Introduction to Plant Pests and Insects

As well as losses to crops caused by diseases, animals such as plant pests, insects and nematodes are significant biotic factors. There are two basic types of feeding behaviour by insect pests.

Some pests, such as aphids, leafhoppers and whitefly obtain their food by piercing plant cells with their mouthparts and the sucking out the cells contents.

Such insects often live in large colonies and cause damage not only by killing plant cells but also as major vectors of plant viruses.

The other type of insect feeding behavour is chewing plant leaves, stems or other parts of the plant. Most of these insects are beetles (Coleoptera), flies (Diptera) or Moths (Lepidoptera) and, in most cases, it is the larvae that do the damage.

Insects feed on seeds or on the inside of the plant stem; the feeding damage on seeds can lead to fungal infection and thus to mycotoxin production.

The larvae of insects feed on the inside of cotton fruiting structures (cotton bolls from which cotton is extracted) causing considerable economic losses; losses are more considerable in developing countries where less insecticides are used.

There are four basic approaches to controlling pest insects. The most widespread method is the application of insecticides, which causes many problems.

As well as the potential hazard to human health, most insecticides are not completely specific to the target pest, also killing non-target beneficial insects; they can enter the food chain causing hazards to birds and mammals.

Furthermore, it is difficult to control pests that feed inside plants, such as stem borers, with insecticides. The continual use of insecticides can lead to the target insects developing resistance to the chemical.

As with plant pathogens, farmers adopt a range of cultural techniques to try to control insect predation.

Attempts to avoid insects are made by planting their crops at times or in places where insect populations are minimal.

Mixed cropping can reduce insect populations; for instance some Andean farmers grow maize and lupins together, which reduces insect damage on the corn.

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Mixed cropping leads onto another approach to pest insect control, that of integrated pest management (IPM). There have been many IPM schemes but relatively few can be regarded as being entirely successful.

This is because of the complexity of the ecosystems that the schemes are attempting to influence and the fact that there are many varying external factors continuously affecting the ecosystems.

Fourthly, as with controlling pathogens, plant breeding has been extensively used in attempts to introduce characters conferring resistance to pest insects into crop plants.

Since plants cannot run away from danger that have developed three types of defence against insects and other herbivores.

Plant secondary metabolites have been adapted to either poison or deter pests to give effectively internal chemical control. However, as many of these may cause damage to humans they have to be reduced or eliminated over the years in breeding programmes.

The structure of plants can provide physical barriers that inhibit pest insects and plants can grow in such a way as to compensate for damage caused by pests. Examples of this are hairy leaves or leaves with glands that exude a stick liquid.

Genetically Modified (GM) Control of Pest Insects (Plant Pests)

One of the two transgenic traits that have been commercially deployed in a widespread manner is that conferring protection against certain insect pests. There are two main modes of operation of these transgenes.

The most frequently used is that which inhibits the insects' digestion processes either by damaging the gut wall or by inhibiting digestion, thereby starving it.

There are two genes used to damage insect pest gut walls, the most widespread being the toxin encoded by a bacterium named Bacillus thuringiensis, termed Bt toxin.

The basic details of Bt and its endotoxin are shown. There are different forms of the Bt toxin protein, each with a different range of target insects. It is important to note that most of these target insects are those that chew plants and usually not those that feed by piercing and sucking.

Another important point is that each toxin is limited in its effective range and is not toxic to most non-target organisms.

Recently, studies have shown that it is possible to modify the Bt toxin molecule to change its target and tailor it to specific requirements.

The transgenic expression of the Bt gene means that the toxin is produced throughout the plant and is very effective against pests that live within the stem or in cotton bolls; thus transgenic expression of Bt gives good levels of pest control in commercially released varieties. It is now being considered for controlling pests in food crops such as rice.

The widespread deployment of Bt toxin-expressing crops has revealed various advantages and also raised a number of concerns. Among the advantages are greatly increased yields, reduced insecticide input and a reduction in the production of mycotoxins. The concerns fall under four headings:

Possible effects to the health of humans and other animals have been studied in depth and, in spite of various reports there is no firm evidence for any bad effect.

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There have been concerns about gene flow to the environment - these will be considered later.

Similarly there have been concerns about effects on non-target species exemplified in the Monarch Butterfly episode.

Once again there is no direct evidence for such effects, especially when one measures them against a baseline of the conventional agronomic practice of insecticide spraying.

There are worries about target insects developing resistance against Bt toxin and there is some recent evidence that this may be happening in limited cases.

The second gene used to affect the insect's digestive system is an agglutinin from the snowdrop (Galanthus nivalis) (GNA) which targets sap-sucking insects.

Although the transgenic expression of GNA shows considerable promise in controlling several important plant pests there have been no commercial releases on it yet.

Concerns have been expressed as to potential hazards to human health and its use has been controversial: for the so-called Pusztai GNA-potato controversy. In view of the "bad press" concerning GNA, the possibilities of using other plant lectins is being explored.

As well as affecting digestion by damaging or blocking the insect gut cell wall, the actual process of digestion can be inhibited. Insects use various enzymes to digest their food in their gut such as proteases that digest proteins.

There are various forms of proteases, the most important ones in insects being trypsins, chymotrypsin, alpha-amylase and cysteine; inhibitors for each of them are known.

Experiments are under way to express these protease inhibitors in plants so that insects feeding on them cannot digest their food. The concerns about protease inhibitors are similar to those on Bt toxin and especially on possible effects on non-target species.

Various other approaches to protecting crop plants against pest insects are being explored. These include the isopentenyl transferase gene from bacteria which affects insect cytokynin biosynthesis, toxins from spiders, scorpions and wasps and proteins from insect endoparasites.

All these are at the experimental stage and after assessing their effectiveness, large-scale safety checks have to be undertaken. Also they will have to be acceptable to the public and some of the toxins mentioned above may prove difficult to "sell".

However, it is possible that some of these genes or yet others may reach the stage of commercial release reasonably soon.

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