THIRTY-SEVENTH CONFERENCE. By B. E. HITCHCOCK Bureau of Sugar Experiment Stations, Mackay

Transcription

1 THIRTY-SEVENTH CONFERENCE THE SOLDIER ELY PEST By B. E. HITCHCOCK Bureau of Sugar Experiment Stations, Mackay Introduction In Queensland the soldier fly Altermetoponia rubriceps (Macquart) (Family Stratiomyiidae) has long been recognized as a pest of cane at Mackay (Jarvis, 1925) and Bundaberg (Mungomery, 1926), but since about 1950 its pest status has been much greater than during the 25 years following its first discovery. In 1966, a second species, subsequently named A. Java James (James, 1968), was found on one farm at Proserpine and another at Mackay. In 1967 the writer found the latter species in many parts of the Mackay district, where its pest status appeared to be similar to that of A. rubriceps, but up to the present only A. rubriceps has been found in south Queensland and at Innisfail. In 1967, the writer began ~omprehensive studies, at Mackay, on the life history and control of both these species. This paper provides an interim report on portion of these studies, and an assessment of the change in the pest status of the soldier fly in the Mackay and Plane Creek districts over recent years. Life History Studies The earliest recorded attempt to determine the life history of A. rubriceps was by Irwin-Smith (1920, 1921) in Sydney in This worker found it possible to rear advanced field-collected larvae through to adults. She was also successful in getting adults to lay eggs and eventually in getting larvae from the eggs, but these newly hatched larvae soon died and she was unable to obtain a complete life cycle. Since that time there have been a number of attempts to breed soldier fly from eggs so as to determine the length of the life cycle, but in each instance the larvae have died in the delicate early stages. Thus when the writer began studies in 1967, little was known of the development of the larvae and the actual length of the cycle and the number of instars involved was purely a matter of conjecture. Since the insect was known to fly annually it was inferred that it might have a one-year life cycle, but field observations had shown that larvae of greatly differing sizes occurred at all seasons of the year, suggesting that more than one generation was present and the cycle might be longer than one year. Moreover, in New South Wales, where the insect commonly occurs in lawns and pasture, two flight periods are recorded, one in the autumn and one about November (Irwin-Smith, 1920) while in Queensland only one flight period, from late March to June, has been observed. In the studies carried out at Mackay the problem has been approached from several different angles so that in the event some approaches failed, others might be successful. The different projects initiated were as follows :- l. Collecting larvae from the field some weeks prior to the start of the flight season and rearing them in individual containers, with the object of ascertaining if some individuals remained as larvae until the

2 208 THIRTY-SEVENTH CONFERENCE 1970 following flight season, thus indicating a life cycle longer than one year. 2. Placing egg masses in large cages, then segregating the larvae into individual containers at about eight months of age. 3. Placing newly hatched larvae in small containers so constructed that the progress of the larvae could be observed with a minimum of disturbance. At about six months or older the larvae were segregated into individual containers. Rearing was carried out in an air-conditioned insectary, temperatures being maintained to conform with those recorded in the soil of a nearby cane block by a soil thermometer. Moisture was kept uniform in all containers by watering to a standard weight. Since sugar cane would have been awkward to germinate and maintain in the small containers, maize was used as the food plant. Reseeding of the containers was timed so that fresh seedlings were germinating by the time the old ones had started to die back. Light was provided by fluorescent tubes, the light level being sufficient to enable the maize to grow quite vigorously until it was stopped by the 12 inch-high gauze covers over each container. Rearing of larvae collected from the field just prior to the flight season was initiated in 1967 and The results have been similar in each instance. The majority of the larvae matured to adult males or females during the normal flight season in the year they were collected. Subsequently, during the spring and summer, mortality among remaining larvae was high, but in each instance some survived and those collected in 1967 and 1968 have produced adults in the year following collection, indicating a life cycle longer than one year. A few adults which had died while still within the pupal case were noted about November among these larvae, but there is no positive evidence from either the field or laboratory, of a November flight such as occurs in New South Wales. The larvae collected prior to the flight season in 1967 and 1968, were mainly those of larger size. Later, improved collecting techniques often disclosed the presence in the field of very large numbers of tiny larvae which, if included in these projects, might have increased the proportion of those which did not attain the adult stage in the year in which they were collected. In November and December, 1968, some 500 larvae of A. rubriceps and a similar number of A. fiva were collected from the field and each larva weighed before being placed in the rearing jars. Larval weights ranged from 78 mg down to less than one milligramme, the latter being far smaller than any larvae collected previously for rearing. Larvae of A. rubriceps which weighed less than 6.0 mg and larvae of A. Jlava which weighed less than 3.0 mg, comprised the bulk of the field population at the time of collection; none of these small larvae gave rise to adults during the 1969 flight season, so it appears likely that these represented a relatively recent generation, probably from eggs hatched in 1968, and also, possibly the slower developing members of that generation (see below). However, during the 1969 flight season the larger larvae yielded adults, males emerging from larvae which had weighed 26 mg or less when collected, females coming mostly from larvae that had weighed over 26 mg. It was concluded that the great variation in size of larvae found among field populations is attributable to three factors, (1) the presence of overlapping generations, (2) the difference in size between the sexes, (3) a wide range of size, at least during the November-December period, between individual larvae irrespective of their potential sex. Rearing of larvae from egg masses placed in large cages was initiated in 1967 and 1968 and the larvae transferred to individual containers at

