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Biological Agriculture and Horticulture, 1991, Vol. 8, pp. 33-52.
Copyright © 1991. A B Academic Publishers. All Rights Reserved. Reprinted with permission.
Nancy M. Endersby and Wendy C. Morgan
Institute of Plant Sciences, Department of Agriculture, P.O. Box 381, Frankston, Victoria 3199, Australia
Current public concern about the possible adverse effects of agricultural chemicals on health and the environment has generated interest in reducing chemical inputs in vegetable growing. Crucifers are important vegetable crops in Australia and are subjected to attack by lepidopterous pests especially diamondback moth (Plutella xylostella (L.)) and cabbage white butterfly (Pieris rapae (L.)). Control options for cabbage pests reviewed here include use of natural insecticides (mainly plant-based). physical barriers. biological control. insect sterilization.. intercropping, companion planting and host plant resistance. Successful reduced chemical insect control should concentrate on combining several Of these options in complete pest management systems.
Current public concern about some chemicals in agriculture causing health and environmental problems, and the requirements of some overseas countries for no chemical residues in agricultural products, has reinforced the need to develop techniques to eliminate or reduce the present level of chemical inputs in vegetable growing. In Australia in 1988, $554) million (Morgan, 1989) was spent on the purchase of agricultural chemicals for insect, weed and ease control of which approximately 20% was for insecticides.
Insect control using insect repelling plants(Smith& Secoy, 1981)or natural products such as sulphur, to control insects and mites, combined with religion, ritual and superstition, began with early civilization and continued through the Middle Ages to the Agricultural Revolution (Conacher 1986). As broad scale agriculture emerged, accompanied by increased insect pest outbreaks, botanical preparations such as nicotine, pyrethrum, hellebore, quassia and derris were used more widely (Ordish, 1967) and commercially manufactured pesticides based on inorganic substances such as copper, arsenic and mercury were formulated.
Further research during the inter-war years and the Second World War led to the synthesis of the organophosphate and organochlorine groups of insecticides (Conacher, 1986) whose residual properties, effectiveness and economic benefits produced an exploitive phase during which insecticides were used routinely on a large scale (McEwen' 1978).
The adverse effects of synthetic insecticides, resulting from their misuse, include human poisonings, destruction of natural enemies of pests, insecticide resistance, crop pollination problems due to honeybee losses, domestic animal poisonings, contaminated livestock products, fish and wildlife losses (Pimentel et of., 1980) and contamination of underground water and rivers.
Further effects occur at the level of the ecosystem where species' diversity may be reduced, food chains may be modified and energy~nutrient cycling patterns can be altered (Pimentel & Edwards, 1982). Destruction of natural enemies of pests may lead to an outbreak of secondary pests necessitating the use of additional chemical treatments (Pimentel, 1985). Problems of resistance, residues and environmental contamination have resulted in a reflective phase of chemical use where, in some situations, reductions in and alternatives to synthetic insecticides are being sought.
There are two ways to reduce synthetic chemical insecticide use. The first is to eliminate these products, replacing them with alternative control methods and the second is to reduce the number of spray applications by spraying when necessary rather than on a routine basis. The latter technique relies on previously developed action thresholds which are based on a measurement of pest density (Baker, 1984). This paper will discuss alternatives to synthetic insecticides which are currently available, or are being developed, for control of lepidopterous pests of crucifers (broccoli, cabbage, Brussels sprouts and cauliflower) and will draw some conclusions.
Crucifers are important vegetable crops in Australia. In 1987/88, 11,279 hectares produced 212,947 tonnes of crucifers (Australian Bureau of Statistics, 1988). In Victoria, there are approximately 3500 hectares of crucifers and in 1987/88 production was 63,716 tonnes (Australian Bureau of Statistics, 1988). The estimated total wholesale value of crucifers produced in Victoria in 1986/87 was $29 million (Waters et al., 1988), which represents almost 23% of the total value of all vegetables (excluding potatoes) produced in Victoria (Australian Bureau of Statistics, 19903.
The main insect pests of crucifers in Victoria are the larvae of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Yponomeutidae), and the cabbage white butterfly, Pieris rapae (L.) (Lepidoptera: Pieridae). The cabbage aphid (Brevicoryne brassicae (L.)) and other lepidopterous larvae including the looper Chrysodeixis argentifera (Guenee) (Lepidoptera: Noctuidae) are incidental pest species. They are all controlled conventionally using routine applications of synthetic chemical insecticides
Cabbage grown in summer (1989) at Frankston, Victoria without insect control suffered severe damage to head and outer leaves and marketable yields were 30-35% less than those with a regular or a reactive (based on larvae numbers) chemical treatment (Endersby & Morgan, unpublished data). Hence insect control is desirable both from an appearance or quality and a yield point of view.
