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Sending out signals to attract insects that eat pests may be the way to sustainable agriculture , says S.Ananthanarayanan.
As crop plants are bred and refined for features of better yields and also to be hardy and resilient, they are losing out on a network of natural safeguards that wild plant varieties employ for pest control. Great effort, expense and damage to the environment then become necessary, for a purpose that the plants had within themselves before they got specialised.
Johan A. Stenberg, Martin Heil, Inger Åhman and Christer Björkman, from the Swedish University of Agricultural Sciences and the Department of Genetic Engineering of the National Polytechnic Institute in Mexico describe in the Elsevier journal, Trends in Plant Science, their study of how wild plant varieties are able to attract and retain insects, which do not harm the plants, but feed on species of pests that do. Even if re-engineering crop plants to get back the lost function is too challenging, the researchers suggest planting wild varieties along with crop plants or spraying crop plants with the active agents that wild plants use.
Numerous studies, they say, have established that wild plants generate volatile organic compounds (VOC) to produce odours that announce to predators that leaf-eating organisms, which endanger plants, have appeared on the scene. As generating VOC consume scarce resources, plants generally produce them at the onset of damage by herbivores. "Even as humans we smell it when our neighbour is mowing the lawn - odors can carry very precise information," says co-author Martin Heil of CINVESTAV-Irapuato in Mexico The VOCs released are of different kinds and can indicate the nature of damage and the type of the attacker, so that specific predators associate odors with the herbivore involved, as an efficient defense mechanism for the plant. And then, to hold the interest of the predators that the VOC invokes and get them to stay on, many kinds of plants also exude nectar on different parts of the plant body. These food rewards are carbohydrate rich, which favour the nutrition needs of predators, rather than the herbivores, and also serve to keep the predators both energetic and to discourage conflict among themselves, the paper says.
In addition to VOC and food rewards, the paper says, plants also attract and maintain the carnivore (as opposed to herbivore) creatures by providing shelter, like cavities for ants or mites, which are more often used by the predators. In general, the paper says, plants that generate extra floral, i.e. other than in the flower, nectars (EFN) and produce large amounts of pollen or provide shelters for predators show stronger resistance to attack by herbivores.
EFNs have also been found to impart advantages even by themselves. For one, they are direct antimicrobials and immediately protect both an injured plant and its neighbours from disease. Volatile Salicylic Acid, for instance, regulates plant resistance to infection and certain forms directly inhibit growth of bacteria and fungi. This versatility of EFN action hence marks EFN as a valuable trait that could be induced in crop plants
Lost protection
While this kind of ‘bio-control’ of pests is observed in plants in the wild, the features seem to have been bred out in domesticated, crop varieties. “Crop domestication aims to enhance the quality of plants for human use,” the paper says. “In addition to yield, domestication most commonly has altered the size, taste, and nutritional quality.…favoring synchronous ripening, homogenous plant sizes…..or other characteristics of relevance for cultivation and harvesting and traits that facilitate transport and storage…. enhanced resistance to pathogens or other forms of stress representan integrated goal in most plant breeding programs,” the paper says. In the process, breeding for the trait of resistance to herbivorous pests has been neglected. The paper, in fact, cites an instance of breeders and governmental agencies having consciously preferred cotton plants that did not exude EFNs because presence of colonies of insects on the plants was considered undesirable!
Many of the traits that confer direct resistance to pests have been counter-selected during domestication, either because they depend on undesirable properties such as bitterness, hairiness, toughness, or toxicity, and thus reduce the quality of the consumed parts, or because they cause a yield penalty because of the metabolic costs of resistance expressionl
The result is that we have huge acreage under crops with high yield and resistance to drought or flooding, traits which benefit crop-eating pests as much as they help us, but without the protective features that were native to the ancestors of the domesticated plants. This has encouraged efforts to put these features back into crop plants, a move towards ‘rewilding’. But features like secretion of a range of VOC or EFN involve multiple genes and the trail of their being bred out, over generations, cannot be retraced. “Whereas disease resistance is frequently based on gene-for-gene resistance, and thus depends on the presence of a single and usually dominant gene, the blends of VOCs that exert biological functions are complex,” say the researchers. Classical breeding strategies, like crossing, mutation, breeding by ‘qualitative trait loci’ are hence unlikely to be successful in getting domesticated plants to regain the lost capacity.
Practical bio-control of pests has thus used methods like conserving the existing population of predators, attracting them from adjacent areas, or physical release of predators onto crop plants infested by herbivores. This approach, however, has not gone far in all areas, because the carnivores tend to disperse to neighbouring ecosystems, or they may starve once the herbivores are consumed, or, if they are used as a preventive, they may need to consume the plant matter for themselves. Artificial release of VOCs to invoke carnivores also presents problems. For one, VOCs are signals not only to carnivores but to herbivores too. The strategy may hence turn counter-productive! VOCs also have a wide range of effects on the plant, some of which may not be desirable. And again, their function depends on a complex of physical conditions, like temperature, humidity, which a wild plant takes into account, but is still not understood for artificial use of VOCs. And finally, VOCs may call in carnivores when there are not enough herbivores, which would affect the fitness of the carnivore and also lead to the carnivores ‘unlearning’ the response to the VOCs.
One way natural VOCs are used to control pests is by ‘intercropping’ with plant species that emit VOCs to mimic herbivore damage of the main crop plant, and hence attract, or ‘pull’ carnivores. Or, there could be plants that exude repellent VOCs to expel or ‘push’ herbivores away to neighbouring fields. Another strategy, of using artificial VOC dispensers has been refined to the ‘attract and reward’ scheme (see pictures), where a combination of VOCs and EFNs provide higher efficiency.
There is still much work to be done and ground to be covered, the authors of the paper say. The existence of the volatile and odor related communications network in ecology has been recognised only some thirty years ago and work larger breeding programmes as well as new methods to leverage biological methods for viable and sustainable pest control need to be developed, they say.
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