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.
|Qualification||Doctor of Philosophy|
|Award date||25 Apr 2003|
|Place of Publication||[S.I.]|
|Publication status||Published - 2003|
- tetranychus urticae
- phytoseiulus persimilis
- predatory mites
- volatile compounds
- jasmonic acid
- plant composition