Introduction

Most people who garden are aware of insects that have been apparently colonised by microbes. The small discoloured carcasses of larvae or adults are unearthed as you dig through soil. One group of organisms that parasitise insects are the fungi. Insects are common pests of agriculture and horticulture. Several fungi, like those seen in parasitised garden insects, have been used to control insect pests.

A diverse array of fungi associate with insects, and the nature of the interaction is also diverse. This section will only deal with pathogenic fungi, but see reference below for a detailed discussion of all types of association.

Biocontrol of insect pests can take one of several different approaches. The most common approaches are inundative application and classical control.

Fungi Used Inundatively

The Fungi

Several species of fungus have a reputation for biocontrol of insects. The fungi come from all divisions. The most commonly used ascomycetous fungi  include the genera Beauvaria, Metarrhizium, Tolypocladium and Lecanicillium. These fungi have been applied inundatively, in the manner of a chemical application, with the goal of killing most, if not all, insects.

The most historically important of the commonly used fungi is Beauvaria bassiana. The Chinese have a long history of successfully culturing silkworms. However, when the industry was taken to Europe, cultures of insects were devastated by Muscadine disease. The causative agent in turn was determined as Beauvaria bassiana, a fungus that grows on and in larvae. Determination by Bassi that the infectious agent was a fungus, was one of the first examples where germs were shown to cause disease. The fungus has since been used successfully to control the larvae of many Lepidopteran insects.

Metarrhizium, also an asexual stage of an Ascomycete, has been successfully used to control spittlebugs in South America, Rhinoceros beetle in the South Pacific islands, and in Australia to control locusts.

Lecanicillium lecanii infects sucking insects such as aphids and scale insects. The fungus is used to control sucking insects mostly in greenhouses where humidity is high. Some plant-sucking aphids also have the nasty habit of transmitting viruses, thus control of the insect and the viruses it transmits are both possible.

Tolypocladium is a fungus found in both soil and water. It has been reported to colonise a wide range of insects, and effectively controls mosquitos in experimental conditions.

Mode of  Attack

Penetration: In general, when a spore comes in contact with the insect cuticle, the spore attaches, germinates, penetrates the cuticle (from outside in, or inside out, depending on whether the spore lands on the outer surface, or is ingested). Penetration may involve physical force, LINK or more commonly physical and enzymic action combined. Inside the haemocoel, the form of the fungus may change to yeast-like LINK, though filaments and wall-less protoplasts are also found.

Secondary Metabolites: Many insect pathogens release metabolites that increase the chance of insect death. A wide range of pathogenicity factors are known and they come from different metabolic pathways. The factors may be toxins, immunosuppressants or antibiotics. Destruxens from Metarrhizium are polypeptides that appear to be directly toxic to the insect. Cyclosporin-like immunosuppressants from Tolypocladium  increase the rate at which the fungus colonises the haemocoel. LINK Better understanding of the formation and mode of action of secondary metabolites will be important if rapid knock-down of insects remains the goal of inundative biocontrol.

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Other Approaches to Insect Control

Classical

The control of pasture cockchafers (Aphodius sp) has been achieved in the Adelaide Hills of South Australia by the release of an isolate of Metarrhizium specific to the host. The larvae of the insect burrow through soil where they feed from roots of pasture grasses and incidentally, contact spores of the fungus. Colonisation of the larvae results in death after several days, with the subsequent release of spores of the pathogen, followed by subsequent cycle of contact, colonisation and insect death.

In principle, the density of the fungus increases when high levels of insect are present in the soil. As a result the insect population declines, enough for most of the larvae to miss contact with the fungus. However, the fungus is sustained at a low density, to become active only as the larval densities increase. The classic pattern of population change is shown in the figure.

Variations:

Several insects are of major concern in Australia. One, Heliothis (Helicoverpa) armigera is a serious pest of many different crops and pastures. In cotton, larvae eat foliage and bore into boles. Heliothis has several stages where it is vulnerable to biocontrol agents: as eggs on the plant surface, as larvae on the plant surface and as long-lived pupae in the soil. Fungi may play a role at each stage. Fungi that live on the leaf surface may colonise eggs. Fungi that live in the leaf may deter or poison larvae feeding from the leaf LINK, or be treated using inundative approaches, and fungi may colonise pupae in soil.

Fungi may also influence the way target plants respond to the insect pest. The first is direct, via release of insect toxins by the fungus. So far, only a few fungi have had their entomotoxins characterised. Indirect interactions are also documented. Induction of a plant response to fungal pathogens is well recognised. It appears that a similar response to insects is induced in plants by some fungi. This process requires considerable research before it can be used in field control of insect pests.

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Conclusion

Insects and fungi interact in various ways. The interactions where fungi kill the insect may be used in specific cases, but our overriding impression is that we know so little about the interactions, we are probably going to find many associations that can be manipulated resulting in a decline of the pest.

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References

Butt TM, Jackson CW & Magan N 2001 Fungi as Biocontrol Agents. CABI.

Lawrence JF, and Milner RJ. (1996). Associations between fungi and arthropods. Fungi of Australia Vol 1B, 137 – 202.

Robson GD, van West P & Gadd GM (eds) 2007. Exploitation of Fungi. CUP

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