The entomofauna of roots of Centaurea maculosa Lam., C. diffusa Lam., and C. vallesiaca Jordan in Europe. Niche separation in space and time

Phytophagous insects representing 4 orders, 12 families and 22 species are associated with the roots of Centaurea maculosa Lam. (21 insect species), C. diffusa Lam. (12 insect species) and C. vallesiaca (11 insect species) in Europe. A total of 192 root samples (110 samples of C. maculosa and 41 samples each of C. diffusa and C. vallesicac) from 71 localities in 8 countries were collected and analysed between 1979 and 1984. Proportions of insect orders in the insect species found on each plants varied considerably between C. maculosa, C. diffusa and C. vallesiaca, but corresponding values for the roots of C. maculosa and C. diffusa showed a concurrent ranking. A total of 30 parasitoid species were reared from these root feeders and parasitisation levels were generally below 10 per cent, but reached up to 45 per cent at two localities. Root feeders (excluding Hemiptera) exploit four distinct root structures on C. maculosa, three on C. diffusa and only two on C. vallesiaca (spatial food niche separation within the plant). Utilization levels of these feeding niches varied greatly between the three host plants. Temporal niche separation does not seem to play a major role in structuring these root feeding insect guilds. Oviposition sites (plants with green leaves) are available throughout the year and larval feeding generally coincides with high levels of water (over 60 per cent) and nitrogen (over 1.8 per cent) content in the roots (autumn and spring). With regard to species introductions into North America for the biological control of these knapweeds, we suggest that at least one (host specific) species should be selected for each of the different root structures. The potential impact of these root feeders on knapweed population growth and equilibrium density is briefly discussed