Abstract
Giant hogweed, Heracleum mantegazzianum (Apiaceae), was introduced from the Caucasus into Western Europe more than 150 years ago and later became an invasive weed which created major problems for European authorities. Phytophagous insects were collected in the native range of the giant hogweed (Caucasus) and were compared to those found on plants in the invaded parts of Europe. The list of herbivores was compiled from surveys of 27 localities in nine countries during two seasons. In addition, literature records for herbivores were analysed for a total of 16 Heracleum species. We recorded a total of 265 herbivorous insects on Heracleum species and we analysed them to describe the herbivore assemblages, locate vacant niches, and identify the most host-specific herbivores on H. mantegazzianum. When combining our investigations with similar studies of herbivores on other invasive weeds, all studies show a higher proportion of specialist herbivores in the native habitats compared to the invaded areas, supporting the "enemy release hypothesis" (ERH). When analysing the relative size of the niches (measured as plant organ biomass), we found less herbivore species per biomass on the stem and roots, and more on the leaves (Fig. 5). Most herbivores were polyphagous generalists, some were found to be oligophagous (feeding within the same family of host plants) and a few had only Heracleum species as host plants (monophagous). None were known to feed exclusively on H. mantegazzianum. The oligophagous herbivores were restricted to a few taxonomic groups, especially within the Hemiptera, and were particularly abundant on this weed.
Highlights
Weed invasion hypothesesThe increased competitive ability of non-indigenous plant species is often attributed to the absence of their specialized natural enemies (Torchin et al, 2001; Mitchell & Power, 2003)
Phytophagous insects were collected in the native range of the giant hogweed (Caucasus) and were compared to those found on plants in the invaded parts of Europe
23 153 found worldwide (Bernays, 2003) and (2) with 5610 phytophagous insects found on beneficial plants of the former USSR (Fig. 3), we find that sap-sucking herbivores constitute an disproportionately large feeding guild on H. mantegazzianum (Fig. 4)
Summary
Weed invasion hypothesesThe increased competitive ability of non-indigenous plant species is often attributed to the absence of their specialized natural enemies (Torchin et al, 2001; Mitchell & Power, 2003). We seek to test the hypothesis that proportionally more species of herbivore specialists are found in the native range of H. mantegazzianum. This will provide a list of associated herbivores and their host range that can be considered in developing a classical biological control programme. We evaluate the niche size (measured as plant organ weight) of H. mantegazzianum and investigate whether any parts are less occupied within the introduced range, making them suitable targets for biological control. These three questions are relevant when discussing why some plants transform into serious weeds in regions to which they have been introduced
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