Contents - LAC Biosafety

insects, and laboratory trials have shown their potential for practical use as
mentioned in Chapter 10.
M.anisopliae has a wide host range, covering species of Coleoptera,
Lepidoptera, Diptera, Orthoptera, Hemiptera and Hymenoptera. In general,
successful infestation by entomopathogenic fungi requires high atmospheric
humidity, perhaps for spore germination, viability and sporulation after the host
is dead. Because of this limitation, successful field control has been achieved
only under some circumstances. Their potential needs to be further explored and
conditions for successful use standardized. There is also scope for isolating more
virulent strains. Beauveria, which can be mass-produced on artificial nutrient
media, has shown potential for control of soil insects like whitegrubs in forest
nurseries in China and India (Speight and Wylie, 2001). Metarhizium has been
found effective against the pine shoot-boring moth Rhyacionia frustrana in Cuba.
It may also have potential against root-feeding termites (see under Eucalyptus,
Chapter 10).
Bacteria
9.3 Overview of pest management options 163
Many species of bacteria infect insects but only a few cause serious
disease. Of these, some like Serratia marscecens, which can cause significant
mortality of Hyblaea puera pupae, as mentioned under teak in Chapter 10,
are also pathogenic to man, and therefore not safe for insect control. Bacillus
thuringiensis, usually abbreviated to Bt, first recognized as a disease agent in
silkworm, has emerged as the most promising bacterium for control of
lepidopteran and some coleopteran pests. The related B. sphaericus is pathogenic
to mosquito larvae and B. popilliae to scarabaeid beetles. Different strains of
B. thuringiensis have been isolated with different levels of pathogenicity to various
insects. Bacillus thuringiensis var. kurstaki has been found the most pathogenic
to lepidopteran larvae.
The incidence of Bt infection in natural populations of insects is not high
enough to cause acceptable levels of mortality and therefore living Bt, unlike
other natural enemies, is not effective for standard biological control practices.
Living Bt is slow to act and is also killed by sunlight. For these reasons, most
common formulations of Bt contain the toxin produced by Bt. It is used for
control of insects in the same way as chemical pesticides are used. Bt toxin,
however, is not harmful to man. It consists of proteins, called delta-endotoxins,
present in large crystals in mature, sporulating cells of the bacterium. After
consumption by the insect, the proteinaceous crystals break down in the high pH
medium of the larval gut, releasing the delta-endotoxins which are further
broken down to toxic protein molecules by the digestive enzymes. The toxins