Previous PageTable Of ContentsNext Page

Pesticide usage

N. Casimir

Chief, Entomlogy Branch, Biological and Chemical Research Institute, Rydalmere

The use of agricultural chemicals, and pesticides in particular, has become a controversial social issue leading to a complex mass of legislation concerned with ensuring that such chemicals are safe to use. In my discipline of entomology, community recognition of, and emphasis on, the twin problems of pesticide residues and environmental pollution have added an extra dimension to our research and advisory programmes. Adding to this scenario, the intractable problem of pesticide resistance by insects and disease organisms, it becomes clear why so much attention is being paid to the development and adoption of pest management approaches aimed at reduction in pesticide use.

There is some danger, however, of us losing sight of the continued and increasing importance of pesticides to agricultural production and, in fact, to our general economic well-being. Pesticides permit full expression of the many improved practices in agriculture and are in no small part responsible for the spectacular increase in food production per unit of land since World War II.

There are many approaches possible for protecting plants and animals from attack by diseases and pests and from competition by weeds, They include biological, genetic, mechanical, cultural and chemical methods. Nevertheless, I regard pesticides as still our most important tool. In many instances it is possible to blend them with other methods into an integrated control approach and this is certainly being attempted on a broad scale throughout Australia. Good examples are the so-called SIRATAC computer-based model for advising growers on spraying procedures against pests in cotton; and the importation from the United States and New Zealand of predatory mites that have developed resistance to commonly used organophosphate insecticides in apple orchards. These efficient predators are released into orchards to control the important pests, 2-spotted mite and European red mite and the programme in NSW has been calculated to save growers in excess of $100 per ha per year in spraying costs. Two things should be noted, however. Both programmes involve very high value crops which cannot be grown economically without highly effective pest control and both rely on the use of pesticides as key factors in the management process. They aim to rationalise and reduce pesticide usage, but would have no future without the availability and use of effective pesticides. They both also require much more knowledge and understanding of biological processes by growers and their advisors, and a willingness to integrate crop protection into an overall farm management system.

Applied research in crop protection has tended to move away from pesticide investigations into these alternative areas such as biological control, breeding of resistant varieties, etc. Whilst I applaud this provision of much needed balance, I feel the pendulum may be swinging too far in that all pesticides are being regarded as suspect and all pesticide investigations as unfashionable. We need more expertise in pesticides and their use, not less. My own group of entomologists in the NSW Department of Agriculture features insecticide resistance studies and the use of laboratory bioassay to examine effects of pesticides on pests as well as on beneficial insects. It is vital that both Government and private institutions maintain a commitment to the rational development of pesticide use if agricultural production is to be maintained at a high level whilst alternative approaches are being sought.

Pesticide use is circumscribed by several considerations. One of these is pesticide resistance which is a common but not inevitable consequence of repeated pesticide usage. Almost 400 species of insects and mites have been recorded as resistant throughout the world whilst the problem has also become increasingly evident over the last decade with fungal plant pathogens, particularly in the case of newly-developed fungicides which are very specific in their mode of action on the fungal cell. Resistance is developed by a process of selective breeding. In a natural population of a pest there is usually an extremely low proportion that is naturally immune to the pesticide. These survive and breed and succeeding generations, following regular applications of the pesticide, tend to contain higher and higher proportions of resistant individuals. The factors conferring resistance in the pest are genetically controlled so that progeny of resistant parents tend also. to be resistant. Resistance may persist for many generations in the field following withdrawal of the pesticide. Cross-resistance may also occur where a pest develops resistance to two or more pesticides after exposure to just one, For example, resistance to dieldrin confers cross-resistance to other related compounds such as chlordane and heptachlor.

It is most important to find out if failure of a pesticide is due to resistance or to other causes and it should not immediately be assumed that the failure is caused by resistance. Often other factors are involved - inefficient, badly adjusted or worn spray machines; poor application procedures; disregard of the instructions concerning concentration, mixing, etc; unsuitable weather conditions during or after application; bad timing of an application; expecting too much of the pesticide, e.g, large numbers of the pest moving into the treated area after a chemical has broken down; deterioration of chemical after lengthy or incorrect storage; poor formulation, although this is fortunately rare.

