RAGWORT BORER

Rotorua Morning Post, Volume 2, Issue 459, 17 February 1933, Page 7

RAGWORT BORER

EFFECT ON WEED INJURES STEM BUT NOT GERMIN ATION OF PLANT. INTE'RESTING NOTES. A recent report in the "Post" of the damage to ragwort caused by a small horer inset in the Te Pu and I Hamurana areas has attracted considerable interest. Specimens of the 1 borer were shown to Mr. C. S. DaT gliesh, fields instructor for the Rotorua district, who stated that the larva was apparently the blue-stem ragwort borer (homoeosoma vagella). The following notes upon the borer \ and its operations were supplied by j Mr. Dalgliesh: — ^ j The caterpillar of the moth homoe- j osoma vagella has been reported as j doing considerable damage to rag- j wort in Kaharoa district and therefore a brief history of this insect may be of interest to r'eaders. The genus homoeosoma is a cosmopolitian one, usually attacking the flower heads of eompositae. H. vagella, however, is noted as a borer in the pithy stem of ragwort (one of the eompositae) the flowerhead being apparently undamaged. Investigations were carried out in Taranaki during 1929 to ascertain the ; extent and damage done, and also to determine whether or not the insect promised to aid in the control of the weed. Description of Insect. Although no eggs have been found, from the manner of entrance into the flower-stalk of the ragwort, these appear to be laid in the axils of the leaves.

When the young larva is hatched, it j immediately starts to bore into the j fiower-stem, leaving a small heap of sawdust-like refuse (frass) at the j entrance to the burrow. Once inside the stem, the larvae make their way downward through the ! pitch toward the crown of the plant. | Besides boring in the pith, the larvae j frequently eat round the outside of ! the flower-stalk at its junctions with ' the crown. The destination of the borers seems to always be in the direction of the pithy crown, which j sometimes, if in sufficient numbers, i they will hollow out. | The larva or caterpillar is approxi- ' mately seven-tenths of an'inch long, plump, and of a bluish colour, and has three darlc stripes running the whole length of the back, the middle | stripe being narrower than the other j two. j Pupation in Stem. | Pupation takes place inside the ' hollowed-out stem. The pupa is of a pretty blue or blue-green colour and is surrounded by a tough white envelop'e distinctly longer and broader . than the caterpillar. The Moth. The expansion of the wings of the ! moth is approximately one inch. The ; body is rather stout. The forewings ( are narrow and of a yellowish-white ' colour with grey markings. The hind wungs are considerably larger than the forewings, of a dusty greyish- j white colour and fringed posteriorly. ! The perfect insect usually appears ; from January to March. Field Investigations. Investigations were made over ; eight farms, and although 80 to 90 and up to 100 per cent. of the plants were affeeted, the growth of ragwort was not materially arrested. From the data gathered it may be said that homoeosoma vagella does not extend to the flower-head, so that the plant can seed just as well as ever. Tests of seed from infested , and healthy plants showed no difference in germinating power, so that the processes of reproduction and dis- ! tribution of the weed would not be hampered. Since this is the case, j the economic importance of h. vagella is very small, and its utility in a campaign for control of the weed is almost negligible. Also, the moth' is • itself controlled by parasites, so that its usefulness is still further reduced.