Populations of the Sweetpotato Whitefly on Cotton Grown in Open-Top Field Carbon Dioxide-Enrichment Chambers
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Atmospheric carbon dioxide levels are rising from the current ambient level of ca 350 ul /liter to 500 -600 ul /liter projected for 50to 75 -years hence. In addition, plant scientists are enhancing the CO2 environment to increase photosynthesis which is currently limited by inadequate levels of CO2. There are many questions as to how increases of CO2 might affect other organisms. The growth and feeding response of the soybean looper, Pseudoplusia includens (Walker), to soybeans grown in controlled carbon dioxide atmosphere was studied by Lincoln et al. (1984). Larvae fed at increasingly higher rates on plants from elevated carbon dioxide atmospheres, suggesting that the impact of herbivores on their host plants may increase as the level of atmospheric carbon dioxide rises. During 1984 we sampled sweetpotato whitefly, Bemisia tabaci (Genn.), populations in field plots ofKeywords:
Carbon respiration
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Plans for sampling sweetpotato whiteflies in cotton were evaluated within 8,000 acres of cotton within central Arizona. These plans were found to be a practical and efficient way to track whitefly populations. In general they should add about 8 minutes to a pest sampling regime. Neither time of day nor sampler experience were found to have a significant effect on the number of whiteflies counted. Therefore, this method provides a common currency for growers, PCAs and others to compare whitefly numbers among fields and through time.
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Plant derived oils (cottonseed and soybean) in greenhouse and field studies were promisingfor sweet potato whitefly, Bemisia tabaci ( Gennadius) control. Cotton seedlings treated with oil in the greenhouse repelled whiteflies up to 7 days and high mortality of treated larvae, pupae and adults occurred. In the field, application of crude cottonseed oil to cotton with high air-blast, ground spray equipment suppressed whitefly populations thoughout the season. INTRODUCTION The sweet potato whitefly, Bemisia tabaci (Gennadius) has become increasingly important as a pest of cotton and other crops in Arizona since 1981 (Duffus & Flock 1982, Duffus et al. 1986). Conventional spray applications have not given effective control (Johnson et al. 1982, Prabhaker et al. 1985, and Dittrich et al. 1985). The sweet potato whitefly produces honeydew which has been implicated in the worldwide problem of sticky cotton, although stickiness in cotton may also result from other causes (Perkins 1987). The need for an effective control agent for the sweet potato whitefly on cotton prompted us to conduct greenhouse and field studies to determine the potential of soybean and cottonseed oils as pesticides. PROCEDURES AND RESULTS Greenhouse bioassays against the sweet potato whitefly and the cotton aphid showed that adult sweet potato whiteflies were repelled (87 %) up to 7 days by a 5% soybean oil treatment to cotton seedlings (Table 1). Egg viability was reduced by 84% with a 10% cottonseed oil spray. Treatment of whitefly larvae with 10% cottonseed oil sprays resulted in a 90% or greater reduction in the number of larvae and pupae (Table 2). Field studies in Israel showed that crude cottonseed oil (5 to 7 %) aqueous sprays applied with high air -blast sprayers that provided under -leaf coverage, effectively suppressed sweet potato whitefly numbers throughout the season on cotton. Results were superior to control provided by insecticides (Fig. 1). Applications of oil alone applied at 120 1/ha at 3to 8 -day intervals with an inflatable sleeve boom sprayer reduced whitefly populations when treatments were initiated in early August before the whiteflies had become too abundant. There were 24 whitefly pupae/leaf in insecticidetreated cotton and 1 whitefly pupa/leaf in the oiltreated cotton in mid -September (Table 3). Results were equally satisfactory when tractor speeds were 8.5 or 3.5 km/hr, delivering 50 to 120 1/ha (5.3 or 12.8 gal/A), respectively. A sampling method for adult whiteflies consisting of hitting the tops of 10 plants over a box with an oilcoated black bottom was developed and can be used to quickly evaluate the need for or the effectiveness of control measures.
