SYMPOSIUM IV. Migration and Dispersal of Tropical Noctuid Moths CONTENTS. 531 Migration strategies and outbreaks of noctuid pests in Australia

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1 SYMPOSIUM IV Migration and Dispersal of Tropical Noctuid Moths CONTENTS ROGER A. FARROW and GARRICK MCDONALD S. J. JOHNSON PETER ONYANGO ODIYO D. J. W. ROSE, C. F. DEWHURST, W. W. PAGE and L. D. C. FISHPOOL 531 Migration strategies and outbreaks of noctuid pests in Australia 543 Migration and the life history strategy of the fall armyworm, Spodoptera frugiperda in the western hemisphere 551 The use of biogeographical techniques in the study of migrant noctuid moths 561 The role of migration in the life system of the African armyworm, Spodoptera exempla CHEN RUILO and A. G. GATEHOUSE BAO ZIANGSHI 571 Research on the migration of oriental armyworm in China and a discussion of management strategy 573 Migration and low population density in armyworm (Lepidoptera: Noctuidae) life histories ABNER M. HAMMOND and HOWARD W. FESCEMYER JEREMY N. MCNEIL D. E. PEDGLEY, M. R. TUCKER and C. S. PAWAR A. G. GATEHOUSE 581 Physiological correlates in migratory noctuids: The velvetbean caterpillar as a model 591 The true armyworm, Pseudaletia unipunctata: A victim of the "Pied Piper" or a seasonal migrant? 599 Windborne migration of Heliothis armigera (Hiibner) (Lepidoptera: Noctuidae) in India 605 Migration and dispersal of tropical noctuid moths: Summing up the symposium 529

2 Insect Sci. Applic. Vol. 8, Nos 4/5/6, pp , /87 $ Printed in Great Britain. All rights reserved 1987 ICIPE ICIPE Science Press MIGRATION STRATEGIES AND OUTBREAKS OF NOCTUID PESTS IN AUSTRALIA ROGER A. FARROW 1 and GARRICK MCDONALD 2 'Division of Entomology, CSIRO, GPO Box 1700, Canberra, ACT, Australia 2601; 2 Plant Research Institute, Department of Agriculture and Rural Affairs, Swan Street, Burnley, Victoria, Australia 3121 (Received 10 March 1987) Abstract Cutworms, semiloopers, budworms and armyworms in the genera Agrotis, Chrysodeixis, Heliothis, Mythimna, Persectania and Spodoptera are important pasture and crop pests in Australia. They comprise both cosmopolitan species, such as A. ipsilon, H. armigera, M. separata and 5. exempta and endemic species such as A. infusa, H. punctigera, M. convecta, P. ewingii and P. dyscrila. Although the cosmopolitan species are major pests of parts of Asia and Africa, they are, with the exception of H. armigera, less important agricultural pests in Australia than their endemic counterparts. The latter are widely distributed outside cropping areas, because they breed on a wide range of native host plants as well as on introduced crops and pastures and also have the potential to invade cropping areas from native habitats. The cosmopolitan species are largely confined to tropical and subtropical summer crops and improved pastures of north and east Australia where chronic infestations often develop although major outbreaks and migration out of cropping areas are rare. Periodic outbreaks of endemic species result from an unusually favourable growth of vegetation in early autumn, following drought-breaking rains in the inland. Moths arrive in the rain-affected areas as a result of migration and concentration by rain-bearing troughs and depressions. Southward migration of their progeny occurs in spring on warm northerly airflows produced ahead of eastward-moving cold fronts and results in invasions of temperate crops and pastures. Migration also occurs in anticyclonic conditions, resulting in an extensive redistribution of populations and is adapted to the erratic distribution of rainfall in inland Australia. Among cosmopolitan species, only Spodoptera spp. cause outbreaks which are infrequent and occur with the onset of summer rains in coastal and sub-coastal areas. Most movements of cosmopolitan species are confined within cropping areas although migration from coastal to inland crops, under the influence of prevailing SE winds, and in frontal systems, troughs and storm outflows, is also suspected. Key Words: Migration, outbreaks, noctuids, Australia, weather, crops, pastures Resume Les larves de Lepidopteres des genres Agrotis, Chrysodeixis, Heliothis, Mythimna, Persectania et Spodoptera sont d'importants predateurs des cultures et des paturages en Australie. Elles englobent des especes cosmopolites, telles que A. ipsilon, H. armigera, M. separata, S. exempta et des especes endemiques, telles que A. infusa, H. punctigera, M. convecta, P. ewingii and P. dyscrita. Bien que les especes cosmopolites soient des predateurs majeurs dans certaines aires d'asie et d'afrique, elles sont, a l'exception de H. armigera, moins importantes comme pestes d'agriculture en Australie que les especes endemiques. Ces dernieres sont largement distributes hors des regions de culture, parce qu'elles se reproduisent sur un grand nombre de plantes indigenes, aussi bien que sur les cultures et les paturages d'especes introduites, et elles ont la capacite d'envahir les surfaces cultivees a partir de leurs habitats. Les especes cosmopolites sont largement resteintes aux cultures d'ete des tropiques et des sous-tropiques et aux paturages ameliores du nord et de Test d'australie ou les infestations chroniques se developpent souvent, quoique les pullulations majeures et les migrations hors les aires de culture soient rares. Des pullulations periodique d'especes endemiques se produisent lorsque le debut d'autumne est exceptionnellement favorable a la croissance de la vegetation, apres les fortes pluies suivant une periode de secheresse. Les lepidopteres arrivent dans les aires