The effects of tomato plant age, leaf age and imidacloprid on thrips probing and host settling behaviors were investigated for two thrips species, the tobacco thrips, Frankliniella fusca (Hinds) and the western flower thrips, F. occidentalis (Pergande) (Thripidae: Thysanoptera). A preliminary sampling experiment indicated that Frankliniella fusca was the more abundant thrips on pre-flowering tomatoes plants. Both thrips species were present and are important vectors of Tomato spotted wilt virus (TSWV). Plant age and imidacloprid effects were evaluated because previous field studies showed TSWV infection in tomato fields was affected by different plant ages and with imidacloprid treatment. Since thrips mortality has not been associated with these effects, plant age and imidacloprid might affect thrips probing and host selection behavior which ultimately could affect the thrips TSWV transmission efficiency. I used DC electrical penetration graph technique (EPG) to quantify thrips probing behavior. Whole plant bioassays were used in all settling experiments because F. occidentalis preferred leaves of whole plants to excised leaves or leaf discs. Males and female F. fusca probed and ingested more and longer on 3 and 4-week-old plants compared to 6 and 8-week-old plants. Female F. fusca probed and ingested more frequently than males in the plant age experiment but not in the leaf age experiment. Frankliniella fusca probed and ingested more frequently on 2 and 4-week-old leaves compared to 1-week-old leaves. Plant age did not affect the probing frequency or duration of F. occidentalis. Both thrips species preferred to settle on 3-week-old plants. Frankliniella fusca preferred to settle on old, 4-week-old leaves. Frankliniella occidentalis showed no settling preference for leaf ages. As imidacloprid concentrations in leaf tissues increased, F. fusca probing frequency and duration declined while F. occidentalis probing frequency and duration increased. In addition, ingestion frequency and duration declined for F. fusca and increased for F. occidentalis as imidacloprid concentrations increased in the plant tissue. Frankliniella fusca avoids leaves with imidacloprid and settles on untreated leaves, while F. occidentalis settles randomly when offered the choice to settle on imidacloprid and untreated leaves.