There are a number of non-synthetic chemical options for control of Lepidoptera and other insect pests of crucifers which include:
1. Non-persistent natural products which may be either a) plant derived insecticides, b) plant derived insect repellents, c) products which cause physical damage to insects, d) insect derived repellents. Physical barriers. Biological control. Insect sterilization. Intercropping. Companion planting. Use of insect resistant plant varieties.
Use of one of these control options will rarely solve all pest problems in all seasons. Ideally many of the options should be combined in a complete pest management system.
Natural insecticide products, many of which are plant-based, have no persistent detrimental effects on the environment as they rapidly degrade to harmless- substances. Some have the disadvantage, however, of killing a wide range of insect species including natural predators and parasitoids. Their insecticidal activity is enhanced by concentrating or purifying the active constituent (Angus, 1968). Natural materials suggested for control of P. rapae and P. xylostella are derris, pyrethrum, nicotine, quassia, garlic, wormwood, rhubarb and tomato leaf sprays (Conacher, 1986) and neem seed extract (Rice, 1989).
Derris dust is a product derived from the root of several species of tropical legumes (Ordish, 1967), especially Derris elliptica (Angus, 1968) and D. trifoliata which grow in tropical Asia and parts of the Congo Basin They are also cultivated in Guatemala and Ecuador (de Wit, 1966).
The active ingredient of derris dust, rotenone, is lethal to a broad range of Insect species, as well as earthworms and fish. It is advisable, therefore, not to use the product near waterways and dams. Rotenone acts as a stomach poison and is effective for approximately 48 hours after application (Conacher 1986). According to Bennett (1988), rotenone has synergistic effects when applied with pyrethrum.
Pyrethrum is extracted from the flowers of Chrysanthemum cinerariaefolium or C. roseum (Conacher, 1986). It kills aphids and caterpillars, but also affects arthropod predators such as lacewings, hoverflies and ladybird larvae (Bennett, 1988). Bees may be spared if pyrethrum is sprayed in the evening (Little, 1989). Pyrethrum is a contact spray with a 12 hour effectiveness.
Quassia is made from the bark and root of Picrasma quassioides, a South American tree. The spray is made by boiling quassia chips in water, straining through a filter and adding a wetting agent (Conacher, 1986, Bennett, 1988 Little, 1989). It is effective against aphids and caterpillars which are small in size and safe towards ladybirds and ladybird larvae. Some disadvantages of using quassia as an insecticide are that it kills the larvae of hoverflies and may taint food crops if used just before harvesting (Conacher, 1986).
An insecticide can be extracted from seeds and leaves of the neem tree (Azadirachta indica A. Juss, Family Meliaceae). Neem trees occur naturally in the hot, dry tropics of southern and southeastern Asia and their seed extract has been used as an insecticide throughout this area for thousands of years (Rice, 1989). The seeds contain a tetranortriterpenoid known as azadirachtin which has deterrent, antifeedant. growth disrupting, anti-ovipositional and fecundity reducing properties on a range of insects (Schmutterer, 1990).
Neem derivatives are most effective as feeding poisons for nymphs or larvae of phytophagous insects, and lepidopterous larvae are very susceptible. This property makes neem suitable for use in pest management programmes because parasitoids and some other natural enemies of pests are spared. The neem insecticide does not have an immediate knockdown effect on pests, but reduces feeding and death occurs within several days (Schmutterer, 1990). The residual effect may persist for two to seven days.
Nicotine, extracted from the tobacco plant (Nicotania tabacum L., Family Solanaceae), is highly toxic to mammals but breaks down within 48 hours (Conacher, 1986). It is also a powerful insecticide towards larvae of Lepidoptera and other pests (Bennett, 1988), but also kills some beneficial insects and earthworms.. Nicotine, applied as tobacco dust, can burn seedlings and carry tobacco mosaic virus. Nicotine preparations are prohibited for use in Victoria and should never be considered for use.
