I've made an insanely oversimplified sketch to show the general loads on something like a weedhopper. All of the attachments are pin joints that can't resist rotation about the pin, and therefore take shear loads only.
It's a simple pin jointed triangle, complicated by the fact that the main element is a long beam extended beyond the triangle, loaded across its axis. Most of the weight of the airplane is on the main lift struts, with mostly a compression force on the root fittings but with some up or down shear as well, depending.
The shear loads on the root fittings depend a lot on where the main lift strut picks up the wing, having an upward/inboard component, downward/inboard component, or neutral/inboard component depending on where the lift strut is attached along the spar beam.
Drag and anti-drag loads on the wing, and pitching moments also add or subtract from the shear loads caused by the lift force on the root fittings. Working it all out gets pretty complicated.
The root fitting loads are usually lower than the strut attach loads out along the wing and you'll often see surprisingly small bolts attaching the wing to the fuselage on high wing aircraft. On little puddle jumper planes like Aeroncas, you'll see fairly beefy 3/16" or 1/2" bolts at the wing to lift strut attach, but itty bitty quarter inch bolts joining the wing to the fuselage.
On a Twin Otter, with strut attachments way inboard at the nacelles, the root shear loads are more substantial and are inboard and down.
On the Weedhopper my guess is the lifting force transmits an upward and inboard shear load to the root bolts and this force varies with drag loads and pitching moments.