Chemical control studies for silverleaf whitefly, Bemisia argentifolii Bellows and Perring, control on cotton showed that fenpropathrin-acephate, fenpropathrin-endosulfan, and endosulfan-bifenthrin mixtures gave adequate control and increased cotton yields were obtained as compared within untreated cottons. Pyriproxyfen, applied biweekly or alternated with fenpropathrin-acephate, Nicotiana, and a fenpropathrin-mycotrol mixture also gave effective control.

Adult lygus bugs, Lygus hesperus (Knight), were collected from alfalfa fields in 11 different cotton producing areas of Arizona. A standardized glass vial method was used to estimate susceptibility of the collected populations to the organophosphate insecticide, acephate (Orthene®), and the pyrethroid bifenthrin (Capture®). Overall, lygus from throughout the state were significantly less susceptible to acephate and bifenthrin in 1995, than in 1994. Resistance of lygus to acephate continues to be widespread and intense, but not uniform in Arizona. In 1995, all populations possessed individuals capable of surviving exposure to vial treatments of 10,000 μg/ml acephate. Lygus bugs from Safford and Maricopa represented the most and least susceptible populations, respectively, to both acephate and bifenthrin. These two populations were tested for susceptibility to nine other insecticides: aldiaarb (Temik®), dimethoate (Gowan Dimethoate E267®), endosulfan (Gowan Endosulfan 3EC®), imidacloprid (Admire 2F®), malathion (Gowan Malathion 8®), methamidophos (Monitor 4®®), methomyl (Lannate LV®), oxamyl (Vydate 3.77L®), apt oxydemeton- methyl (Metasystox-R SC®). The Maricopa population was significantly less susceptible to six of these insecticides. Our findings support the hypothesis that the intensive use of pyrethroid and organophosphate insecticides for whitefly control in cotton has selected for resistance in lygus. This result portends increased problems with lygus control in the future, points to the need for developing new tools for controlling lygus bugs in Arizona cotton, and underscores the urgent need to find alternatives to the current heavy reliance on insecticides for managing whiteflies in cotton.

Three resistance monitoring methods were tested to evaluate their relative reliability, discriminating ability, convenience, and practicality for monitoring insecticide resistance in Arizona whiteflies. Adult whiteflies were collected from the field and tested in the laboratory with three methods: leaf disk, sticky trap, and vial. Each method was evaluated against two populations divergent in susceptibility using a mixture of Danitol® + Orthene® and two single chemicals, Thiodan® and Danitol®. The Yuma population was relatively susceptible and the Gila River Basin population highly resistant. Correlations of field efficacy and leaf disk bioassays were conducted with the Yuma population and a comparatively resistant Maricopa population. At each location egg, immature, and adult whitefly densities were monitored before and after Danitol® + Orthene® treatments and resistance estimates were also monitored in the populations using leaf disk bioassays. Our results illustrated that the leaf disk method had the greatest discriminating ability between susceptible and resistant populations. The results also indicated that the vial method was the most practical, and that the sticky trap method was good at discriminating between populations that have large differences in susceptibility. The field efficacy trials indicated results from leaf disk assays reflected what had occurred in the field.

A cotton management program in the Imperial Valley, CA was designed to reduce pink bollworm, Pectinophora gossypiella (Saunders), populations. The program established I March as the earliest planting date, 1 September for defoliant or plant growth regulator application and 1 November for cotton stalk destruction and plowdown. In-season gossyplure-baited pink bollworm male moth activity monitoring and immature green cotton boll inspections for larval infestation were encouraged as decision making aids to determine the need for additional control action. Male pink bollworm moth catches in gossyplure-baited Lingren and delta sticky traps were significantly reduced each year from 1990 to 1994 following the initiation of the management program in 1989. Fewer larvae per cotton boll occurred in the years from 1990 to 1992. Fiber quality of commercial cotton sampled was also improved from 1989 to 1994, as compared to the 1984 to 1988 average. Cotton production, in general, was reduced during 1989 to 1994 in areas surrounding Imperial Valley and may have contributed partially to reduced populations in Imperial Valley.

Bolls from transgenic cotton, NuCOTN 33 (Delta and Pine Land Co.) containing the Bollgard TM gene (Monsanto Co.) and from the parental cultivar DPL-5415 were examined for mature larvae of the pink bollworm (78,240 total bolls). Bolls from five paired fields were collected in one study (Queen Creek, Buckeye, and Gila Bend areas) and a composite of 10 fields of each cultivar were collected in a second study (Paloma Ranch area). Bolls were incubated for 2 weeks (dissected late season) or dissected to find mature larvae, respectively. Collections of 100 or 80 bolls per field were made weekly or biweekly from July through November, 1995. Numbers of pink bollworm larvae were very low in all fields through August and thereafter increased steadily in the control fields. Numbers of larvae found in transgenic cotton were extremely low or non -existent throughout the season, even in fields which were adjacent to heavily infested control fields. These results show that NuCOTN 33 retained a high degree of efficacy for preventing development of mature pink bollworm larvae (diapause larvae) during the late season. Most important, these data provide baseline information against which efficacy in subsequent years can be compared.