The only way that resistance can be proved conclusively is by comparing the response of the suspected resistant strain to that of a susceptible strain (not previously exposed to pesticides) of the same species under controlled conditions, i.e. laboratory tests. In the Entomology Branch at BCRI pesticides are treated against a wide range of pest insects and mites and beneficial species. The testing programme has resulted in the detection of new resistances in several major pests including sheep blowfly, spotted alfalfa aphid, green peach aphid, two-spotted mite, moth pests of stored products and banana weevil borer. The well-equipped laboratory is recognised as the resistance testing authority in Australia for sheep blowfly, 2-spotted mite, lucerne aphids, green peach aphid and heliothis, and provides a diagnostic testing or monitoring service for many major insect and mite resistances. Such programmes are most important as an essential precursor to rational decisions about alternative strategies and rapid diagnosis can save an industry a great deal of money in wasted chemical.

There is one basic rule with pesticides in relation to resistance - avoid unnecessary use. If resistance does occur the simplest procedure is to use an alternative registered pesticide to which there is no cross-resistance. Proven strategies either to prevent or delay the development of resistance do not exist at present, either for plant pathogens or insect pests. Theoretically the rotation of different chemical groups should delay resistance but long-term experimentation under practical conditions is almost impossible to carry out. The problems of implementation are formidable.

The agro-chemical industry and farmers are faced with an immediate need to formulate strategies that will extend the life of a valuable pesticide and that will provide adequate crop protection should resistance develop suddenly. Whether these goals are all achievable or even compatible is not clear. One approach used with plant pathogens is to recommend the use of mixtures of chemicals with different modes of action and this policy has been adopted, for example, with benomyl. Benoniyl is the best known of the benzimidazole groups of fungicides to which resistance by the serious brown rot fungus of peaches has developed. There is, however, a strong body of opinion amongst entomologists that mixtures of insecticides may well be no better, possibly worse, than using two insecticides sequentially. Whatever is done by way of manipulating usage of a pesticide is really only a delaying action in most cases and, in the long term, the reduced use of pesticides in conjunction with management practices, biological control, etc, must be encouraged to decrease the chances of developing resistance.

I can best illustrate the tactical use of pesticides by referring to a number of specific pest problems.

(1) Stored Grain Pests

The insect problems that beset the grain industry in the mid- 1970’s demonstrate the consequences of relying too heavily on one pesticide over a long period, Malathion was used to treat all grain on receival at bulk storages in NSW for 12 years. It provided unprecedented control of grain insects until several pest species became resistant. The biggest problem to emerge was the cross-resistance of the lesser grain borer to other alternative organophosphate (OP) insecticides. Because of this it is now probably the most important pest species in stored grain in Australia. Since 1976 it has been necessary to include pyrethrum, pyrethroids (e.g. bioresmethrin) or carbaryl in grain protectant mixtures with an OP insecticide. There was consequently a steep increase in the cost of grain protectants (by a factor of x7 in just three seasons). A panel of entomologists and industry representatives is also working continuously on the testing of new chemicals as replacement materials.

A much greater cost is foreshadowed in plans to combat future resistance problems by storing grain in an inert atmosphere (nitrogen) or with refrigerated aeration, or by heat disinfestation of grain as an alternative to fumigation at terminals. The new grain protectants and new methods of fumigation are buying time for the construction and capital outlay necessary to implement non-chemical methods, These latter will, indeed, require a costly restructuring of the grain storage industry.

(2) Sheep Blowfly

Blowfly strike costs the sheep industry about $55 million annually through losses and management costs. Some forms of strike, such as breech strike, can be prevented by good management such as crutching and mulesing. Insecticides are, however, still needed to combat body and po11 strikes. These represent only 9-11% of strikes in dry weather but the proportion rises to

25% in wet weather and in moister districts.