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Cottonseed
Gossypium
Honeydew
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In conjunction with the 1992 cotton variety testing program (see Silvertooth 1993), 12 medium maturity varieties were evaluated for the presence of sweetpotato whitefli.es (SPWF). Three sites (Queen Creek, Maricopa, Coolidge) were selectedfor expression of a full range of infestation intensity. Total immatures per square inch ranged from less than 8 up to more than 8(X), depending on site, sample date and variety. The results indicated that there were significant effects ofvariety on SPWF numbers; however, the ranking of varieties was not always the same nor significant. Most varieties performed comparably, but one consistently had more SPWFs than the remaining varieties (chi 1 35). Comparative results from the three sites indicate that there may be separate preference and performance components which lead to the development of an infestation. Lea f hairi.ne.ss was quan tijied front samples at one site, and the relationship of this factor to whitefly susceptibility is discussed. Two varieties, co1135 and stv453, were found to be significantly more hairy than the remaining varieties. The results provided here represent a preliminary evaluation of the data. Further analyses will attempt to relate various crop production and agronomic factors to SPWF susceptibility (e.g., plant height, fruit retention, height:node ratio, canopy closure, growth habit, maturity, yield, crop protection regimen). One fact was clear, however. None ofthe varieties evaluated here demonstrated resistance to SPWFs and certainly not to the extent that a producer could eliminate substantial risk of infestation through variety selection. Introduction Integrated pest management in any cropping system is knowledgeintensive and is dependenton a thorough understanding of how control tactics influence pest dynamics. Some tactics are preemptive (e.g.,proper cultivar selection), while others are remedial (e.g., chemical control). This study is focused on the impact of cultivar selection on the late season dynamics of the sweetpotato whitefly, Bemisi.a tabaci (Germ.). This information coupled with other critical studies ofcrop -insect interactions will help build the foundation of knowledge necessary for the development ofan economically and environmentally rational management plan for the sweetpotato whitely (SPWF). 'This study was conducted in part in response to critical questions about the susceptibility of various cotton varieties to SPWF. Host plant resistance is a mainstay of most successful IPM programs around the world. It offers the producer the opportunity to prevent insect infestations simply through the appropriate selection of seed. Unfortunately, little is presently latown about the status of currently grown varieties to this strain of the SPWF (strain B), nor the potential underlying mechanisms of tolerance, avoidance, or resistance. A statewide cotton variety testing program was conducted in 1992 under the coordination of Dr. J. Silvertooth. Numerous accounts of the specific methodology used to conduct this test are outlined elsewhere in this volume. This study sought to compare whitefly infestations among the medium maturity cotton varieties used in the testing program. Furthermore,an effort was made to select sites and times that would exhibit the full range of infestation severity in order to infersome characteristics of SPWF preference versus performance.
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RAPD -PCR was used to detect differences at the DNA level between the A and B forms of B. tabaci. All twenty of the RAPD primers tested distinguished readily between the forms. These primers also distinguished between the forms at the egg and nymph stage. Genetic similarity statistics indicate that these two forms of B. tabaci were no more closely related to each other than to bayberry whitefly (Parabemisia mvricae) or bandedwinged whitefly (Trialeurodes abutilonea). DNA from B. tabaci collected from 11 locations around the world was also analyzed. These insects could be classified into six distinct groups, suggesting there are more than two variant forms of B. tabaci. A comprehensive examination of all variant types of B. tabaci will have to be conducted before a concise definition of the taxonomic relationship between the 'A' and 'B'forms can be determined.
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Why did this insect seem to suddenly appear? There are a number of opinions. One is that the several warm winters the Southwest has experienced recently has allowed the insect to invade more northerly areas than it usually occupies. This species seems to be distributed worldwide between the 30° parallels but it may extend its distribution northward when weather conditions permit. It has been an injurious pest of cotton in Israel for a number of years. Another idea is that the extensive use of pyrethroid insecticides may have permitted the insect to increase. Dan Gerling, UC Riverside, made detailed studies of this and two other species of whiteflies on cotton at Indio and Westmorland, CA from 1962 to 1965 and concluded that damage often follows after insecticide application (Ann. Entomol. Soc. Amer. 60:13061321). Natural control factors played an important role in keeping the whitefly populations at low levels. The buildup in 1981 was so widespread that it is difficult to associate it with a particular insecticide group. Also, whiteflies were very severe in some of the fields treated with pink bollworm pheromones and thus had not been treated with the normal doses of pyrethroids or other insecticides. Others believe that stub cotton may have provided overwintering sites and a good host for the spring development. While this is true, areas in California such as the Coachella Valley, far from any stub cotton fields, had very severe whitefly infestations by early August 1981 (as they did in 1963). As for overwintering, adult whiteflies are present on many of the weeds in the Phoenix area in early January 1982. It now remains to be seen whether they will persist the whole winter and whether the eggs they have laid will develop.
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Overwintering
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Sweetpotato Whitefly (Bemisia Tabaci Gennadius) Population Relationships to Cotton Yield and Quality
Sweetpotato whitefly (SPWF) Bemisia tabaci Gennadius strain B has been a devastating pest of cotton in Arizona and California in recent years. Management systems involving cultural procedures, SPWF population monitoring crop sanitation, crop sequencing chemical control and other technology are developing slowly. SPWF population information in relation to cotton yield and quality losses are urgently needed Preliminary 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 in 1993. The results suggest significant correlations for numbers of SPWF per leaf disc from cotton leaves vs. cotton yield and lint stickiness. Cotton lint yield was negatively correlated to all stages of SPWF populations (0.783 or higher). Lint stickiness was high positively correlated to SPWF populations (0.707 or higher) and cotton defoliation was positively correlated to SPWF populations (0.876 or higher).
