ARGENTINE STEM WEEVIL IN RYEGRASS

Press, Volume XCIX, Issue 29222, 4 June 1960, Page 8

ARGENTINE STEM WEEVIL IN RYEGRASS

Susceptibility Of Varieties

QF the ryegrasses Italian and short rotation are the most susceptible to Argentine stem weevil on the basis of studies made at Lincoln by the substation of the Entomology Division of the Department of Scientific and Industrial Research, but in spite of the weevil, Italian produced a greater quantity of dry matter per plant than the other varieties apart from the new variety long rotation which is not yet released.

The studies at Lincoln were made over two generations of weevil on plots of 1340 plants of each of long rotation, perennial, short rotation and Italian ryegrasses. Measurements were made when the weevils had reached their peak feeding capacity in August-September and in January-February. Over the two generations an average of 18 per of long rotation tillers were infested, 31 per cent, of perennial. 71 per cent, of short rotation, and 92 per cent, of Italian. For the two generations in order of time. 27 per cent, and 9 per cent, of long rotation tillers were affected. 42 per cent, and 20 per cent, of perennial. 79 and 63 per cent, of short rotation, and 93.5 and 90.6 per cent, of Italian. Certain Death The significance of weevil infestation in a narrow ryegrass tiller is that it will certainly die. It is common for nine and up to 17 eggs to be laid in a ryegrass tiller but normally not more than two grubs survive per tiller. Cocksfoot can be quite heavily infested and in recent trials 53 per cent, of cocksfoot tillers were so affected, but the more robust tillers of this species are rarely killed unless the infestation is of the order of more than six weevils per tiller. Up to 47 eggs have been seen on a cocksfoot tiller and there have been up to 21 grubs to a tiller.

An interesting aspect of the investigations was that dry matter yield for each variety on a per plant basis—that is the grazing value of the plant—at the end of February after the trial plots had been subjected to the two generations of weevils was 27 grams for long rotation, 13 grams for perennial. 174 for short rotation, and 19> for ftalian.

Great Bulk Commenting on these results, Mr J. M. Kelsey, officer in charge of the sub-station, said that even with a 92 per cent, infestation ftalian ryegrass had turned out the second best producer indicating the tremendous bulk of feed it produced. In the South Island, he said, it was used for quick growing feed for periods of not more than two years. It would be interesting to speculate on how long it would last if it could be freed of the weevil, and when it had such a high production value it was obviously worth seeking out an economic method of control of the weevil.

After five or six years’ work in this field, Mr Kelsey said he felt fairly certain that the short life of short rotation—from three to seven years—was the result of the stem weevil.

Perennial, with a 31 per cent. Infestation, was the lowest producer of them all and long rota-

tion was far superior to any other variety. With its high palatability as well, long rotation promised to have a real place in pasture lands in the future. Occasionally, there is a third generation of the weevil in a year in May-June, but none was observed this year and Mr Kelsey said that in examination of more than 300 paddocks in six years third generations had been observed on only three occasions, but as it did not have a true hibernation the adult of the species could, under favourable conditions, come out to feed in even mid-winter.

Control Mr Kelsey said that more time had to be spent in developing an economic control for the weevil It was known that DDT, lindane, and dieldrin would control the adults moving on the surface oi the ground, and on plants, and the right conditions for control treatments were also known, but it was not possible to keep the insecticide on the ground, as rain washed it in beyond the quarterinch depth to which the weevils penetrated. Ip. the South Island the periods for control measures were the last two weeks in August and the first two weeks of December—no sooner because rain might wash the insecticide in beyond the effective position on the surface and no later, because once the eggs had been laid nothing could be done to control the grubs. The recommended application of insecticide was the normal dry mix DDT super, including 21b of p.p.i. DDT to the acre, as for grass grub control. Reinfestation Because the adult weevils would fly on any sunny day,and also in warm, dull weather, the possibilities of reinfestation of a pasture were considerable, and treatment in August did not confer protection against the second generation.

Mr Kelsey said that the weevil was widespread in the North Island and in Canterbury, Marlborough, and Nelson, and while farmers in Otago and Southland claimed that they did not have it, it had also been observed there during grass grub sampling The most obvious symptom of weevil infestation in a pasture was a white, wisp-like twist of cellophane on the tip of the grass leaf, left after the adult weevi had taken out the green matter. This could be seen throughout the year, but was most common in September-October and De-cember-January. It was not important in itself, apart from being an indicator that the weevil was present in the pasture. The actual damage was done by the grub in the tiller.