Suppose a horizontal disc fixed in the center with a vertical shaft passing perpendicular to the plane, is rotating at some angular speed and there is an insect sitting initially at center. The insect then starts moving radially outwards. From conservation of angular momentum of insect+disc system, we can state that the angular speed of disc reduces as the moment of inertia of the insect with respect to the axis increases. My question here is that, if we see only the disc without the insect as the system(or as a single rotating object), the angular speed of it changed and hence, some external torque has definitely acted upon the disc. What is the force that produced this torque?
I tried to think about it - If we say that the insect is moving along $x$ axis and the axis of rotation is the $z$ axis, the weight of the insect should not cause any torque because the cross product of distance and insect's weight lies in $y$ axis while we need a torque about z axis. Therefore, there must be a force along the y axis, what can be such a force? Can anyone help me out in analyzing the motion and dynamics out here?