3 1970 THIRTY-SEVENTH CONFERENCE 209 about eight months of age, when they were from two to six millimetres in length. In both 1968 and 1969, adults were obtained from some of these larvae at one year of age. Most of these adults were males, but females were also obtained. However, the vast majority continued as larvae, indicating a life cycle longer than one year. Unfortunately, all the 1967 larvae died before maturing, although some survived into the 1969 flight season and reached what is thought to be the final instar. The generation which hatched from eggs laid in 1968, will reach two years of age in the autumn of The third project, in which larvae have been reared in small containers so that more detailed observations of their development were possible, was very difficult to initiate, but since satisfactory techniques have been developed, it has proved the most rewarding of the three projects. Initially newly hatched larvae are caged in polythene tubes three inches long by one inch in diameter which have been filled with soil and maize seed germinated therein. Portions of the sides of the tubes are removable so that the progress of the larvae can be observed and recorded at frequent intervals, with a minimum of disturbance. Because the tubes are small these observations can be made under a microscope, which is very necessary because of the small size of the larvae. Using these containers it has been possible to observe feeding and moulting and determine instars. The feeding was most interesting since it showed conclusively that the larvae cause considerable mechanical damage to roots. The mouth parts include two strong hooked mandiblelike processes, which curve downwards. These are used to excavate cavities in roots, into which the larva inserts its head and imbibes the juices in the root. Second instar larvae have also been observed to break off bunches of root hairs with these processes. Having seen the larvae excavating roots in the laboratory it was not hard to find cane roots in the field, in infested stools? with similar excavations, although disturbance of the stool usually displaces the larvae from any such root excavations. In the laboratory, roots heavily excavated usually die distally from the damage and in the field it is commonly found that heavily infested stools have many old dead roots with an abundance of small holes in them. In young fresh cane roots, soldier fly excavations differ from those made by nematodes in that the tissue is removed and the edges of the holes are clear cut, while nematode punctures only become holes when the tissue around them dies and collapses. However, in dead roots one could not be confident that soldier fly were responsible unless some fresh roots with clear cut excavations were also present. There are some other small soil insects which excavate roots in a similar manner to soldier fly, but their numbers are usually small and in the writer's experience the presence of such excavations on fresh roots has usually been followed by the finding of soldier fly larvae. The determination of the number of instars is an important aspect in the study of the life cycle of any insect. These studies have indicated that there are nine larval instars in larvae which reach the adult at one year of age and at least ten instars in those with a longer development period. The duration of the first larval instar is about one week in both species, but subsequently there is a good deal of variation between individuals, even from the same egg mass and reared in the same container. Evidently even at this early stage there is a differentiation between those individuals which will mature to the adult in one year and those which will develop more slowly. These studies were initiated with larvae which hatched in late May

