Investigation of Australian Soldier Fly in Opotiki District

New Zealand Journal of Agriculture, Volume 77, Issue 2, 16 August 1948, Page 161

Investigation of Australian Soldier Fly in Opotiki District

T rich farmlands of the Opotiki district, in the Bay of Plenty, are widely known for their heavy yields of cob maize and for their high-producing pastures of ryegrass, white clover, and paspalum. Within the past few years, however, maize after lea has often been sickly, stunted, and low in yield, and some pastures have deteriorated sadly. The cause of this is thought to be the larva ot an Australian soldier fly which is present in the soil at Opotiki but is not known to , 7 . . iLi rv 1 j occur elsewhere in New Zealand.

By

E. R. MARRYATT,

Fields Instructor, Whakatane.

HOW this fly came to Opotiki and why it is confined to that locality are at present, unknown, but the extent of its damage and ways of combating it are being investigated. There is no record of its having caused very serious trouble in Australia, where it is regarded as a minor pest of sugar cane, so alarmist reports about it should be treated with reserve pending investigation. There is no evidence in pastures of browning off or dying of plants such as is caused by the New Zealand grass-grub, and such evidence might be expected were - the insect really a serious pest in pastures.- On the contrary, because the fly goes through its larval stage insidiously sucking the sap from the grass roots, the damage to pasture production may still be severe but not as noticeable as that done by the root-devouring and soil-altering grass-grub. -. Dr. D. Miller, Director of the Entomology Division of the Cawthron Institute, has identified the fly as M etop onia rubriceps, Macquart. A record, of this fly at Opotiki by J Muggeridge was published in 1944 in the “Journal of Science' and Technology,” Volume 26, number 4 (Section A), pages 190-192. Description of Insect “ The larva is biscuit to soil coloured except immediately after moulting, when it is a pale cream. It is sausageshaped and up to 10 millimetres long.

It has shallow grooves round its body and sparse hairs or bristles. The skin is very tough, feeling rather like rubber when a live larva is pinched between thumb and finger. The female fly is about the same length as the common house fly except when ready to lay, when her body is greatly extended but very slender. She has an amber-coloured head and knees and elsewhere is black. The male is shorter and entirely black. The wings are highly veined and the flight is rather slow and usually close to the ground. An Australian entomologist, IrwinSmith, states that two broods of the fly appear annually and that the larval stage exceeds six months and may exceed a year. In February and March empty pupal cases vertical in the soil with the open end flush with the surface have been found at Opotiki, and, as decomposition of these thin shells could be expected to be fairly rapid, emergence of the fly was assumed to have been recent. That was confirmed in April on subsequent visits, when many flies were observed on the wing and fewer larvae than formerly were found in certain soils where the larval population was known approximately. Odd flies were caught earlier in the autumn, and even in November, December, and January. Occasional ones have also been seen in motor-cars in the town of Opotiki when the cars

had just arrived from other districts, which indicates an easy means of transporting the fly elsewhere. Habits of the Larvae The larvae or maggots are found usually within the top 2 or 3in. of the soil, where the roots .of plants are most numerous. They may be found at greater depths in the early larval stages, and on the lighter soils of the Opotiki hills they may be found at greater depths than in the soils of the Opotiki flats. They appear at first to be immobile, but obviously can move about in the soil quite well by means of their segmented bodies and bristles. ' v ' 2-bi-Larvae have frequently been - discovered adhering strongly to the roots of maize and pasture plants by their jaws, and from the point where they are attached to the tips the roots are plainly dead. Usually, however, breaking the sod to look for the larva disturbs and dislodges it from its exact position, so that all that can be seen is that the maggot is among the roots. Because it is also found distributed throughout the soil where, temporarily at least, there are no living plants, there is evidence to show that: the larva does not live entirely or continuously on sap from living plants. How long it can exist without ; this support is not known. The larvae have been found in large numbers on the light soils of the ridges near Opotiki and they also occur on some, parts of the Opotiki flats. None has been found. further away than about seven miles from Opotiki.

Causes of Damage Some farmers of this district now expect poor results from maize sown immediately after old pasture because of a high infestation of larvae in the soil. These farmers claim that. better results are obtained in - subsequent years. Apparently pastures are not seriously infested until, after... their fourth year down, when, it is stated, production is noticeably reduced. The fly mates, lays her eggs, and dies soon after their emergence, so tending to reproduce her kind in the approximate area of her own pupation. That may explain why the infestation of a pasture increases with its age. Very high larval counts of 300 to 400 to the square foot have been obtained in some of the oldest pastures. In a crop of sickly maize after old pasture two plants lifted and examined had 90 and 150 larvae among their roots.

All failures of maize and other crops and all pasture deterioration at Opotiki are not attributable to the fly. Good ..crops of maize are still taken after old pasture on some farms. Some Opotiki farmers complain that crops from paspalum pastures which three or four years ago hid the fences when hayed are now not worth cutting; that is true, but continuous haying could cause it to happen, though the soldier , fly may aggravate results of this kind of mismanagement. On well-managed pastures the fly may fairly take the bulk of the blame. The twin cress problem at Opotiki is probably partly caused by pasture deterioration which has been accentuated by the fly. Official Investigation The alleged damage caused by the fly has been under investigation for some time by the Entomology Division of the Department of Scientific and Industrial Research in conjunction with the Fields Division of' the Department of Agriculture. The aims of these investigations are to discover the .extent of the damage and how to combat it. In pastures the damage is difficult to determine, as it will probably be a slow change over several years. Fall-ing-off in the production of a grazed pasture, though very noticeable to the farmer who has to find feed for his cows, is difficult for. him to demonstrate to occasional visitors. Attempts to kill the larvae in plots on infested pastures and to compare , the growth

on maggot-free and maggot-infested pasture have failed so far because all the insecticides used either affected the growth of the pasture or failed to • kill the maggot. Experiments with Maize Investigations into the damage caused by the. insect on maize presented fewer difficulties and already' have produced some interesting information. Mr. J. Baird, of Kukumod Hill, made available for experimental work a field which was ploughed out ,of maggot-infested and badly-deteri-orated pasture and sown to greenleed. oats in the autumn of 1947. The oats i failed, apparently because of a high population of larvae in the soil. Half of this failed crop of oats was ploughed 'early and the other half was ■ not ploughed until a week before being sown to maizethree months later. No cultivation was done during these three months because the weather; was too wet. - The strike and establishment on the two halves of the field were not noticeably different, but up to February the growth of maize in the early-ploughed half was much better than that on the late-ploughed half; from February onward that difference was not apparent. A comparison by weight of the yield of cobs on each half of the field at plucking time will be . necessary to determine if the wide difference in growth ,in the first three months seriously affected the yield. The maggot population was about 85 per cent, lower, in the earlyploughed half. Certain insecticides

which were effective against the larvae on the late-ploughed half produced no apparent improvement in growth of maize on the early-ploughed half, as it was already growing to its capacity. Four different and likely insecticides were used in the hope that at least one of them would kill the larvae without affecting the maize and so give a comparison of maize on maggot-free and maggot-infested soil. Two of these insecticides greatly reduced the larval population and on these plots the maize was at least 18in. higher than on the untreated plots on the late-ploughed half and similar in height to that of all the maize on the

early-ploughed half. The same results were obtained in the three replications of the treatments. Because of their high cost the insecticides used in this - trial are not a practical solution to the problem, and great interest is therefore being taken in the good results from early ploughing in reducing the maggot population. How this has come about is not known, but the objective was to ‘ attempt to kill the maggot by starvation and exposure to birds. A check-up on maize crops grown in the first year after grass around

Opotiki this year has tended to confirm that where the land was. ploughed out of grass at least three months before being sown to maize the grub has caused little damage. However, 1 very few farmers had ploughed as early as that. One farmer reported that early ploughing failed to give him satisfactory results some years ago. About 25 per cent, of these first-year crops were failures and about 15 per cent, partial failures this year. The larval population in Mr. Baird’s field in which the experiments were carried out was of medium intensity. There was stunting but no deaths of plants, as there were in many other crops in the locality. e Results of Investigations Investigations to date have revealed that Metoponia rubriceps is a serious pest of first-year maize at Opotiki, causing in 1947-48 at least a 25 per cent, loss of first crops of maize after grass. Other crops known to be severely affected are green cereals and kumikumi. No exact information has yet been obtained about how serious the pest is in grassland at Opotiki. An indication that the insect may be controllable by cultural methods has been obtained, but the prospects of controlling it chemically are not bright. Investigations are continuing. Farmers are recommended to plough infested pasture at least three months before sowing to any other crop. A summer fallow and cultivation with light tine harrows are likely to be even better, as larvae may then be destroyed by drying out or by insect-eating birds. Acknowledgments The pioneering work of certain local farmers who discovered the maggot to be the cause of some of their troubles and who persisted in endeavouring to secure official investigation is acknowledged. These Messrs. L. Holman and T. I. Steel, > and an Opotiki chemist, Mr. J. D. Clarkdeserve the thanks of Opotiki farmers and of the writer.

Special Weather Forecasts for Farmers

UP-TO-DATE weather information • is most important to the farmer at. such times as shearing, haymaking, and lambing, and though a check can b 3 kept by listening to the regular broadcast forecasts, these are often not sufficiently specialised to meet individual needs. Such needs are best met by a request telegram to “Weather,” Wellington (the telegraphic address of the Meteorological Office), which will receive the attention of the forecaster. It should be realised, however, that these telegrams should reach “Weather,” Wellington, early enough to ensure that a reply is not delayed by closing of the local telegraph office or any other contingency. The reply will be brief and sent “collect.” There are many kinds of telegrams that may be sent . according to the needs of the farmer concerned, but in all cases farmers would materially assist the forecaster by giving the actual dates for which the forecast is required and the particular kind of weather, which is of special importance; for example, strong winds, rain, snow,, or cold changes. It would also be very helpful if the

nature of the operations being undertaken could be briefly indicated; for example, shearing, haymaking, or lambing. ‘ In cases where the name of the telegraph office of origin does not give an adequate indication of the situation of the farm, a brief note on location is useful. This is clearly important if the farm is near mountain ranges or is at a considerable altitude. Telegrams Should be Explicit It will be realised that it is hardly possible to give a brief reply to a telegram reading: “Please send weather prospects next few days. Smith, Tauranga,” as “weather prospects” does not i indicate the particular weather elements which are important and the time period is only vaguely indicated as “next few days.” It would clearly lead to a more definite and more concise reply if the telegram were worded: “Please send prospects for rain* 15th and 16th. Smith, Tauranga.” In this case the reply could be simply: “No rain expected. Weather.” \ Another type of request which • is

often used to safeguard farming operations is that requiring information to be sent under certain specified conditions, as in the following: “Please send warning of cold changes during December. Cloudy Valley, altitude 5000 ft. Jones,” or “Please advise when three days without rain are expected. Smith, Tauranga.” . ' The period of validity of most weather forecasts is normally . taken to be 24 to 36 hours, and though under favourable weather conditions this can sometimes be extended up to three days, any . longer period must be regarded as too uncertain for practical use. - When asked for longer-period forecasts, forecasters generally will give as long as is reasonably certain and then indicate the uncertainty of the remaining period in the telegraphic reply. For example, in reply to a telegram - reading: “Any rain Monday to Saturday,” the reply could be: “No rain Monday, Tuesday, Wednesday—tain thereafter.” . For their part, farmers can give considerable assistance to the Meteorological Office by forwarding details of peculiarities in weather in their district, which can then be investigated.