arrosees par les pluies, du fait de trous d'air et depressions atmospheriques porteurs de pluie, qui en provoquent la concentration et le migration. Us produisent une nouvelle generation qui migre vers le sud au printemps, transportee par les vents chauds qui se forment a l'avant de fronts froids, et qui se deplacent vers Test. Ainsi se produit l'invasion des cultures et paturages des zones temperees. Les migrations ont aussi lieu en conditions anti-cycloniques causant une redistribution importante des populations. Ceci est une adaptation a l'irregularite de la repartition des pluies a l'interieur en Australie. Parmi les especes cosmopolites, seule les spodopteres causent des pullations, mais rarement, a l'arrivee des pluies d'ete dans les regions cotieres et sub-cotieres. Les deplacements des especes cosmopolites sont generalement confines aux aires de culture bien que les migrations vers l'interieur a partir des cotes soient egalement suspectees, en cas de de vents de sud-est dominants et dans les fronts atmospheriques, les trous d'air ou les courants orageux. INTRODUCTION widespread outbreaks of armyworms and cutworms in both crops and pastures can be distinguished from Noctuid moth larvae have had a substantial influence chronic infestations of budworms and semiloopers on crop and pasture production in Australia since which develop more locally in high value crops, often farming was introduced with European settlement where chemical control is operating. The continuing early in the last century (Oliff 1890, Tryon 1900, spread of cropping and pasture improvement into the Froggatt 1907, Smith 1933). Sporadic and often inland and the establishment of irrigated cropping, 531

3 532 ROGER A. FARROW and GARRICK MCDONALD Endemics Heliothis punctigera (Wallengren) (Native budworm) Agrotis infusa (Boisduval) (Black cutworm) Mythimna convecta (Walker) (Common armyworm) Table 1. Migrant noctuid pests of Australia Cosmopolitans "Sibling pairs" Heliothis armigera (Hubner) (Cotton bollworm) Agrotis ipsilon (Hufnagel) (Greasy cutworm) Mythimna separata (Walker) (Northern armyworm) Agrotis munda Walker (Brown cutworm) Neocleptria punctifera (Walker) (Lesser budworm) Persectania dyscrita Common (Inland armyworm) Persectania ewingii (Westwood) (Southern armyworm) Others Mythimna loreyimima (Rungs) (Sugarcane armyworm) Spodoptera exempta (Walker) (Day-feeding armyworm) Spodoptera exigua (Hubner) (Lesser armyworm) Spodoptera litura (Fabricius) (Cluster caterpillar) Spodoptera mauritia (Boisduval) (Lawn armyworm) Chrysodeixis argentifera (Guenee) (Tobacco looper) Chrysodeixis eriosoma (Doubleday) (Green looper) particularly in subtropical areas, favours continuing change to the distribution, abundance and economic importance of noctuid pests in Australia. The development of programmes to forecast noctuid pest outbreaks to reduce farm costs has been proposed in Victoria by McDonald (1986). This review discusses the current state of knowledge of a major requirement in effective forecasting an understanding of the migration strategies of the noctuids themselves in relation to outbreaks. PEST SPECIES Major outbreaks of caterpillars, which are usually preceded by invasions of moths, are caused by the armyworms, Mythimna convecta (common armyworm), Persectania dyscrita (inland armyworm), P. ewingii (southern armyworm), Spodoptera exempta (day-feeding armyworm) and S. mauritia (lawn armyworm); the cutworms Agrotis infusa (brown cutworm) and A. munda (black cutworm); and the budworms Heliothis punctigera (native budworm) and Neocleptria punctifera (small budworm). Chronic infestations develop in crops in Spodoptera litura (cluster caterpillar), Agrotis ipsilon (greasy cutworm), Heliothis armigera (bollworm etc.) and semiloopers in the genus Chrysodeixis (Table 1). Larvae of other migratory noctuids are also important pests of crops, fruits and vegetables, and include species of Achaea, Anomis, Earias, Mods and Othreis, but they rarely occur in outbreaks. Persectania is the only genus of these noctuids that is endemic to Australasia although there are a number of species in the genera Agrotis, Heliothis and Mythimna which are also restricted to Australia. No species in the cosmopolitan genera Spodoptera and Chrysodeixis are known to be confined to Australia. Of particular interest is the existence of three pairs of sibling species of endemic/cosmopolitan origins which have overlapping distributions in Australia. These are: (endemic species first) A. infusa I A. ipsilon; H. punctigera IH. armigera; and M. convecta/m. separata (Table 1). MIGRATION AS AN ADAPTIVE STRATEGY In temperate latitudes of Australia, noctuid development is affected by cold conditions, which limit or terminate development and activity in winter, and by dry and hot conditions which limit food supply and survival in summer. In tropical and subtropical latitudes, drought and heat are the main limiting factors to development and survival. Noctuids may tolerate cold in situ or they may die out in cold areas which are recolonized in more favourable times, or adults may actively emigrate to milder climates where populations can survive. With respect to drought, noctuids may similarly aestivate, die out, or emigrate to more favourable environments. In cropping systems, the availability of suitable breeding sites is often more restricted, in time and space, between rotations of different crops and fallows of varying suitability for noctuid breeding. Short-range movements by noctuids are often adequate to exploit this environment. Several strategies to cope with seasonal adversities can be distinguished among Australian noctuids. The endemic species of Agrotis, Heliothis and Persectania overwinter in high latitudes as larvae and pupae, and in the case of Heliothis in diapause (Cullen and Browning, 1978) which prevents premature emergence of adults in autumn. This ensures that the bulk of the population passes the winter in a cold tolerant stage rather than as a vulnerable young larva. Nevertheless, adults often appear in crops in spring prior to local emergence or in numbers greater than predicted for local emergence (Wilson, 1983), suggesting that immigration is occurring from lower latitudes where emergence took place at an earlier date. For example, immigration from the mainland explains early flights

