Natural born killers - insect viruses
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.
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.
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.
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.