The ornamental design for an insert for an insect trapping device, as shown and described.
FIG. 1 is a perspective view of an insert for an insect trapping device embodying our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is bottom view thereof.
In the drawings, the broken lines depict environmental subject matter only that forms no part of the claimed design. The dot dash dot broken lines depict the boundary of the claim that forms no part thereof.