Biocontrol - the future

With the issue of pesticide residues never far from the front page, biocontrol methods are becoming a must for everyone in the sector.

There is considerable pressure on growers to reduce or eliminate the use of pesticides in crop production systems, because of concerns about the effects of pesticide residues. Although biocontrol is an attractive option, the development and implementation of commercially viable systems often takes years.

Potential biocontrol agents include microbial agents (viruses, fungi, bacteria), nematodes, insects and mites. Some of these can be applied in a manner similar to insecticides, while others require more complex application strategies. It is also possible to exploit natural enemies that are already resident in an area.

Many biocontrol agents are subject to regulation in the UK. As with pesticides, all microbial biocontrol agents must be registered with the Pesticides Safety Directorate before they can be applied, while predators, parasitoids and insect pathogenic nematodes that are not endemic in the UK are regulated through ACRE (the Advisory Committee on Releases into the Environment). At present, very few microbial control agents are registered in the UK. This article focuses on the future direction of biocontrol and discusses developments that are particularly relevant or promising.

As biocontrol agents are used more widely there will be an increasing need to ensure that they are compatible with one another. There may be opportunities to use several different biocontrol agents against a single pest species to get more reliable control. For example, insect pathogens, which kill their hosts relatively quickly, can be used to supplement pest control with predators and parasitoids. In a recent collaborative project with Stockbridge Technology Centre, funded by the Horticultural Development Council, researchers at Warwick HRI used a commercial preparation of the fungal pathogen Beauveria bassiana, together with the predatory mite Phytoseiulus persimilis, to control spider mite on tomato. Spider mite is normally controlled using applications of the predatory mite, reinforced with sprays of selective acaricides. However, differences between predator and prey in their rates of establishment and development have resulted in a strong dependency on acaricides and led to the development of resistance.

The research showed that Beauveria was effective at all stages of the spider mite life-cycle, and was compatible with the use of Phytoseiulus, although this is not yet available in the UK.

In situations where two or more species of arthropod natural enemy are used, issues of compatibility can sometimes be overcome by the use of supplemental or alternative food sources. Research at Warwick HRI has shown that the provision of pollen can prevent the predatory bug Orius laevigatus from eating the predatory mite Neoseiulus cucumeris. However, some pests also feed on pollen and future research will focus on how to provide supplemental foods that support natural enemies without benefiting pests.

Computer programs that model insect development and behaviour can be used to great effect in research on biocontrol. They can be used to forecast the potential pest load experienced by a crop, identify natural enemies that can be combined together and understand how natural enemies move through crop canopies. By combining modelling approaches with experimental work, it should be possible to develop robust biocontrol strategies.

The value of such models is that they enable researchers to screen many different strategies and identify those that should be tested. For example, recent modelling research at Warwick HRI demonstrated the predatory mite Neoseiulus cucumeris and the predatory bug Orius laevigatus could be combined to give rapid control of western flower thrips, Frankliniella occidentalis, despite the fact that the bug will eat the mite. The effectiveness of this combination has now been confirmed experimentally.

Modelling research has also shown that the structure of the crop canopy plays an important role in the ability of predators to locate pests. Computer programs are being used to understand how connections between plants in the canopy affect the movement of natural enemies. The more connections in the canopy, the quicker the predators can find the pest. However, as the complexity of the canopy increases, so does the time taken by predators to locate their prey. Understanding how the relationship between plant connectivity and plant structure affects predator movement will be crucial to the development of rapid and robust biocontrol strategies.

The use of predators and parasitoids to control pests is now relatively commonplace in protected crops such as tomatoes and cucumber. However, there is still much to be done before similar systems are developed for crops grown outdoors. This is mainly because of the large differences between the two growing systems. Biocontrol agents that are introduced into glasshouses are effectively contained and are not exposed to the dramatic changes in temperature and moisture. In addition, there are few alternative sources of food, so they are forced to eat the target pest. In contrast, natural enemies in field crops usually have a wide choice of prey.

Predators, parasitoids and pathogens of arthropod pests are naturally widespread in the environment, and there is growing interest in how they can be exploited for biocontrol. Strategies that promote the activity of these natural enemies are called conservation biocontrol. There is also increasing interest in manipulating local habitats, using mulches, wildflower strips or beetle banks, to enhance the abundance of naturally occurring predators, parasitoids and pathogens of pests. However, little is known about whether and how these manipulations lead to increased pest control. Researchers at Warwick HRI are planning to investigate how the composition of mulches and wildflower strips can be altered to increase the abundance of natural enemies that are specific to key pest species, such as aphids, and whether natural enemies in the wildflower strips move into the crop to provide increased pest control.

Some species of insect pathogenic fungi cause natural epidemics in insect populations. In the southern US, a fungus causes natural epidemics in aphids on cotton plants. Researchers have now developed an aphid fungus prediction service for cotton growers using simple field sampling techniques. The service tells growers when a fungus epidemic is going to happen, allowing them to withhold sprays. The service is operated by two people and is estimated to save US cotton growers some $30 million a year. In the UK, natural epidemics in aphids are often caused by the fungus species Pandora neoaphidis. If the triggers for these epidemics were understood they could be used to form an aphid pest management strategy.

Although biocontrol can be highly effective, it can be difficult to make biocontrol agents available to growers. Biocontrol is often more expensive than the use of chemical pesticides. However, when used as part of an Integrated Pest Management (IPM) strategy, biocontrol can help to deliver sustainable methods of pest control, whereas sole reliance on chemical pesticides can lead to problems such as the development of pesticide resistance. Biocontrol was first adopted mainly as a response to widespread development of pesticide resistance, but it has now been shown to provide other benefits, by allowing the use of bumble bees for pollination and improving crop quality.

Unfortunately, it can be difficult to commercialise biocontrol agents. The costs of registration for microbial agents in the UK are very high, deterring companies from pursuing registration. In a new initiative, scientists at Warwick HRI and colleagues in the Politics Department of Warwick University are conducting research into the regulatory system for microbial agents. The aim is to identify ways regulatory systems can be made more innovative, in order to benefit growers, regulators and the public.