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.