Oxalic acid is the active ingredient in a rhubarb insecticidal spray and may be extracted by boiling the leaves. The resulting extract is improved if mixed with soap (Bennett, 1988) and is useful against aphids (Little, 1989). Tomato leaves contain a substance similar to nicotine which may be extracted by a similar method (Conacher, 1986).
When using alternative products, attention should still be paid to safety precautions and withholding periods. It must also be ensured that they are registered for use on food crops.
Insect feeding damage to crucifers can also be reduced by repelling pests rather than by killing them. Abivardi & Benz (1984) studied plant extracts as feeding repellents (antifeedants) against Pieris brassicae (L.), the large white butterfly. They found that extracts of Mentha piperita L. (peppermint),
Angelica archangelica L. and Eucalyptus sp. significantly reduced larval ceding in glasshouse experiments. Melissa officinalis L. (common balm) and Artemisia absinthium L. (wormwood) also reduced feeding, but to a lesser degree. Concentrated extracts of these species may have potential as sprays against P. rapae.
The following plants were found to have an oviposition deterring effect on P. rapae in experiments performed with leaf extracts in a dual choice chamber (Lundgren. 1975):
Lycopersicon esculentum Mill. (tomato)
Sambucus nigra L. (elder)
Thymus vulgaris L. (thyme)
Sylvia officinalis L. (sage)
Artemisia absinthium L. (wormwood)
A. abrotanum L. (southernwood)
Allium cepa L. (onion)
These species also have potential as insect control preparations.
Eighty eight plant species are reported to be insecticidal to P. xylostella and many of these also have repellent properties (Morallo-Rejesus, 1986). Most belong to the families Asteraceae, Fabaceae or Euphorbiaceae.
Diatomaceous earth is made up of the silicified skeletons of microscopic marine organisms (diatoms). It punctures the cuticle of some insects, causing death by dehydration and also affects pest movement, breathing and digestion. Conacher(1986) recommends its use against molluscs, aphids and other soft-bodied insects and larvae. Diatomaceous earth may be applied as a dust or as a spray in water with a wetting agent. Safety precautions should be taken when using this product as there is a risk of silicosis if the dust is inhaled
Soap sprays act by disrupting the cuticle of soft-bodied insects and larvae The insecticidal activity of several soap products against cabbage pests was evaluated by Koehler et al. ( I983a) in the United States. Their results indicated that soaps must be applied frequently to prove effective.
Treatment with one soap, which is also available in Australia, significantly reduced numbers of P. rapae larvae and damage to cabbage, but compared with untreated cabbage depressed yield (Koehler et al.. 1983a). However trials performed on a larger scale by the authors at Frankston, with soap on cabbage, resulted in no reduction in feeding damage and final yields equivalent to those obtained with no insect control. Insect feeding damage to the cabbage head was significantly worse, using soap, than damage on heads receding no insecticides (Endersby & Morgan, unpublished data).
Conacher (1986) gives a recipe for 'bug juice' which is a spray consisting of liquefied insect pests. This method aims to spread natural pathogens, cause pheromone confusion in the pest and attract. natural predators. Little information about the efficacy of this technique has been published.
The use of synthetic pheromones for monitoring pest populations and as a confusion technique has potential in pest management. Sex pheromone of P. xylostella has received much research attention (Chow et al.. 1974) and Chisholm et al. (1979, 1983) have described trapping systems and lure formulations which could be used to estimate adult populations. Chisholm et al. (19843 have studied mating disruption via male disorientation in P. xylostella using combinations of pheromone components.
Physical methods of insect control such as manipulation of temperature or relative humidity, physical shock and electric discharge may kill pests or make their preferred micro-environment inaccessible. Some of these methods have been adopted, to an extent, in the food storage industry but have not been applied to field crop management (Banks, 1976). There are some physical control techniques however, which have potential for use with vegetable or row crops.
Talekar et al. (1986) found that sprinkler irrigation applied to cabbage for five minutes at dusk throughout the life of the crop physically disrupted diamondback moth flying activity and oviposition and drowned larvae and adults. Such a modification of a cultural practice could be a valuable component of a pest management system.
The use of lightweight netting row covers, as a barrier to oviposition, is another effective non-chemical insect control technique. Row covers are mainly used to extend the growing season and by protecting against frosts provide early vegetables by decreasing time to maturity (Mansour, 1989). They are also effective as barriers against P. rapae and P. .xylostella (Endersby & Morgan, unpublished data) and possibly aphids.
