7 Biological control

The biological control literature is extensive, but the most practical and useful publication is the handbook by Harley and Forno (1992).

7.1 Forms of biological control

Biological control can be of three main types:

· Conservation, involving habitat manipulation to encourage populations of natural enemies or environmental factors (fire, flood) which kill the invader or seriously reduce its competitive ability.

Biological control is targets the reproductive stage of a plant's life-cycle rather than its vegetative stages. This is generally thought to be more promising (S. Neser pers. comm. 1993).

For forest vegetation management the use of mycoherbicides appears to have much potential (cf. Markin & Gardner 1993). The use of specific rust fungi is being investigated by bodies such as the International Mycological Institute, Egham, U.K. Fungal pathogens are no widely considered for biological control in conjunction with insect agents or where the use of insects has been unsuccessful (Barreto et al. 1995, Evans et al. 1995).

There are two other methods of biological control, though neither are usually classified as such.

Use of other plant species. There is potential in some situations for the use of native or "non-invasive" plant species to suppress the alien plant. Experiments using native and alien trees to suppress gorse (Ulex europaeus) are underway on Maui and Hawaii islands in the Hawaii chain. Native Acacia koa appears to be effective within five years. In the long term these methods may be more effective than herbicides, which provide quick suppression but are not a permanent solution (Tulang 1992).

Use of livestock. Goats are used to control Psidium guajava and Schinus terebinthifolius on a ranch in Hawaii. Cattle can be used to control some species by high stocking rates. The Voison or short-duration grazing system is useful; grazing, hoof action and dung can break up the vegetation sufficiently to allow desirable species to establish, although in many cases this disturbance would facilitate the invasion of alien plants (Bredhoff 1992).

Biological control is a very complex undertaking in which numerous factors need to be taken into account so we can only outline the main features here.

7.2 Effectiveness

The success rate of biological control has been about 50% according to two studies. Julien (1992) reported a 47% success rate (partial or complete control) worldwide. In Hawaii biological control with insect has been used against 21 plant species and in 8 cases control was very good, partially successful in 3 cases and failed in 10 cases (insects either failed to become established or had no significant effect) (Markin et al. 1992).

Predictions can be made about the likely effectiveness of biological control by consideration of the weed's abundance in its native range. If it is common or weedy in its native range there is little prospect of classical biological control being effective in a new range (Scott & Panetta 1993). Taxonomic isolation from crop plant is an important criteria for selecting weeds for biological control programmes.

For weeds as a whole, many established insects fail to build up sufficient populations to harm their hosts due to interference from local predators and pathogens. Therefore, assessments of the diversity and abundance of native ants and other predatory insects may give an initial indication of the chances of success (D. Gardner pers. comm. 1994).

Biological control may not be as effective against scattered plants in forest because of insect survival and dispersal difficulties. Most successful programmes have been against rangeland or farmland weeds (Markin and Gardner 1993).

In the past most successful programmes have been against perennial plants. In tropical areas this may not be a serious limitation as many annuals grow throughout the year and so may be equally susceptible to control? (C. Riches pers. comm. 1992).

Markin & Yoshioka (1992) devised a method for comparing the chances of a successful biological control programme against different weeds. Each of 20 factors is scored on a scale 0 to 5, a factor that could hinder or deter a programme scores 0 and one that simplifies or contributes significantly to its success is given the weighting 5.

There is little point in eliminating a weed (by biological control) if another weed takes its place; for example, a successful biological control programme eventually eliminated Lantana camara from large areas in Hawaii, only to have some of it invaded by Schinus terebinthifolius (Andres 1977).

7.3 Cost and time span

Biological control is not cheap as proper screening is essential. Andres (1977) estimated the total cost of a biological control programme to range from US$1-2 million. Harris (1979) estimated that between 19 to 24 scientist years would be needed per weed, costing between US$1.2-1.5 million and would require the establishment of 2.3 agents.

A facility in which insects can be quarantined whilst their biology and feeding preferences are studied must be available. There were only 11 in the U.S.A. and the simplest cost about US$200,000 to build in 1977, with operating costs of over US$100,000 a year (Andres 1977).

Harris (1979) gave the following average periods of time needed for the development of an agent. Constructing or gaining access to a quarantine facility takes about 2 years; evaluating 10 or more insect species can take 3-5 years; the establishment of say 5 insect species then needs a rearing facility to propagate the insects, taking 2-3 years. Thus a programme may take 7-10 years altogether.

The time period over which biological control agents take to have an effect (if at all) varies greatly. In Hawaii (with a good climate) success occurred in about 3-10 years, but it took up to 60 years for Lantana camara (Andres 1977).

7.4 Environmental impact

Although biological control has been considered a more ecologically acceptable method for the control of invasive organisms than chemical methods, this view has recently been questioned (Simberloff & Stiling 1996a, 1996b) as the biological control of a number of animals has resulted in disasters including the extinction of a number of endemic taxa. The use of biological control agents to control weeds can be considered much safer although earlier programmes have caused problems. In the 1960s an insect introduced to West Africa to control Lantana camara attacked sorghum, but after this the introduction of systematic screening has meant that there have been no major incidents reported since then. Adult insects have been known to start eating native plants once the alien has been eliminated, though in no case to date has appreciable damage been caused to the native species (Andres 1977). But the element of risk is still present, and it would be wrong to assume that the impact of biological control on the native system can ever be fully predicted. Risks can only be reduced using control agents with high host specificity. Therefore, the legislative position in many countries, properly restricts the scope of this control method.

7.5 Conflicts of interest

If remaining blocks of natural habitat are small, the chance that biological control could be carried out without conflicts with other land uses in the intervening areas declines. Biological control programmes against plants have sometimes been restricted to anti-reproductive agents because the plant had economic value in other locations (S. Neser pers. comm. 1993).

In poorer countries the use of biological control rather than manual control favours scientists and technicians, many from richer countries, instead of local people who would be employed to carry out manual eradication. Therefore, careful political and socio-economic analysis of this option is required and it is crucial that the decision is taken at an appropriate local level.