Why are topical treatments or surface sprays largely ineffective against developing *A. obtectus* larvae?

Answer

The entire life cycle occurs inside the seed

Because the entire life cycle, save for adult emergence, occurs inside the bean seed, topical treatments or surface sprays are largely ineffective against the developing larvae feeding internally.

Why are topical treatments or surface sprays largely ineffective against developing *A. obtectus* larvae?
insectclassificationWeevilbean