What's the best type of wood to use for building a structure from segments roughly 1.5X1.5mm square in cross-section, as far as strength to weight ratio is concerned?

The convention has been to use balsa for all the sizes, but I've been wondering if a more dense type such as spruce or pine would be more efficient for smaller sizes such as the case above.

Bamboo is not allowed.

if you can get osme willow, that wolud be good, quite light, reasonable strength, flexible

Model aircraft makers usually use sitka spruce for wing spars because of its strength and lightness.

balsa wood is the strongest wood for its weight. but its fragile... and is easy to damage with anything. as for the most chemicaly resistant wood its Osage-Orange

Hardwoods are much tougher than pine or spruce. What are you making?

Lava

Supposedly, a half-meter high tower to support 15 kilograms. (33Lbs)

pine will do for that easily