Dieldrin, which provided excellent long-term control for some years, is an example of a treatment that became unacceptable on two counts - the development of high order resistance by the fly and the problem of undesirable residues in wool and meat. It was replaced by diazinon but resistance of a rather lower order eventually developed to it and to other OP’s and to butacarb. Quite recently a new chemical, vetrazine, which belongs to a chemical grouping containing herbicides rather than insecticides and is an insect growth regulating substance, has been found to be very effective and is being widely used. No resistance has been detected as yet but it is probable that this will eventually occur.

Research at Rydalmere involves the maintenance of susceptible and resistant strains of flies for regular evaluation against new candidate chemicals and for more basic studies of biochemical mechanisms of resistance. Other laboratories are examining alternative approaches such as genetic manipulation of flies or breeding of sheep less susceptible to fly attack, but insecticides remain an important factor in strike prevention.

(3) Lucerne Aphids

Two exotic species spotted alfalfa aphid and blue green aphid -. appeared in Australia for the first time in autumn 1977. They spread rapidly and devastated large tracts of lucerne. The industry was dependent almost entirely on the one highly susceptible lucerne variety - Hunter River. Other pasture legumes were also affected, Substantial programmes were instigated by the affected States and CSIRO following detailed discussions, A first concern was to test and have registered effective insecticides to relieve the immediate problem. The main programme, however, involved the integration of pesticides with introduction of parasites and disease organisms from overseas, utilisation of the many predators already present in Australia, the breeding and importation of lucerne varieties resistant to the aphids, and ecological studies of the aphids themselves. In combination, these programmes have been successful in reducing the problem to manageable proportions and they provide a good example of a rational, modern approach to a new pest problem.

Pesticides were an important part of this approach. Chemicals such as demeton-.S-methyl, pirimicarb, dimethoate, thiometon, and disyston were recommended because they were either more active against the target pest than the beneficial species, or could be applied at rates low enough to provide reasonable aphid control without grossly upsetting the parasite/predator complex; or were specific to the aphids because of the mode of application, e.g. disyston granules applied to the soil. The pesticides bought time for the development of other methods such as the breeding of resistant lucerne varieties. The whole programme was threatened by the discovery in March 1979 of spotted alfalfa aphids on a few farms at Tamworth that were highly resistant to all the recommended chemicals. Similar resistances occurred at Armidale, April 1980, and by early 1981 this resistance was widespread in South Australia and throughout NSW. Fortunately the early diagnostic work had quickly established a suitable alternative - chlorpyrifos, which has been used successfully since on resistant populations. Some critics suggested at the time that the use of low rates of specific insecticides had hastened the onset of resistance but it is interesting to note that the original occurrence at Tamworth followed very frequent application of demeton-S-methyl at rates at least 4x those recommended. Benefits derived from the use of several different control methods and integrating them with a sensible approach to pesticide use were very evident,

(4) Plague Locusts and Grasshoppers

These pests pose a different set of problems from those discussed before. Pesticide resistance is barely an issue because only a relatively small proportion of the total population is ever treated at any one time. The swarms formed by plague locusts are used as specific targets for spraying, often from aircraft, as part of organised control campaigns. Overall strategy is dependent on sophisticated survey methods involving satellite imagery and use of helicopters, etc, as well as an efficient information service. Attempts are made to locate and treat potential outbreak sources (often in remote, difficult terrain) before they can develop into a full-scale plague.

The choice of pesticides depends on a number of factors in addition to efficacy. They must be relatively safe for handling by a wide variety of persons; they must have no residue problems in foodstuffs or meat and they must not result in significant environmental side effects; they must be relatively cheap and be capable of being stored for long periods without deterioration; and they must have the appropriate physical properties for use as ultra-low volume aerial applications. Fenitrothion and, to a lesser extent, malathion are the main chemicals being currently used. Plague locust control is the field which has seen the most significant developments in modern spray application technology, involving the use of special formulations and manipulation of droplet sizes for different circumstances.

