Fighting the foe and befriending the friend

by Sarah Reynolds

In a natural environment, plants and insects tend to be closely associated and insects, even in a natural setting, can be very damaging to certain plant species. Plants have therefore evolved a number of defence mechanisms to try and prevent or minimise this damage. In order to overcome these defence mechanisms, insects also evolve and there is a constant co-evolution, and therefore a balance, between insect and plant populations. When man interferes by simplifying the natural environment with agricultural crops, and by improving individual plant species for his own purposes, it is only the insects which can then evolve. This is why insects are winning the 'arms race'. In an agricultural setting, it is the job of the plant protection specialists to redress the balance.

Integrated pest management (IPM) is recognised as the most sensible approach to controlling pests on agricultural crops in both developed and developing countries. In many ways, developing countries have the most to gain. Heavy reliance on chemical control is too costly for countries short of foreign exchange and, too frequently, its use is damaging to the health of the user and to the health of the environment. While few would suggest that chemical control should not be used under any circumstances, most people agree that greater emphasis should be placed on cultural practices which reduce damage from pest attack, biological control and plant breeding for resistance. The term 'integrated' pest management implies the need to understand the interactions between plant, pest, pest control, and environment or agricultural setting. How can pest management be integrated effectively unless these interactions are fully understood.

Two heads better than one - provided they study together

Two of the major disciplines that have an effect upon pest control are, by and large, pursued independently and single-mindedly; plant breeding and entomology. Plant breeding for resistance to insect pest damage is only one of the many routes to the objective of breeding better varieties for farmers. But how seriously do plant breeders consider the mode of action, and the effect, of the host plant's resistance to insect pest attack? Even more important, and even more likely to be neglected is the effect of the host plant's resistance on the insect pest's natural enemies.

Entomologists approach the challenge of insect pest damage to crops by studying the life cycle of the insect pest, its predators, parasites and pathogens, and they use this knowledge to reduce crop damage. But do entomologists take the specific variety of the crop they are trying to protect sufficiently into consideration? Scientists are discovering that host plant resistance and biological control may, contrary to expectation, interact to the disadvantage of the farmer rather than to his or her benefit.

Your friendly natural enemy

Decades of plant breeding have failed to produce varieties totally immune to insect damage but high levels of resistance in some crop varieties to some insect pests have been achieved. However, high levels of resistance may not be the best objective. If the majority of the insect pest population fails to survive, the population of that pest's natural enemy will also decline. The farmer loses both friend and foe. Moreover, high levels of resistance, particularly to such insects as leafhoppers, white flies and aphids, quickly breaks down. This is because new insect biotypes develop from those insects that survive despite the host plant's resistance. Populations of these surviving insects can multiply very quickly and the virulent insect pest destroys not only the farmer's crop but years of plant breeding effort and cost.

Biological control seems to work most effectively where host plant resistance is only partial. Some resistance is valuable because it lowers pest populations to a level at which a predator natural enemy can make a real impact on crop yields by reducing damage to an acceptable minimum. It is important that sufficient numbers of bath the pest and its natural enemy remain at the end of the crop season so that the balance between plant, pest and predator is such that the farmer derives maximum benefit as early as possible in the next season.

These interactions are complex and depend upon many factors. Pests which encounter crop varieties that are less favourable to their taste may 'wander around' far longer from hatching to feeding site, presenting an agreeably slow-moving food source for a predator. On the other hand, a virus disease may be spread more quickly if the host plant is resistant to the insect which transmits the disease. This is because the insect moves quickly from plant to plant as it seeks a more favourable feeding site, spreading the virus as it goes. The architectural structure of a plant may be significant. For example, insect pests that encounter a smooth and shiny leaf may be more likely to fall off the host plant, to be consumed by predators on the ground. A hairy leaf may have the same effect. But a hairy leaf or stem may also make it more difficult for a carniverous natural enemy to move about in search of an insect that is devouring the plant under a hairy canopy of protection.

Plant breeders, as well as entomologists, need to understand these interactions if IPM is to be successfully applied. To ignore them may result in plant breeders discarding material with characteristics of use to the natural enemies of crop pests and, therefore, indirectly of use to farmers. Why destroy a pest with an insecticide when, with constructive help from plant breeders and entomologists, your friendly natural enemy can do it for you, for no cost, season after season?