Silverleaf whitey, Bemisia argentifolii Bellows and Perring, adults and nymphs were significantly reduced season -long in cotton plots treated with fenpropathrin plus acephate on 3 occasions (15 July, 2 August and 29 August). Thermodetector sticky cotton ratings were significantly reduced in insecticide-treated plots compared with untreated plots. Heavy rains reduced cotton stickiness in all plots.

Growers, Pest Control Advisors and Cooperative Extension, University of Arizona personnel coordinated areawide pest management activities in an area near Gila Bend, AZ to maximize the effectiveness of strategies to control pink bollworm and whitefly. Data on insect populations and pesticide applications is held within a database that is shared with cooperators on a real time basis. Control measures are discussed and common goals reached for reduction of pests within the area. Assessments from growers support the whitefly survey activities of the personnel. Fields were surveyed once per week. Data describing the population is faxed or phoned to the Pest Control Advisor and remedial action implemented at their discretion. Cooperative Extension personnel suggested pesticide use patterns to reduce resistance of whitefly and checked for field populations using University of Arizona recommendations. In 1995 an areawide pin head square program was followed based on Heat Units After Planting for timing pink bollworm susceptible stage of the cotton plant for each field and combined with the Heat Unit Model for pink bollworm emergence to determine percent emergence of the population. In 1996, many of the growers planted genetically engineered cotton and used lures to reduce pink bollworm and used the insect growth regulators under the Section 18 for whitefly control.

Silverleaf whiteflies (SLW), Bemisia argentifolii Bellows and Perring, have been devastating pests of cotton and other crops in Arizona and California in recent years. Studies with cotton insecticide treatments initiated each week from shortly after cotton seedling emergence to late in the cotton season were conducted at the Irrigated Desert Research Station, Brawley, CA. The results suggest action thresholds in relation to cotton yield of 0.22 SLW nymphs/cm² of leaf disc, 0.64 eggs/cm² of leaf disc or 2.22 adults/cm² of yellow sticky card surface.

Management of whiteflies with two insect growth regulators was compared with standard practices in grower managed cotton near Gila Bend, AZ. The IGRs, Knack (pyriproxyfen) and Applaud (buprofezin) were tested in a randomized complete block experiment with seven replicates. University ofArizona recommendations were followed to time insecticide applications. Following IGR applications, the nymphal populations remained near or below action thresholds (≤ 0.5-1.0 large nymphs per 3.88 cm² disk) from early August through early October. The standard practices treatments maintained the nymphal population through only early September, when populations sharply increased IGR treatments resulted in adult populations below University of Arizona action thresholds nearly as long as the nymphs. There was an adult population peak that followed a nymphal peak near the middle of September.

Two insect growth regulators (IGRs) that are selective against whiteflies (Aleyrodidae) became available for the first time in 1996 to Arizona cotton growers under emergency exemption. These IGRs were studied in a commercial -scale whitefly management trial (178 acres) in 1996. The trial was designed to evaluate provisional whitefly recommendations. Three sets of factors were tested in a 48 plot factorial design: application methods, thresholds for initiating IGR use, and insecticide regimes. Ground (broadcast at 15 gallons/acre) and aerial applications (5 gallons/acre) were roughly equivalent over a wide range of variables examined (whitefly populations, number of sprays, cost, and yield). Under the higher population densities, ground applications sometimes suppressed whiteflies to a greater extent than aerial applications. The rapid advance of the population resulted in the initial triggering of all thresholds within just five days. No consistent trend in population suppression was seen for the thresholds tested (0.5, 1.0 and 1.5 large, visible nymphs per 3.88 sq cm leaf disk located between the major and first, left lateral vein of the fifth main stem node leaf below the terminal). The control cost for the highest threshold was significantly less than for the middle threshold, but not for the lower threshold. Under emergency exemption, each IGR may be used only once per season. The sequence of use did not result in any consistent advantage in population suppression, cost, number of sprays needed, or yield. The IGR regimes were in general more efficacious, less disruptive, and less costly than the conventional insecticide regime. There were significantly fewer sprays needed by the IGR regimes compared to the conventional regime. All regimes successfully controlled whitefly populations for a 12 week period and cost significantly less than conventional programs tested in 1995 (Ellsworth et al. 1996a). IGRs are effective, long-lasting, and less environmentally disruptive alternatives to conventional insecticides. They reduce the risk of secondary pest outbreaks and pest resistance, and increase the opportunity of natural enemy conservation.