Integrated Environmental Biotechnology 255found to be necessary, are improvements to the physical formulation of the crop
spray; dry, flowable formulations being an advance on the original wettable pow-
ders. Increasingly, nematodes especiallySteinernemasp., are demonstrating great
potential as biological control agents applied as sprays. It is anticipated that
they will complementBacillus thuringiensisin extending the spectrum of pests
controlled by ‘environmentally friendly’ means (Knight, R., Koppert Biological
Systems, personal communication).
There are otherBacillus species which have also been used effectively as
microbial insecticides. These areBacillus sphaericus which produces a toxin
more potent but more specific than Bt, andBacillus popillaewhich although
it does not produce a toxin, kills its host by weight of bacterial numbers. The
latter is active against Japanese beetle, while the former is quite specific against
mosquito larvae. Both the mosquito larvae andBacillus sphaericusabound in
heavily polluted water such as cesspits where the bacterium may exert control on
the proliferation of mosquitoes. A different approach to microbial pesticides has
been to examine the exploitation of Baculoviruses. The drive to use Baculoviruses
as a means of biological pest control has dominated its research in the past, but
currently these viruses are being recognised more as vectors to express proteins
of various origins at a very high level indeed and so have become enormously
useful tools in the major branches of biotechnology. Several Baculoviruses are
registered for use in the USA as insecticides against maize bollworm (Heliothis
zeaSNPV), gypsy moth (Lymantira disparMNPV) Douglas fir tussock moth
(Orgyia pseudotsugataMNPV) and in the UK against pine sawfly (Neodiprion
sertiferMNPV) and pine beauty moth (Panolis flammeaMNPV). Members of
the Baculovirus family, also called Nuclear Polyhedrosis Virus for reasons which
become clear with a knowledge of their replication cycle, have been isolated from
Lepidoptera, Homoptera (aphids and their relatives) and Diptera. The infectious
cycle passes through a stage where several virus particles are bound together
in a large crystal of protein. This protects the virus particles until the crystal is
ingested by an insect where enzymes in the gut digest this polyhedrin protein
releasing the viruses. These enter the insect’s cells where they are uncoated,
make their way into the cell nucleus, and the viral replication cycle commences.
Some 12 hours after initial infection, virus particles are released which spread
the infection to neighbouring cells. By 24 hours post infection, the protective
polyhedrin protein coded for in the viral DNA, is being produced in sufficient
quantities to start assembling the crystal structures. By this time all the insect
tissues are suffering severe damage such that at the time of death, the insect
is effectively a mass of virus particles surrounded by the insect’s cuticle. This
cadaver is eaten by birds, and consequently may be spread some distance. The
virus is carried intact in the bird’s gut protected in the polyhedra, which is
resistant to digestion by the enzymes found in avian gut. Polyhedrin protein is
made in enormous quantities in the infected cell, but since its only known function
is to protect the virusin vivo, in the wild, then it is redundant when culturing the