The sweet potato whitefly (SPW), Bemisia tabaci, has become a serious cotton pest in the arid southwest. Control measures are difficult because the immatures develop on the undersides of the leaves and aerial applications are usually ineffective in delivering control agents to the leaf undersides and lower leaf surfaces. Also, there are several generations of SPW in a season and SPW populations appear to build resistance to conventional pesticides quickly. Control measures are needed that overcome these problems. We tested the insect growth regulator, buprofezin (Applaud) with the adjuvant Comate as a possible spreader agent, the synthetic pyrethroid fenpropathrin (Danitol) with the organic phosphate acephate (Orthene) as a possible synergist, and a cotton seed oil (Natur'l Oil) against SPW.

The tests were conducted in the late season of 1990 at 3 locations: Maricopa Agricultural Center (MAC), Maricopa, AZ, Yuma Valley Agricultural Center (YVAC), Yuma, AZ, and Irrigated Desert Research Station (IDRS), Brawley, CA (Danitol/Orthene were not tested at MAC). Plots were 0.016 acres, with 5 replicates per treatment in a random block design. Cotton varieties were : MAC DPL 90, YVAC - DPL 64, and IDRS - DPL 794. AI Rates were: buprofezin 0.38 lb./acre, Danitol 0.20 lb./acre, and Orthene 0.75 lb./ acre. Natur'l Oil was used at 2 %, 3.5 %, and 5 % V/V at YVAC and 2 % V/V at MAC and IDRS. Comate was used at 1 % V/V.

Application was made with a Solo backpack gasoline driven mist blower with an upturned nozzle with throttle delivery of about 180 mph. SPW immatures were sampled by taking whole leaf, half leaf, and 4 10-mm disks/leaf. Ten leaves were sampled per replicate (50 leaves per treatment). Immatures were classified as small, large, pupae, or emerged, and were recorded as dead or alive. Adults were sampled by sticky pan counts at all 3 locations and by leaf counts at MAC.

SPW populations were the lightest at MAC, followed by IDRS, and were extremely heavy at YVAC.

Buprofezin suppressed SPW large larvae to 40 % that of the control and pupae to 20% at MAC (10 days after treatment (DAT)) and to 28 % and 25 % respectively at IDRS (19 DAT); likewise at YVAC, suppression of large larvae and pupae was to 25 % and 24 % of SPW numbers in control plots.

Control with Danitol and Orthene together was synergistic at YVAC with a suppression of large larvae and pupae to 37% and 18 % of SPW numbers in control plots. However, only one application was made with Danitol/Orthene and by DAT 13 at IDRS and DAT 26 AT YVAC the SPW numbers rebounded. For example. at YVAC (DAT 26), SPW numbers in the Danitol/ Orthene treated plots were 3 times greater than in control plots.

At MAC (DAT 10 after a second treatment), Natur'l Oil at 2 % reduced numbers of large larvae by only 25 % but pupae were suppressed to 22% of SPW numbers in control plots. At IDRS, 2 % Natur'l Oil reduced numbers of large larvae and pupae by only 29 % and 7 % respectively (DAT ll,following second treatment). However at YVAC, 5 % Natur'l oil suppressed SPW large larvae and pupae to 35 % and 25 % respectively.

The adult counts did not appear to reflect the changes in SPW immature population levels. This was probably due to adult movement in and out of the plots. None of the agents gave what would be typically considered to be good control. However this work showed some efficacy for all of them. It appears that multiple treatments are required. The MAC and IDRS locations received 2 applications and the first of these was early enough to act against the very low populations before the August population build up. The single application at YVAC was against SPW populations that had already built up to extremely high numbers. For example, single leaf disk samples (10 mm diam) had immature counts higher than a thousand. Moreover, the sugar content of the fibers from honeydew was 3-4 % whereas 0.3 % is considered an economic threshold.