4 210 THIRTY-SEVENTH CONFERENCE 1970 or early June, The incubation period of the eggs at that time was ten to twelve days, although it may be as short as six to seven days in the warmer months of March and April. Since there was a large number of larvae in each container records of individual larvae were not possible and the practice was simply to record the numbers and sizes of each instar present at each inspection, inspections being carried out at intervals of three days. Characters to separate the various instars were determined by using larvae found actually in the process of moulting and a key was worked out to separate the various instars. It was also found possible to separate the larvae of the two species by a constant character which appeared in all instars. The more rapidly developing larvae reached the sixth instar by November, entered the seventh and eighth instars during the period from January to April, the ninth instar in May or early June and adult males appeared in late June. On the other hand, some of the slower developing larvae were still in the second instar in November, 1968, and many did not reach the fifth and sixth instars until June, 1969, when one year old. By January, 1970, when these slower developing larvae were 18 months old, most had reached the ninth instar and three (one A. rubriceps and two A.flava) had moulted further to a tenth instar. In addition there were a few A.flava still in the seventh and eighth instars and one in the sixth. In 1969 the project was repeated along the same lines with larvae hatching in that year. The results so far are essentially the same. A few larvae reached the fifth instar as early as September but most did not do so until October and November and in January, 1970, a few had reached the seventh and eighth instars. In January, 1970 the slower developing larvae were in the third, fourth and fifth instars. It is concluded that a one year life cycle for some soldier flies has been established and that probably this is more common among males than females. There is also evidence that for the majority the life cycle is longer than one year, but whether this is two years will not be established until adults have been bred from larvae at present under study. Although characters to separate the various larval instars have been worked out, determination of individual larvae is often not easy and whether a total of nine larval instars for one year larvae and ten or more for those with a longer cycle, is correct, will require confirmation. Additional instars among those larvae with the longer life cycle is not unexpected and has precedents among other insect species, but it is also not uncommon for males to have fewer instars than females and in this particular project we have so far bred males only from larvae in which the number of instars was limited to nine. Pest Status The change in the pest status of soldier fly, from that of minor importance in the nineteen twenties and thirties to major importance in the nineteen fifties and sixties, is well known. The role of soldier fly as a major cause of poor ratooning at Bundaberg was first postulated in 1956 and confirmed during the next few years, during which chemical control methods were tested in field trials. Farm applications of insecticide for soldier fly control were first carried out at Bundaberg in 1961 and at Mackay in At Mackay the area being treated annually had reached 5,149 acres by 1966 and has ranged as high as 6,910 acres in subsequent years. By 1969 there were about twenty-four thousand acres of cane in the district which had been treated against soldier fly, involving 32 per cent of the 1,438 growers in the district.

5 THIRTY-SEVENTH CONFERENCE 21 1 Information on insecticide applications for the control of soldier fly in the Mackay and Plane Creek districts, is recorded by the respective Cane Pest and Disease Control Boards which, in recent years at least, have handled virtually all the chemicals used. Because of the cost of the treatment most farmers seek advice from pest control officers before initiating treatment on their farms, so that the proportion that may have been treated needlessly is low. Thus the records of insecticide usage provide a reasonable basis for identifying farms which have become infested. However, once an infestation has been found and treated on a particular farm, the farmer in subsequent years will probably treat other fields on the farm as they come up for replanting. This is a wise procedure, but in the records it inflates the total area treated each year without indicating whether new farms are involved or not. Thus the number of farmers treating for the first time in each successive year gives a much better indication of the rate of increase in the incidence of soldier fly than the total number of acres treated alone. Table I shows the total number of acres treated and the numbers of Plane Creek a Plane Creek Fig. \--Total number of acres treated and the number of farmers treating for the first time each yes.

6 212 THIRTY-SEVENTH CONFERENCE 1970 TABLE I-Total acres treated for soldier fly control and number of growers treating for the first time, in each annual planting farmers treating for the first time each year since treatment commenced at Mackay and Plane Creek. The same information is illustrated more graphically in the histograms in Figure 1. In the Mackay district (Figure 1) the area treated rose steadily from 130 acres in 1962 to 1,590 acres in In 1966 there was a big jump to 5,169 acres followed by a steady rise to a peak of 6,910 in 1968 and a small fall to 5,750 acres in The numbers of fresh farms being treated each year present quite a different picture. There is a slow rise from 20 farms in 1962 to 33 in 1965 and in 1966 a large increase to 167 farms, which matches the big increase in acres treated in that year. However, 1966 was the peak year and subsequently the numbers of fresh treatments fell to 34 and 49 in 1968 and 1969, respectively. The fields treated in 1962 and 1963 were mostly those where poor growth associated with soldier fly had been occurring for some time. With the recognition of the pest status of soldier fly and development of control measures these farmers were naturally the first to take action. Apart from one farm at Baker's Creek and three near Walkerston, all the early treatments were near Finch Hatton and it was first thought that soldier fly were absent from other parts of the district. However, as pest control officers and farmers gained experience in recognizing the presence of soldier fly, new infestations were found in parts of the district where they had not previously been known to occur. Thus by 1965 there was a very keen awareness of the soldier fly problem among farmers throughout the district with the result that many more infestations were identified and substantial areas were treated in Much of this area could have been infested for a long time and the great increase in treatments in 1966 was probably due to a wider recognition of the pest rather than a sudden spread of soldier fly. In the Plane Creek area (Figure l), treatment began in 1965 with 194 acres, rose rapidly to a peak of 985 acres in 1968 and fell to 616 acres in The numbers of farmers treating rose from three in 1965 to 42 in 1966, recognition of the pest being no doubt stimulated by the wide publicity the soldier fly problem was receiving in the Mackay