Aphid attack and associated virus transmission is reduced in some horticultural crops using aluminium foil mulches (Gibson & Rice, 1989). The surface of the mulch prevents flying aphids from settling, as light wavelengths reflected are different from those reflected from crop foliage which normally attract aphids to land. Alternatively, yellow plastic mulches covered with an adhesive or a contact insecticide could be used to attract and destroy aphids.
Many insecticides not only kill the target insect but also destroy its natural enemies. When the use of insecticides is reduced, the level of natural biological control by parasitoids and predators will increase. Such an increase is advantageous, but natural biological control rarely produces a very high level of control-in crops with relatively short growing seasons. Even so, natural levels of biological control can be used as a component in a pest-management system.
P. rapae populations in Canberra have ant predation as a substantial mortality factor (Jones, 1987). Predation, by two species of Iridomyrmex was found to have a patchy distribution in space and time, but was density dependent occurring at a higher rate where larval density was high. P. rapae individuals parasitized by the braconid wasp Apanteles glomeratus (L.) were preyed upon more heavily by ants than were unparasitized larvae. Later instar larvae (III-V) were also taken more often than earlier instar larvae.
Pimentel (1961) observed predators from the following families attacking first and second instars of P. rapae. P. xylostella and T.ni in Ithaca, New York: Coccinellidae (3 spp.), Syrphidae (4 spp) and Chrysopidae (> I sp). These predators however were not abundant and mainly attacked aphids. Spiders were relatively abundant and appeared to be effective control agents Predators observed eating P. rapae eggs in Columbia, Missouri (Parker, 1970s included ants, Chrysopa larvae, trombiculid mites, Lygus adults and nymphs and coccinellid adults and larvae.
Baker (1970) studied bird predation of P. rapae and P. brassicae in a rural garden and an allotment in Britain. House sparrows (Passer domesticus L ) and garden warblers (Sylvia borin Boddaert) were observed feeding on eggs of P. rapae. The main avian predators of P. rapae larvae observed were the House Sparrow, the Great Tit (Paws major Praz.) and the Blue Tit (Paws caerulus Praz). The song thrush ( Turdus ericetorum Turton ) took fourth and fifth instar larvae. The different bird species were observed to search different parts of the cabbage plants, for example, Starlings (Sturnus vulgaris L.) may tear leaves from the cabbage heart to reach the larvae inside.
Some Australian insects such as the tachinid parasitoids Compsilura concinnata (Meigen), Paradrino laevicula (Mesnil) and Winthemia lateralis (Macquart) (Diptera: Tachinidae) are parasitoids of P. rapae (Cantrell.1986).
Wilson (1960) detailed the history of insect introductions to Australia for biological control of P. rapae and P. xylostella. Three species of wasp Apanteles glomeratus (L.), Apanteles rubecula Marsh (larval parasitoids) and Pteromalus puparum (L. ) (a pupal parasitoid), have been introduced to control P. rapae. Additional wasp species which parasitize P. xylostella have been introduced from New Zealand and from Italy.
Waterhouse and Norris (1987) recommended introduction of the following wasp species for control of P. ~-xylostella if they are not already present Diadegma eucerophaga (Gray. ), Apanteles plutellae Kurdj., Diadromus collaris (Grav.) and Tetrastichus sokolowskii (attacks larvae or prepupae)
Yarrow (1970) bred eight species of parasitoids from larvae of P. xylostella in south eastern Queensland. Over two years the parasitoids had no effect on reducing economic damage to cabbage and only consistently reached parasitism rates over 50% in the second year.
Goodwin (1979) recorded an average rate of parasitism of P. ,xylostella over four successive cabbage crops in Victoria, as 49%. The three main parasitoid species were D. eucerophaga D. collaris and Diadegma rapi (Cambridge) Also present were six minor parasitoid species and one hyperparasite. Apanteles spp. averaged only 2.5% of total parasitism and appeared erratically. The indigenous parasitoids were also seen to be dominated by superior numbers of the introduced species. Further research on D. eucerophaga is required to determine the factors which permit its success under variable field conditions and its potential role in programmes for P. xylostella involving selective use of pesticides (Goodwin, 1979).
Hamilton (1979) monitored the seasonal abundance of P. rapae and P. xylostella and their diseases and parasitoids in New South Wales. He found that within the population of P. rapae adult activity commenced each year in mid-August and ceased for a short time in mid-winter. The population was not reduced to below damaging levels despite parasitism by A. glomeratus and P. puparum. Crop damage is not prevented by A. glomeratus as it does not kill the larva until the final instar and is essentially a parasitoid of P. brassicae (Wilkinson, 1966).
