CONTROL OF WEEDS BY INSECT PARASITES

Press, Volume CIII, Issue 30562, 3 October 1964, Page 21

CONTROL OF WEEDS BY INSECT PARASITES

The chances of success in the control of weeds in New Zealand by biological methods mostly outweighed the risks, and the future for the biological control of weeds was assured, said Dr. J. M. Hoy, of the Entomology Division, Department of Scientific and Industrial Research, Nelson, in the New Zealand “Science Review.” He gave further details in an interview with a reporter of “The Press.”

Dr. Hoy described progress in the control of six weeds gorse, manuka, ragwort, St. John’s wort, Mexican devil weed, and sweet briar. The gorse seed weevil, Apion ulicis, had been successfully introduced and was now widely established, said Dr. Hoy. It had been chosen in preference to something which destroyed the plants because at the time of the introduction of the weevil gorse was used in some areas for stock fattening and was still being planted for hedging. Because of changing circumstances, gorse was now regarded as a primary weed of great importance and attention was therefore being given to the possibility of introducing insects capable of doing direct damage to the foliage and stems and/or roots. MANUKA On manuka, Dr. Hoy said the plant had ceased to be a conspicuous feature of the vegetation of Canterbury, and in certain areas in the

province was now a rare plant because of the depredations of insect parasites of the genus Eriocoecus, selfintroduced from Australia. The parasites were held in cheek, however, practically throughout the North Island and in wetter districts of the South Island by a fungus, Myrangium thwaitesii. Other species of Eriocoecus than the two already established had been examined in Australia for their capacities in checking manuka, and one showed much promise, Dr. Hoy says, but its reaction to M. thwaitesii was unknown: Manuka parasites were unlikely to be introduced intentionally until there was a general concensus as to the desirability of biological control of manuka. While farmers mostly regarded it as a dangerous weed, soil conservationists feared undesirable consequences if it were removed from certain sites. Ragwort had been successfully infested by the ragwort

seed-fly in the Putaruru district, but opinions differed as to the insect’s efficiency in controlling the weed, said Dr. Hoy. Hormone sprays offered a simple alternative to biological control of ragwort in intensively farmed areas.

In certain back-country areas where the ragwort was gaining ground, such as certain mountain valleys in the Haast area, hormone spraying was impracticable and biological control seemed on general grounds to offer a better alternative; the seed-fly was unlikely to be useful in the Haast area, however, and no other parasite was commercially available. The matter was still under investigation. ST. JOHN’S WORT St John’s wort had been attacked by two different, insects—the leaf-eating beetle and the gall midge. The beetle was introduced into the Awatere Valley in 1943 and later into other areas; in the Awatere it had given useful and continuing control, but results had been less spectacular elsewhere. Recent attempts to introduce a related species had not so far been successful. The gall midge,. which had been used with good effect both in Australia and North America, had recently been established in New Zealand and promised successful control of the weed. DEVIL WEED Mexican devil weed occupied extensive areas in Northland and the northern Coromandel Peninsula. The stem gall-fly, introduced into certain parts of Coromandel in 1958, had dispersed rapidly and indications were that this would prove the most spectacular success of biological control to be seen in New Zealand. The gall-fly had now been introduced also into several district in Northland, where it was again spreading rapidly. SWEET BRIAR The sweet briar had increased rapidly in density and distribution in the South Island hill country since the rabbit population was reduced. Insect control was hazardous because of the economic importance of several close relatives of the briar. Investigations had reached the stage where certain parasites might be introduced into New Zealand on a quarantine basis, but the project was held up by the lack of suitable controlied-enylron-ment quarantine space.