Another severe pest of pastures and crops in the Tablelands and Slopes districts of NSW is the non-swarming wingless grasshopper. Changes in agricultural practice sometimes alter the status of insects as pests. Wingless grasshopper, once a sporadic pest of native pastures, is tending to become a chronic and serious pest of improved pastures almost certainly due to the incorporation of legumes into the pasture. Clovers are a favoured host of this species.

Unlike plague locusts, wingless grasshoppers do not readily form swarms and usually occur at moderate to high densities over large tracts of country. Broadly speaking, the insecticides effective against locusts are also effective against wingless grasshopper but the tactical problems of use are quite different. A growing number of landholders is not prepared to spray on a whole property basis because of both cost and environmental considerations. There is evidence, at least in some seasons, that a better tactical use of insecticide is not to wait until after the hoppers have dispersed widely (usually in December) before spraying, but to spend some time earlier in the season (October/November) to locate and treat rather more dense patches of younger hoppers which tend to occur on similar parts of properties each year.

Another approach being taken in a detailed study of factors that influence changes in numbers of wingless grasshoppers is an investigation of the biological control agents that occur naturally in the field. A parasitic nematode has been found that exerts a high degree of control in some areas under some weather conditions. We are examining the possibility of manipulating this as a form of biocide.

(5) Earth mite, lucerne flea, and pasture cockchafer

These species are common pests of annual improved pastures and newly sown crops in the southern highlands and south west of NSW, I will discuss them under the one heading as they often occur in combination, As with the wingless grasshopper, the use of superphosphate on improved pastures and increased stocking have increased the pest status of the black-headed pasture cockchafer.

The main thrust with recent investigations has been to provide recommendations using chemicals that do not present residue problems, combined with cultural practices that reduce pest numbers, e.g. elimination of weeds in the case of red-legged earth mite and lucerne flea. Biological control has also been pursued by CSIRO for these latter pests by the introduction of predatory mites. The effects of these mites are still being assessed,

A major problem has been that DDT, extremely effective against red-legged earth mite, may result in unacceptable residues in animals and animal products. It is no longer registered in NSW for use on pastures. A similar but much less serious problem occurs with lindane, used for cockchafer control. Alternative chemicals are now available.

(6) Caterpillar plagues

Caterpillar plagues have been recorded since last century in south western NSW. There may be a complex of species involved, including the well-known cutworms and armyworms. Similar plagues occur in other semi-arid regions of the world where it is thought depletion of pastures over a long period is more due to protracted droughts, overstocking, rabbits, fires, etc, than to caterpillar plagues. In the short term, of course, severe pasture losses occur by feeding caterpillars, especially if associated with drought.

The problems posed are very complex and there is no easy solution. Our current investigations are concentrating on control with insecticides, which are really only viable in a high-value crop or pasture situation, Again the search is for cheaper, more effective, and non-residue producing insecticides.

Earth mites, lucerne flea, pasture cockchafer and caterpillars are such common problems in the local region that I have included a more detailed statement with the summary of this talk, which I understand is available to all attending the conference. I hope it will answer any queries you may have on these pests.

These examples of common pest problems, mainly of pastures and field crops, indicate that major concerns in the use of pesticides are pesticide resistance, undesirable pesticide residues in treated foodstuffs or meat products, and environmental pollution. Pest management programmes which integrate pesticide use with alternative, non-chemical methods or even replace pesticides with other methods offer the best way of overcoming or circumventing these problems. Some spectacular successes have been achieved, particularly with biological control, but it is unlikely that more than a small percentage of our pest problems can be solved without the use of pesticides and this is particularly so with the more sporadic pests or pests of broad-acre farming such as pastures and many field crops. We, as crop protection researchers, are endeavouring to see that pesticide use in such situations involves the least hazard in terms of undesirable side effects. This does often mean, however, turning away from cheap, easy solutions to problems. Unfortunately, increased costs may be one of the prices we have to pay to meet increasingly stringent restrictions on pesticide use.

