Prof Don Hendry
In common with all other life forms, insects suffer from viruses that cripple and kill. However, these viruses have long been regarded as the "Cinderellas" of virology. They have no medical importance, as in general viruses that cause disease in insects do not do so in humans. Admittedly, insects do transmit viruses that cause illness in humans (think of mosquitoes and yellow fever!) but the insect in such cases is usually unaffected. Agriculturally they are of little interest, not being pathogenic for crop plants. As a result, they have been studied little apart from a small community of virologists interested in the fundamental properties and uniqueness of these viruses.
One of the most exciting areas of research is the use of insect viruses for pest control. At South Africa's Rhodes University, where there is a long tradition of research on insect viruses dating from 1969, insect viruses have been found to have tremendous advantages over chemical pesticides. Viruses are exquisitely target-specific, thereby not harming beneficial insects, and they are naturally-occurring products that do not affect the food chain.
We are looking at using a virus to control a major pest, the false codling moth or Cryptophlebia leucotreta. The moth causes economic problems, injuring a range of crops throughout sub-Saharan Africa, including citrus fruit grown in South Africa. Not only does the insect damage citrus crops pre-harvest, its phytosanitary status (pest-free status) is such that the detection of a single larva in fruit destined for export can result in rejection of the entire consignment. Control of this insect is complicated by the fact that it is what is known as a cryptic (non-obvious) pest, in that the moth lays one or a few eggs on the surface of the orange. On hatching, the larvae very quickly burrow into the orange and are then essentially inaccessible. There is thus a very small window of opportunity between hatching and burrowing.
The moth virus that the Rhodes team are working with is known as Cryptophlebia leucotreta granulovirus (CLGV). It belongs to a family of insect-pathogenic viruses (the Baculoviridae) that have large genomes of double-stranded DNA (the genetic material - deoxyribonucleic acid) consisting of about 100 genes or more. In addition, each virus particle is individually embedded in a large protein crystal (or granule) which protects the virus when it is outside the insect and exposed to the environment (see images below). When an insect swallows this virus, usually from the detritus of larvae already killed by it, the virus's protective granule is dissolved by the alkalinity of the gut.
Released, the virus particles infect the intestinal cells, eventually spreading throughout the insect and causing morbidity, appetite loss, flaccidity and then death. Unfortunately, this can take 72 hours or more, during which time the insect can still do considerable damage. Much current research is aimed at accelerating the lethal effect of these viruses.
In South Africa, Cryptophlebia leucotreta granulovirus (CLGV) was first observed as a lethal infection in a laboratory colony of the false codling moth maintained by Capespan (now Citrus Research International or CRI). In collaboration with CRI, Rhodes University has purified samples of the virus, examined them using electron microscopy, and extracted the DNA by removing the protein components with alkali and phenol. By cutting the viral DNA with a range of restriction endonucleases (enzymes that each cut DNA at a unique sequence on the DNA) and separating the resultant fragments by electrophoresis through an agarose gel, we have shown that our virus is distinct from a CLGV strain isolated some time back in the Cape Verde Islands.
Large amounts of the South African strain of Cryptophlebia leucotreta granulovirus have been produced at Citrus Research International by including CLGV-SA in the artificial diet on which the laboratory colony of false codling moths is fed, and then harvesting the infected larvae. After being purified, the moth virus is formulated to contain a sticking agent as well as an ultraviolet protectant to delay its inactivation in sunlight once used in the field.
Laboratory trials were performed on fruit infected with insect eggs to determine how much virus to apply. Field trials were then done by spraying formulated CLGV-SA on selected orange orchards where false codling moth is a problem. The sprayed orchards showed a significantly lower insect infestation and fruit damage compared to unsprayed orchards. Insect mortality occurred in as little as two days. While not yet rivalling the speed of chemical pesticides, the virus presents nevertheless a feasible alternative.
Work overseas has indicated that knocking out certain genes of the virus, by gene manipulation, has the paradoxical effect of accelerating insect mortality. These viral genes are thought in nature to delay the death of the false coddling moth, thereby permitting more virus to be produced. Work is now in progress at Rhodes University to locate and characterise selected genes of CLGV-SA.
An attractive feature of insect viruses as biopesticides is that they are capable of mutation and change, unlike chemical agents which are immutable in the face of insects becoming resistant (an ever-increasing problem facing pesticide manufacturers). Insect viruses can themselves change thereby presenting the insect with an "altered" virus against which its resistance is ineffective. In the light of our increased concern about environmental issues and chemical pollutants, insect viruses have a bright future for pest control, particularly if used in Integrated Pest Management programmes together with other pathogenic microbes.