Only a small proportion of dipterans are herbivorous. Most agromyzids are family-specific, and the larvae often feed on one or several plant genera belonging to the same plant family. Melanagromyza heracleana Zlobin is a species which was recently found on H. mantegazzianum in the Caucasus region and was new to science (Zlobin, 2005). The stem-feeding larvae are very common in the Caucasus but do not seem to have a negative impact on H. mantegazzianum (Hattendorf, 2005). It is possible that this species is not specialized in feeding only on giant hogweed, even if it has so far only been found on this species. Feeding larvae of this species generate white mines inside the hollow stem, a few millimetres wide (Fig. 11.8). This small fly develops slowly and only one generation per year is produced. The pupa overwinters inside the stem. The close relative Melanagromyza angeliciphaga Spencer is found on giant hogweed, but on other genera of Apiaceae as well (Spencer, 1972, 1976). Most impressive are the feeding traces of the agromyzid Phytomyza sphondylii Goureau, which makes leaf mines of about 2 cm wide and 7-10 cm long. Its larvae develop into shiny black pupae from May to June and July to September. Despite the fact that these mines are rather large, P. sphondylii is responsible for less than 1% damage to giant hogweed in England (Spencer, 1972, 1976; Sampson, 1990).
The 6-7 mm long larva of the carrot fly Psila rosae (Psilidae) is a root miner, found often on giant hogweed, but it also feeds on other Apiaceae (Sheppard,
1991). It hibernates as a larva and has 2-3 generations per year. Particularly the second generation is known to cause great damage to crops such as carrots. When 132 apiacean species and subspecies were tested against the carrot fly, 78 of them were proved to be new host plants, 27 were confirmed as hosts and 27 failed to support carrot flies (Hardman and Ellis, 1982).
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