Combining the cinnabar moth (Tyria jacobaeae) and the ragwort flea beetle (Longitarsus jacobaeae) for control of ragwort (Senecio jacobaea): an experimental analysis

Published source details James R.R., McEvoy P. B. & Cox C. S. (1992) Combining the cinnabar moth (Tyria jacobaeae) and the ragwort flea beetle (Longitarsus jacobaeae) for control of ragwort (Senecio jacobaea): an experimental analysis. Journal of Applied Ecology, 29, 589-596.
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Summary

Common or tansy ragwort Senecio jacobaea is a widespread weed in the US Pacific Northwest. A field experiment tested the independent and combined effects of cinnabar moth Tyria jacobaeae larvae and the ragwort flea beetle Longitarsus jacobaeae on ragwort control. These insect herbivores feed on different stages of the host-plant and at different times of the year and were introduced to North America as biological control agents.

Study site: The study was undertaken in a 0.9 ha meadow in Cascade Head Scenic Research Area on the coast of Oregon, USA. Both potential ragwort biocontrol insects had been introduced to the area by the Oregon Dept. of Agriculture between 1978 and 1980. Their populations increased and from 1981-83 ragwort declined to 3% of its former density.

Experimental design: Fifteen (0.5 x 0.5 m) experimental plots were covered in 61 x 61 x 61 cm frames covered with nylon mesh (0.6 x 1 mm) screens. Ragwort was grown in a glasshouse and 2 months later in February 1988 transplanted into the plots and thinned to eight per plot (32 large plants/m²) in summer. One year later plots were assigned one of five treatments (replicated three times):

i) No insects introduced (control)

ii) Cinnabar moth only: cages opened 6 June to 27 July (flight period); flowering ragwort defoliated by hand in July to simulate moth larval feeding

iii) Cages opened to allow beetle access, closed from 6 June until 27 July to exclude moths

iv) Cages opened to allow beetle and moth access; ragwort defoliated by hand in July to simulate moth larval feeding

v) Open controls: no cages to measure any effects of cages; ragwort defoliated by hand in July to simulate moth larval feeding

The number of cinnabar moth larvae was insufficient to cause significant damage, hence hand-defoliation. Flea beetles also entered the plots prior to removing exclosures therefore, such plots were therefore treated with carbofuran insecticide.

Ragwort survival and harvesting: Ragwort survivorship, number of leaves and capitula in the winter and once every 2 weeks in spring and summer present. Two replicates were harvested (March and August) to determine effectiveness of beetle exclusion and measure root crown (March), biomass (August) and capitula number and seed number.

Beetle extraction: beetles were extracted from plants using Tullgren funnels over 7 days. Beetles were counted, and plants were further dried and then weighed to obtain dry biomass.

Flea beetles reduced vegetative ragwort densities by 95% and flower production by 39%, as compared to those in control plots. Damage by cinnabar moths was simulated, due to lack of natural colonization, by removing all leaves and capitula, but plants were able to regenerate some foliage and flower. This treatment reduced capitulum production by 77% and the number of achenes (seeds) per capitulum by 15%.

Flea beetle damage (in combination with simulated moth defoliation) reduced the ability of flowering ragwort plants to compensate for defoliation and defloration to the extent that capitulum production was reduced by 98% and no viable achenes were produced.

Caging effects: Caging effects were minimal other than increasing flowering plant biomass by 37%.

Conclusions: These findings support a strategy of complementary biological control of ragwort using these two insect species which attack different stages and at different times, thereby reducing the number of invulnerable life stage and temporal refuges for the host. Further trials would be useful to assess the true impact of cinnabar moth larvae rather than using the defoliation technique used here.

Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://links.jstor.org/sici?sici=0021-8901%281992%2929%3A3%3C589%3ACTCM%28J%3E2.0.CO%3B2-S

For a review of biological control of ragwort see:
Roberts P.D. & Pullin A.S. (2005) Effectiveness of the control of ragwort (Senecio) species: “Can biological control by the use of natural enemies effectively control Senecio jacobaea (common ragwort)?” Systematic Review No.5b. Centre for Evidence-Based Conservation, Birmingham, UK.