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Bore holes are visible at the base of flower buds, the latter being hollowed out. Bracteoles are spread out and curled downwards. Leaves and shoots may also be consumed by larvae. Larger larvae bore into maturing green bolls; young bolls fall after larval damage. Adults lay fewer eggs on smooth-leaved varieties.
Young fruits are invaded and fall; larger larvae may bore into older fruits. Secondary infections by other organisms lead to rotting.
Eggs are laid on the silks, larvae invade the cobs and developing grain is consumed. Secondary bacterial infections are common.
Larvae feed on the developing grain, hiding inside the head during the daytime. Compact-headed varieties are preferred.
Foliage, sometimes entire small plants consumed; larger larvae bore into pods and consume developing seed. Resistant cultivars exist.
Flower buds and flowers bored by small larvae, may drop; larger larvae bore into locules of pods and consume developing seed. Short duration and determinate varieties are subject to greater damage. Less-preferred varieties exist.
Leaves, sometimes flowers attacked by larvae; severe infestations cause defoliation. Less preferred varieties exist.
H. armigera is a pest of major importance in most areas where it occurs, damaging a wide variety of food, fibre, oilseed, fodder, commodity and horticultural crops. Its major pest status is rooted in its mobility, polyphagy, high reproductive rate and diapause, all of which make it particularly well adapted to exploit transient habitats such as man-made agro-ecosystems. Its predilection for harvestable parts of essential food and high-value crops like cotton, tomato, pulses and tobacco confers a high economic cost to its depredations. The high level of control required under these circumstances, and the absence, in most situations, of adequate natural control means that chemical, or at best integrated control methods usually need to be adopted.
In view of the need to make use of and exploit the existing spectra of natural enemies and to reduce excessive dependence on chemical control, particularly where there is resistance to insecticides, various IPM programmes have been developed in which different control tactics are combined to suppress pest numbers below a threshold. These vary from the judicious use of insecticides, based on economic thresholds and regular scouting to ascertain pest population levels, to sophisticated systems, almost exclusively for cotton, using computerized crop and population models to assess the need, optimum timing and product for pesticide application. The SIRATAC system, developed in Australia during the 1980s, and its subsequent derivatives fall into this category (Room, 1979, 1983; Hearn et al., 1981). A major constraint to the development of IPM for H. armigera, particularly on cotton, has been the need to deal with a complex of pests where control needs may be irreconcilable, as for example in the characteristics of the cotton plant which can either be unfavourable to H. armigera or to jassid pests in terms of leaf hairiness, and in the withholding of early season applications to encourage the build-up of natural enemies against the need to control sucking pests which can be severe on young plants.
Owing to its strongly dispersive habit, efforts to regulate the influx of H. armigera into crops is generally not a viable option. Some cultural methods, such as an enforced 'close' season, may be regarded as regulatory, but to be effective these will depend on strict compliance, geographical isolation and the absence of a significant alternative wild host population in the area.
Another aspect of regulatory control is in the use of insecticides against which H. armigera has severe incipient resistance, and of 'hard' insecticides which are particularly damaging to natural enemies. An example of this is the resistance management strategy developed in Australia, where the use of pyrethroids was confined to particular phases in the cotton-growing season, principally to minimize selection for resistance.
Cultural Control and Sanitary Methods
Cultural manipulations of the crop or cropping system and land management have been tried as tactics to manage H. armigera populations. Trap cropping and planting diversionary hosts have been widely applied and recommended in the past, although with limited success. In the case of cotton, the diversionary hosts maize and sorghum had too short an attractive period to sustain populations; the tendency of these and earlier-planted crops to augment or create infestations were major disadvantages. The importance of ploughing cotton stubble to reduce overwintering populations of pyrethroid-resistant H. armigera was stressed by Fitt and Forrester (1987), and post-harvest cultivation to destroy pupae of bollworms has received considerable attention in the USA. However, all in situ cultural control tactics (including area-wide management of early season populations on wild hosts, as advocated by several workers in the USA for American species; Stadelbacher, 1982), and the concept of a close season during which food plants are denied for over one generation, would seem to be largely invalid where the immigration of adults into the protected habitats is the key consideration.
One indirect cultural method which could be included under this heading is the regulation of crop agronomy, variety (such as the okra-leaved varieties of cotton), spacing and fertilizer regimes to render the crop, and thus target larvae, more accessible to insecticides or microbial formulations applied by conventional means.
