Cotton aphid, Aphis gossypii, and silverleaf whitefly, Bemisia argentifolii are two sucking insect pests that frequently occur in the San Joaquin Valley at economically damaging levels. Whitefly growth regulators Knack^® and Applaud^®, which were registered under Section 18 in 1997 in California, were being tested in 1996 in some large-scale field tests where it was observed that the insect growth regulators (IGRs) were possibly providing some aphid control in addition to the whitefly control. In order to evaluate the IGRs' effects on cotton aphids a series of controlled greenhouse experiments were done that examined the efficacy of applications before and after aphid infestation on aphid nymphs and adults. Knack was effective at reducing the number of offspring produced by adult cotton aphids as well as reducing the survival of nymphs to adulthood, particularly of the very youngest of aphids. Applaud also reduced the number of offspring produced by adult aphids but the magnitude of this effect was much less then it was with Knack. Knack was effective at reducing survival and reproduction when applied to cotton aphid nymphs at one tenth the label rate.