4 Migration of noctuid pests in Australia 533 Generation O Parental-EMIGRATION A F, ---* MIGRATION Fig. 1. Migrant strategies of noctuid moths in relation to overwintering capabilities, (a) Cold tolerance and diapause. (b) Cold susceptible and non-diapause. of P. ewingii in Tasmania (Martyn and Hudson, 1953; Helm, 1975; Fig. la). The cosmopolitan species of Agrotis, Chrysodeixis, Mythimna and Spodoptera are absent from southern Australia in winter (Fig. lb) and, with the exception of Chrysodeixis, are rarely common in the south at any time. It is not known whether the immature stages of these moths are less tolerant to cold or if lack of a diapause prevents the bulk of the population entering the coldest months at a cold tolerant stage. However, both M. separata and A. ipsilon breed in New Zealand (Watson and Hill, 1985) in a climatic regime similar to that of Tasmania, well to the south of their known breeding limits in Australia. In sub-tropical areas of Australia, the compolitan species occasionally appear in numbers greater than predicted from local breeding, suggesting immigration (Persson, 1977). Less is known of the adaptive strategies for coping with drought and heat in summer. Experimental evidence suggests that endemic species like A. infusa, A. munda and M. convecta do not tolerate high temperatures and/or a dry temperate summer (Common, 1954a, 1958; Persson, 1977; Smith and McDonald, 1986). A. infusa is univoltine and migrates to the summits of mountain ranges in south-east Australia in spring where it aestivates (Common, 1954a), unlike all other species in this genus in Australia which continue to breed during summer. In summer, surveys suggest that moth populations are aggregated in areas of green vegetation where it has recently rained (Smith and Caldwell, 1947). occurs during migration, due to the passage of a cold front, for example, the migrants may be returned to their departure point. Trivial and migratory movements can be considered as extremes of a continuum (Farrow and Daly, 1987) whose scale depends on the behaviour of the moths, the spatial distribution of desirable habitats and atmospheric conditions. Several types of movements are readily identifiable in A. infusa, for example. Initially there is a migratory movement from the western plains to the tablelands, in relation to synoptic weather, described later. Movements then become limited to the biological boundary-layer as when moths move through the eucalypt forests to progressively higher altitudes. A reverse movement to the plains occurs in summer, but its stages are not well documented (Common, 1954a). In this review we define migratory movements as those in which moths ascend into the planetary boundary layer at dusk and fly in the geostrophic wind at m above the ground (Fig. 2). LIMITING EFFECTS OF CLIMATE AND WEATHER Rainfall is the dominant climatic factor limiting plant growth in Australia. More than 80% of the land area receives less than 600 mm of rain annually (Fig. 3a), compared with evaporation of mm over the same period which limits its effectiveness in producing plant growth. Run-off is also a major source of moisture for the growth of vegetation in the areas such as the flood plains of the Channel country of south-west Queensland. Rainfall in the inland varies seasonally from winter maxima in the south (Mediterranean climate) to summer maxima in the north (monsoon) (Fig. 3b). Rainfall is higher and more evenly distributed and evaporation is lower along the east coast. Consequently, potential winter breeding areas are usually confined to cool southerly latitudes and to milder east coast habitats, while summer breeding areas are confined to the tropics and east coast. This situation is in complete contrast to that of eastern China and USA where, for example, over the same latitudinal range, summers are hot and wet and winters very cold and often dry. In the Australian inland, rainfall is highly variable in TYPES OF MOVEMENT There is no single satisfactory definition of migratory movements. Although trivial movements are generally separated from migratory movements on behavioural grounds, this distinction is often far from clear in highly mobile noctuids. For example, if feeding and oviposition sites are widely separated by unfavourable habitat (e.g. in crop mosaics or when nectar sources in non-cultivated land are separated from larval host plants in crops or pasture), what would be defined as appetitive flight could extend to a protracted movement of the kind usually associated with migration between habitats. On the other hand if moths emigrate on a night when a 180 wind shift PREVAILING WIND DIRECTION -* Short range movements in crop ^ Long range movements above crop but within Biological Boundary Layer s Migratory movements in Planetary Boundary Layer and Geostrophic Layer Fig. 2. Diagram of movement levels over a crop mosaic.