The planting of crop varieties that are resistant or tolerant to H. armigera has received major attention, particularly for cotton, pigeonpea and chickpea. This is a tactic of considerable importance within IPM systems. Many crop species possess some genetic potential which can be exploited by breeders to produce varieties less subject to pest damage; this can take the form of antibiosis (unpalatability), antixenosis (non-preference) and tolerance. However, where there is a pest complex, interactions may not always be favourable. For example, fewer eggs were laid on plants having the glabrous leaf character in cotton, however both larval survival and susceptibility to jassid attack were higher. Varieties of chickpea, groundnut and pigeonpea showing varying degrees of resistance have been developed at ICRISAT in India, some of which have been successfully used by farmers.
In recent years, genetic engineering techniques have enabled genes carrying the toxic element of Bacillus thuringiensis to be introduced into crops such as cotton and tomato. Although the technique is still very much in its early stages, transgenic crop varieties offer considerable promise for use in IPM systems against H. armigera. As with the use of all resistant crop varieties, however, care still needs to be taken to avoid excessive selection pressure against the resistance factor, so that in such systems a mixture of both resistant and susceptible varieties is often recommended to lessen this.
While IPM strategies are generally geared to provide a regime in which maximum feasible advantage is taken of local biological control agents, their unassisted suppression of H. armigera populations to below an economic threshold without the use of insecticides would be a major advantage, both in ecological and economic terms, particularly if this was sustainable. To this end, substantial efforts have been made either to introduce exotic natural enemies or to augment existing populations of parasitoids and predators to achieve satisfactory levels of control. Because of the need to produce very large numbers of parasitoids or predators simultaneously and economically, emphasis has been placed on Trichogramma spp. which are most amenable to mass rearing. Although these and a number of other parasitic species have been field evaluated against H. armigera, results have not so far been encouraging, especially in agrosystems where insecticide applications against H. armigera or other pests are consistently necessary.
There have been attempts to enhance mortality due to natural enemies by the introduction of species that might complement existing natural enemies or be superior to them (reviewed by Waterhouse and Norris, 1987). Attempted introductions have included parasitoids of Heliothis virescens and Helicoverpa zea from the Americas as well as species from other parts of the range of H. armigera. Few of these have been successful. Trichogramma pretiosum and T. perkinsi from the USA are reported to have become established in Indonesia and South Africa, respectively. Other successful establishments are: India (Chelonus blackburni, Eucelatoria bryani, both from the USA, and Bracon kirkpatricki from Kenya); Fiji (Cotesia marginiventris, also from the USA); New Zealand (Glabrobracon croceipes from the USA); Western Australia (Cotesia kazak and Hyposoter didymator, both from Europe). None of these introductions appears to have had a significant beneficial impact. However, the introduction of Cotesia kazak from Greece into New Zealand, where there were no native parasitoids of this pest, resulted in substantial parasitism but because of the low tolerance for insect damage in tomato crops, insecticides are still needed.
The relative specificity, potential activity, environmental safety and immunity to insecticides have made microbial pesticides a favoured component of IPM strategies, and considerable efforts have been made to develop the most promising agents, Bacillus thuringiensis and Helicoverpa armigera nuclear polyhedrosis virus (HaNPV) into commercially viable products. Present and active under natural conditions, both these agents, but particularly HaNPV, have some impact on H. armigera populations, although seldom reaching the epizootic proportions necessary to achieve effective control. Field tests with artificially produced Bt and HaNPV have so far had only limited success, mainly because of rapid degradation by UV light, insufficient titres ingested by larvae, and lack of virulence. However work is continuing to overcome these constraints stimulated by increasing resistance to insecticides and awareness of the environmental threats they pose.
The whole subject of biological control of H. armigera is treated in considerable detail in King and Jackson (1989).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
H. armigera, like its close relatives H. zea and Heliothis virescens in the New World, is a pest of major importance in most areas where it occurs, damaging a wide variety of food, fibre, oilseed, fodder and horticultural crops. Its considerable pest significance is based on the peculiarities of its biology - its mobility, polyphagy, rapid and high reproductive rate and diapause make it particularly well adapted to exploit transient habitats such as man-made ecosystems. Its predilection for the harvestable flowering parts of high-value crops including cotton, tomato, sweetcorn and the pulses confers a high economic cost, and socio-economic cost in subsistence agriculture, due to its depredations. However, regional and even relatively local differences in host preference can give rise to differences in pest status on particular crops; this was shown by populations in northern and southern India where severe infestations of cotton are only a relatively recent event.
H. armigera has been reported causing serious losses throughout its range, in particular to cotton, tomatoes and maize. For example, on cotton, two to three larvae on a plant can destroy all the bolls within 15 days; on maize, they consume grains; and on tomatoes, they invade fruits, preventing development and causing falling.