Hamilton (1979) suggested that A. rubecula which was not present in the study area would prove more effective in reducing crop damage as it kills the larva in the fourth instar. Solitary endoparasitoids such as A. rabecula disrupt their hosts' functioning sooner than do gregarious endoparasitoids (e.g. A. glomeratus) because they reach their required weight more quickly, presumably due to their lower nutritional demands (Smith & Smilowitz, 1976). They are likely to be more useful as biological control agents than gregarious endoparasitoids (Parker & Pinned, 1973).
A gregarious egg parasitoid of P. rapae Trichogramma evanescens Westwood (Hymenoptera: Trichogrammatidae) has been released in North America (Parker, 1970), West Germany (Gras et al.' 1981) and the USSR (Bondarenko. 1982). The parasitoid occurs naturally in Europe. After its release in Missouri (Parker, 1970), egg parasitism ranged from 20-75% per host generation. Parasitoids such as T. evanescens which kill the pest at the egg stage could be a useful component in a pest management system.
Parasitoids of P. xylostella only killed at the pupal stage reducing the future adult population. Although parasitism levels were sometimes high, significant plant damage still occurred. However, levels of disease and parasitism recorded by Hamilton (1979) were significant and considered sufficient to form the basis of a pest management programme for crucifers in the Cumberland county area of New South Wales.
Attempts to improve the level of biological control have been made by using supplemental host and parasitoid releases where 1. initial host densities were low preventing rapid increase of parasitoids required for control of subsequent populations and 2. parasitoid densities were low due to hyperparasitism and winter mortality (Parker, 1970; Parker, 1971; Parker et al. 1971; Parker & Pinnell, 1972). A system of manipulating host and parasitoid population densities may be necessary on a variety of crops where the main obstacle to effective control by parasitoids is host discontinuity (Parker et al. 1971).
Fungi, viruses and bacteria which are pathogenic towards insects have potential as biological control agents:
Some fungi, mainly from the order Entomophthorales, are insect pathogens and are effective in periods of prolonged dampness. Overwintering and soil inhabiting insect life stages are most susceptible (Elliot & de Little, n.d.). Fungi usually infect insects by penetration of the body cavity via the integument and then proliferate, filling the body with hyphae (Steinhaus,
Larvae and pupae of P. xylostella have been attacked by several species of fungi including Erynia blunckii (Lakon) and Zoophthora radicans (Brefeld) (Phycomycetes: Entomophthoraceae) and in some situations high levels of control have been achieved (Wilding, 1986).
Wilson (1960) noted the discovery of a virulent virus disease of P. rapae larvae. Experimentation with the granulosis virus (P. rapae GV) has been performed by Jaques (1970,1977) in Canada. Application of granulosis virus (GV) to soil rather than to foliage allows greater persistence, increase in virus concentration and recurring epizootics in subsequent crops in the same area (Jaques, 1970). Virus residues in soil are not harmful due to their host specific nature, however, GV may infect the parasitoid A. glomeratus if ingested by the host within less than five days after parasitoid oviposition (Levin et al., 1981). It was also found that A. glomeratus adults can transmit GV from infected to healthy P. rapae larvae in the laboratory (Levin et al. 1979)
Although further research and product development is required, arthropod viruses have potential as components of an integrated pest management system. This is because they are selective control agents and may be used without harming the environment, against pests including those which have developed insecticide resistance (Falcon, 1976). The main problem with using viruses as insecticides is their sensitivity to UV light and their slow rate of kill but using new techniques of gene manipulation it may soon be possible to engineer insecticidal genes into viruses to increase their speed of action (Christian & Oakeshott, 1989).
The microorganism, B. thuringiensis Berliner (a complex of strains), may be used as a biological control spray for lepidopterous larvae including P. rapae and P. xylostella. The bacterium produces a combination of toxaemia and septicaemia in the larvae (Angus, 1968). An infected caterpillar ceases to feed soon after ingestion of the bacterium and dies after three to five days (Brunner & Stevens, 1986) dissolving into a soft, black mass with a liquid exudate (Bennett, 1988). Larvae may become infected by ingesting spores from such cadavers or from the faeces of other infected larvae (Wilding, 1986), but epizootics (disease outbreaks) are rare (Fuxa, 1987).