5 534 ROGER A. FARROW and GARRICK MCDONALD I SEASONAL RAINFALL ZONES LOW RAINFALL HIGH RAINFALL r ~ A Summer dominant ^ ^ Summar dominant^ f\"^ Uniform ^ Uniform \ AVERAGE DAILY MINIMUM TEMPERATURE JULY < 5 C Fig. 3. (a) Median annual rainfall, (b) Seasonal rainfall, (c) Rainfall variability, (d) Isotherm of 5 C average July minimum temperatures, (e) Isotherm of 35 C average January maximum temperatures. time and space, the variability reaching an extreme in doptera and Chrysodeixis and M. separata and H. south-west Queensland (Fig. 3c). The average rainfall armigera develop later in the season, when temperatures are higher, following the onset of the of these areas is often a meaningless value since the weather experienced comprises an irregular sequence summer rains (Persson, 1977; Broadley, 1978; Hitchcock, 1969; Wardhaugh et ai, 1980). The endemic of droughts, extending for many months and even years, and floods in which the average annual rainfall species of Mythimna is, however, not cold-tolerant may fall over a period of a few days. and does not commonly breed in the southern winter Noctuid survival may also be affected by extremes rainfall area of Australia (McDonald and Smith, of temperature. Potentially unfavourable areas for 1986). Survival in these colder regions is only likely winter breeding in cold susceptible species and for if pupation occurs before mid-winter which depends summer breeding in heat intolerant species are shown on a late summer/autumn egg-laying, although the in Figs 3d-e, where arbitrary limits have been set at dry conditions of that time rarely favour survival of 5 and 35 C, respectively. young larvae (Smith and McDonald, 1986). In winter, larvae are commonest in subtropical Australia Endemic noctuids, namely, A. infusa, A. munda, M. convecta, P. ewingii, P. dyscrita and H. punctigera where winter rains are unreliable and often localised. exhibit their greatest breeding potential in areas with A. munda, H. punctigera and N. punctifera also autumn rains, following the breaking of summer breed widely in winter in the subtropics if rainfall is droughts, whereas the cosmopolitan species of Spo- adequate. In contrast, P. ewingii is widespread in the

6 Migration of noctuid pests in Australia (a) Well above average Well below avera (b) Below average Fig. 4. (a) % Deviation of annual rainfall in south-west Queensland (b) Distribution of annual rainfall deviations and and monthly deviations for cooler regions of southern Australia throughout the year, its northern distribution, at about 32 S being limited by warmer temperatures (McDonald and Smith, 1986; Fig. 3d). Major outbreaks of endemic species of armyworm and cutworm and, incidentally, the Australian plague locust, followed by spectacular invasions of the south-east have followed widespread, droughtbreaking rains in the inland in 1907, 1931, 1936, 1947, 1954, 1973 and 1983, shown in Figs 4a-b. The outbreak and subsequent invasion of noctuids from western New South Wales in spring 1980 can be related to drought-breaking rains which affected this area in the preceding autumn (Fig. 4c). Drought

7 536 ROGER A. FARROW and GARRICK MCDONALD Mitchell grass Astrebla Blue grass Dicanthium Salt bush and tussock grass Atripjex,StipJ [ Woodland, Scrub and desert grasses Fig. 5. (a) Croplands of Australia, (b) Grasslands of Australia. appears to depress natural enemies allowing relatively uncontrolled population increases in the noctuids once rain has fallen (Markovitch, 1957; McDonald and Smith, 1986). Outbreaks of common armyworm in Queensland in 1931, 1938, 1948, 1952 (May and Passlow, 1954) and 1978 (Broadley, 1979) do not show such a marked relationship with annual rainfall departures. The cosmopolitan species are commonest in the warmer, higher rainfall areas of the east coast. In the armyworms, S. exempta and S. mauritia, outbreaks have been associated with drought-breaking rains in coastal and subcoastal Queensland and New South Wales (Smith, 1933; Weddel, 1936; Rand and Wright, 1978; Broadley, 1978; Anon, 1981a). Rainfall is the most plausible explanation for the differences in distribution between the endemic and cosmopolitan sibling species in Australia. A. ipsilon, M. separata and H. armigera are major pests of crops, particularly maize, in New Zealand and are widely distributed throughout most of the cropping belt in summer (Watson and Hill, 1985). In equivalent high latitudes of Australia, these species are either absent or limited to moist coastal pockets and irrigation areas. New Zealand experiences appreciably higher summer rainfall and less evaporation than comparable parts of Australia. This provides these essentially warm-climate species with a reservoir of hosts for surviving summer in New Zealand, similar to the situation in Asia and north America, but usually absent in Australia. LIMITING EFFECTS OF CROPS AND PASTURES Cropping areas represent about 2.75% of the total land area of Australia, equivalent to about 21 m ha. They are concentrated in temperate areas of the south-east and south-west and largely comprise winter cereals. Cropping is extremely restricted in tropical Australia (north of 23 S); small areas of sugarcane production occur along the east coast (0.4 m ha) and scattered areas of summer crops occur on the tablelands and highlands in the higher rainfall areas, extending into subtropical Queensland (0.9 m ha). Several small irrigation areas producing cotton and rice and a range of other summer crops have been developed in the tropics and include schemes at Emerald, the Burdekin (in progress) and the Ord River (Fig. 5a). Larger