An invasive new biotype of the tomato/potato psyllid (Bactericera [Paratrioza] cockerelli [Sulc.]) (Homoptera: Psyllidae) recently has caused losses exceeding 50% on fresh market tomatoes in west North America. Despite these extensive losses, little is known regarding the threshold levels at which populations must be suppressed in order to prevent economic losses. A series of experiments were therefore designed using combinations of two common tomato cultivars (QualiT 21 and Yellow Pear), five pest-densities (0, 20, 30, 40 and 50 nymphs/plant), and three feeding-duration (5 days, 10 days, and lifetime) treatments to test the relative importance of pest density, feeding period, and cumulative psyllid-days to establish economic threshold levels for psyllids. The cultivars differed considerably in their response to the toxin injected by the psyllid nymphs. ’Yellow Pear’ plants could recover from feeding by up to 40 nymphs for as long as 10 d, whereas ’QualiT 21’ plants were irreparably damaged by densities of 20 nymphs feeding for only 5 days. On ’Yellow Pear’, all plant measurements such as the number of yellow leaves and plant height were significantly better correlated with cumulative psyllid-days than with either pest density or feeding duration. On ’QualiT 21’, all plant measurements other than the number of yellow leaflets and leaves were significantly better correlated with pest density than with feeding duration or cumulative psyllid-days, and pest density was a better predictor of psyllid damage. Potential reasons for the variable responses between cultivars and the implications for psyllid sampling and integrated pest management are discussed.