7 1970 THIRTY-SEVENTH CONFERENCE 213 district. However, the areas treated by individual farmers must have been small since a total of only 318 acres were treated in that year. Perhaps this indicates an initial caution until farmers could judge the effectiveness of the treatments for themselves, and may explain why the total area treated increased rather sharply during the following two years up to Overall the picture is much the same as at Mackay, with the numbers of fresh farms treating falling sharply subsequent to 1966, while the total area treated each year has remained high. Whether the lower figure in both districts, for acreage treated in 1969, as compared with the Fig. 2-Location of farms treating for soldier fly at Mackay and Plane Creek.

8 214 THIRTY-SEVENTH CONFERENCE 1970 preceding two years, indicates a downward trend for the future, remains to be seen. Clearly the rate of increase in the incidence of soldier fly, as indicated by the numbers of fresh farms treating each year in the Mackay and Plane Creek districts, is much less than might be inferred by reference to the total area treated each year alone. Nevertheless, the increase in the last two years has amounted to some 11 1 farms which is appreciable, especially since these are mostly new infestations, as few farmers with significant infestations have failed to treat after the publicity given to soldier fly in recent years. The question of whether the vast increase in the incidence of soldier fly during the last few years is due to spread from limited areas, or whether this native insect has always occurred in small numbers throughout the district and has only recently started to build up to pest proportions, has been the subject of much discussion. In an attempt to clarify the matter the location of all farms treating for soldier fly were marked on a map of the Mackay and Plane Creek districts, symb~is being used to designate which year the respective farms first began treatment. Figure 2 is a simplified version of this map and shows where the early treatments against soldier fly were carried out, between 1962 and In the Mackay district the area around Finch Hatton has been known for soldier fly infestations for many years and only four farms outside this area were treated in 1962 and Three of these were at Walkerston and one at Baker's Creek. In 1964 and 1965 the majority of fresh farms treating were still around Finch Hatton, but there were a number as much as ten miles east of the old area. At Walkerston only one further farm began treating, but across the Pioneer River there were quite a number of new farms stretching over the ten miles from the river to Habana. In the same years treatments were also begun at Calen some 30 miles north west of Walkerston and 20 miles north across a mountain range from Finch Hatton. In 1966, farms which treated for the first time were located in all parts of the district and from that year onward there has been no appreciable part of the district which is not affected. Thus if the increased incidence of soldier fly from 1962 to 1965 was due to spread from the areas around Finch Hatton and Walkerston, then the insects covered a phenomenal distance in a very short space of time. Moreover, if they were spreading outwards from these localities, it is surprising that they did not appear on more farms close to the old infestations. In none of the many flights observed at Mackay have the adults shown themselves as very active fliers. Usually they fly only short distances and remain well below the tops of the cane or other vegetation occurring in the vicinity of the particular infestation. If the flight occurs from a fallow or newly planted cane, the flies may rise some feet from the ground if disturbed and travel perhaps 20 yards, but this appears the maximum. At Bundaberg adults have sometimes been observed to swarm above the tops of large cane, and observations have been made of adults travelling over fallows with the wind. Although such mass movements have never been authenticated at Mackay, it would seem that they are possible. However, since the prevailing winds during the flight season at Mackay are southerly, that would not favour the movement eastward from Finch Hatton, and the infestations around Walkerston would seem too small to provide enough adults to infect farms stretching from the Pioneer River to Havana. Southerly winds might carry adults north from Finch Hatton, but there is a high mountain