One of the disadvantages of chemical insecticides is that they are a crude weapon in the fight against pests, causing untold lateral damage. But it seems that plants themselves induce a form of chemical control for their own protection in a way that is far more sophisticated that the simple release of distasteful toxins. Whenever plant cells are broken down, chemical volatiles are released. Newly mown grass, with its unmistakable smell, is an example that most people are familiar with. But in addition, some varieties of plants, when (and only when) attacked by a herbiverous insect pest, have the ability to release additional chemicals which can be picked up by any organism in the environment.

A chemical cry for help

Carniverous insects can pick up the signal and identify it as indicating a potential source of food. In effect, the plant is issuing a cry for help or, in other words, doing what we as humans do: 'making friends' with the enemy of the enemy. What is even more remarkable is that the chemical composition of the volatile released by the plant can vary depending on the insect species which is causing the damage, thereby ensuring that only the natural enemy of that particular insect will be attracted to the plant under attack. Plant varieties vary in their ability to use this sophisticated form of protection but it is an aspect that must be of considerable interest, especially to those forward-thinking plant breeders sympathetic to the concept of IPM. If the effectiveness of this chemical signal is included among the selection criteria used by plant breeders to evaluate new crop varieties, some of the work of pest control could be undertaken by the plants themselves. This would reduce the need for further human intervention for pest control during the crop's growing season with, for example, commercial insecticides.

It can take many years for a plant-breeding programme to produce results in the form of varieties available to farmers but the benefits of better cooperation between plant breeders and entomologists will surely come in time. However, it would be wrong not to pursue the opportunities that are available today. There may be varieties already developed and available to farmers which have the valuable characteristic of being able to release their own SOS signal. All that is necessary is to identify which they are. Even without specially developed varieties, there are many other IPM strategies which can be put in place which enhance the biological control of insect pests by their natural enemies.

One of the main issues which concerns the agricultural research community is how to ensure that the fruits of their research are implemented at farmer level. Scientists can no longer wash their hands of this responsibility because the funding of their research programmes increasingly depends on it. Since IPM is an approach to pest control rather than a technology, much depends on education. In some ways, farmers are ahead of the scientists because, to a certain extent, they are already practising IPM. Naturally, farmers grow their crops in a way that, to the best of their knowledge, will reduce yield losses from pest and disease. Their survival has depended upon their doing so. Building upon this knowledge, and understanding the mechanisms involved, should help farmers avoid practices that are counter-productive, such as unwittingly destroying natural enemies of pests.

Farmers' field schools

There is a natural tendency for farmers to believe that all insects are pests and that the larger the insect, the more damage it does. There is often little understanding of the role of natural enemies. In southern Ghana, a scheme to help farmers understand the role of pests on their rice crops has resulted in the setting up of farmers' field schools. Teams of five farmers met once a week for a complete season in order to study the insect pest complex and the effect on yields of an IPM approach to crop protection. Control plots, on which insecticides are sprayed at the first sign of pest damage (in accordance with usual practice) are grown alongside IPM managed plots. Pest levels, natural enemy levels and the health and yield of the rice crop are monitored throughout the season.

Insects found on the crops are caught and caged together so that farmers can tell by the survivors, which are the pests and which are their predators. Healthy plants are deliberately cut down by as much as half in order to simulate pest damage. The ultimate yield is then measured to assess whether such pest damage would have had a serious effect. Farmers discover that the yields are much the same from both the IPM and the control plots. However, the cost of inputs on the IPM plots is much lower and farmers learn that IPM can result in significant savings.

The situation on farm is, of course, very different and very much more complicated than the artificial, isolated environment of a research institute. Nevertheless, there may be varieties which are currently in use which interact positively with natural enemies. Others may be far less efficient or may interact negatively. Although the agricultural setting is complex, there are still fundamental interactions which take place and which are important to understand and appreciate if yield improvement and security of food production is to be achieved in the future.

It was to provide a forum to discuss these ideas, within the context of smallholder farming systems in Africa, that the CTA sponsored a seminar entitled Insect pest control for smallholders: integrating biological control and host plant resistance. This took place in Addis Ababa, Ethiopia from 9 to 13 October 1995. The seminar was organisad by CTA with the Institute of Agricultural Research, Ethiopia, and the International Institute of Biological Control (IIBC). Participants included plant breeders and entomologists from 28 countries and they debated a specially prepared paper on the seminar topic, written by M.B. Thomas of Imperial College and J.K Wasge of the IIBC.

It may be many years before the goal of sustainable food security is achieved, and it will take much more than an integrated approach to plant breeding and pest control management to achieve it. But it makes sense to make use of all allies in the fight to protect crops against insect pest damage, whether those allies are the natural enemies of the pest, or even the plant itself. S.R.