2 MATERIAL AND METHODS
2.1 Characterization of the electrical signalling potential of
Bt and non-Bt cotton plants Bioassays to record electrical signalling
of cotton plants were conducted in the laboratory. The experimental
design was a randomized block design, and each treatment was repeated 10
times. The measurement of electrical signals was made on the plant
surface. A technique was used to detect electrical signalling potential
differences over long periods. At the time of the electrical signal
measurements, the cotton plants were placed in a Faraday cage to ensure
electromagnetic isolation of the environment at 26±1°C, with a relative
humidity of 60±10% and a 12-h photophase.
Measurements were made using electrodes consisting of a 0.25 - 0.5 mm
diameter silver lead wire chlorinated in 3 M KCl solution. Five
electrodes were used, four of which were inserted in different
arrangements along the stem of cotton plants. The fifth electrode is the
reference electrode and was inserted at the base or at the stem apex.
The electrodes are connected to a four-channel data acquisition system
with a built-in amplifier (World Precision Instruments Lab-Trax-4 / 24T
model) that is connected to a computer with LabScribe version 3.0
software that decodes the signal (Zawadzki et al., 1995, with
adaptations).
The recordings of electrical activities in cotton plants were performed
continuously for three days. The following variables were obtained:
amplitude and duration of the signal, number of signals generated, time
of signal concentration and frequency of signals generated by cotton
plants. The electrical signalling profile was contrasted between Bt and
non-Bt cotton plants infested with those not infested with aphids
(A. gossypii ). Bt and non-Bt cotton cultivars were planted in
plastic pots containing soil conditioning substrate
(Forth®) and kept separately in cages under the same
climate conditions mentioned before.
2.2 Dispersal pattern of A. gossypii in Bt and non-Bt
cotton plants Bioassays were performed to study aphid behaviour and
associate it with data obtained from electrical signalling bioassays. A
randomized block design with four treatments was used: a1 . Bt
cotton plants infested with 30 aphids/plant; a2 . Bt cotton
plants infested with 60 aphids/plant; a3 . non-Bt cotton plants
infested with 30 aphids/plant and a4 . non-Bt cotton plants
infested with 60 aphids/plant, distributed in 10 blocks. Bt and non-Bt
cotton cultivars were planted in plastic pots containing soil
conditioning substrate (Forth®) and kept in the same
climate conditions mentioned before.
Aphid infestations were performed on Bt and non-Bt cotton that reached
the six-leaf stage. After infestations of the Bt and non-Bt cotton
plants with A. gossypii , the number of aphids was recorded in the
within-plant regions at 0 (immediately during infestation), 24, 48 and
72 h after infestation. The insect within-plant distribution of each Bt
and non-Bt cotton plant was analyzed at three positions: bottom, middle
and top.
To evaluate aphid dispersal behaviour as a function of cultivars (Bt and
non-Bt cotton) and aphid densities, the negative binomial distribution
parameter k was used. There are three basic spatial pattern
distributions: random distribution, regular or uniform distribution, and
aggregate or contagious distribution. This parameter k is an
indicator of uniform distribution, where when k tends to zero,
the distribution is highly aggregated, k ranging from 2 to 8
indicates moderate aggregation, and values greater than 8 (k> 8) indicate that the distribution is random (39). Thek values were estimated by the method of moments.