Insects and insect control

In recent years, there has been a considerable shift away from the chemical approach to insect control. This is often explained in terms of the increasing cost of developing new formulations to combat elevated levels of resistance within several insect populations. At the same time, the accessibility of international travel combined with progressive reductions in agricultural trade restrictions have contributed to the increased risk of introducing previously unfamiliar insect species to working farms, and gardens throughout the country. In their efforts to control troublesome insect populations, organic gardeners and primary producers depend upon a restricted range of natural products and control strategies. These can be grouped within four broad categories.

Bolstering plant resistance

The first of these include all of the efforts which seek to improve the plant’s biological resistance and resilience against insect attack. This is generally achieved by encouraging natural soil fertility and promoting a diversified ecosystem. The positive relationships between healthy soils and healthy plants provide an underlying framework for the organic approach to insect management. It is reasonable to assume that the organic emphasis on soil quality will result in improved nutrient dynamics which subsequently enable the plant to resist many forms of insect attack. The lower incidence of pest infestation on organic farms and gardens is also attributed to the biological diversity which encourages natural insect predators and resource competitors. It is important to avoid the situation where one plant species predominates over a large section of the garden. This encourages insects to establish resident populations.

Physical deterrence

The second category of insect control measures is based upon physical deterrence. In many situations, it is appropriate to remove problem insects from the garden. Destructive invaders like snails, slugs, and caterpillars can be physically removed from plants and collected in plastic buckets containing methylated spirits or saline solution. There are plenty of screens, traps, water sprays, and other devices suitable for the organic garden. Commercial insect screens and finely meshed gauze nets can be used as protective barriers against medium and larger sized insects such as fruit fly, locust, and caterpillars. When properly secured, the screens also prevent access to birds and rodents. The negative consequence of widespread insect screening can be the restriction placed on useful pollinators like bees and honeyeaters.

Many types of trap are used to capture insects. Fruit flies can be captured in plastic bottles or closed funnels containing baits which mimic the signals of sexual attraction. Slugs and snails will drown in plastic basins containing several centimetres of beer at the bottom. Traps like these can help to control insect numbers but may also be used to monitor seasonal variations in the population. This information can be used to establish crop rotations which strategically disrupt breeding and activity cycles. Sprays of water are suitable for physically washing mites, aphid, and other minute insects from foliage. Electronic or mechanical timers which initiate periodic jets will restrict most of their opportunistic movement and behaviours. Most garden centres stock a range patented devices, some of which are designed with the organic gardener in mind. Good examples are the copper strips which can be used to control slugs and snails in most suburban gardens. Individuals who loathe shopping may prefer to depend on the old fashioned remedies. One of my favourites is the use of crushed eggshell or finely ground bone fragments to protect seedlings from slugs and snails.

Natural predators

Natural insect predators provide another type of control measure. There are numerous predatory organisms which can significantly limit the destructive effects of plant feeding insects. It is possible to encourage beneficial predators into a garden but, ironically, this requires viable populations of the pest species to sustain and hold them. From an ecological perspective, the aim is to establish an acceptable equilibrium between these two populations, rather than attempting to eradicate one of them.

Typical examples of beneficial predators include birds, lizards, spiders and a host of carnivorous and parasitic insects: ladybird, lacewing, praying mantis, hoverfly, wasp, predatory mite and nematode. The presence of these species suggests that a complex, self-regulating ecosystem is established. To maintain this, a varied range of plant habitat and sheltering zones are required. Birds prefer trees and open areas; lizards seek grassy cover and hollow logs. The predatory insect species usually depend on nectar or fruit bearing plants to attract their prey.

Biological control

Biological control is also dependent upon identifying species which prey or compete against an established insect pest. Unlike natural insect predators, the majority of organisms used as biological control agents are manipulated by some human intervention. This may involve breeding programs in which an introduced species is released in populations of sufficient size to impact upon the nuisance insect. Biological controls in agriculture are frequently established within the field of microbiology. Various microorganisms such as bacteria, fungi, viruses, and protozoa can be manipulated to improve aspects of the organic ecosystem. These interventions will be ecologically sensitive and expensive, so are usually restricted to specialist growers with sufficient resources and government approval. At the lower end of the scale, many organic gardeners are familiar with the microbial based insecticide known as Bt. This product is developed from selected strains of Bacillus thuringienis bacterium, which releases a natural toxin killing many species of caterpillar and other destructive larvae within several days.

In some situations, an insect population may eventually adapt and become resistant against the individual control strategies described here. Experienced organic gardeners will routinely vary their approach, particularly during prolonged breeding cycles and periods of seasonal acclimatisation when normal resistance is diminished. Dependent upon local growing conditions, there may be opportunities to combine several of the outlined strategies for insect control.