Biorational

Integrated pest management (IPM) involves the use of many techniques, including biological control, to provide effective control of crop 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'.

Bollworm Round-up

Few insect pests attract more attention from pest managers than Helicoverpa and its relatives which attack fruits and bolls in a range of subtropical crops around the World. Here we report some recent developments in their biorational management.

From Australia: Tests...

Scientists at the Cooperative Research Centre (CRC) for Tropical Pest Management (CTPM) in Queensland are developing the use of viruses and parasitoids as part of an IPM package to reduce reliance on conventional insecticides for control of Helicoverpa in sorghum and grain legumes. The research programme includes the development of rapid diagnostic techniques for diseased and dead Helicoverpa, and exploration of the interactions between the viruses, parasitoids, Helicoverpa and host plant.

The diagnostic tests are being developed for two parasitoids (the egg parasite Trichogramma australicum and the larval parasite Microplitis demolitor) and two viruses (a nuclear polyhedrosis virus (NPV) and an ascovirus transmitted by M. demolitor). The benefit of these tests is the rapidity with which the results are obtained; results are available within six and 24 hours for the egg and larval parasitoids, respectively. This timely information will help farmers improve their management decisions. The results from a single sample of Helicoverpa larvae will tell them the level of each natural enemy present, and this may enable them to delay or even avoid using chemical insecticides altogether.

The tests will also aid studies being conducted on the impact of the natural enemies of Helicoverpa in a range of crops and to develop their roles in an IPM strategy. A mass production strategy for the NPV is being developed to provide enough viral material for the production of a biopesticide. The diagnostic test will ensure that genetic changes which might affect the efficacy of the biopesticide are detected, thus providing quality assurance. It can also be used to check naturally occurring viral material for efficacy.

Source: Newsletter from the CRC for Tropical Pest Management, IPM News, Autumn 1997 and Spring 1998.

IPM Strategies...

Promising results have been obtained from work being conducted in Queensland by Brad Scholz at Toowoomba Department of Primary Industry in collaboration with Chris Monsour of CTPM to develop IPM tactics for control of Helicoverpa armigera in maize. They compared the effects of (1) spraying four times with the synthetic pyrethroid deltamethrin after silking had commenced, (2) spraying twice with Bacillus thuringiensis (Bt) after silking had commenced, (3) making three inundative releases of Trichogramma nr. brassicae after silking had commenced and applying NPV twice, and (4) releasing Trichogramma as the sole management tactic. Lowest cob damage was recorded in the plots where NPV or Bt were used. There was no evidence to suggest that the inundative releases of Trichogramma increased egg mortality. There were, however, very high levels of egg parasitism owing to natural popula-tions of the egg parasitoids Trichogramma pretiosum and Trichogrammatoidea bactrae. The application of chemicals disrupted the effectiveness of these parasitoids, and also greatly reduced populations of predators (primarily pirate bugs, ladybirds and spiders). The combination of natural enemies and applied pathogens provided good control of Helicoverpa in maize, and is worthy of on-farm testing.

For further information contact: Brad Scholz, Queensland Department of Primary Industries, PO Box 102, Toowoomba, Queensland 4072, Australia

E-mail: ScholzB@prose.dpi.qld.gov.au

Marking NPVs...

One problem with NPVs is that they act slowly compared with insecticides; for example, the cotton bollworm Helicoverpa armigera can inflict significant damage on cotton before an NPV takes effect. As one part of the development of commercially viable viral insecticides, genetically engineered NPVs with enhanced insecticidal potential (speed of action) are being developed by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) in collaboration with Crop Care Australasia and Zeneca Agrochemicals Ltd (UK).

However, safety testing of such organisms must necessarily be stringent to take account of factors such as host range, environmental persistence, virus dispersal, trans-missibility and genetic recombination. As a precursor to this process, a naturally occurring NPV of H. armigera has been genetically marked so its movements in the environment can be more easily tracked, and this may be released at the Australian Cotton Research Institute in Narrabri (New South Wales) in 1998 if approved by the Genetic Manipulation Advisory Committee - and this would be the first release of a genetically modified virus in Australia. It is anticipated that studying the movement of this virus in the field will provide information useful in planning future field testing of modified NPVs.

...And Lastly, the Forecast

A collaborative Australian project between the Helicoverpa Inland Research Group, the CRC for Sustainable Cotton Production and the CTPM aims to develop a country-wide forecasting service for Helicoverpa based on geographic information system (GIS) analysis and satellite imagery to map breeding areas, and insect development and wind-assisted migration modelling to estimate emergence times and migration patterns. It will forecast migration of Helicoverpa spp. (punctigera and armigera) moths from winter breeding areas of inland Australia to summer cropping areas of eastern Australia.

The system will provide farmers and pest managers with long- and short-term forecasts of early season pressure from immigrating Helicoverpa. The timing and intensity of this varies from year to year, so the forecasting system aims to provide information that can increase the frequency of successful pest management decisions. The plan is to launch the service on the Internet, but the information will also be available by poll fax service. Long-term forecasts, giving predictions a few months in advance, are already available but this new system is designed to provide more precise information on migrations with one-day forecasts planned. Farmers will be able to use the information to plan when to survey for pests as well as when to expect pesticide resistance and when to spray.

This forecasting service will form part of a Heliothis Information Service, which was the subject of a workshop held in Brisbane in July, 1997. The proposed service would provide information on the current activity of both Helicoverpa species, research results, educational material on pest biology and recommended manage-ment strategies, as well as the forecasts. Current losses to Australian agriculture from Helicoverpa damage are estimated as at least Au$ 225 million per year, and if insecticide resistance were to increase this could reach Au$ 900 million. CTPM resource economist David Adamson predicts that the service could return at least Au$ 16 for every Au$ 1 invested. Participant support was unanimous, and efforts are now underway to secure trial funding.

Source: Newsletter from the CRC for Tropical Pest Management, IPM News, Autumn 1997 and Spring 1998. Also see the Internet: http://www.fassbinder.ento.ctpm.uq.edu.au/forecast/intro.html.

From India: NPVs...

A technology already becoming popular with farmers is being developed by the International Centre for Research into the Semi-Arid Tropics (ICRISAT) in Hyderabad to combat Helicoverpa armigera (here referred to as chickpea pod borer). The IPM strategy aims to combine the use of a neem-based insecticide and a host-specific NPV. In on-farm trials, NPVs were used to kill larvae, and the dead larvae were then crushed and mixed with water as a cheap insecticide. Farmers were given training in monitoring pest populations and recognizing threshold levels, and in preparing the viral insecticide. This was applied when population levels reached a threshold level. The results indicated that the NPV treatment is more effective than traditional and pesticide-based control methods, and work is continuing on integrating the botanical insecticide and the virus into a user-friendly package.

For further information see: Ecofriendly check to chickpea pod borer. International Agricultural Development, July/August 1997, p. 23.

...And from the USA: a Natural Insecticide

Recent advances in gene mapping techniques may result in a new method of pest regulation for the corn earworm/cotton bollworm, Helicoverpa zea, one of the most prolific pests of maize and other crops in the USA, according to a report in Agricultural Research. A natural pesticide, maysin, was first identified in the silk of a primitive race of maize by US Department of Agriculture - Agricul-tural Research Service (USDA-ARS) scientists at Tifton, Georgia in 1979. Maysin binds amino acids in the insect gut and effectively starves the insect.

Now new techniques in genetic mapping and manipulation have created a real possibility of building in resistance by boosting the production of maysin by maize plants. A gene has been identified that regulates more than half the amount of maysin produced, and ARS researchers at Tifton and at Columbia, Missouri are looking to increase the expression of this gene and to identify and manipulate other pathway genes.

If it works, insecticide use in maize could be reduced by up to 85%. Furthermore, by keeping H. zea populations in maize low, the numbers of this polyphagous pest that move from maize into other crops including cotton as the growing season progresses could also be reduced.

Source: Cooke, L. (1997) Maysin - a natural insecticide from corn silk. Agricultural Research 45(6), p. 20.

SIT Zaps Tsetses in Zanzibar

A model project begun in 1994 by the International Atomic Energy Authority (IAEA), the Food and Agriculture Organization of the United Nations (FAO) and the Tanzanian Government has successfully used SIT (the Sterile Insect Technique) to reduce dramatically the populations of the tsetse fly Glossina austeni on Zanzibar's main island, Unguja, and looks set to be able to announce its complete eradication - the Organization of African Unity (OAU) have said that "eradication appears inevitable".

Although G. austeni was not detected in Zanzibar until 1945 when it was trapped on Unguja, the tsetse-transmitted disease trypanosomiasis has been known to be present in livestock and wild animals on the island since early in the twentieth century. The constant threat of this disease had made cattle rearing almost unviable in some parts of the island. Glossina austeni does not attack humans but does attack livestock and wild animals, consistent with the epidemiology of trypanosomiasis in Zanzibar. It has remained the only species of tsetse found in Zanzibar, and both tsetse and the associated trypanosomiasis are confined to Unguja. An FAO project on Unguja in 1986-93 failed to eradicate tsetse with a combination of insecticide on cattle and artificial attractant devices, but a substantial reduction of the fly population was achieved.

At the inception of the SIT project, this insecticide/attractant strategy was continued to maintain the suppression of the population until sufficient sterile tsetse males were available for SIT, which relies on swamping the wild population with artificially released sterile males to reduce the fecundity of the wild females. The key to success is the production of huge numbers of (male) insects which are then sterilized with gamma-radiation and released. The FAO/IAEA laboratory at Seibersdorf, Austria maintained a back-up colony and initially shipped excess pupae to the project's sterile male production facility at Tanga on the mainland of Tanzania. But since 1996, all sterile male tsetse released in Zanzibar originated from the Tanga facility which now produces up to 250,000 puparia per week.

Initial releases of up to 20,000 sterile males were made on the ground in one heavily infested area, Jozani Forest. Test releases of sterile males from light aircraft were initiated in August 1994. Routine mass-releases twice weekly by air were pursued from May 1995, initially in the south of the island but later extending north, to allow more complete coverage of the entire tsetse-infested area on the island. Releases reached 40,000 per week in May 1995 and 100,000 per week in August 1996, by which time the releases were covering potential and known tsetse habitats over virtually the entire island. This targeting of the entire breeding population, the areawide concept, is crucial if eradication is to be complete.

Numbers of tsetse caught in the treated area fell dramatically within the first few months after releases began, and levels of sterility increased; in early 1995 about 25% of trapped females were sterile, reaching 60% later in the year. By 1996 captures of fecund wild female tsetse had become sporadic with none caught for several weeks at a time. As of mid September 1997, more than 52 weeks (one year) had gone by with no wild tsetse flies trapped.

For disease incidence, there are no directly comparable before-and-after data, but estimates before sterile releases began had indicated that trypanosomiasis prevalence in herds on Unguja averaged 17-25%, and in a few herds levels of up to 80% were found. Monitoring the level of trypanosomiasis using sentinel herds of cattle began in February 1994 in north Unguja and was extended to the south of the island in the November. By May 1996 levels had been reduced to 1.2% in sentinel herds in south Unguja, a marked reduction from the beginning of the experiment, and by September 1996 it reached zero. In July 1997, only one case of trypan-osomiasis was found in sampling of more than 1000 cattle.

Sterile male releases are scheduled to be discontinued in December 1997, but entomological and veterinary monitoring will be pursued for some time. Already, livestock production in Zanzibar is staging a recovery and is becoming more profitable. Farmers are approaching the Government for more productive cattle crosses between native zebu and exotic breeds.

For Zanzibar, tsetse and trypano-somiasis appears to be a problem of the past. But in many regions of sub-Saharan Africa tsetse flies not only infest huge areas but aggressively invade new areas under agricultural production, an advancement which must be halted. In view of the Zanzibar results, tsetse SIT is now being considered in a number of affected countries as a new tool of great potential for integrated areawide campaigns against tsetse and the trypanosomiasis problem. Among several options one is now being pursued: the Ethiopian Government has approached the IAEA to comple-ment their national tsetse/trypano-somiasis intervention methods with the SIT in a joint effort with other partners to eradicate tsetse flies from 25,000 km² of land with high agricultural potential in the southern Rift Valley. Preparations for the first phase of this programme which include the establishment of a national capacity for tsetse SIT and a feasibility demonstration over a 5000 km² area are underway.

For more information contact: Udo Feldmann, e-mail: U.Feldmann@iaea. org

Double Trouble for Medflies

Promising results from a collaborative project involving the governments of Guatemala and the USA, and the International Atomic Energy Authority (IAEA) have indicated that SIT (the sterile insect technique) and augmen-tative releases of a larval parasitoid of medfly (Mediterranean fruit fly: Ceratitis capitata) may have a synergistic effect on medfly populations in Guatemala. This offers the possibility of an alternative to insecticides for reducing medfly populations in advance of the release of sterile flies in this strategically important area for medfly control. However, given the scale of the area to be treated, for the method to be successful the comple-mentary rearing and releasing of huge numbers of parasitoids followed by similarly huge numbers of sterile male medflies is required, necessitating a vast combined rearing capacity.

Ceratitis capitata infests more than 350 species of fruits and vegetables and represents a barrier to trade and agricultural development in the tropics and subtropics because of quarantine measures imposed on its host crops. A number of parasitoids have been identified from medfly, and a guild established in Hawaii success-fully halved medfly populations there. Both SIT and augmentative releases of the braconid parasitoid Diachasmimorpha tryoni were shown to suppress populations even further, but concurrent releases of the parasitoid and sterile flies achieved the greatest reductions in medfly populations.

