June 1997, Volume 18 No. 2

Biorational

Integrated pest management (IPM) involves the use of many techniques, including biological control, to provide effective control of pests with minimum harmful side effects. Those techniques which are compatible with the use of biological control or have little impact on natural enemies have been described as `biorational'.

Intercropping That Does Protect Cereals

Some striking results from field trials in Kenya indicate that intercropping cereals with forage crops may be a viable way for subsistence farmers to protect their crops from the ravages of stem borers, and laboratory in-vestigations are beginning to eluci-date the mechanisms of this.

Lepidopteran stem borers, part-icularly the indigenous noctuid Busseola fusca and the introduced Asian pyralid Chilo partellus, are serious pests of cereal crops in East Africa. They cause severe crop losses and even crop destruction despite the existence of a large complex of natural enemies, including two larval para-sites, the indigenous Cotesia sesamiae and the introduced species C. flavipes, and the pupal B. fusca parasite, Dentichasmias busseolae.

The Gatsby Charitable Foundation, which finances collaborative projects between international research centres in Africa and advanced research centres in the UK, has been financing a programme to look at ways of controlling stem borers in subsistence cereal cultivation in East Africa. The programme, which John Pickett of the Institute of Arable Crops Research at Rothamsted, UK describes as putting a scientific basis to intercropping, involves teams of scientists from the Kenya-based International Centre for Insect Physiology and Ecology (ICIPE) led by Z. R. Khan, the Kenya Agri-cultural Research Institute (KARI) Regional Research Centre at Kitale headed by Reuben Botaki, and IACR led by John Pickett.

ICIPE have created an extensive germplasm collection and field herbarium of forage plants, and these have proved to have very different susceptibilities to stem borer attack. By bringing together the resources provided by this collection and the expertise of IACR as a major centre for statistically based field trials, Lesley Smart of IACR and Z. R. Khan were able to design field trials to test the effects on stem borer populations of interplanting maize with some of these forage plants. The trials were conducted at two sites in western Kenya: at ICIPE's Mbita Point Field Station close to Lake Victoria and at KARI, Kitale in the highlands.

Many previous studies have indicated that intercropping can produce a dilution effect for crop pests, but the results of these trials indicated something more complex and more interesting. Some species, such as the legume Desmodium sp. and molasses grass (Melinis minutiflora) actually drove off the stem borers, and the repulsion was sufficiently strong to reduce substantially stem borer populations in the maize intercrops. Other species, such as Napier grass (Pennisetum purpureum) and wild sorghum (Sorghum sudanensis), acted as trap crops; they proved more attractive than maize to stem borers, and markedly reduced populations in the cereal crop. In the Napier grass system, the stem borers moved into the forage intercrop and oviposited, but, significantly, the larvae failed to mature; parasitism levels in the stemborer populations appeared normal, however, and this is still being studied.

Other unexpected but useful side-effects of some intercropping regimes were also observed; for example, Napier grass was highly effective in preventing lodging in the tall maize varieties grown at Kitale.

Work in the laboratory is underway to elucidate the mechanisms being exploited and to identify the semio-chemicals responsible. Antennal electrophysiology by Christine Woodcock and Lester Wadhams and gas-chromatographic mass spec-trometry (GCMS) by John Pickett at IACR, together with behavioural, GC and further GCMS studies by Ahmed Hassanali and Wilber Lwande at ICIPE have identified some of the compounds involved in the attraction/repulsion of stem borers. Behavioural responses to plant-produced compounds have also been identified in the parasites, notably to compounds produced as a result of stress or damage.

The goal of this analytical work is not only to be able to exploit the chemicals for crop breeding, but also to provide a marker of resistance for quality control, for example when multiplying up seed. The next stage in the project is to look at intercropping maize and sorghum with both trap and repellent crops together, and to work out the best system and ratio for this. Also, to look further into the role of parasites in the system: their responses to the semio-chemicals and ways of manipulating this.

All of the intercrops above can be used as forages in a mixed cropping system, although livestock may need a period of acclimatization to some of them. Subsistence farmers in this part of East Africa may be cultivating plots as small as 0.5 ha, but the strategy of these intercropping systems can be applied equally well to small or larger farms.

Farmers have already been making visits to the field sites, and, according to John Pickett, their enthusiasm has been spectacular. Certainly, the results so far suggest that such intercropping systems may provide the basis for creating a robust and sustainable system for mixed cereal and forage subsistence farmers in East Africa.

