The sweet potato whitefly (SPWF) Bemisia tabaci (Gennadius). has become a serious problem of a number of agricultural crops in the southern tier of states in the U.S. In the southwestern U.S. it seriously affects summer crops such as melons and cotton, and fall, winter and spring vegetable crops such as lettuce, broccoli and cauliflower. Since this insect has no overwintering resting stage a succession of host plants is necessary in order to span the gap from cotton season to cotton season. This study characterized seasonal population trends of SPWF in cotton and then identified subsequent hosts which were important in the overwintering survival of this insect. The "off-season" hosts included certain weeds as well as cultivated crops and all appear to be important in the seasonal population dynamics of this whitefly.

Sweetpotato whitefly (SPW), Bemisia tabaci (Gennadius), exhibited high levels of flight activity during daylight hours from July to September. Flight activity in a fallow field in Arizona in late August began as early as 0600 h, peaked between 0800 and 0900 h, and decreased thereafter during the day. Flight activity in cultivated cotton, lettuce and alfalfa fields also occurred throughout the day in early and late September. Few SPW were caught between 1900 and 0700 h. Peak numbers were caught on sticky traps prior to 1200 h in Arizona and Israel. Numbers of adults caught on sticky traps decreased from 0700 to 1000 h and with increasing wind velocity. Precopulatory pairing behavior occurred as early as 0700 h and increased gradually to 0900 to 0920 h, when 48% of the adults observed were paired, and decreased thereafter.

Four field experiments were conducted to evaluate efficacy of a number of registered or experimental insecticides against the sweet potato white fly. Several materials gave encouraging results under unusually high population densities. Among the more effective treatments in one or more tests were: Capture, Danitol, NTN 33893, pyriproxyfen and SN 85292, and combinations of Capture +endosulfan, Capture +Ovasyn and Danitol +Orthene.

A treated -vial technique was used to bioassay insecticide susceptibility of sweet potato whitefly populatons occurring on different hosts and at different locations in southern Arizona. All insecticides tested proved to be efficacious against the sweetpotato whitefly. Combinations improved efficacy, for example, Orthene to Danitol and DEF to Baythroid. Three important factors appeared to affect efficacy: 1) geographic site; 2) host plant of SPWF; and, 3) time of season. Further detailed experimentation needs to be done to elucidate the reasons for this.

A field study was conducted during 1991 to evaluate the effectiveness of Physalis wrightii Gray, as a trap crop for Bemisia tabaci (Gennadius) when grown with the commercial crop Gossypium barbedense L. Aldicarb at two rates was used in P. wrightii for control of B. tabaci. Three sampling methods: pan counts, leaf turns, and leaf counts measured B. tabaci numbers in both crops. Aldicarb treatment in the trap crop did significantly affect B. tabaci populations in comparison with untreated P. wrightii (P= 0.000, df =2, 240). Differences in numbers of adults observed on the two hosts were not significant. Analysis of aldicarb metabolite residues by high pressure liquid chromatography exhibited a pronounced dose effect (P= 0.001, df =2, 22) in the trap crop leaf tissues. The chemically treated trap crop did significantly protect the cotton crop from larger and more rapidly developing sweetpotato whitefly populations when compared to the untreated check.

Liriomyza leafminers have occurred in Arizona for several years. It has only been recently that a new species, Liriomyza trifolii, has been reported on vegetable crops such as melons and lettuce. During the 1991 growing season, L. trifolii was observed causing damage to cotton for the first time in Central Arizona. Infestations occurring in Coolidge, had reached damaging levels and required control, whereas populations occurring in Yuma were very low. The impact of management and environmental factors responsible for the outbreaks of L. trifolii on cotton are discussed.

The sweet potato whitefly can have an impact on cotton by reducing yields through direct feeding damage, by transmitting the cotton crumple leaf virus during feeding and by the production of large amounts of sticky honeydew that interferes with the harvesting and ginning process. Data on whitefly populations collected weekly from 938 yellow sticky traps near cotton fields have been entered into a geographic information system (GIS) database. In general, whitefly populations were high near cotton fields in the Yuma area before July 6th. They rose rapidly in central Arizona between July 6th and July 20th. During the month of August, counts continued to rise in central Arizona, particularly in western Pinal County. Populations began to fall during October. Whitefly populations in eastern La Paz County were slower to develop than in other areas in western Arizona. Whitefly populations in Graham and Cochise County were not significant throughout the growing season. Cotton crumple leaf virus was observed in parts of central and western Arizona.

An analysis of weather conditions prior to and during the severe sweet potato whitefly (SPWF) outbreaks of 1981 and 1992 in Arizona revealed some striking similarities. Both bad outbreaks were preceded by 1) weak monsoons during the summer preceding the outbreak (1980 and 1991); 2) winters with very warm minimum temperatures and no incidence of extreme cold; and 3) springs with above normal temperatures and precipitation. The analysis also revealed the presence of above normal minimum temperatures during most winters since SPWF became an important pest in the Desert Southwest.

Fixed precision sequential sampling plans are reported for egg and nymphal stages of Bemisia tabaci on cotton. These plans were developed based on detailed examination of within -leaf, within plant, and within field distributions of egg and nymphal (crawler through pupae) stages during the 1992 growing season in central Arizona. The most efficient sample unit for eggs and nymphs was determined to be a single 3.88 cm2 circular plug taken from the basal portion of the 2nd sector of the 5th mainstem node leaf (counted from the terminal). Tentative plans are presented relating cumulative counts to sample size for fixed levels of precision.

Both registered and experimental insecticides were evaluated for effectiveness against the sweetpotato whitefly in several field experiments at Yuma, Arizona. Best results were obtained with combinations of two insecticides, particularly a pyrethriod and an organophosphate, rather than with individual materials. Results of these experiments indicate that unusually heavy infestations can be currently controlled even though sustained use of these insecticides would probably lead quickly to the development of resistance.