As B. thuringiensis must be ingested to kill the larvae, leaf coverage plays an important role in its effectiveness (Bryant & Yendol, 1988). The bacterium produces spores which give rise to vegetative cells and protein parasporal inclusion bodies (crystals) which are toxic to most lepidopterous larvae. The combined action of spores and crystals is essential for greatest efficacy, so the bacterium is cultured in the laboratory on an artificial medium to produce a preparation containing a concentrated amount of both spores and crystals (Angus, 1968).
Fisher and Rosner (1959) tested the toxicology of the insecticide and concluded that it is harmless towards mammals. Steinhaus (1959) has also cited theoretical and experimental evidence which suggests that varieties of B. thuringiensis are unlikely to mutate into forms pathogenic towards vertebrates. The bacterium can also be used for control of lepidopterous pests without adverse effects on their natural enemies (Sivapragasam et al., 1988).
Preparations of B. thuringiensis are used extensively on cole crops in Canada against P. rapae, P. xylostella and T. ni, providing equivalent control to that of chemical insecticides (Sears et al., 1983; Jaques & Laing, 1984). B. thuringiensis is also one of the two most widely used products for control of these three species on sauerkraut cabbage in New York State (Eckenrode et al., 1981).
In trials undertaken at the Vegetable Research Station, Frankston, applications of B. thuringiensis var. kurstaki (Sandoz Crop Protection, a.i. 16000 I.U./mg) timed according to a modification Of Baker's (1984) action thresholds, controlled P. rapae and P. xylostella and produced cabbage yields equivalent to those obtained using regular applications of conventional insecticides (Endersby & Morgan, unpublished data).
B. thuringiensis insecticides often contain spreaders, stickers and anti weathering agents to ensure the spores adhere to the surface of the crop and remain accessible to feeding larvae. Tompkins et al. ( 1986) found that when combined, reduced rates of B. thuringiensis and nuclear polyhedrosis viruses provided better control of T. ni and P. rapae than normal rates of either.
The efficacy of B. thuringiensis var. kurstaki and P. rapae granulosis virus can also be enhanced by mixing with low concentrations (0.1 or 0.25 of full rate) of. the synthetic pyrethroid, permethrin, providing equivalent control to that obtained with permethrin at full rate (Jaques, 1988). Use of such mixtures is an effective way to reduce use of synthetic insecticides. There are few reports of development of resistance to spores and crystals of B. thuringiensis. However, rapid development of a resistant strain of Plodia interpunctella (Hubner) (Lepidoptera: Pyralidae) (McGaughey, 1985) and Cadra cautella Walker (Lepidoptera: Pyralidae) (McGaughey & Beeman 1988) was noted in a stored grain environment where B. thuringiensis remains stable for a long period of time.
It is estimated that B. thuringiensis accounts for less than I per cent of world insecticide sales (Jutsum et al., 1989). Commercial potential of microbial insecticides is likely to increase in the future. Current research by some agrochemical companies aims to isolate strains showing improved environmental resistance and activity. Genetic engineering of gene codes for microbial toxins into more efficient 'carrier' microbes and directly into crop plants is also being studied (Jutsum et al., 1989).
There are numerous biological chemicals (biopesticides) in the early stages of investigation which have potential to be mass produced in a bacterial or cell culture system using genetic engineering (Hogan, 1990). These include semiochemicals such as pheromones and antifeedants as well as biological compounds involved in physiological reactions including behaviour' for example' enzymes, hormones and neuropeptides.
The sterile insect technique has mainly been used against Diptera (Lindquist' 1986). The main principle of releasing sterile male insects into the natural population is that these insects mate normally with females which lay infertile -eggs (Smith et al., 1964). For this to be achieved, the ideal chemosterilant should produce lethal mutations in sperm which prevent development of the zygote. The monogamous female's normal mating requirements will then be met, but no progeny will arise (Smith et al., 1964). With polygamous species at least some potentially successful-matings would be replaced by sterile matings. If sterile females are released, they also contribute to a reduction in the natural population as males which mate with them are removed from the reproductive population for those matings (Richardson et al., 1982).
Sterile males must be at least as sexually competitive as normal males (Smith et a/.' 1964), but this may be difficult to achieve. Some mass reared insects may have reduced fitness compared with those in the wild (Bush & Neck, 1976) and other insect species are not suited to laboratory rearing. Admm~stration of chemosterilants to insects in food or by topical application s another area where further study is needed (Smith et al., 1964).
The chemosterilant should degrade rapidly after sterilising the insect to avoid its movement into the ecosystem via excretion, consumption of the insect by other organisms or decomposition of the dead insect. If safe, selective methods of bringing the chemical to the pest are devised, then chemosterilants may eventually be used to induce sterility in natural insect populations (Howland et al., 1966).
Cook and Hooper (1974) measured the sterilising effects of an alkylating agent or aziridine compound, phenyl metepa, on adults of P. xylostella. Males were sterilised after three hours exposure to the residue of 1.57 mg/cm2 phenyl metepa. No effects on longevity were observed and matings by these males had no effect on the fecundity of the females. However, metepa was not recommended as an ideal chemosterilant for use with natural populations as it is toxic to mammals and causes high mortality in female diamondback moths before sterility is achieved. More potential chemosterilants have to be evaluated if chemosterilization is to be employed as a control method.
Intercropping is the practice of' increasing crop diversity' by growing more than one plant species in a field to overcome insect pest outbreak problems associated with monocultures. Dempster (1969) studied the effects of weed control in brussels sprouts on P. rapae and found that weeds provide a habitat for predators of the caterpillar. However. yield reduction due to weed competition outweighed the advantageous effects of insect control obtained in the weedy plots. A less competitive plant species (for example' Trifolium spp.) could be used under the crop to provide habitat for predators with the added benefits of weed smothering and nitrogen fixation.
Buranday and Raros (1975) compared the abundance of adults and oviposition of P. xylostella in a cabbage field and in a field with cabbage and tomato intercropped. Both factors were lower in the intercropped field and it was suggested that volatile compounds emitted by the tomatoes repelled the adult moths. The recommended planting pattern is two cabbage rows between two rows of tomato. The pest control benefits' with respect to reduction in larval feeding damage, were not assessed as plots were sprayed regularly with B. thuringiensis, masking any affect of tomato on larvae. In another study,: numbers of P. xylostella larvae and pupae were reduced by intercropping cabbage with tomato, barley, dill, garlic, oats or safflower (Talekar et al., 1986).
Kenny and Chapman (1988) assessed an intercrop of cabbage and dill (Anethum graveolens L.). The number of cabbage aphids on cabbages planted near dill was lower than those planted without dill. Results for numbers of P. rapae and P. ,xylostella and damage measurement were inconsistent due to low pest populations. Competition from dill was found to reduce yield, but a different planting arrangement could overcome this problem.
Companion plants may be defined as plants which, when grown in the company of another, improve its performance. Insect repelling plants are one example of a companion plant. Much anecdotal information exists about the compatibility of different plant species. Observation that some plants grow well when planted close to other plants of a different species has resulted in active companion planting, particularly in home gardens. To date, few scientific studies of companion planting have been made anti those which exist have been performed on a small scale.
Some plants are thought to have insect repellent properties towards cabbage white butterflies and other pests of crucifers. The following herb species are listed by Little (1989) as cabbage insect pest repellents: sage (Salvia officinalis L.), rosemary (Rosemarinus officinalis L.), hyssop (Hyssopus officinalis L.). thyme (Thymus vulgaris L), dill (Anethum graveolens L ), southernwood (Artemisia abrotanum L.), mint (Mentha spp.), tansy (Tanacetum vulgare L.), chamomile (several genera) and orange nasturtiums (Tropaeolum minus L.). As well there are some economic crop species which are insect repelling, such as celery and tomatoes (Philbrick & Gregg, 1982 Little. 1989; Kenny & Chapman. 1988).
In a study of the effect of companion planting on cabbage pests (Latheef & Irwin, 1979) in the U.S.A., no significant differences were observed between treatments and the control with respect to number of eggs' larvae and pupae and no reduction of insect damage was indicated. The companionate plants used were French marigold (Tagetes patula L.)' garden nasturtium' pennyroyal (Mentha pulegium L.). peppermint (Mentha piperita L.) garden sage and thyme. Many of these herbs have only been resorted n~ h~vino Ah; repelling properties.