irrigation areas occur in temperate latitudes in the Namoi, Murrumbidgee and Murray Valleys of New South Wales and Victoria. This limited area of cropping is in complete contrast to south-east China, for example, where more than 50 m ha of rice, wheat, maize, soy and cotton are grown under subtropical conditions. In the United States more than 19% of the land area is arable and is dominated by maize production. In contrast, Australia is endowed with extensive pastoral areas which cover about 60% (468 m ha) of its surface area. Some of the most productive grasslands occur on the self-mulching clays and black earths of Queensland and New South Wales (Fig. 5b). A wide range of broad-leaved annuals and perennials also grow in these grasslands, particularly in drier areas and during winter (mostly Compositae and Chenopodiaceae). In the south-east and southwest, pastures have been extensively improved by the introduction of exotic legumes and grasses, a practice which is extending to tropical areas. Productivity of crops and grasslands is highly dependent on rainfall. In temperate areas the main growth periods of exotic pastures are in late autumn and early spring: in winter temperatures are too low, in summer it is generally too dry. In subtropical areas, exotic pastures and winter crops will develop all winter if rainfall is adequate, but most growth occurs in summer. Native grasses of the inland require high temperatures and hence, only respond rapidly to summer and early autumn rains (Christie, 1975). By comparison, growth of broad-leaf herbage in pastures is dependent on winter rain (Noble and Crisp,

8 1979). Grass-feeding noctuids, particularly the armyworms, M. convecta, P. ewingii and P. dyscrita, are strongly adapted to breeding in native grasses within and well beyond the cropping zone. P. ewingii has a southern distribution and breeds without obvious preference in native and improved pastures and in cereals. P. dyscrita occurs in more northerly inland regions (Common, 1954b), although rarely in Queensland, and is largely restricted to native and improved pastures (McDonald and Smith, 1986). M. convecta is more opportunistic and breeds on cereals and on native and introduced grasses in both sub-tropical (Broadley, 1979) and temperate regions. A. infusa, A. munda, H. punctigera and N. punctifera also breed widely in native pastures on a range of broad-leaved plants, but rarely on grasses (Common, 1958). A. munda and H. punctigera also breed in winter, spring, and summer broad-leaved crops and vegetables in temperate latitudes (e.g. peas, lucerne, lupins, chickpeas and tomatoes). The vegetation responds quickly to droughtbreaking rains, creating a patchwork of green and dry areas which are readily detectable by ground and aerial surveys. Preliminary surveys indicate that M. convecta larvae, for example, are only found in green vegetation patches produced by rain or run-off (McDonald, 1986). The cosmopolitan species, H. armigera, A. ipsilon, M. loreyimima, M. separata, S. litura, S. exigua and Migration of noctuid pests in Australia 537 Chrysodeixis spp. are almost entirely pests of crops and vegetables. Only outbreaks of S. exempta and S. mauritia are occasionally reported from pastures, usually of introduced species like Pennisetum clandestinum (kikuyu) and Paspalum dilitatum (paspalum), which are dominant grasses of improved pastures along the east coast (Anon, 1981a; Broadley, 1978; Weddel, 1936). Before the establishment of summer cropping in the inland over the past 30 years, species like H. armigera, S. exigua, S. litura, A. ipsilon and Chrysodeixis sp. were only reported from coastal and subcoastal Australia (I.F.B. Common, pers. commun.), where summer crops like maize had been established since European settlement. With the spread of cropping in the inland, all of these species have become established in such areas, mainly in subtropical and warm temperate regions. Only M. loreyimima and M. separata have remained pests of coastal crops such as sugar-cane and maize (Hitchcock, 1969). Outbreaks of cosmopolitan species in crops are confined to S. exigua and S. exempta. They have only been reported in S. exigua for 1967/8 and 1978/9 from a range of crops (Anon, 1969, 1980a). Outbreaks of S. exempta in crops and pastures have occurred more frequently in coastal and sub-coastal Australia (Weddel, 1936; Hitchcock, 1969; Broadley, 1978). The remaining cosmopolitan species tend to occur in chronic infestations in crops rather than in outbreaks. The level of infestation was sufficient in at least one area, the Ord River Irrigation Scheme of tropical western Australia, to cause abandonment of cotton growing (the species responsible were H. armigera and S. litura). H. armigera, economically one of the most important insect pests in Australia, has been rarely collected outside crops either as larva or an adult, although studies on adult trapping in progress indicate that it may disperse more widely than previously supposed (P. Gregg, pers. commun.). In dry periods, crops often present the only green vegetation suitable for noctuid breeding because surrounding pastures and herbage may be quite dry. It is not surprising, therefore, that summer crops form such an attractive breeding habitat for some endemic and most cosmopolitan species of migratory noctuids in Australia. The latter group exploit the limited extent of summer cropping areas in Australia by largely restricting their migratory ambit within such areas. SYNOPTIC WEATHER AND MIGRATION Synoptic weather provides the key to understanding the dynamics of migrations and of the outbreaks that may precede them. The weather pattern on the night that an individual moth emigrates largely determines the destination of the migrant and can be regarded as a stochastic event. Australia is primarily a region of atmospheric subsidence and a strong anticyclonic ridge separates the tropical SE trades from the temperate westerlies (Fig. 6a). This ridge, with its prevailing light and variable winds, is essentially a barrier to movement between tropical and temperate latitudes. Its axis shifts less than 10 south and north in summer and winter respectively. Weather systems progress eastwards across Australia, taking the form of anticyclonic cells separated by depressions to the south and troughs to the north. Cold fronts extend north from depressions, which are generally centred in the southern oceans, and separate warm tropical northerly airflows from cooler polar southerly airflows, circulating around the high pressure cells (Fig. 6a). Warm fronts and warm air sectors are never present in these systems, in contrast to those in the northern hemisphere. The discontinuity separating tropical and polar air is very sharp and northerly winds accelerate ahead of the cold fronts, often exceeding 50 km/hr at 100 m altitude at night and may extend over 1000 km of latitude in southern Australia. Because of the regular eastward movement of the systems at km/hr, these belts of northerlies rarely affect a given location for more than 48 hr. When a cold front crosses a given site, winds back to strong south-westerlies, which are often shortlived in inland areas (<24hr), and then moderate and back with the approach and passage of the next anticyclonic cell. In the tropics the north-west monsoon is shallow and shortlived (January-March) and rarely penetrates further than 18 S, although tropical depressions and troughs may extend as far as 30 S and occasionally bring unseasonal rains to the inland. The winds associated with such systems rarely extend outside subtropical Australia and present little opportunity for long-range displacement, comparable to the frontal systems described previously (Fig. 6b), although they could be of considerable importance in concentrating noctuid migrants in areas of the inland where rain is likely to fall. In January 1972, an exceptional monsoonal surge penetrated deeply into central and eastern Australia, bringing widespread drought-breaking rains. This coincided with large

9 538 ROGER A. FARROW and GARRICK MCDONALD Fig. 6. (a) Generalised pattern of synoptic weather, (b) Surface streamline chart showing penetration of monsoon in summer. catches of Heliothis at light and the first appearance of insecticide resistance in H. armigera in the cottongrowing areas of eastern Australia (Goodyer et al, 1975; Kay, 1977), following its original detection in the Ord Irrigation Area (Fig. 5a) in Western Australia (Wilson, 1974). In outbreak years, when large populations of noctuids are produced as a result of favourable autumn and winter rains in the inland, southward invasions of moths occur on northerly airflows ahead of cold fronts (Fig. 6a). Influxes of A. infusa, A. munda, H. punctigera (Anon, 1980b) and M. convecta (Smith and Donald, 1986) have been reported between August and November in southern mainland states, Tasmania (Drake et al, 1981; Helm, 1975), and even New Zealand (Fox, 1978). Radar studies have confirmed the existence of southward movements of noctuids across Bass Strait into Tasmania (Drake et al, 1981) and transmigration across Yorke Peninsula in South Australia (Drake and Farrow, unpublished data). Radar studies in inland Australia (Drake and Farrow, 1985) have, however, additionally shown that noctuid migration is a regular nightly event in spring, with moths migrating in a range of directions (not only southward) corresponding to the geostrophic wind flow (Fig. 7). Northward migrations of moths were observed in the wake of cold fronts in September 1980 and corresponded to unexpected influxes of H. punctigera in cotton-growing areas in the northeast of the state of Narrabri (Wilson, 1983). A source was known for these moths in inland New South Wales where a large caterpillar outbreak had been reported earlier in winter, comprising a range of noctuid pests (Anon, 1980b). Invasions of the south-east on northerly airflows also followed this outbreak (Anon, 1981b) suggesting that the geographic location of emigrant source areas is of greater importance in determining the destination of immigrants than the weather systems themselves. The radar study also showed that a systematic rotation of the migration direction of insect targets occurred between the passage of successive cold fronts, in relation to the wind shifts described previously (Fig. 7). This results in an extensive redistribution of migrating insects, including the noctuids, which were thought to be the dominant large insect flying in September Such behaviour is of considerable adaptive significance as it enables at least some of the dispersing moths to locate habitats suitable for breeding, particularly when these are widely separated in unfavourable seasons (Drake and Farrow, 1985). The northerly and south-westerly airflows of temperate latitudes present the greatest opportunity for net displacement because of the strength of the upper winds in the vicinity of the weather fronts. Aerial densities are generally highest on northerly airflows in the vicinity of fronts. This can be due to increases in Fig. 7. Diagrammatic trajectories of potential migrant noctuids between the passage of successive cold fronts over central western New South Wales.

