So, what's the problem
Common tansy (Tanacetum vulgare) is a Eurasian plant species whose wide native distribution ranges from western Europe eastwards to Japan. It was introduced to North America for medicinal purposes in the 17th century. It has since become an invasive weed and is declared noxious in several US states and Canadian provinces. Common tansy is unpalatable to cattle and horses and therefore especially problematic in pastures and hayfields where it significantly reduces fodder production.
What is this project doing?
In 2006, a consortium of Canadian and American funders was established and CABI's weed team at our centre in Switzerland was asked to start a biological control programme.
A thorough survey of the published literature revealed 169 insect species and one mite that attack Tanacetum spp. in Eurasia. Of those, 30 species only attack plants in the Tanacetum genus. We prioritised a dozen candidate insects, based on their potential host range (the plants they can develop on) and damage to the target weed.
We carried out surveys across Europe to find and collect these species. We then established rearing colonies and tests to confirm their host specificity (a guiding principle for biological control is a biological control agent should not damage plants other than the target weed).
We have done this for eight potential biological control agents: the stem-boring cerambycid beetle Phytoecia nigricornis, the leaf-feeding chrysomelid beetles Longitarsus noricus, Cassida stigmatica and Chrysolina eurina, the stem-boring weevil Microplontus millefolii, the shoot- and flower-feeding gelechiid moth Isophrictis striatella, the leaf, stem and flower gall midge Rhopalomyia tanaceticola and the shoot-tip-boring moth Platyptilia ochrodactyla.
Host-specificity tests conducted in 2006–2014 with P. nigricornis, L. noricus, Cassida stigmatica, I. striatella and R. tanaceticola showed that they are not sufficiently specific for field release in North America.
Tests with the stem-mining weevil M. millefolii showed more promise. Exposing weevils first to a single test species or the target weed (no-choice testing) and then a test species together with the target weed (multiple-choice testing) revealed only three test species that occasionally supported egg laying and larval development. Since there was very little attack on these test species compared with T. vulgare, we agreed with our North American counterparts to continue our investigations.
Work with the shoot-tip-boring moth P. ochrodactyla has been hampered by its complicated life cycle. Females lay their eggs in summer between the individual flowers of the compact flowerhead. The following spring, the larvae burrow short mines into newly growing shoots. Their feeding can reduce growth of young shoots by over 70%. No-choice tests (where the moth is exposed to only one test species or the target weed) in cages revealed that a few non-target species can support larval development. In an open-field test, larvae were only found in T. vulgare. We plan to conduct additional open-field tests to further investigate whether non-target plants are attacked by P. ochrodactyla when no flowering T. vulgare are present.
Work with the leaf feeding beetle Chrysolina eurina started in 2016 and results of no-choice tests suggest that the host range of C. eurina is mostly limited to Tanacetum species. We plan to conduct multiple-choice tests to evaluate whether the North American native T. camphoratum and T. huronense will be attacked under these conditions.