Abstract: The effects of planting dates, lines of grain amaranth and their phenological stages on Cletus fuscescens populations were studied during the growing seasons of 2009 and 2010 in the experimental field of National Horticultural Research Institute, Ibadan. Lines ‘P 373’ (Amaranthus hypochondriacus), ‘Montana-3’ (Amaranthus cruentus) and ‘D 136- 1’ (Amaranthus hybrid) were transplanted at 3 weeks old on three different planting dates which were at 2 weeks interval in a randomized complete block design replicated three times. Cletus fuscescens population levels started increasing when seeds began to form on the head of the amaranth and population levels generally peaked with 6.33 bugs in 2009 and 7.84 bugs in 2010 when the seeds were completely formed but milky. The individual effects of lines, 507.15 (2009), 539.55 (2010) and phenological stages, 116.52 (2009), 291.26 (2010) of grain amaranth significantly (p>0.05) affected Cletus fuscescens population levels with planting dates, 822.17 (2009), 1116.47 (2010) being the predominant factor. The interaction between planting dates and phenological stages (33.54) of grain amaranth was observed to be the most important factors in the first growing season: 2009 while in 2010, planting dates and lines was the most important interaction (35.36) affecting the population build up of Cletus fuscescens on grain amaranth. Planting of early flowering and maturing line early in the growing season assisted the critical phenological stage “milky seeds” to escape the high population levels of bug infestation in the field.