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Fiber crop
Bt Cotton
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Surveys of whitefly parasites in cotton showed few or none were present during July and October in some areas, preliminary observations of sticky traps show that large numbers of parasites were present in some fields during October. Introduction In another report of a whitefly survey of cotton fields herein, sweetpotato whitefly, Bemisia tabaci (Gennadius), parasite populations in cotton fields in the Coolidge area were very low during July and October. No parasites were collected from fields during mid -July from Avondale to Wadell. Some of the sticky traps used to monitor whitefly populations during 1992 were saved and are just now being checked for whitefly parasites. Preliminary data show that parasites were very abundant in some cotton fields during October. Materials and Methods Adult SPW were monitored with yellow sticky traps (30.5 x 15 cm) wrapped around 7.6 cm diameter cylinders. Traps were attached vertically to 1.2 m sticks and placed at a height of 61 cm just outside 938 randomly selected cotton fields throughout Arizona. After the traps had been examined for whiteflies, selected traps were examined and the total number of whitefly parasites was counted. 1 USDA, ARS, Western Cotton Research Laboratory, Phoenix, AZ 85040. 2 Southwest Boll Weevil Eradication Program, Phoenix, AZ 85282.
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Deltapine 50 (DP -50) and Stoneville 506 (ST -506), short season cultivars of upland cotton, Gossvpium hirsutum L. , were grown under weekly or biweekly irrigation schedules in 0.2 ha plots in a split plot design at Maricopa, AZ. The seasonal average numbers of sweetpotato whitefly eggs and nymphs were 24% greater on leaves of plants irrigated biweekly. The leaves of ST -506 had 26% greater numbers of eggs and nymphs than did leaves of DP -50. Samples of lint from the two cultivars irrigated biweekly had 32 % more sugar than did lint from the cultivars irrigated weekly (weekly = 0.28 ± 0.02% , biweekly = 0.41% ± 0.03% sugar). Our results indicate that the numbers of immature sweetpotato whitefly on cotton plants can be reduced by 47% by selecting a less susceptible cultivar and avoiding plant water stress. Introduction Agricultural research efforts are directed toward water conservation through improved applicationmethods because of the high costs of irrigation water. Cotton producers are forced to use the least amount of irrigation water practicable; delaying irrigation is often used as a means to control plant growth. Delaying or reducing the amount of water applied frequently results in severe plant water stress. Little data have been published on the effects of plant water stress on the populations of insects associated with cotton. Materials and Methods Experimental design was 2 x 2 factorial arranged in split plots and with three replicates. Plots were planted 17 April 1992 at the University of Arizona Maricopa Agricultural Center (MAC) in central Arizona. Whole plot treatments were weekly or biweekly furrow irrigation and early or late irrigation termination. Split plots were planted to one of two shortseason cultivars, Deltapine 50 (DP -50) or Stoneville 506 (ST -506). Whole plots were 12 rows (rows spaced on 1.02 m centers) x 170 m; split plots were 12 rows x 85 m. All plots received biweekly irrigations of about 13 cm of water and half of the plots of each cultivar received an additional irrigation of about 8 cm of water between biweekly irrigations (weekly irrigation). Irrigation began in all plots on 5 June and continued to 14 Aug. All plots received the same fertilization and cultivation practices during the season. One application of acephate was made on 27 July to control lygus bugs, Lygus hesperus Knight.
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Fiber crop
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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.
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Fiber crop
Bt Cotton
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Seven insecticides were evaluated in nine treatment combinations for efficacy against the sweetpotato whitefy (SPWF) and the cotton aphid (CA). Five different classes of chemistry were represented by these compounds, which were compared to an untreated check. The infestation was characterized as severe (> 3(X)SPWFs /sq. in. ) and included a substantial number of CAs atthe beginning of the evaluation ( >90 /leaf). Three applications were made in August after the onset of 'stickiness. For SPWFs, three treatments compared favorably with the check, but only after three applications Orthene +Danitol, Capture alone, and Capture +Ovasyn. Intermediate control was achieved with Endosulfan +Ovasen. Rankingsfor CA control were dissimilar with Vvdate +Asana, Endosulfan +Ovasen, and Ovasen alone consistently peiforining better than the check. Vvdate alone, Endosulfan alone, and Orthene +Danitol were intermediate in CA control, but also significantly different from the check. Capture alone and Capture+Ovasyn which performed well for SPWFcontrolwas not efficacious against CAs. Indeed the two single pyrethroid treatments (Capture: A.sana) failed to achieve any degree of CA control.
Endosulfan
Whitefly
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