of the volatile mixture was carried out in a gas chromatograph. This made it possible to selectively remove compounds from the mixture. Before the volatiles were tested on bioactivity in an olfactometer, the compounds were revolatilised by thermal desorption and stored in a Teflon bag. Subsequently, the Teflon bag was pressurised and a continuous flow of volatiles was led to the olfactometer.Validation of the method showed that most of the investigated compounds that varied in boiling points and in chemical nature showed a high recovery (80-100 %). Only compounds that had a relatively high boiling point (> 300 °C) or contained a phenolic group showed low recoveries (30-50 %). The biological activity of the volatiles emitted by T. urticae -infested lima bean leaves ( Phaseolus lunatus ) and the compounds methyl salicylate and (3 E )-4,8-dimethyl-1,3,7-nonatriene were successfully tested after being processed with the method. An advantage of the method is that volatile blends can selectively be manipulated. Besides, solvent introduction into the bioassay can be circumvented. After processing and revolatilization of the mixture, a known concentration of the volatiles can be led to the bioassay. In potential, the method can be useful to determine biologically active compounds from complex mixtures in the future.</p

Verdedigingsmechanismen van planten in een tritroof systeem

The spider mite Tetranychus urticae Koch is a serious pest in field crops, glasshouse vegetables and fruit crops. It is a generalist herbivore with several hundreds of host plant species. Phytoseiulus persimilis Athias-Henriot is one of its natural enemies. Investigations of the tritrophic system of plant, T. urticae and P. persimilis will contribute to a better knowledge about the direct and indirect defence defensive strategies of plant species. Host plant acceptance by the spider mite T. urticae, as a measure of the plant’s direct defence, was investigated for eleven plant species. The degree to which the spider mites accepted a plant was expected to depend on differences in nutritive and toxic constituents among plant species. At the level of plant species, a large variation in the degree of acceptance by T. urticae was found. Except for ginkgo (Ginkgo biloba) most plants were accepted or well accepted by the spider mites. At the level of plant family, four plant species from the Fabaceae were compared to four plant species from the Solanaceae. It was shown that all species from the Fabaceae were accepted by the spider mites for feeding, while plant species from the Solanaceae varied in spider mite acceptance from well accepted (tobacco: Nicotiana tabacum) to poorly accepted (sweet pepper: Capsicum annuum

Differences among plant species in acceptance by the spider mite Tetranychus urticae Koch

The spider mite Tetranychus urticae Koch has a broad range of host plants. However, the spider mite does not accept all plants to the same degree because of differences in nutritive and toxic constituents. Other factors, such as the induction of secondary metabolites, the morphology of a leaf surface and the presence of natural enemies, also play an important role in plant acceptance. We compared plants from various families in their degree of acceptance by the spider mite, to get an indication of the plant's direct defence. Glycine max (soybean), Humulus lupulus (hop), Laburnum anagyroides (golden chain) and Nicotiana tabacum (tobacco) were highly accepted by the spider mites. Different glandular hair densities among tobacco cultivars did not affect their suitability towards spider mites significantly. Solanum melalonga (eggplant), Robinia pseudo-acacia (black locust), Vigna unguiculata (cowpea) and Datura stramonium (thorn apple) were accepted by the spider mites to a lesser degree. Vitis vinifera (grapevine) was poorly accepted by the spider mite. It might be that the food quality of the leaves was not high enough to arrest the spider mites. Also, Capsicum annuum (sweet pepper) and especially Ginkgo biloba (ginkgo) were poorly accepted by the spider mite, probably because of the presence and concentration of certain of the secondary metabolites in the leaves. The spider mites accepted all the plants belonging to the Fabaceae for feeding, but those belonging to the Solanaceae showed a larger variance in spider mite acceptance varying from well accepted (tobacco) to poorly accepted (sweet pepper)

Differences among plant species in acceptance by the spider mite Tetranychus urticae Koch

The spider mite Tetranychus urticae Koch has a broad range of host plants. However, the spider mite does not accept all plants to the same degree because of differences in nutritive and toxic constituents. Other factors, such as the induction of secondary metabolites, the morphology of a leaf surface and the presence of natural enemies, also play an important role in plant acceptance. We compared plants from various families in their degree of acceptance by the spider mite, to get an indication of the plant's direct defence. Glycine max (soybean), Humulus lupulus (hop), Laburnum anagyroides (golden chain) and Nicotiana tabacum (tobacco) were highly accepted by the spider mites. Different glandular hair densities among tobacco cultivars did not affect their suitability towards spider mites significantly. Solanum melalonga (eggplant), Robinia pseudo-acacia (black locust), Vigna unguiculata (cowpea) and Datura stramonium (thorn apple) were accepted by the spider mites to a lesser degree. Vitis vinifera (grapevine) was poorly accepted by the spider mite. It might be that the food quality of the leaves was not high enough to arrest the spider mites. Also, Capsicum annuum (sweet pepper) and especially Ginkgo biloba (ginkgo) were poorly accepted by the spider mite, probably because of the presence and concentration of certain of the secondary metabolites in the leaves. The spider mites accepted all the plants belonging to the Fabaceae for feeding, but those belonging to the Solanaceae showed a larger variance in spider mite acceptance varying from well accepted (tobacco) to poorly accepted (sweet pepper)

Qualitative and quantitative variation between volatile profiles induced by Tetranychus urticae feeding on different plants of various families

Many plant species are known to emit herbivore-induced volatiles in response to herbivory. The spider mite Tetranychus urticae Koch is a generalist that can feed on several hundreds of host plant species. Volatiles emitted by T. urticae-infested plants of 11 species were compared: soybean (Glycine max), golden chain (Laburnum anagyroides), black locust (Robinia pseudo-acacia), cowpea (Vigna unguiculata), tobacco (Nicotiana tabacum), eggplant (Solanum melalonga), thorn apple (Datura stramonium), sweet pepper (Capsicum annuum), hop (Humulus lupulus), grapevine (Vitis vinifera), and ginkgo (Ginkgo biloba). The degree to which the plant species produced novel compounds was analyzed when compared to the odors of mechanically damaged leaves. Almost all of the investigated plant species produced novel compounds that dominated the volatile blend, such as methyl salicylate, terpenes, oximes, and nitriles. Only spider mite-infested eggplant and tobacco emitted a blend that was merely quantitatively different from the blend emitted by mechanically damaged or clean leaves. We hypothesized that plant species with a low degree of direct defense would produce more novel compounds. However, although plant species with a low direct defense level do use indirect defense to defend themselves, they do not always emit novel compounds. Plant species with a high level of direct defense seem to invest in the production of novel compounds. When plant species of the Fabaceae were compared to plant species of the Solanaceae, qualitative differences in spider mite-induced volatile blends seemed to be more prominent in the Fabaceae than in the Solanacea