Similar trials were conducted in San Jose, U.S.A. by Koehler et al./.,(1983b) using anise (Pimpinella anisum L. ), basil ( Ocimum basilicum L. ), thyme' sage nasturtium marigold, catnip (Nepeta catania L.) and summer savory (Satureja Montana L.). Herbs were planted in a ratio of 4:1 cabbage. Significantly fewer eggs of P. rapae were deposited on cabbages growing near anise. Several species (nasturtium. marigold and catnip) reduced numbers of P. rapae larvae by a small amount' but damage levels were not reduced. Substantial reductions in cabbage yields due to competition with companion plants negated any beneficial insect repelling effects.
Latheef and Ortiz (1983a) concluded that interplanting collard plants (Brassica oleracea cv Morris heading) with a mixture of herbs, encouraged oviposition on collards by P. rapae. A mixture of herb species was used in an attempt to increase any insect repellent effect and to disorientate insects preventing location of the host plant. Collards in treatments containing herbs consistently received more eggs than collards in monoculture.
There were inconsistencies observed, however, with respect to oviposition host preference (Latheef & Ortiz, 1983b). Hyssop, recognised as an insect attractant by Philbrick and Gregg (1982), Woodward (1986) and Hylton (1974) was most attractive to ovipositing P. rapae in summer and wormwood (Little, 1989) was most attractive in autumn. Conversely, in spring and autumn, Latheef and Ortiz (1983a) found that of the herb species tested; wormwood was least attractive to oviposition.
Further studies of companion planting on a large scale are required to assess the potential of herbs as crop pest repellents.
Cultivation of plant varieties which have some degree of resistance to insect attack minimises the need for insecticide applications. Radcliffe and Chapman (1966) performed field studies on commercial cabbage cultivars in Wisconsin and discovered that a colour related factor appeared to be important in determining host preferences for P. rapae. Red cabbage varieties were less susceptible to oviposition than any of the green varieties evaluated. Differences in susceptibility to oviposition by P. rapae were also observed between the green cabbage varieties. Radcliffe and Chapman (1966) could demonstrate no correlation between size and ovipositional preference and concluded that susceptibility may rely on genetically determined chemical or visual stimuli.
Dickson and Eckenrode (1975) undertook genetic studies of cabbage cultivars in a search for resistant sources from which they could transfer resistance to T.ni (the cabbage looper) and P. rapae into desirable cabbage and cauliflower lines. They found that environmental factors and degree of maturity influenced resistance factors, but also selected several varieties as potential resistance breeding stocks.
Use of host plant resistance in a pest management system, however, does not have to involve many years of plant breeding and screening (van Emden, 1982). Elimination of highly susceptible varieties and use of existing varieties with some degree of resistance will reduce the need for insecticide application as will adjusting the growing season to avoid periods of intense insect attack. Creighton et al. (1981) used a combination of cabbage cultivars with a degree of insect resistance and B. thuringiensis treatments to obtain a level of control of P. rapae, T. ni and P. xylostella which was better than either method alone.
There are a number of alternatives to synthetic insecticides available for control of lepidopterous and other pests of crucifers. Some such as the plant based products may be used in a similar way to insecticides but their use still requires careful consideration as some may have detrimental effects on nontarget insects, insect predators other arthropods and fish. It is also sound practice to use when necessary rather than as a preventative measure to maximize efficacy and to delay establishment of resistance.
Some of the options such as biological control with predators and parasitoids involve an acceptance of low levels of the pest larvae to maintain predator populations. The future use of microbial biological control agents will overcome this as they can be applied when and where the pest occurs.
Techniques such as the use of cultivars with some resistance to pests can be adopted with little change to conventional methods of' growing vegetables. Many of the other options, however, are valuable as components of' a pest management program which would involve changes in cultural practices.
Few of the available non-chemical insect control methods have been adopted by farmers (Vereijken 1989). There are a number of reasons for this: some non-chemical control methods are complicated and time consuming being perceived as less reliable, professional and/or effective, and often as more expensive than insecticides. Some of them require more thought and planning than a regular spray programme. In general chemicals are effective and farmers are accustomed to using them. These reasons are not insurmountable, but probably require a more intensive extension of' research.
There are still many situations where insecticides are presently required: (i) where no alternatives are known or have been tested; and, (ii) in cases of very severe outbreaks. In these instances we should endeavor to use insecticides more efficiently by calibration of spray equipment by correct timing of' applications and by use of' the recommended rate.
Further research is required into developing pest management systems in particular areas using alternatives to synthetic insecticides. Future research into reducing chemical use is likely to involve genetic engineering of plant varieties to enhance resistance to pest attack, development of' biological controls and testing of action thresholds.
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