Bt Research 2015, Vol.6 No.2 1-10
ISSN 1925-1939
6
We for the first time clearly demonstrated that
Bt
can
efficiently be used for controlling mites (Jisha et al.,
2014). Prompted by our previous studies (Smitha et al.,
2013a, 2013b), the focus of this work was to check
whether the
Btk
-toxin (crude mixture of a few
vegetative cells, endospores, crystals, debris) is
suitable to combat
A. guerreronis,
the highly
damaging coconut pest
.
One might think that
A.
guerreronis
could be controlled by spraying water on
the crown of the infested palms (or naturally by rain).
Such methods will not be effective because, in such
environment, they will be hiding safely within the
perianth of nuts (characteristic cryptic habitat) where
the reach of water is not possible. Thus, soon after
rainy season, their rapid multiplication occurs. This
trickiness of the mite attracted us to adopt a suitable
strategy,
i.e
., to apply crude solid-fermented matter
within the leaf pockets of the coconut palm at the
onset of the rainy season; because the leaf pockets
harbor traces of litter and dust, which would act as the
solid matter for the growth of
Btk,
as in the potato
flour supplemented preparation, that has already been
demonstrated (Smitha et al., 2013a). The copious
number of endospores in the solid-fermented matter
already deposited in the leaf pockets would germinate
during the raining environment, which would facilitate
the rapid spread of motile
Btk
all over the crown of
coconut palm. Eventually,
Btk
would also reach at the
hide-outs of the cryptic
A. guerreronis
thereby killing
them, once the toxin is released upon the death of
Btk
cells. The rapid spread of these mites from one palm
to the other is mainly through air currents during dry
season and also by crawling or by phoresy (
e.g
.,
carried on insects or birds that visit palm flowers)
(Haq et al., 2001).
Literature describes no consistent mode of application
of
Bt
-toxins against insects, yet
Bt
products have been
in commercial use for over 50 years. Toxins of
Btk
have widely been used to control the forest pests such
as the gypsy moth, spruce bud worm, the pine
procesionary moth, the European pine shoot moth and
the nun moth (Gui-ming et al., 2001). Direct feeding
of crude pellet containing
Bt
-toxin (Fadeland Sabour,
2002), sprays (Mulligan et al., 1980), in the form of
pollen diet (Buchholz et al., 2006), application of
fermented broths (Brar et al., 2006) are the normal
modes of applications being practiced in toxicity
assays against various insects. But, none of these
techniques seem to be suitable to combat
A.
guerreronis,
because these mites are microscopic,
air-borne and highly sensitive to moisture
.
Moreover,
they reside inaccessibly inside the perianth (cap) of
the coconut buttons. Thus, the efficacy of starchy
fermented matter (powdered) was employed, which
would naturally stick to the surface of the coconut
buttons proving suitable environment for feeding
(ingestion upon sucking the juice from the tender part
of the button), avoiding the requirement of an
adhesive as in normal applications. This technique is
the first of its kind and was found effective for
combating
A. guerreronis.
Upon application of
Btk
-toxin, the mortality rates of
various insects have been demonstrated by various
authors (Aranda et al., 1996; Heckel et al., 1999). We
demonstrated a lower probit value (1.639 µg/cm
2
) for
the crude
Btk
toxin in solid-fermented matter, which
indicates the high efficiency of the preparation.
Treatment of 0.02 ha pasture plots with
Bt
H14 (1
kg/ha) resulted in effective control of
Aedes
spp. and
Culex tarsalis
. An aerial application on 12 ha duck
club pond with
Bt
H 14 (1 kg/ha) resulted in a 99%
reduction of
C. tarsalis
, apparently without adverse
effect on predator populations (Mulligan et al., 1980).
Crystals purified from
Bt
HD1 and HD73 were found
highly toxic to tobacco hornworm larvae (50% lethal
dose, 0.01 μg per third instars larvae) and crystals
purified from
Bt
HD1 were toxic to meal worm larvae
(50% lethal dose 4 μg per second instar larvae) (Zhu
et al., 1989). In these examples,
Bt
-toxins acted on the
actively feeding larvae of susceptible species by a
mechanism which involved consumption and
proteolytic processing of the toxic protein followed by
binding to, and lysis of midgut epithelial cells (Grove
et al., 2001). Unlike in the above and many other
studies (Broderick et al., 2009
) -
where larvae were
the primary target - in the present study, 100%
mortality of the adult mites was noticed in 24 h of
application at a concentration of 1.88 μg/cm
2
of crude
matter,
i.e
., for a heterogeneous population led by 20
adult mites. Furthermore, data clearly indicate that the