Plutella Resistance to Bt Toxins

Research published in the Proceedings of the National Academy of Science, USA, led by Bruce Tabashnik at the University of Hawaii in collaboration with the Biotechnology Research Institute (Montreal, Canada) and Clemson University (South Carolina, USA) has suggested that resistance to the widely used biopesticide, Bacillus thuringiensis (Bt), could evolve much faster than previously anticipated. There are many different Bt toxins produced by numerous strains of the soil bacterium, and it has been assumed or at least hoped that resistance to each of them would have to evolve independently and would be a complex and long-term process. Last year, Plutella xylostella (diamondback moth) became the first pest to develop Bt resistance in a field population, and now Tabashnik has discovered that, in this species at least, a single and apparently common recessive gene confers resistance to four different Bt toxins (Cry1Aa, Cry1Ab, Cry1Ac and Cry1F).

Tabashnik performed crosses between a field population that had been heavily exposed to Bt toxins and a susceptible laboratory population with a long history of non-exposure. He found that the offspring of survivors of exposure to just one toxin were resistant to all four, the results suggesting that the resistance may have been the con-sequence of a single gene mutation. He also found that 21% of individuals in the susceptible laboratory population were heterozygous for the multiple resistance gene, compared with an expected frequency of 1 in 10,000.

The implications of these results for the future of transgenic crops are immense. Biotechnology has facilitated the transfer of Bt genes into many crops in an effort to confer pest resistance on them. The Bt toxins Cry1Ab and Cry1Ac were the basis of pest-resistance in transgenic maize and cotton, respectively, planted for the first time last year in the USA. Some larval survival of Helicoverpa zea occurred in cotton in both the USA and Australia (see BNI 17(4) and `The Trials of Transgenic Cotton', this issue) and, although survival may have had nothing to do with resistance to Bt this time, in the long term larval survival could become a significant factor in the selection for more Bt-resistant pests. Furthermore, the discovery of multiple resistance is significant for the management of polyphagous pests such as H. zea, a pest in both cotton and maize; exposure of a population to the one transgenic crop could result in selection for cross-resistance to the other. The need for the development of effective resistance management strategies would appear to be vital and urgent or the valuable resource of Bacillus thuringiensis toxins in pest control may be lost.

These thoughts will have been in the minds of those who attended the US Environmental Protection Agency (EPA) public meeting on resistance management plans for transgenic pest-resistant crops in Washington DC on 21 March. The meeting sought comment on whether such prog-rammes should be a voluntary or mandatory condition of registration. There are some signs that the danger of the development of pest resistance is being taken seriously by both government and pesticide companies in the USA, and, for example, the inclusion of refugia non-transgenic crops in cropping schemes is com-mon. Resistance management plan requirements are being made of some US EPA approvals. Monsanto has been granted approval for maize hybrids expressing the YieldGard gene for lepidopteran pest resistance, but this is restricted in the southern USA where cotton is also grown, and where cotton bollworm/corn ear-worm (H. zea) movements between cotton and maize need to be studied further. Monsanto was required to implement a resistance management plan by 2000 in its transgenic maize crops, but they have opted to instigate it from the outset.

The Trials of Transgenic Cotton

Some 8% of the Australian cotton farmers growing Monsanto's trans-genic pest-resistant cotton have experienced fruit loss, and some larval pest survival has been noted. Monsanto have suggested that cold weather may have inhibited the expression of the Bacillus thuringiensis gene which gives it resistance to the cotton bollworm (Helicoverpa zea), and have lowered the population thres-hold for supplementary spraying. Gary Fitt of the CSIRO Cotton Research Unit says that the strategy for managing H. zea on transgenic cotton has always assumed that some larvae will survive and that, overall, the use of pest-resistant transgenic cotton crops in Australia has reduced pesticide applications by 70%.

This mirrors events in the USA last year where some 13% of the cotton-growing area in the southern states was planted with Monsanto's Bollgard cotton, targeted principally against the major cotton pest there, Heliothis virescens (tobacco budworm) but also Helicoverpa zea (cotton bollworm) and pink bollworm (Pectinophora gossypiella). In some areas, and in particular Texas where 2% of the crop area was planted with transgenic cotton, there were outbreaks of the secondary pest H. zea to which the transgenic cotton is less resistant. Although this species is common in cotton its numbers are rarely high enough to cause signifi-cant damage, and pyrethroid and other conventional insecticides used to control H. virescens are normally effective against it. There were queries over the efficacy of expression of the B. thuringiensis gene, the development of resistance in H. zea, and even the provenance of some of the seed (see BNI 17(4)). Monsanto say that in the cotton-growing belt as a whole, 60% of farmers did not use conventional insecticides in transgenic crops at all last year, and most others only sprayed once. The results for the year have indicated that cotton yields across the USA were up; in Alabama which had the highest area percentage of transgenic cotton at 77%, the yield was the second-highest ever recorded. Countrywide and overall, farmers reported 7% higher yields in trans-genic crops compared with con-ventional crops, while pesticide applications in the cotton sector were down by 34.6%, and by 48.5% late in the season; this decrease was attrib-uted partly to the use of transgenic varieties, but also to lower areas of cotton planted last year together with lower pest pressure than the previous season. A survey by Monsanto of 2200 cotton growers who used the new hybrids found 80% of them to be pleased or very pleased. Monsanto are increasing the range of transgenic cotton products available in the USA this year, with glyphosate-resistant Roundup Ready being added to nine pest-resistant Bollgard varieties, and there will also be a small amount of seed expressing both traits.

Meanwhile the US Environmental Protection Agency are in the process of deciding whether or not to renew a conditional registration which would allow Rhone-Poulenc's Buctril (bromoxynil) to continue to be used in Calgene's herbicide-resistant transgenic BXN cotton varieties, but they are facing opposition from an environmental group, the Environ-mental Defense Fund, who claim that bromoxynil is more hazardous than previously recognized. Calgene's subsidiary Stoneville Pedigree Seed plans field trials this year of transgenic varieties combining resistance to Buctril with resistance to pests from Monsanto's Bollgard gene.

Food for Beneficials

A novel way of conserving beneficial insect species in cotton by giving them a food supplement is being developed in Australia by Robert Mensah of NSW Agriculture together with Rhone-Poulenc Rural Australia Pty Ltd. Discovered by Mensah, `Enviro-feast', is made up of complex carbo-hydrates and proteins; it is an attractant, but, depending on the concentration used, can also be a food source to sustain beneficial insects between pest outbreaks. Mensah says that the product attracts a wide range of beneficial species including `lacewings, ladybirds, big-eyed bugs and damsel bugs', and works well in combination with a permanent refugia crop, such as lucerne, inter-planted with the cotton. Field trial results indicated that when the Envirofeast/refugia system was used in combination with nuclear poly-hedrosis viruses (NPVs) against Helicoverpa species in cotton, the number of synthetic insecticide sprays could be reduced from eight to two without sacrificing yield.

For more information contact: Robert Mensah, NSW Agriculture, Australian Cotton Research Institute, PMB Myall Vale, Narrabri, NSW 2390, Australia.

Tel: +61 67 991525;
Fax: +61 67 931186;
E-mail: robertm@mv.pi.csiro.au

European Union Maize

Following the European Union's authorization of transgenic maize from Swiss-based Novartis (formed by the merger of Ciba and Sandoz), some European farmers may be planting the engineered hybrids this year. Owing to the incorporation of a gene for Bacillus thuringiensis toxins, this transgenic maize has resistance to the European corn borer, Ostrinia nubilalis. It is also glufosinate-resistant and contains an ampicillin-resistance trait, the result of a beta-lactamase enzyme marker gene. Ironically it is this gene, originally incorporated to provide an indication of which plants had taken up the pest- and herbicide-resistant genes, that has caused most of the controversy as both scientists and politicians have failed to agree over whether or not there is a risk of the gene being passed on to animals and man. Novartis are hoping to sell the seed in France, Italy, Spain, Portugal and southern Germany where the pest can cause 20% losses, but although the German government may approve planting of transgenic maize this year, France and Italy have both banned its cultivation, and the chairman of the French government's advisory committee on biomolecular engineering, Axel Kahn, has resigned arguing that the French government are being inconsistent. In addition, while the UK government has cleared four lines of transgenic maize for import and use in processed human and animal feed, Austria and Luxemburg have banned such im-ports altogether.

The situation is very different in the USA where corn-borer resistant maize was introduced last year, and there has been a large increase in the number of transgenic hybrids on offer. Farmers who planted them last year were reported to be impressed by their performance as conventional hybrids were hit heavily by pest damage. Novartis have 22 NK and Maximiser hydrids this year, while Mycogen are offering seven NatureGard hybrids based on the Novartis germplasm.

Aubergines Out

A transgenic aubergine crop trial being conducted by the Indian Agricultural Research Institute (IARI) in New Delhi was ordered to be destroyed by the Indian Department of Biotechnology, who said that the trial was unauthorized and cont-ravened safety guidelines, according to a report in Nature (2 January 1997). The Department argued that the crop, which contained Bacillus thuringiensis genes intended to confer insect pest resistance, should have been tested in greenhouse trials first, and that the field trials constituted a danger to conventional aubergine crops of contamination by insect- or wind-borne pollen from the transgenic crop. The leader of the IARI team, R. P. Sharma, said that the transgenic crop was protected from insect pollinators by a giant mosquito net and no other aubergines were growing in the vicinity. Fears have also been ex-pressed that details of investigations being conducted with the gene at 10 other institutes are lacking.