Spiromesifen is a novel insecticide (belonging to the new chemical class of spirocyclic phenyl-substituted tetronic acids) with a unique mode of action. Laboratory and field experiments were conducted to test the efficacy of this insecticide against the greenhouse whitefly, Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) on strawberry, Fragaria ananassa (L.). Laboratory experiments showed that spiromesifen at 0.5 and 1.0 μg·mL−1 a.i. inhibited egg hatching by 80% and 100%, respectively, whereas at concentrations of 3.1, 3.0, and 10.0 μg·mL−1 a.i., this insecticide, respectively, killed 100% of the first, second, and third instar nymphs. Much lower toxicity to adults was observed. Field trials revealed that application of spiromesifen reduced the whitefly egg numbers by 61% to 80% from 2 to 3 weeks posttreatment in comparison with the pyriproxyfen treatment, whereas the application lowered the egg numbers by 34% to 73% from 2 to 5 weeks posttreatment compared with the buprofezin treatment. In comparison with pyriproxyfen treatment, spiromesifen application decreased the numbers of immature whiteflies by 29% to 92% from 1 to 6 weeks posttreatment. The effect of spiromesifen on reduction of immatures was similar to that of buprofezin. Also, the efficacy of spiromesifen on suppression of adult numbers was comparable to that of pyriproxyfen or buprofezin. Spiromesifen shows promise for inclusion in integrated greenhouse whitefly management programs and insecticide resistance management programs on strawberry.