Red-legged earth mite, Halotydeus destructor (Tucker)

Lucerne flea, Sminthurus viridis (.Linnaeus)

Black-headed pasture cockchafer, Aphodius tasmaniae (Hope)

The red-legged earth mite and lucerne flea are very well-known exotic pests of pastures - particularly annual improved pastures - and newly sown crops, on the Southern Tablelands and in the southern Riverina of New South Wales. Both pests gained entry into southern Australia many years ago and also occur in Western Australia, South Australia, Victoria and Tasmania. They were first noted in southern New South Wales in 1930 (red-legged earth mite) and 1935 (lucerne flea).

The black-headed pasture cockchafer is a very well-known native pest of annual improved pastures on the South Coast, Southern Tablelands and southern and south western Slopes of New South Wales. Although larvae were recorded attacking natural pastures near Cooma and at Bombala in 1903 and 1906 it was not until the late 1930’s and 1940’s - when use of superphosphate, sowing of improved pastures and increased stocking became increasingly more common - that widespread and recurrent damage occurred. Black-headed pasture cockchafer is also present in Victoria. Tasmania, South Australia and the South Island of New Zealand.

The habits, importance and control of these three pests have been investigated over a long period by various State Departments of Agriculture (including New South Wales), the Waite Agricultural Research Institute and the CSIRO. Practicable and economical control measures have been developed except for situations when the topography creates problems of its own, e.g. steep rugged slopes.

Red-legged earth mite and lucerne flea can seriously damage and occasionally kill (e.g. 1979 and 1980) establishing pastures and crops particularly if plant growth following emergence is retarded by cold or dry weather. Pastures and crops are most vulnerable during the first month or so after germination and can be protected by seed treating, or boom spraying the seedlings as soon as they emerge and also a wide strip of plant growth around the perimeter of the crop or pasture. Very low (29 to 36 g active constituent per hectare) rates of non- residue problem insecticides are used for broad area spraying.

Weeds, especially capeweed, Arctotheca calendula (L) Levyns, are important breeding sites for red-legged earth mite and lucerne flea. Clean fallowing in autumn and the elimination of weeds around the perimeter of crops and pastures will reduce pest numbers. Perennial pastures and reseeding annual pastures can be protected, if necessary, by spraying them about a month after the first good falls of rain in autumn. Heavy rainfall stimulates hatching of the over-summering eggs and treatment about a month later will eliminate the first generation immature stage mites and lucerne fleas.

Black-headed pasture cockchafer larvae damage pastures by feeding on the surface pasture growth from autumn until early or mid-spring and also by covering plants around the openings of their tunnels with excavated soil. Damage is usually most severe in annual improved pastures which are more than three years old. In some years (e.g. 1980) when larvae are present in very high numbers and the autumn “break” is delayed and then followed by a cold spell, the loss of winter feed due to black- headed pasture cockchafer may be extremely serious.

A widespread exotic bdellid mite - the pasture snout mite, Bdellodes lapidaria (Kramer) - gained entry into mainland southern Australia more than 50 years ago. It prefers to feed on young immature lucerne fleas and, depending upon the season, sometimes causes an appreciable reduction or almost complete elimination of the pest.

Commencing about 17 years ago the CSIRO introduced two mites, a bdellid - the spiny snout mite, Iveomolgus capillatus (Kramer) and an anystid, Anystis sp. - for the biological control of lucerne flea and red-legged earth mite, respectively. Releases and field studies were initially made in Western Australia. Subsequently, the anystid mite was released in Victoria in 1976. This work is long-term and it will be some years before the influence which these mites have on lucerne fleas and red- legged earth mite becomes clear.

Introduction of a cropping phase between short-term (2 or 3 year) reseeding annual improved pastures, or sowing mixed perennial pastures and regulating their grazing during sumner may markedly reduce losses caused by the black-headed pasture cockchafer. In some years, protracted spells of wet weather or short periods of very wet weather in autumn, winter or spring may substantially reduce larval numbers through the combined effects of drowning, larvae fighting amongst themselves on the soil surface and outbreaks of a fungal disease, Cordyceps aphodii (Matheson).

