| Summary |
Biological control is one of the tools used for integrated pest management of invasive plant species but it is not without risks to native plants. I researched the non-target impacts of the biological control agent, the seed head weevil Larinus planus (Coleoptera: Curculionidae) on the Great Lakes dune and cobble shore endemic threatened thistle, Cirsium pitcheri (Asteraceae). Pitcher's thistle is an herbaceous perennial monocarpic plant with no means of vegetative reproduction, relying solely on seed set for population persistence. The seed head weevil is univoltine and lays eggs in thistle heads. The developing larva chews the ovules or seeds before emerging as an adult to overwinter in leaf litter. I repeatedly surveyed Pitcher's thistle plants from three populations in northern lower Michigan for impacts. The insect oviposits on thistle heads from mid-June to early July, before C. pitcheri flowering. Heads that received oviposition were on average 12-14 mm in diameter. Approximately 32% of the 1,695 heads surveyed had oviposition. A subset of dissected heads had 56% weevil egg mortality. With weevil survival, the number of filled seeds was reduced by 62%. A generalized linear mixed binary logistic model reported date of oviposition and size of heads as significant predictors of oviposition on heads. I tested the effectiveness of an organic insect deterrent, Surround[registered] WP, in reducing impacts on C. pitcheri. The kaolin clay is mixed in water and then applied to the plant create a protective film after drying. I first needed to confirm that kaolin clay did not negatively impact pollinator visits, which are important for seed set. To test this, I performed simultaneous 10 min observations on kaolin clay treated plants and untreated plants. Kaolin clay did not deter insect visits, affect species richness of visiting insects or change the length of the visit. To test the effectiveness in deterring the weevil I applied either kaolin clay or a water control to pairs of heads at Petoskey State Park. The clay-treated heads had significantly fewer oviposition holes, and were less likely to have oviposition holes at all. There was no significant difference in the mean number of chewed seeds between treatments; however, there were significantly more filled seeds in heads treated with kaolin clay than in water-treated heads. I recommend the application of kaolin clay to reproductive Pitcher's thistle plants mid-June to early July every 3-7 days on heads not yet flowering. Reduction of impact from Larinus planus is critical for conservation of this species and may also be important for the entire network of insect flower visitors in the dune ecosystem. Pitcher's thistle flowers from late-June to early August. Many species of insect from several taxonomic orders visit Pitcher's thistle flower heads indicating a generalist pollination syndrome. Generalist plants are often important floral resources that maintain plant-pollinator network structure, potentially as keystone species. I hypothesized that Pitcher's thistle is an important floral resource for the flower-visiting insects during its flowering period. To test this hypothesis I performed insect visitor observations on all the insect pollinated plants in randomly selected plots in the dunes of Sturgeon Bay, Wilderness State Park, MI. C. pitcheri received more visits and had more visitor species than any other plant in the network by a large margin. I used R Bipartite package to calculate species-level network metrics such as species strength and weighted connectedness and betweenness. I consistently found Pitcher's thistle to have the highest scores. The index d' rated Pitcher's thistle as the most generalized, i.e., received the most insect visitors. C. pitcheri also had a disproportionate effect on the flower-visiting insect fauna, relative to the abundance of its floral resources (number of open flowers, number of plants observed). Other plants in plant-pollinator networks have been shown to be important, perhaps keystone species, using this technique of species-level network metrics. Pitcher's thistle is an important species for the plant-insect network of the dune ecosystem. Insects and the flowering plants they visit and should be prioritized for conservation. |
