Work with C. peyerimhoffi (the seed feeding weevil) started in 2008. Progress was slow initially as only few weevils could be collected in the field in Italy. In the meantime a successful rearing colony at our lab was established. In 2012, we continued with no-choice oviposition (egg laying behaviour on the target weed and test species) and larval development tests. Tests were established with 32 test plant species, 20 native to North America, including one federally listed endangered species. Apart from dyer’s woad, eggs were found in five test species, including the closely related European I. glauca. In subsequent development tests, with two of the five species, plus eight species on which eggs had been laid in previous tests, none of the exposed test species supported larval development, confirming the narrow host range of C. peyerimhoffi. We will continue these tests in 2013. In addition, we will make an effort to determine the proportion of seeds damaged by adult feeding, and are planning to establish a multiple-choice field cage test for the species where the weevils laid eggs under no-choice conditions.
We have now exposed a total of 97 test species to C. rusticus (the root-crown feeding weevil), over half of which are native to North America. Of these, six species in four genera, all native to North America, have so far supported development to adult of C. rusticus. However, since many plants died and had to be dissected prematurely (before mature larva could develop) we need to carry out more tests, especially with perennial and biennial native North American species.
In two open-field tests, in which plant species attacked under no-choice conditions were exposed, adults of C. rusticus only emerged from dyer's woad. However, again many test plant species died prematurely. In 2011, we therefore established an open-field test, where plants were dissected for eggs and first instar (life stages) larvae. Although the majority of eggs (97%) were laid on dyer's woad, two test species (the native North American Caulanthus crassicaulis and Lepidium densiflorum) were slightly attacked. We assume that this was influenced by placing weevils onto the exposed test plants and not in between plants. The test nevertheless showed that potential non-target attack by C. rusticus under open-field conditions is a very rare event. A similar test was established in autumn 2012. Final results will only be available in 2013.
No-choice larval transfer tests with P. isatidis (the stem-mining flea beetle) between 2006 and 2012 revealed a broad larval host range. However, multiple-choice field cage and open-field tests revealed much more restricted female oviposition behaviour. In an open-field test established in autumn 2010, only one adult of P. isatidis emerged from a test plant (the European Barbarea vulgaris) apart from dyer's woad. We therefore think we should continue work on P. isatidis. Since larval transfer tests omit the egg laying behaviour of females, we established no-choice oviposition (egg laying) and larval development tests by placing egg laying females onto individually potted gauze covered test or control (dyer’s woad) plants in autumn 2012. In addition, we established another open-field test together with C. rusticus (see above). Results of both tests will only become available in 2013.
In 2011, an impact experiment was established, transferring three different densities of P. isatidis larvae (0, 30, 60 and 90 per plant) onto potted dyer’s woad plants. Shoot height was reduced by about 22cm at the highest density, while shoot base diameter, biomass and seed production were not significantly decreased. Although seed production was reduced by 58% at the highest density compared to control plants, results were not statistically significant because of high variability. The accelerated phenology of dyer’s woad due to an unusually hot spring is thought to have hampered the impact experiment with P. isatidis and we are therefore going to repeat this test. The experiment nevertheless gave an indication of how P. isatidis is likely to reduce the shoot height and seed production of dyers woad. Our previous observations show that very early P. isatidis attack can completely stunt plants and more or less inhibit seed production.
In autumn 2008, we established an experiment to quantify the field impact of , C. rusticus, P. isatidis and interspecific plant competition on dyer’s woad. Herbivore damage to plants was mostly accounted for by C. rusticus and was significantly reduced on plots sprayed with insecticide. Plants on unsprayed plots produced more but shorter and thinner shoots than plants on sprayed plots. In 2011, seed output was reduced by 72% - very similar to seed reduction in a pot impact experiment conducted with C. rusticus. We found that interspecific competition significantly reduced survival of dyer's woad and therefore plant numbers, the proportion of plants reaching the reproducing stage and the total number of seeds produced per plot. In 2011, there were hardly any dyer’s woad plants left on un-weeded plots. Data collection therefore concentrated on weeded plots, and the experiment was terminated in summer 2011.
Approximately a year and a half after this experiment was set up; few seeds of dyer’s woad were left in the soil seed bank, indicating that the seed bank of dyer’s woad is rather short lived, at least under our experimental conditions. This was confirmed by the fact that few new seedlings emerged in spring 2010.
In Idaho, USA, a similar experiment was established in autumn 2009. First results indicate that a higher number of seedlings germinated in spring than in fall, while in Southern Germany, a similar number of seedlings germinated in fall and spring. Similar to Southern Germany, interspecific competition in Idaho reduced survival, the proportion of reproducing plants and seed output. Instead of damage to the plants by insects, the experiment in Idaho investigated the effect of the native rust fungus Puccinia thlaspeos. However, since only a few plants were infected with the rust no quantitative data on its impact on dyer’s woad could be collected.