The medfly, previously eradicated from Mexico, now poses a serious threat to both Mexico and the USA. An international organization rep-resenting Mexico, the USA and Guatemala (MOSCAMED) has been responsible for some 20 years for maintaining a barrier along the mountainous Guatemala/Mexican border against the northward spread of the medfly, and this has tradition-ally relied on a combination of malathion baits to reduce populations followed by sterile insect releases. The MOSCAMED programme has involved the development of the most sophis-ticated and largest rearing facility in the World, currently able to produce 1200 million sterile medflies each week, including 120 million of a male-only strain. This rearing capability should be able to incorporate without problem the rearing of large numbers of medfly parasitoids.

A number of medfly parasitoids have established in Mesoamerica, but the rate of parasitism is low and sporadic. Preliminary studies identified D. tryoni as the most promising parasitoid for augmentative release in the Guatemalan highlands, but because of the mountainous terrain the develop-ment of an aerial release method was seen as critical to the success of any parasitoid mass release programme: the method adopted which gave good survival involved releasing chilled parasitoids in paper bags from an altitude of some 100 m.

In the experimental study, parasitoids were released at 1000-8000 individ-uals/ha/week onto a plot inside an area where sterile male medflies were also being released (at 4000 flies/ha/week). Medfly populations within this area, on plots with and without parasitoids, were compared with those of untreated plots. In this region coffee is planted extensively and is the dominant host of medfly.

Analysis of samples taken from the coffee indicated that there was indeed little endemic parasitism in the area, but where aerial drops of parasitoids had occurred parasitism rates of 15-84% were recorded during the peak period for medfly populations which was 11-18 weeks after parasitoid release. There was evidence of the parasitoid dispersing up to 1.5 km from the drop sites. Medfly popula-tions were significantly lower in both treated plots than in the control. Although there was no consistent difference in medfly population levels between sites with and without parasitoids, significantly fewer adults were caught in the parasitoid-augmented site during the medfly peak population period and it is suggested that this was the result of parasitoid activity.

For further information contact: John Sivinski, USDA-ARS, Gainesville, FL 32608, USA

[E-mail: jsivinski@gainesville.usda. ufl.edu]

or

Felipe Geronimo, USDA/APHIS/PPQ Methods Development, Guatemala Station/La Aurora Lab, Guatemala City, Guatemala [E-mail: ppq@guate.net].

Viral Genes in Transgenic Plants: a Cause for Concern?

Evidence is gradually accumulating from laboratory studies and field observations which casts doubt on the safety of plants carrying viral genes. It suggests that the possibility of recombination to form new and potentially dangerous forms is more likely than previously thought, and that there is a real risk of naturally occurring viruses being able to acquire genes from transgenic crops.

Using biotechnology, crops are given viral genes which confer resistance to the virus the genes come from, but according to a report in New Scientist (16 August 1997), the US Department of Agriculture (USDA) is increasingly concerned that this genetic material may recombine with wild viruses to create new hybrids and diseases, and USDA has outlined possible restrictions which may have to be introduced on genetically engineered crops. Possible moves being considered include restricting the length of gene sequences which can be introduced into crop plants and outlawing the use of genes that make functional proteins.

The report also describes work con-ducted by D'Ann Rochon of Agriculture Canada. She infected plants with a cucumber mosaic virus which lacked the gene for a protein that enabled it to move between cells, and then took a similar gene from another virus and inserted that into the plants too. She found that recombination occurred readily between the two viruses in a matter of days: one in eight of her test plants had functioning mosaic viruses, and this could only have been as a result of recombination. Traits such as these which affect the ability of a virus to move between cells or infect new plants are amongst those regarded as potentially dangerous.

Proponents of biotechnology have argued that such an event is unlikely to occur in nature. However, evidence from Africa would seem to question this. A new viral strain of cassava mosaic virus (CMV) has emerged in Uganda with devastating results: according to a report in New Scientist (30 August 1997) entire crops of this staple food have been lost and economic losses are estimated to be some US$ 60 million. Two related but distinct forms of this whitefly-transmitted disease (African CMV and East African CMV) have been known for a long time. According to a report by Bryan Harrison and colleagues in the August 1997 issue of the Journal of General Virology (78, p. 2101) the new strain contains DNA from both; it is almost identical to East African CMV except for the central section of the coat protein which comes from the African CMV. The authors say that this is the first example of interspecific recombination between identified plant viruses that has gone on to cause a major new epidemic, and that the new virus probably arose in the late 1980s in a plant infected by both strains of CMV.