Army Caterpillar Damage on East Coast Hill Country

New Zealand Journal of Agriculture, Volume 98, Issue 4, 15 April 1959, Page 329

Army Caterpillar Damage on East Coast Hill Country

By

C. J. HAMBLYN,

Fields Superintendent, Department of Agriculture, Palmerston North

IN the late autumn of 1956 and again in the autumn of 1957 the army caterpillar appeared in devastating numbers on east coast North Island hill country. During these two autumns the caterpillars infested upward of 120,000 acres of steep hill pastures, stripped the country bare of green leafage, and left it open to invasion by such weeds as thistles, dandelion, and hawkweed. The army caterpillar, which is often referred to as army worm, is the caterpillar stage of a common native night flying moth. It has often devastated such crops as barley, maize, wheat, and oats in summer or early autumn, and in a number of districts has severely damaged pastures on river flats and swamp areas, usually after a summer flood followed by warm, muggy weather. . In such visitations they have appeared in tall, rank pastures, which the moths have evidently found to be favourable places for egg laying, as they do the long grass of headlands and also the crops they attack.

THE recent outbreaks described in this article were on a much bigger and more widespread scale than hitherto recorded in New Zealand. Districts Affected The caterpillars severely damaged some 3,000 acres of hill country pastures in Poverty Bay in 1956, but did not reappear in this area in 1957. Wairoa County, which experienced the most widespread infestation in 1956, when some 20,000 acres were ravaged, had less than 10,000 acres affected in 1957. In central and southern Hawke’s Bay, . however, where the damage in 1956 was confined to about 3,000 acres of coastal land, the area affected increased to some 30,000 acres in 1957, including many areas much further inland. A few small isolated outbreaks were recorded in the Wairarapa in 1956, but in the autumn of 1957 more than 40,000 acres of coastal hill country was laid bare by very heavy infestations of the caterpillar. In line with the unpredictable nature of these appearances there were no reports of the presence of the caterpillar in' large numbers in the autumn of 1958 anywhere on the east coast and it is quite possible that it may be a long time before they reappear in this country on the scale of 1956 and 1957. Life History of Insect Unfortunately the life history of the moths, of which the so-called army worm is the larval or caterpillar stage, has not been studied fully in the field nor have the origin and course of a typical army caterpillar outbreak been carefully followed and recorded. Two different insects appear to be involved, Pseudaletia separata and Persectania aversa, both of which belong to the very large family of night flying moths the Noctuidae. The life cycle as far as is known is typical of this type of moth. The moths mate soon after emergence from the chrysalids in late spring and early

summer and the females lay up to 500 eggs in the folded blades of grass or cereal crops. The eggs under favourable conditions soon hatch, and the larvae, at first small greenish caterpillars, proceed to feed and grow in the area where they are hatched. They pass through a series of up to six moults, shedding the complete skin. When the caterpillars occur in very large numbers it seems that their feeding activities foul the feed and this causes them to start moving as an army in search of fresh clean feed. They like to climb up the stems and leaves of the plants they are feeding on, and they eat mainly at night, hiding at soil surface during the day. Their food requirements are quite

small in the early stages, but when reaching maturity they can eat their own weight or more in a single night. When fully fed the caterpillar goes into the chrysalis or resting stage, from which the adult moth emerges. Under favourable climatic conditions two or three full cycles are passed through in a summer and in the absence of, or when protected from, any or all of the natural controlling agents such as birds, insect parasites, bacteria, and fungous or virus diseases which attack the caterpillars, the population of moths can be built up enormously in a favourable season. When these moths proceed to lay eggs in hundreds the stage is soon set for a mass emergence of hungry caterpillars should favourable autumn weather cause these eggs to hatch. If the insects can overwinter successfully, the moth population at the beginning of the next season can again be very big and new massive caterpillar infestations likely. The fact that two seasons of widespread damage such as those recently experienced have been followed by a year of freedom from the pest suggests that the controlling agents can be quick and very effective in keeping the pest from becoming an annual scourge. Features of Infestations The recent visitations, beside being the most widespread and heaviest infestations of pasture lands by the army caterpillar so far recorded in this country, were remarkable in several other respects. Before the 1956 outbreaks were started, apparently by favourable autumn weather causing a mass hatching of eggs, there must have

been an unusual build-up in moth numbers during summer and some prodigious egg laying in the long roughage which they found suitable high in the hills. They always chose the sunny faces and they chose, for some unknown reason, areas of unimproved or very little improved hill country where there was plenty of roughage and cover for the eggs and the young caterpillars. The first sign of trouble in the autumn of 1956 was the appearance about mid April of . large patches of pasture which had turned brown high

in the otherwise-green hills. The hills were generally green this autumn because of good summer and early autumn rains. These brown patches grew bigger every day and investigation showed that millions of army caterpillars, which had apparently started from small pockets and patches of roughage on the sunny slopes, were on the move eating out the green leaf of the grasses and chewing into the crown. They did not eat all the herbage, but their heavy onslaught on the plants so reduced vitality that the remaining leafage rapidly turned brown and eventually died off. As the caterpillars moved outward and downward on the hills the areas of depleted pasture spread until in many areas the whole of the sunny faces of the. hills showed the effect of their passage. The caterpillars prefer plants of the grass family, including the cereal crops, and the way in which they avoided patches of dominant clover pasture and even isolated clover plants was really remarkable. Their dislike of the clovers was perhaps the reason for their appearance only in unimproved pastures and for their not invading improved pastures adjoining the areas they were feeding in. Thus areas of sown pasture showing high fertility with plenty of clovers were not affected. They also confined their feeding almost entirely to the sunny faces and generally the shady faces were not depleted. The caterpillars continued active in the autumn of 1956 until well into June and in some areas up to the end of June. The 1957 outbreaks differed somewhat from those of 1956 in that

the first appearances were some 2 weeks or more earlier, and because the country was much drier than in 1956, the typical browning of green herbage was not nearly so noticeable. The uneaten herbage did change colour, however, and the damage to plants was much more severe, a big proportion being killed. Nature of the Damage The autumn and early winter of 1956 were very favourable to pasture growth,. and though the caterpillars cleaned up a considerable amount of cattle feed, they also cleaned out and opened up big areas of accumulated roughage on many of the unimproved hills. In some districts, notably Wairoa, the loss of cattle feed called for an adjustment of winter stocking, but generally pastures recovered well and no great harm was done. The position was very different, however, after the much more extensive infestations in such districts as Hawke’s Bay and the Wairarapa in the autumn of 1957. The caterpillars then became active after a prolonged dry spell which had already badly dried the pastures. Their activities, though starting earlier, persisted well into June, by which time the debilitated sward was unable to recover because of cold, wet conditions. Then in late spring, when such areas normally make good growth, drying winds took their toll, and the summer and autumn of 1957-58 were the driest in these coastal areas for many years. The effect of the drought on the feed position in the autumn and winter of 1958 on east coast hill country was bad enough, but on the big areas already depleted by the ravages of the caterpillar the result was really serious. On the majority of hill country farms from the Wairarapa to Poverty Bay reductions in livestock carried were necessary, particularly of cattle, because of the exceptionally low rainfall and prolonged droughty conditions, but where the caterpillar had also affected pastures the reductions had to be much heavier than would have been necessary otherwise. In many areas a further problem arose in that the opening up of the sward resulted in heavy infestation of large blocks of hill country by thistles of various sorts, flat weeds, and other weeds. Remedies There are at least two remedial measures to combat the caterpillars when pastures are attacked. They can be stopped in their tracks as it were by spraying with several of the new insecticides including DDT and dieldrin and either killed off completely or prevented from moving on to fresh areas. On areas fed over by

the caterpillars the sward is opened up with much bare soil and these conditions are very suitable for successful oversowing, especially with clovers. Several important features of the two large-scale outbreaks on east coast hill country either prevented successful use of either of these remedies or where they were used made the results generally uneconomic. In the 1956 outbreaks most of the damage had been done before farmers realised what had happened. However, quite a number were successful in stopping further movement on to fresh areas by aerial spraying round the perimeter of the advancing caterpillars. A band of some 50 to 60 ft. was sprayed with one pass of the aeroplane or helicopter. About half the band was applied on the advancing caterpillars and the rest to the pasture ahead. One of the main considerations in such cases was the fear that the caterpillars would move on to and clean out improved pastures in the valleys and lowlands adjoining the hills. As already stated, they did not do so even where no spraying was done. Oversowing of Affected Areas Only a few farmers took advantage of the conditions left by the caterpillars by oversowing with clovers and topdressing affected areas. Most hill country men considered it too late to oversow because it was well into June before the caterpillars stopped feeding. However, because of the favourable winter and spring those who did oversow were generally successful in getting a strike and satisfactory establishment. The 1957 outbreaks were generally a little earlier and much more spraying was done, but here again by the time it was decided to act the caterpillars had already advanced over big areas of country and the cost of spraying was very high relative to results in saving feed. Some quite successful oversowing was also done in some districts, but the extremely adverse conditions. of the following spring, summer, and autumn killed off most of the young clover plants. The fact that most of the country affected had never been topdressed before made the establishment of oversown clovers still more difficult so late in the season. The successful establishment of oversown clovers by one or two farmers who risked oversowing and topdressing infested areas in mid April when the caterpillars started to show up, and some 6 to 7 weeks or more before their active feeding stopped, indicated that this could be done safely, and so give the clovers much better conditions for establish-

mem. The difficulty, of course, was in deciding what area to oversow, since no one knew just how far the caterpillars would go unless they were stopped by spraying with insecticide. EffecHveness of Spraying Though the spraying was effective, it was difficult to decide on the value of tackling such widespread and massive invasions of pastures. There is no doubt that the sprays used and the method of application were effective in stopping the further spread of the caterpillars, but the spraying was far too late, and the cost, relative to the type of feed saved on unimproved hill country, unduly high. But here again it was not known that the caterpillars would not move on to and clean up more valuable pastures adjoining the hills. Effective sprays applied by helicopter or aeroplane per acre were: 1J pints of 15 per cent dieldrin in 5 to 10 gallons of water, more water being used according to the rankness of growth; 2 pints of 20 per cent DDT in similar quantities of water. The dieldrin was much quicker in action than the DDT, but the latter was quite effective. . When the caterpillars were inactive for some days the insecticides took longer to act. Possibilities of Early Spraying The fact that the main hatchings of eggs, which gave rise to the immense populations of voracious caterpillars on the move in search of fresh feed, took place in relatively small patches of over-rank grass high in the hills some 2 or 3 weeks before the movement began suggests that had any of these infestations been noted and the danger appreciated at this early stage spraying of such areas would have been much less costly and far more effective in preventing damage. It would have been possible, also, had the early incidence of abnormal numbers of the small caterpillar been reported, to have given farmers in districts with similar climatic and farming conditions a warning to investigate the hills on their properties. This should certainly be the procedure when infestations start in valuable crops such as barley, wheat, oats, and maize, where early action by spraying can save thousands of pounds. It is quite possible, however, that by the time another visitation of the type and magnitude of those of 1956 and 1957 occurs again on the hill country of the east coast the lessons learnt on how best to combat this pest and remedy its damage will have been forgotten.