Insects that affect the quality of seeds in three seed stands of Pinus engelmannii Carr in the state of Durango, Mexico, were identified and quantified. Ten trees at each of the three sites were randomly selected, and the seeds were collected from each of 10 fully developed cones. From the total seeds harvested, 400 were randomly selected and subjected to X-ray analysis to quantify whether seeds were filled, empty, or damaged by insects. The main factors that affected seed quality were poor pollination and pest attack, especially by Leptoglossus occidentalis Heidemann. Most seeds were damaged by pests at El Encinal (42.8%) and fewest at Las Cumbres (19.4%). Damage caused by the insect Cydia latisigna Miller occurred more frequently at El Encinal (5.0%) and less frequently at Las Cumbres (2.2%). Megastigmus albifrons Walker was found most frequently at La Florida (1.1%); at Las Cumbres, no damage was caused by this insect. The greatest damage caused by Leptoglossus was observed at El Encinal (1.3%), and minor damage occurred at Las Cumbres and La Florida (0.5). Most damaged seeds were at El Encinal (61.7%), while least were at Las Cumbres (22.8%). These results showed the seed bug (L. occidentalis) is the agent that causes major damage as seeds develop.