Monetary losses result from the direct reduction of yields and from the cost of monitoring and control, particularly the cost of insecticides. In Australia, Wilson (1982) estimated total Australian losses at $A 23.5 million; with increases in the prices of insecticides and the replacement of the cheaper pyrethroids with more expensive alternatives to counter pyrethroid resistance, Twine (1989) has estimated that costs in Queensland alone would have increased to about $A 25 million annually.
In India, where H. armigera commonly destroys over half the yield of pulse crops, pigeon pea and chickpea, losses were estimated at over $US 300 million per annum (Reed and Pawar, 1982), while in the late 1980s losses of both pulses and cotton were estimated to exceed $US 500 million, with an additional $US 127 million spent on insecticides on these two crops annually (KN Mehrotra, Indian Agricultural Research Institute, New Delhi, unpublished data, 1987/88). Following the rapid upsurge of pyrethroid resistance, and reduced effectiveness of other insecticide groups in H. armigera (Dhingra et al., 1988; McCaffery et al., 1989) these figures will certainly need to be revised upwards.
Oerke et al. (1994) reported that H. armigera is an economically important pest or a key pest in Africa, Asia, Europe and the former USSR, and Oceania. Previously, Ridgway et al. (1984) had reported also that H. armigera was partly responsible for a major portion of cotton crop losses.
In Africa, H. armigera can reduce yields substantially. In the Côte d'Ivoire, between 1978 and 1983, cotton crop loses in the south of the country were primarily due to H. armigera and were ca 60% (Moyal, 1988). In Zimbabwe, potential crop losses due to H. armigera were 1175 kg/ha (Gledhill, 1976). While H. armigera has now been contained as a pest on cotton in Zimbabwe, it is important in Tanzania where the economic loss of cotton was estimated at over $US 20 million (Reed and Pawar, 1982).
In Andhra Pradesh, India, problems in controlling H. armigera were first encountered in 1987. More than 30 insecticide treatments were applied, yet the average yield fell from 436 kg/ha in 1986/87 to 186 kg/ha in 1987/88. This was a reduction of 61% (Armes et al., 1992). In Thailand, H. armigera has been the principal cotton pest since the mid-1960s. Losses due to H. armigera were at least 31% in 1975-79 (Mabbett et al., 1980). In China, losses due to H. armigera larvae increased with plant age. Crop losses were substantial regardless of soil fertility (Sheng, 1988). The damage threshold, 7.5 kg/ha, was reached at 35 egg clusters/100 plants. Integrated pest management reduced H. armigera infestations from 1.6 to 0.1% in Jiangsu between 1976 and 1982 (Jin, 1986).
In the EPPO region, H. armigera is of great economic importance in Israel, Morocco, Portugal, former USSR and Spain, and of lesser importance in the other countries where it is established. Despite extensive spread in Greece, H. armigera only causes periodic damage to cotton.
Chickpeas and Other Crops
In India, chickpea is the most important pulse crop and is grown on 7.3 million hectares in various agro-climatic conditions. Although its yield potential is 2.5-3 t/ha, the average yield is only ca 0.8 t/ha. The extent of losses caused by H. armigera varies from region to region and depends upon climate and crop intensity. However, a monetary loss of 203 crore rupees annually is estimated.
Changes in sowing date have had a considerable influence on pod damage and seed yield of chickpea. Pod damage due to H. armigera increased as sowing dates grew later. At five different sowing dates, % pod damage was 5.8, 8.1, 14.9, 18.2 and 26.2% while corresponding seed yields of 2452, 2409, 1859, 1439 and 1010 kg/ha, respectively, were recorded. The co-efficients of correlation between sowing date and pod damage and between pod damage and seed yield were significant (Saxena et al., 1998). The larval population of H. armigera on chickpea was ca four times higher at dense spacing (33 plants/m²) than at wide spacing (3 plants/m²) (Yadava et al., 1998).
Chickpea yields have been shown to increase following control treatments. The application of nuclear polyhedrosis virus reduced larval populations by 26.8% and pod damage by 36.6% and increased yields by 72% compared with untreated plots (Bhagwat and Wightman, 1998).
Damage has been reported in India on potatoes, sunflowers, Guizotia abyssinica, pigeon peas and cotton. Crop losses of 10-100% have been estimated for potatoes in India. In studies over three seasons, between 1982 and 1985, on four varieties average losses of 0.34% were recorded. Based on the average potato yield for India of 15.8 t/ha, the loss rate was 2.1% (Parihar and Singh, 1988).
An outbreak of this noctuid occurred on young Pinus radiata in New Zealand in 1969 and 1970, when the larvae consumed more than 50% foliage of about 60% of trees.