A knapweed biological control perspective

Diffuse and spotted knapweed (Centaurea diffusa Lam. and C. stoebe micranthos (Gugler) Hayek) are Eurasian plants that devastate dry and mesic North American grasslands. They have a mutualistic association with arbuscular mycorrhizal fungal (AMF) phylotypes with hyphal links to nearby plants and a nutrient flux to the strongest sink, usually knapweed. They displace many AMF beneficial to grass and affect knapweed nutrient allocation, biology, knapweed insects and probably root necrosis and emergence of ant buried seed. AMF determined nutrient root or shoot allocation determines nutrient shoot and root allocation and the benefit to root or seed-head insect species and whether C. diffusa is an annual–biannual or a semelparous perennial needing 5 or more years to flower. Both knapweeds do well without its AMF phylotypes without competition in fertile soil. In grass in Eurasia, they have a community of seven seed-head species segregated by head development stage. Prolonged seed dormancy buffered knapweed decline that resulted in release of a surfeit seed-head species. The presence of an eliasome on the seed and vigorous seedling clumps suggests burial by myrmecochorous ants with AMF supplied carbon supporting their growth. The root species community is segregated by habitat, climate, root part, and size. With larval induced compensatory growth and AMF nutrient sharing, the growth of plants with and without a larva was the same. On feeding completion, a nutrient out flux from the attacked plants reduced growth; but without killing. This needs a dual species or a repeated single species attack. Root species packing increases knapweed utilization; but the four approved species are insufficient for maximum utilization. Two additions are suggested. The aim of the paper is to provide enough understanding of the AMF and its plant and insect interactions to facilitate knapweed biological control and avoid past mistakes.