A LOOK AT THE PROBLEM OF PORINA

Press, Volume CVII, Issue 31440, 5 August 1967, Page 10

A LOOK AT THE PROBLEM OF PORINA

In Canterbury the immediate major problem is resistance of grass grub to D.D.T. From a control point of view porina is not a major problem. Porina damage has been accentuated this year because of highly favourable climatic conditions creating an abundance of rank pastures.

The writer of the accompanying article is Dr. R. P. POTTINGER, of the agricultural zoology department at Lincoln College. It is the second and final part of a review of grass grub and porina which he began last week. Today, Dr Pottinger deals with mainly porina and has a few suggestions to make about what farmers can do on badly damaged country.

Diagnosis of porina caterpillar presence has been made too late on many farms and as a result many pastures have been lost or inefficient control obtained. Porina Caterpillar Biology Porina caterpillar damage in Canterbury is caused by a complex of three species, for practical purposes not readily distinguishable from each other. All have one generation each year, but due to differences in the time of year at which the respective adult stages occur, feeding damage by larvae of these species is staggered over the late summer, autumn, winter and spring periods. The species concerned are:

1. Wiseana cervinata (early flying species) which is present as an adult moth in Sep-tember-October. Its caterpillar stages cause extensive pasture damage from late summer through Into winter. 2. Wiseana umbraculata (middle flying species), which mainly flies in November-De-cember and feeds as a caterpillar later into winter. 3. Wiseana despecta (late flying species), which flies in January-February and causes pasture damage in late winter and siting. This species is more abundant in wetter regions or areas and is of consequence in some plainsland pastures only after one or two wetter than average summer seasons. By far the most common and important species is the early flying porina whose life cycle is summarised in the accompanying diagramThe adults are heavy bodied, drab coloured Insects about one inch long. Their body colour is variable, but they are brownish coloured and the forewing is characterised by having close to its anterior margin a dark strip fringed anteriorly with white. The adults are attracted to light and bang against windows early in the evening from the end of September into November. Peak flights occur in the third week of , October. They do not feed and only live one to four days, but in that time each female may lay from 400 to 2300 eggs , on the ground surface as she ' crawls or flutters among the vegetation. The females ap- ' parently have no preference for the type of vegetative cover, but they tend to prefer to lay eggs on rank vegetation. It is thought that most females remain in the field in which they emerge, but at Lincoln College a newly-sown pasture was lost in one year. In this case the population originated entirely from moths migrating Into the paddock from other sources. The eggs are very small, less than half a millimetre in diameter, oval and creamy white when first laid, but turn black within a day. They require moist conditions throughout a three to fiveweek incubation period and suffer high mortality if exposed to relative humidities less than 70 per cent The young caterpillars, which are inconspicuous, live above ground for a period of four to six weeks, beneath litter and vegetation, before excavating tunnels into the ground from January on. They are particularly

susceptible to dessication and their survival is enhanced by rank pasture growth. Their tunnels may have a diameter of up to three-tenths of an inch and reach six to 12 inches. The caterpillars shelter in them during the day but emerge at night and graze grasses and clovers. Porina masks its tunnel entrance by placing the tunnels under overhanging vegetation and covering them with fine silken webbing which supports plant litter and/or soil particles. A protective runway, camouflaged in the same way, often leads two to three inches out from the entrance to where the caten>illar is feeding. Porina castings of fine loose earth, which cover burrow entrances, are highly characteristic, but can be readily distinguished from the smooth, shiny castings of earthworms, because of their crumbly, non-glossy appearance and tendency to dry more quickly than other soil particles or castes. The caterpillars grow to a length of 2}in to 3in by the end of May or June. Dorsally they are a dirty olive green colour, but lighter ventrally. They are rather limp and flabby to handle, do not coil up and besides having three pairs of thoracic legs have five pairs of “tube” legs on the abdomen.

The pupae appear in early spring, are reddish brown in colour, up to ljin long and and retain movement up and down their tunnels. They wriggle to the entrances of their tunnels when the moth is ready to emerge at dusk. Damage Porina caterpillars are defoliators and when present in populations of less than two per square foot rarely kill plants or cause extensive damage. Populations between two to seven per square foot may cause patches of a pasture to be reduced in vigour or even die, whilst populations in excess of eight per square foot can virtually wipe out a sward.

Damage becomes apparent with the first late summer and autumn rains when pastures fail to make satisfactory growth. In the early stages, examination of the sward will reveal small circles of grazing around the tunnel entrances. These small areas of damage enlarge and join together, and if grazing pressure is maintained by the caterpillars on the growing points of plants they die in early winter and the sward takes on a brown or blasted appearance. Badly affected pastures open up, make poor spring growth and are invaded by weeds. Unlike grass grub damaged plants, plants damaged by porina retain their roots and cannot be lifted from the ground. Damage may continue all winter, but most attack due to early flying porina ceases in August Generally, porina damage : Is most prevalent after wet summers and/or In pastures which have been kept rank over the summer period. Farmers who take small seeds crops, farm extensively and are understocked, particularly in wet seasons, should be con- , tinually on the look out for , the first signs of damage in , February, The last two sunt , mers have been wetter than , usual and were ideal for build i up of porina populations. As ■ well they favoured some build ' up of the later flying species

to occur. “Double” attacks of porina, although widespread, are probably not as common as some people think. Lucerne stands are less susceptible to damage than pastures, but in bad porina

years severe damage to lucerne stands can occur.

The major signs and symptoms of attack for porina can therefore be summarised as follows:

(1) Caterpillars eat grass and clover off at ground surface, but the roots remain

(2) Attack Is more widespread over a pasture than initial grass grub attack and pastures may be lost within one or two years of sowing down.

(3) Caterpillars construct vertical burrows in the soil and these are covered by silken webbing and loose crumbly castings. (4) Damage first becomes apparent in February and continues all winter. (5) Surface examination will reveal young caterpillars up to three quarters of an inch long under surface debris and vegetation in Decem-ber-January, whilst digging samples with a spade from February on will reveal the pest concerned and allow a rough estimate of the population to be made.

(6) The ground remains firm underfoot (7) Affected pastures make poor autumn and spring growth, die out under heavy infestation and are prone to weed invasion. Control Good preventive and curative control of porina caterpillar could be obtained with dry mix and wet mix D.D.T. superphosphate before its use was prohibited. The newer pelleted formulations of D.D.T. are not effective In the control of porina, and at

present control of this pest relies on a curative rather than preventative approach. Several organophosphate materials are available as sprays or pelleted formulations and include trichlorfon, diazinon and fenitrothion. Because of their higher cost compared with D.D.T. and short residual life after application, their use as a curative ; rather than preventative in- ■ secticlde cannot be over em- . phasised. When a curative ap- . proach is used it must be r realised that correct timing . of insecticide' application is I imperative. i Used as sprays, all the I above organo-phosphates are I effective. The rates recom- > mended vary from half to > one pound of active ingre- ! dient per acre. If these i materials can be applied in I February or early March, I half to three-quarters of a I pound of insecticidal material r may suffice per acre. If - treatment is delayed one to i one and a half pounds of ins secticide an acre may be re--1 quired. Trichlorfon is the ' cheapest but breaks down - within two days of applica- * tion, whereas the other two ’■ remain effective on the ; sward for a week or more.

Spray formulations rely on porina Ingesting the insecticide on the foliage and their efficiency is impaired by rain within two days of application and heavy frost restricting caterpillar feeding activity subsequent to application. In summer and early autumn, trichlorfon spray is probably the best choice for obvious reasons. Later on in autumn the longer life materials, diazinon and fenitrothion would be a more logical choice. In winter.

however, application of spray materials is risky and it is safer to use granular preparations of any one of these organo-phosphate insecticides. Spraying could be resumed in spring for late flying porina. If spray applications are made in winter, control operations may have to be repeated. Pellets are costlier than spray formulations and as earlier spraying is cheaper and more efficient it is important that farmers are able to recognise porina in February or early March, assess the problem if it exists and take action immediately. Farmers should inspect every pasture on their farm at this time of the year, and in particular pasture? which they have been unable to keep grazed low during the summer or have taken seed crops from. A farmer cannot afford to wait until the damage is visible to him and hence beyond repair in May or June and then expect to recoup his losses by expensive applications of these materials. Many pastures treated this winter were already ruined before treatment Early treatment ensures success and adequate winter feed.

Any discussion on porina caterpillar control would not be complete without some comment on grazing management In a recent trial at Lincoln College it has been amply illustrated that grazing pasture throughout November, December and January can reduce a potentially severe population to a low level. In an adjacent area kept for seed a population of eight caterpillars a square foot completely eliminated the sward, whereas the grazed area which supports a population of one and a half cater-

pillar* a square foot is still surviving at the present time.

Farms which are heavily stocked, while still prone to porina attack, are less likely to suffer severe porina damage than lightly stocked farms. Heavy stocking means more winter feed conservation in the form of hay or silage, and this means more lucerne which is less prone to attack and fewer swards continuously rank through the summer months.

Light land fanners who are understocked must always be on the lookout for porina in February, particularly in wet years and in pastures which have had toppy summer growth.

These are the major control procedures necessary to combat porina. It is not a problem if diagnosed and approached properly. Other control procedures have been attempted in the past, including the use of light traps but with unconvincing results. Like grass grub porina has a range of natural enemies but these do not appear to affect economic control so that chemicals and grazing management must remain as our main lines Of defence. Future Action On many farms, particularly those on light-medium land or better, several courses are at present open to the farmer to help him recoup some of his losses and recover his former position. Sowing cash crops and alternative forage crops is a real prospect at present as the dry winter has provided good soil working conditions and the absence of cover on dam-

aged pastures means that quick cultivations are possible. If more feed is required over the next year sod seeding damaged swards with Manawa and Paroa ryegrasses should be of considerable assistance. Sod seeding should be done in August and in grass grub areas D.D.T. should be surface broadcast at the same time. In porina damaged swards treated this way, if late flying caterpillars are not present organo-phos-phates may or may not need to be applied next February or March. If a decision is made to spring sow more grass now, early cultivation is essential and sowing must be completed by the end of September. Argentine stem weevil damage could occur this summer on some areas treated this way. On light land, cultivation for next year’s winter feed could be started with the long-term aim of increasing the acreage of lucerne which is more tolerant to damage by both pests. Several cash crops could be sown now. Spring wheat can still be sown in the next two weeks, but on lighter country it may pay to grow feed or malting barley, or field peas. In some areas crops of linseed or garden peas could be sown. When applying insecticide always make sure that the applicating equipment is in perfect working order to ensure complete coverage of the sward. There is ample evidence, in the way of missed patches and strips, of inefficient spray and pellet application in every district in Canterbury.

To ensure control of both pests and to check previous control operations In the case of grass grub, farmers should examine every paddock on the farm In February and early March. The only sure way to make a correct diagnosis is to dig soil samples, and carefully examine them. All pastures treated with insecticide should be examined again after treatment