Blue-green aphid review

Press, 18 February 1977, Page 6

Blue-green aphid review

Farm and station

Although the blue-green aphid has apparently disappeared from the areas in which it was abundant earlier in the season, it was, in fact, still present in low numbers everywhere and seemed to be increasing again slowly. Mr T. E. T. I rought. an entomologist in the insect control group of the Ministry of Agricul- , ture, said this week.

In a review of th I behaviour of the pest i the province this seasor i he said this week that i: • some areas where the pes I was not present earlie ‘ there were high in I festations now. This in dicated that a build-up o j predators, such as lady , birds, and disease, couli i have been a factor in th' I crash of the populations but now as the predator , started to find places ti ; spend the winter the pres ! sure on the aphids wouk ; diminish and Mr Trough 1 said he believed that thesi populations would build u{ . again. ' The importance of ; : build-up late in the curren | season was difficult tc ; gauge, but he believed from experience here late ; last season and in the I North Island, that a build. up could be serious if luI cerne stands were no; I grazed. Last year heavy in- , festations in March ' and I April caused yellowing and ; die-back. If the pest, under these conditions, was not controlled, early growth in the spring might also be affected. Although conditions for harvesting hay this season had often been difficult, growing conditions had also been good all the year. Thus damage and yield reduction by the pest were often not obvious. However, all of the trials had shown a loss of 20 to 25 per cent of hay where aphids were in high numbers and in these conditions spray treatments were definitely beneficial on a cost basis. Mr Trought said that farmers who had observed the dramatic reduction in populations of aphids may either have been glad they did not spray or may have felt that they had wasted their money if they had sprayed. But he said he believed that the pest would be important again next season and if there was a dry spell in the spring or earlv summer it could cause greater losses than this season. “I would suggest that a wait-and-see policy next year could result in serious losses in production,” he said. From one season’s work he said that they did not know enough about the effects of grazing in the presence of aphids on losses in production of lucerne. Trial results had shown early season losses of 15 to 20 per cent as a result of the lower populations present under grazing management. Thus even under some forms of grazing regime spraying might be necessary for maximum economic production. There was evidence that heavily infested growing lucerne was partly unpalatable to stock. Hay from infested plots had also been set aside for comparisons later this year of the digestibility and palata-

ie bility of this material and n non-infested hay. i, Although spraying against the aphid where the lucerne was for hay 5t had been shown to be fully r justified, the use of granui- lar insecticides drilled with i- the seed to protect the >f seedlings was more difficult to justify. In trials d where the seedlings had e been artificially inoculated >, with the pest yield in's creases of 190 per cent had o been obtained where seed- ;- lings had been protected 1 and cut at the time of the t usual first grazing. e However, this season 3 flights of aphids early in the period had not been a sufficient to cause a heavy t infestation on the seedlings 3 and growing conditions had I, been good. ' In another season, not " only might the flights be " earlier but drier conditions might cause a greater setback from lower inj festations. According to the . season a spray treatment . could be used as, and if, necessary. In white clover seed crops Mr Trought said that trials had shown a loss of 17 to 20 per cent of seed in heads which came to maturity in the late November - December period. Again, this year, the extended flowering period of white clover would probably compensate for this earlier loss. Yields of the whole crop had still to be taken. Next year yields might be largely from the earlier developed heads, and spraying when aphid numbers on the seed heads started to rise would be justified. No-one would be more pleased than himself if the pest was never to raise its ' head again, Mr Trought i said, but his forecast about it was pessimistic.

Mr Trought said that there had also been a number of inquiries about the control of the sitona weevil, another pest of lucerne which was reported early last year to have been found in many places on Banks Peninsula. He had seen a fair amount of damage caused by it but what it meant in terms of yield loss he was unable to say. The larvae feed on the root nodules of the plants and adults on the foliage causing a scalloping of the leaves. Mr Trought said that his work had been concerned with trying to find a chemical that would control the sitona and the blue-green aphid at the same time and in a situation where bees were also at risk. He has come up with a combination of trichlorfon and bromophos, which he

says does a fairly good job against both pests at the same time and which when applied in the evenings is safe for bees. The suggested rates were a kilogram of trichlorfon and 0.28 kg of bromophos to the hectare, but this week a trial was to be put down; in which these rates were to be reduced to see if a lower rate was satisfactory. At the rates suggested he said that the cost of the materials only was about $l5 per hectare, which was too costly if it was not certain whether the treatment was necessary or not.