Infestations are normally distributed patchily and early detection will enable control measures to be applied while the larvae are small and have caused little damage. Inspect likely infested areas - pastures infested the previous year or pastures which had little surface cover in summer - in April and May for recently hatched actively feeding larvae and, if necessary, treat the affected areas promptly.

DDT was widely used for many years, as a foliar and base soil spray, to control red-legged earth mite in pastures and crops and was highly effective at comparatively low cost. However, it can cause problems of unacceptable residue in animals and animal products. DDT usage was recently reviewed by the Registrar of Pesticides and it is no longer registered for these purposes in New South Wales. Alternative non-residue treatments to DDT for red-legged earth mite control include azinphos-ethyl, methidithion, omethoate, phosmet and dimethoate. The New South Wales Entomology Branch was one of the first to seek for non-organochlorine alternatives to DDT for red-legged earth mite control.

Lindane has been used since the early 1950’s to control black- headed pasture cockchafers in pastures. It may still be used provided that dairy stock or animals being finished for slaughter do not graze the treated pasture. However, lindane usage is currently being reviewed by the Registrar of Pesticides. Fenitrothion is the recommended alternative to lindane for control of larvae of the black-headed pasture cockchafer.

Caterpillar Plagues in South Western New South Wales

Plagues of caterpillars periodically cause widespread and very serious damage to native grazing plants and shrubs in south western New South Wales. The plagues may occur in either late summer-autumn (e.g. 1973) or winter-early spring (e.g. 1980) and have been recorded since well before the turn of the century.

The most damaging species in late summer-autumn is usually the weed webworm, Loxostege affinitalis (Lederer). Plagues in winter-early spring are composed of a number of species including the common cutworm or bogong moth, Agrotis infusa (Boisduval), banded saltbush moth, Anthela denticulata (Newman), pasture day moth, Apina callisto (Walker), brown pasture looper, Ciampa arietaria (Guenee), a noctuid, Euplexia nigerrima (Guenee), a noctuid, Neocleptria punctigera (Walker), inland armyworm, Persectania dyscrita (Common), and southern armyworm, P. ewingii (Westwood).

Caterpillar plagues are not unique to south western New South Wales. They also occur in other semi-arid regions of the world such as the western United States. Long-term detailed studies in semi-arid areas of south eastern Australia have shown that grazing land depletion is mainly due to periodic protracted droughts and to overstocking, rabbits, fires and overgrazing around key areas such as watering points - and not to the caterpillar plagues. There is no doubt, however, that plagues of leaf-eating caterpillars can seriously damage extensive areas of vegetation and as the plagues are associated with drought periods the additional loss of forage can be very important.

There is no simple solution to the problem posed by caterpillar plagues. World-wide evidence indicates that spasmodic caterpillar plagues have been, and always will be, a normal occurrence in semi-arid regions. Because the plagues are induced by weather conditions which initially favour egg laying and survival of the early stage caterpillars but then became unsuitable for the natural control agents - predatory birds, disease and predatory and parasitic insects — there is no likelihood of progress with research into biological control methods. Consequently, our continuing research programme concentrates on control with insecticides, which can only be applied in high—value crop and pasture situations.

If graziers were able to inspect pastures in February—March and also June—July, infestations could be detected early —while the caterpillars were small and little damage had occurred — and spot treated. Careful examination of the lower parts of plants and the soil surface around the base of plants is necessary to find the small caterpillars.

Late stage caterpillars often migrate from places where the eggs are laid and the young caterpillars develop into adjacent, uninfested areas. Broad area treatment of “marching” caterpillars is not economic. However, the progress of migrating caterpillars into high—value areas can be greatly impeded by running 25 to 30 cm deep furrows across their line of advance or around their breeding areas. If the steep side of the furrow is away from the approaching caterpillars, the furrow will assist in concentrating the pests and they can be killed there. Sometimes stock water channels may also act as barriers to the movement of “marching” caterpillars.

Cutworm, armyworm and webworm caterpillars are regarded as very serious pests of pastures and crops. Research is continuing to find cheaper and more effective chemical control agents than those currently available.

Previous PageTop Of PageNext Page