9 1970 THIRTY-SEVENTH CONFERENCE 2 15 range between there and Calen. It is concluded that while soldier fly adults may spread infestations quite readily from field, it is very unlikely that they could have spread infestations throughout the Mackay district between 1962 and In the Plane Creek district the cane land occurs in irregular areas along each side of the Bruce Highway from Flaggy Rock in the south to Alligator Creek north of Sarina. The overall length of the district is some 50 miles and the southerly sections are frequently broken up by areas of forest. Treatments began in 1965 with three farms near Carmila West, but the 42 farms treating for the first time in 1966 were situated in all areas from Flaggy Rock to Alligator Creek. In subsequent years new treatments have been similarly distributed. It seems more likely that the big jump in the number of farms treating in 1966 was due to widespread recognition of the pest rather than a sudden spread of soldier fly over a distance of some 45 miles, even though this was in the direction of the prevailing winds. The conclusion that soldier fly may have been present in many parts of the Mackay district for a long time, rather than having only recently spread from areas around Finch Hatton and Walkerston, is reasonable when one remembers that these are native insects with various host plants other than sugar cane. Although our knowledge of the biology of soldier fly is still too meagre to explain conclusively why such scattered populations should suddenly start to build up to pest proportions, we do know of a number of factors which could have played a part. These are concerned with cultivation, soil moisture, disease and parasites. It has been shown in field experiments that cultivation of the surface soil during the autumn, when the adults are emerging, can have a drastic effect on the population by burying the pupae so that the adults cannot escape from the soil, and by exposing the eggs to the heat of the sun. Cultivation also dries out the surface soil and it has been shown, in the laboratory, that adult females are unable to bury their eggs properly in loose dry soil, and eggs exposed in this way did not hatch. Thus it is possible that the change from horses to tractors some 20 years ago, with consequent less cultivation of young cane, and the modern tendency to minimum cultivation of young cane and less ploughing in fallows, has favoured the breeding of soldier fly. Certainly the recent increase in the practice of growing more than two ratoon crops could be a factor in the build up of the pest. In recent years the irrigation of young cane has been adopted on over 20 per cent of farms in the district and it has been suggested that this may have favoured soldier fly, particularly the eggs. Laboratory experiments have shown that, provided there is some surface moisture at the time of oviposition, subsequent complete drying out of the soil does not effect viability of the eggs. However, in the containers with completely dry soil the young larvae moved to the bottom of the jars as soon as they hatched, while in jars with moist soil the larvae remained near the surface, indicating that moist soil was preferred. In the field the yellow soldier fly, Altermetoponia jlava, seems to prefer the drier, well-drained soils while the black soldier fly, A. rubriceps, can thrive in soils that are waterlogged for weeks at a time during the wet season. However, in the laboratory neither species appeared affected when reared in soils near saturation, although both died quickly when kept completely dry, so that evidently both are susceptible to extreme dryness, and could be favoured by irrigation during the dry time of the year. Not much is yet known about the incidence of disease among

10 216 THIRTY-SEVENTH CONFERENCE 1970 soldier fly, but it has been discovered that they are sometimes widely infected by the green muscardine fungus, Metarrhizium anisopliae Metsch. What environmental conditions favour the spread of this fungus and whether certain larval instars are more susceptible than others, is not known, but usually such diseases spread best in moist conditions. However, even in the dry spring months larvae from some fields have been found to be heavily infected. One very puzzling feature is that, in spite of the thousands of soldier fly larvae that have been field-collected and reared in cages, no parasite or predator has been detected. That a high degree of biological control must take effect in nature on at least some occasions, can be deduced from the build up of infestations that has taken place as the result of experimental applications of insecticides, at rates too low to control the soldier fly, but apparently sufficient to destroy parasites and/or predators (Moller and Mungomery, 1963). Acknowledgments The author wishes to express his appreciation to Mr. S. Greenaway and Mr. P. J. Amiet of the Mackay Cane Pest and Disease Control Board and Mr. E. D. Cran of the Plane Creek Board, who went to considerable trouble to extract the data on insecticide usage from their records. Mr. R. M. Bull, Entomologist at the Southern Sugar Experiment Station, co-operated in much of the life history studies. REFERENCES Irwin-Smith, V. (1920). Studies in life histories of Australian Diptera Brachycera. Part 1. Stratiomyiidae. No. 1 Metoponia rubriceps Macquart. Proc. Linn. Soc. N.S.W., xlv : Irwin-Smith, V. (1921). Studies in life histories of Australian Diptera Brachycera. Part 1. Stratiomyiidae. No. 2 Further experiments in the rearing of Metoponia rubriceps., ibid., xlvi : James, Maurice T. (1968). A new stratiomyid pest of sugar cane in Australia (Diptera: Stratiomyidae). J. Aust. ent. Soc. 7 : Jarvis, E. (1925). Dipterous larvae attacking roots of cane. Qd. agric. J., New Series, XXIV: Moller, R. B. and Mnngomery, R. W. (1963). Some insect pests adversely influencing ratooning in south Queensland. Proc. int. Sug. Cane Technol., l l : Mungomery, R. W. (1926). The soldier fly and its association with cane. Qd. agric. J. 26: