Classic biological control offers a tested, inexpensive and pesticide-free approach to battling alien pathogens…
Bugs such as the Winter moth and Asian Chestnut gall wasp are the bane of valuable trees and the people who rely on and benefit from them.
Insect pests damage around 35 million hectares of forest each year, with particularly catastrophic impacts recorded when non-native species arrive in ecosystems where they have no natural enemies. The scale of impact is increasing with growing international trade and the effects of climate change.
Fortunately, over the last decades, the global community has accumulated considerable knowledge on the application of biological pest control. The introduction of natural enemies of invasive species from their country of origin has proven an effective tool to combat their expansion.
FAO’s new Guide to the classical biological control of insect pests in planted and natural forests distils information in a clear and concise way to assist forest managers in developing countries develop effective pest-control programmes.
“Classical biological control is a well-tried, cost-effective approach to the management of invasive pest species,” says Hiroto Mitsugi, Assistant Director-General of FAO’s Forestry Department.
For example, the introducing of Torymus sinensis, a parasitoid specific to the Asian chestnut gall wasp in China – which spread over Europe and decreased wood yields by 40 percent and nut yields by over 80 percent – proved effective, spreading on its own and killing more than three-fourths of the targeted galls, while leaving other native wasps alone.
The introduction of two parasitoids to the European winter moth – which during the early 20th century went on to ravage North America’s oak forests and cherry and apple orchard with tree mortality rates as high as 40 percent – have collaborated to contain the pest, one particularly effective during outbreaks and the other a persistent predator at lower densities.
Classical biological control does not eradicate an invasive pest species but works to establish a permanent, self-sustaining population of natural enemies that will disperse and suppress a pest population or reduce the speed at which it spreads. When successful, it enables reductions in the use of insecticides, with corollary benefits for human and environmental health.
The guide was launched during the Sixth Mediterranean Forest Week in Lebanon, and offers a host of case studies ranging from the coconut rhinoceros beetle – which devours coconut and oil-palm trees in the Pacific region – the great spruce bark beetle that migrated from Siberia to Western Europe -the Orthezia bug – an omnivorous insect that made it to the small South Atlantic island of Saint Helena and began to decimate gumwood trees – and several pathogens that are increasingly impacting eucalyptus trees.
In all cases, control efforts need to be based on scientific knowledge – often more accessible in a pest’s homeland compared to where it is causing problems – and full risk assessments, the protocols for which are explained in the new guide, along with biosecurity principles to be followed when scaling up interventions and while monitoring effectiveness. Also critical is robust communication to all stakeholders from the outset of a planned intervention.
Classical biological control is also useful for orchard trees, as demonstrated with the mango mealybug. Originally from Southeast Asia, it showed up in West Africa in the early 1980s, where it began sucking the sap of mango trees – highly valued for fruit and shade – secreting a substance that fosters mould and limits photosynthesis, leading to yield losses of as much as 89 per cent in Benin. Hostile parasitoids in the bug’s native habitat were identified and introduced, leading to spectacular results, with cost-benefit ratios for sub-Saharan Africa calculated at 1:808 in terms of the fruit’s value alone.