10 Migration of noctuid pests in Australia 539 the number of moths taking-off on such nights; greater numbers of adults being produced in northern source areas; and concentration of migrants by atmospheric processes in the vicinity of fronts. Takeoff levels at dusk are generally higher when a front is approaching than during periods of settled weather. Temperatures are usually above-average on such nights and tend to persist during the night, particularly at the height of the inversion where the bulk of the moths are migrating (Drake, 1984). While dusk temperatures are frequently below the threshold required for take-off in temperate source areas in spring, this is not the case in sub-tropical source areas, suggesting that elevated temperatures per se are not the only factor responsible for the increased migratory activity observed during the passage of frontal systems. The duration of noctuid migration is generally shorter in settled weather as night air temperatures tend to fall more rapidly than in unsettled weather (Drake and Farrow, 1985) although again threshold effects are probably not involved. In tropical Australia, small scale disturbances, associated with local storms, are more frequent in summer than the large, synoptic scale events described previously. Concentration of migrant populations by storm outflows has been observed on many occasions in Australia (Schaefer, 1976; V. A. Drake and R. A. Farrow, unpublished observations) and is known to play an important role in the development of outbreaks of S. exempta in Africa (Rose et al., 1987). A general downwind deplacement of moths on a prevailing SE airflow has been observed at an inland site at Emerald (lat. 23 S) at all levels by radar and night vision devices (V. A. Drake, R. A. Farrow, W. W. Wolf and A. N. Sparkes, unpublished observations at Emerald, Australia). Such movements could account for the rapid colonization of new cropping areas in the inland by coastal populations of cosmopolitan noctuids. When troughs develop in the tropical interior there have been several reports (unpublished) of moths suddenly appearing in large numbers at light in the core of the convergence. These moths breed with the rains associated with such troughs as described in the case-history presented below. Because of the unreliable rainfall of the inland and the likelihood of both local and widespread extinctions of moths, immigration from the coastal populations may be essential to maintain populations in the arid zone. This source may be a more important origin for moths, breeding with the occasional autumn and winter rains at these latitudes, than the offspring of the populations that migrate to southern Australia. When offshore winds occur in Queensland (e.g. against the prevailing direction of S.E. trades) migrant noctuids have been detected arriving at an island in the Coral Sea 450 km from the mainland (Farrow, 1984). They included A. ipsilon, H. armigera, H. punctigera and S. mauritia plus a range of other migratory species. The chances of migrants locating an island measuring only 400 x 150 m is very low, suggesting that substantial numbers of emigrants must be leaving Queensland's shores. There is no reason to suppose that similar movements do not occur from coastal areas overland, as suggested previously. A CASE HISTORY A HYPOTHESIS FOR THE CAUSES OF THE OUTBREAKS OF N. PVNCT1FERA AND M. CONVECTA IN 1983 The Australian continent experienced a widespread and severe drought from late 1981 until March Heavy soaking rains in mid-march broke the drought over most of the continent. A huge caterpillar plague of N. punctifera was reported in early April 1983 from south-west Queensland and western New South Wales which must have been produced by a large parent population, laying eggs during the heavy rains in the preceding month. The source of these moths, which are widely distributed in the inland, was initially obscure. An examination of rainfall records for January 1983, when the parent moths would have been produced, showed that the only area with any significant rainfall was in central Western Australia in the Gibson desert, 1600 km west of the outbreak area of caterpillars. A very heavy local fall occurred from a rain depression derived from tropical cyclone "Jane" (Fig. 8a). This rain would have produced a rapid response in ephemeral broad-leaved plants and succulent shrubs, the preferred hosts of N. punctifera. The hot summer temperatures, relative absence of parasites and predators in these ephemeral environments, and unlimited food would have provided ideal conditions for the development of a single generation of N. punctifera from a small parent population. Adults would have emerged in early March when north-west winds of km/hr, associated with the rain-bearing trough, were established over central Australia (Fig. 8b). As a result of the drought-breaking rain associated with the trough, suitable hosts were available for immigrant moths resulting in the caterpillar plague. During early May, various reliable reports confirm the presence of an overwhelming population of F 2 N. punctifera adults (Table 2). The drought-breaking rain of 1983 also had a major impact on populations of grass-feeding noctuid larvae, particularly M. convecta. Unlike TV. punctifera, M. convecta did not have the benefit of establishing a large pre-winter F t population. The January rains in Western Australia fell in areas where grasses are uncommon. The widespread rains of mid-march to April encouraged rapid grass growth throughout eastern Australia, providing conditions suitable for the high multiplication of a winter population of M. convecta. A major outbreak was recorded in October/November 1983 throughout most of the cereal-growing regions of south-eastern Australia, extending as far north as southern Queensland. Using a phenological simulation model based on field temperatures (Smith and McDonald, 1986) it was possible to demonstrate that these outbreaks arose from moth flights and egg-laying during early September. On 25 August, an 80 km/hr north-westerly wind stream provided a means of transportation of moths from winter-breeding grasslands in the north, to colder regions of the south (Fig. 8c). The simulation model also indicated that under warm northern conditions, egg-laying in early April, after the droughtbreaking rains, would have given rise to a new generation of adults towards the end of August (Table 2).

11 540 ROGER A. FARROW and GARRICK MCDONALD ^ 1 Well above average Well below ave MIGRATION M. Above average Below average RAIN IN MID MARCH Fig. (a) Distribution of annual rainfall deviations for January (b) Surface streamline chart for 18 March (c) Synoptic situation for 25 August DISCUSSION AND CONCLUSIONS Major outbreaks of noctuid pests in Australia are largely restricted tofiveendemic species which breed in autumn in the native vegetation of the semi-arid zone, when conditions are favourable, and invade crops and pastures in the agricultural areas of the south in the following spring. The cosmopolitan species, including those which have a counterpart endemic sibling species, rarely cause outbreaks in Australia because populations are restricted to areas of reliable summer rainfall, tropical and subtropical climates, and introduced crops where the opportunities for population growth are limited, compared with populations in Asia, America and Africa. The extension of summer cropping and irrigation in tropical and subtropical inland Australia has been accompanied by an increase in the abundance and distribution of cosmopolitan species like H. armigera, A. ipsilon and S. litura, although the migratory movements in these species are poorly understood. In temperate latitudes, regular migrations of the endemic noctuids from low to high latitudes occur each spring. This appears to lead to an accumulation of populations in southern Australia where reproduction in summer is limited by the dry conditions usually experienced in the winter rainfall areas of these latitudes. In outbreak years, large numbers of moths are reported washed up on beaches following migration out to sea. Continental scale movements of migrants into supposedly ecological dead ends has been called the pied-piper effect by Rabb and Stinner Table 2. A chronological record of meteorological and entomological observations in Jan 1983 unseasonally heavy rain in Gibson Desert, Western Australia 18 Mar 1983 heavy rains in central and southeastern Australia; 80km/hr westerly and northwesterly winds 3 Apr 1983 caterpillar plagues throughout south-west Queensland and western New South Wales (non-graminaceous feeders) 23 Apr 1983 heavy rains in central and northern Queensland 5 May 1983 N. punclifera moths observed in abundance in Queensland and western New South Wales 20 May 1983 caterpillar plague in central Queensland, abruptly disappearing by about 25 May 25 August 1983 M. convecta moth invasions recorded in south-east Australia; 90 km/hr north-westerly winds recorded overnight

12 (1979). Some authors such as Walker (1980) consider that the migration of moths from low to high altitudes in spring presents an evolutionary dilemma in the absence of a defined return migration in autumn. We believe the problem is non-existent if we consider poleward migrations as part of a general strategy for survival in ephemeral habitats, whether in native vegetation or in crops. The direction and distance of movements depends on the weather pattern on any given night. For example on six out of seven nights winds may transport migrants in a variety of directions over a range of distances as wind vectors change with the passage of weather systems. On the remaining night a short-lived poleward surge of warm air will transport migrants over longer range to more distant habitats, only some of which may be favourable for breeding. Although offspring from these breeding areas may never return to the centres of breeding at lower latitudes, the advantages gained by colonizing fragmented ephemeral habitats by windassisted movements must frequently outweigh the losses caused occasionally by movements into ultimately unfavourable areas. Populations of a noctuid that lays 2000 eggs, of which only 2% survive to adult, can sustain 95% adult losses during interhabitat movements and still maintain the same population size. Baker (1978) has proposed that moths select a preferred wind direction to migrate, flying poleward in spring and equatorward in autumn. There is no evidence from radar studies (Drake and Farrow, 1985) that migrant moths are specifically orientated in a way that would support this hypothesis. As yet there is no evidence in Australia that greater numbers of moths migrate northwards, in the cool polar air in the wake of cold fronts, in autumn than in spring. The development of new cropping areas in tropical Australia, particularly where irrigation has been established, has always led to a rapid build-up of noctuid pests even in geographically isolated developments. In the Ord River Scheme, H. armigera, S. litura and M. separata quickly built up to levels in a range of crops which put many growers out of business despite frequent applications of insecticide. The limited opportunities for cropping in tropical Australia suggest that it is unlikely that our cosmopolitan pests, with the possible exception of some of the species of Spodoptera, will form outbreaks comparable to the endemic species but will remain chronic pests within the cropping areas. The erratic nature of rainfall in the remote semi-arid areas of inland Australia is certain to cause uncontrollable outbreaks of endemic species to continue and a forecasting system is clearly desirable for farmers operating in the cropping areas of south-east Australia. 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