Wasps are unlikely to win many popularity contests and genetic engineering has plenty of detractors too, but both offer very efficient ways of controlling a major pest affecting maize crops in Europe.
Their usefulness is explained in a new guide, Non-chemical control of corn borers using Trichogramma or Bt maize , produced by ENDURE’s maize case study team. As the title suggests, the guide examines non-chemical controls for corn borers, the larvae of which can cause crop losses of up to 30% in badly infested plots. Part of the From Science to Field series, the guide provides practical, scientifically sound advice particularly useful for agricultural advisers and extension services.
Written by researchers from Switzerland, Spain and France, the guide focuses on non-chemical ways of dealing with the European corn borer (Ostrinia nubilalis or ECB) and the Mediterranean corn borer (Sesamia nonagrioides or MCB).
ECB is widespread in Europe (and indeed is found in Canada and as far west as the Rocky Mountains in the USA), with the small nocturnal moths laying clusters of between 10 and 40 eggs on the lower surfaces of maize leaves. Larvae chew leaves and tunnel in the stems of maize plants, weakening them and causing them to break. Furthermore, maize ears can also be damaged and the wounds caused by feeding leave crops vulnerable to fungal diseases. The pest has one generation per year in Northern Europe, and two to three generations in Southern Europe.
In Mediterranean countries, MCB may cause the most serious corn borer damage. Its lifecycle is similar to that of ECB, but females lay their eggs between the sheath and the stem of maize plants. Larvae enter the stem just after hatching, protecting them from chemical insecticides.
And chemical insecticides are not a straightforward solution for ECB either, say researchers, despite the fact that between 0.7 and 0.9 million hectares of maize crops are treated with insecticides against corn borers every year across the European Union.
One problem is the fact that contact spray insecticides or on-plant micro-granulates are only effective if applied before ECB larvae enter the maize stems. Furthermore, maize plants will often be 1m tall or even higher at this stage, making special equipment necessary. In addition, commonly used insecticides are known to have adverse effects on non-target arthropods, including natural enemies and pollinators.
A century of Trichogramma
ENDURE’s researchers have identified two efficient alternatives to chemical controls for corn borers. One of these, using the release of Trichogramma wasps, is a biological control that is also proving useful for protecting other crops such as apples and vegetables, and has a long history in agriculture.
Trichogramma species are microscopic wasps - less than 1mm in size or smaller than the full stop at the end of this sentence - that have been used to seek and destroy the eggs of the most destructive caterpillar pests for more than 100 years. More than 140 species of Trichogramma have been identified around the world, and they are found in almost all habitats. Less than a dozen species have been mass-reared and used in crop protection programmes, a process that began in the early years of the last century.
In countries such as the USA, interest in using Trichogramma faded somewhat with the creation of effective chemical pesticides, says Professor Allen Knutson, of Texas A&M University System and author of The Trichogramma Manual . Interest remained strong in countries such as the former Soviet Union, where more than 700 Trichogramma factories were once in existence, though these have largely disappeared over the past 20 years.
Trichogramma species are currently released on more than 10 million hectares each year, according to Professor Jeff Bale of the UK’s University of Birmingham and his co-authors, in their paper Biological control and sustainable food production , published in Philosophical Transactions of the Royal Society. These releases are used to combat, for example, pests in vegetables, cereals and cotton in Russia, and maize, cotton, sugar cane and tobacco crops in Mexico.
Trichogramma actively seek out and parasitize ECB eggs. Put simply, female wasps search for ECB eggs and then lay an egg inside. These develop into larvae which feed on the ECB eggs and thus kill them. It is a tactic that is easy and economical to apply, say ENDURE’s researchers. Unfortunately, MCB eggs are not attacked by Trichogramma , but by other egg parasitoids which cannot be mass reared and this means other tactics are needed where MCB pressure is high (see Bt maize below).
ENDURE’s researchers identify Trichogramma brassicae as the most effective species and the guide includes tips on choosing the best time to release them. Release is a simple process: cards containing Trichogramma eggs are attached to maize plants with each farmer or worker able to treat an area of between three and five hectares an hour.
ENDURE’s researchers suggest that between 100,000 and 225,000 wasps be released to counter the first generation of ECB, using 25 to 50 release points per hectare. Wasps can also be used against the second generation of ECB. In areas of high infestation, warm temperatures and taller plants, the release of between 225,000 and 600,000 wasps from 50 release points may be necessary.
Farmers, of course, will have two burning questions before adopting this tactic: is it effective and is it economical? According to ENDURE’s researchers, the answer is yes on both counts. As long as manufacturers’ recommendations are followed, efficacy is comparable to the use of chemicals, they say, with more than 75% of ECB eggs commonly parasitized and killed. In areas subject to second and third generations, good control of the first generation is crucial to achieving the best overall results.
In terms of costs for farmers, this will depend very much on which country the farm is based, the distribution system for Trichogramma and the doses required. In France, for example, they calculate that it will cost a farmer between €35 and €40 per hectare, not including labour costs, to treat first generation ECB. Treating the second generation will cost between €45 and €55 per hectare. Costs for chemical insecticides average between €20 and €40 per hectare in France.
So far, the adoption of Trichogramma for control of ECB has mainly been in France, which is Europe’s largest producer of maize, Germany and Switzerland, with wasps released on around 150,000 hectares per year.
Bt maize: controversial but very effective
While biological control is broadly welcomed, the same cannot be said of genetically engineered crops, which remain banned in several European countries. Nevertheless, Bt maize was grown across some 108,000 hectares in Europe in 2008, Spain accounting for three-quarters of the total.
The reasons for its successful adoption in Spain (and widespread popularity in the USA) are not hard to find. Bt maize has been genetically engineered to produce an insecticidal protein from the bacterium Bacillus thuringiensis (hence the Bt ). B. thuringiensis is a naturally occurring bacterium of particular interest to scientists and the chemical industry as it forms crystals known as cry toxins, which are effective in killing certain pests such as lepidoptera (moths and butterflies) while harmless to other creatures, such as humans and farm animals.
B. thuringiensis has been used to control insect pests since the 1920s and is currently used to produce chemical insecticides such as Dipel and Thuricide. In the case of Bt maize, the insecticide is produced over the whole growing season in the whole plant, providing efficient control (almost 100%) of stem borers such as ECB and MCB.
Bt maize has been approved in the European Union since 1998, though its cultivation is currently prohibited in countries such as France, Hungary, Austria and, most recently, Germany, where a ban was introduced this year.
ENDURE’s researchers have examined many laboratory and field trials which, they say, reveal no detrimental effects of Bt maize on beneficial arthropods, such as natural enemies, soil organisms or pollinators. They further note that the currently available Bt maize varieties produce low toxin concentrations in pollen, minimising the risk for moths and butterflies outside the maize field. Furthermore, because maize originates in America it has no wild relatives in Europe and so there is no risk of out-crossing.
To ensure the coexistence of conventional cultivars with Bt plants, researchers note that minimum distances, set by each country, must be respected by farmers and that a certain percentage of conventional maize must be sowed to reduce the likelihood of resistance developing. In Spain, for example, the percentage is 20% for any field larger than five hectares.
In terms of costs, farmers have to pay a ‘technology fee’ for Bt maize seeds which varies according to the seed company and region. They note that in the Lleida region of Catalonia, Spain, where there is medium to high corn borer pressure, Bt maize is €40 to €45 per hectare more expensive than conventional maize. However, farmers do not incur any extra costs for labour, machinery or chemicals.
For more information
If you found this article interesting, you may want to consult the following articles from the ENDURE archives: