How pyrethrum can both attract and deter insects
Pyrethrum Tanacetum cinerariifolium and some other species in the genus Tanacetum are perennial species that produce pyrethrins that are used as natural pesticides, especially in organic agriculture.
The ecological significance of pyrethrin production is known to be both protection of the plant against insects and, as pyrethrin accumulate in flowers and seeds, protection of offspring against fungal attacks. However, Pyrethrum is also insect pollinated probably primarily by thrips and like other flowering plants the flower is designed to attract pollinators. Pollination is known to be poor in general, however. Thus, an interesting combination arises, as flowers are both made to attract pollinators and at the same are loaded with pyrethrins that deter insects.
It has been hypothesized that this represents a so-called push-pull strategy by the plant, in which an ecological balance between sufficient reproduction and annual survival is reached. Roughly this idea entails that these flowers are cleverly designed to attract sufficient pollinators but simultaneously deter potential herbivores so that reproductive success is not diminished. Understanding this dynamic is ecologically highly relevant as it leads to better understanding of why flowers are designed the way they are, but is of major agricultural importance as well, because pollination is also correlated to pyrethrin content and therefore economically important.
An experiment was conducted in which different Pyrethrum accessions with unknown variation in Pyrethrin content were grown and were either exposed to thrips or not. Flowers were collected both at the pollination stage (stored in alcohol) and at the ripe stage for seed experiments. The question to be addressed here is if there is correlation between pyrethrin content and the number of thrips on the flowers and also whether there is a relation between the presence of thrips and pollination success.
Experiments will be conducted to see whether:
- the presence of thrips has influenced the number of fertile seeds, and
- the pyrethrin content has affected the thrips numbers.
In addition we have a suite of accessions from a wild sister species with a known variation in pyrethrin contents and some distinct differences with pyrethrum. These are growing in a field plot and can be characterized in various ways. Several types of measurements or small experiments are possible.