Diprion pini is an insect with complete metamorphosis, changing from a caterpillar larva via a pupal stage to a winged adult.

Last-stage larvae can be up to 25 millimetres (0.98 in) long; the head is brown, while the body is light green with black spots.

Young caterpillars start feeding on the old needles near where they hatched, leaving the vascular bundles uneaten.

[2] Outbreaks of D. pini are eruptive, rising rapidly to large infestations after long latent periods; this makes monitoring difficult.

[2] Climate change appears to be allowing the pest to damage forests increasingly far north, such as in Scandinavia.