Low dosages of contact insecticides to combat whitefly in greenhouses

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Chemical Applications via Shira's Foggers

The feature of being able to deliver a water soluble chemical incorporated in the fog droplets results in the deposition of the chemical being determined by the physical characteristics of the fog. These characteristics result in the chemical being deposited in a very thin layer on all surfaces of the objects coming into contact with the fog. The water carrier (the droplets) rapidly evaporates and the object being treated by the fog, be it a flower, potato, melon or poultry chick, is left quite dry.

An Ultrasonic Fogging Device for Managing Bemisia argentifolii (Homoptera: Aleyrodidae) in Greenhouse Vegetables (You can watch USDA's article in its July 1998 ARS journal at: Watch Out, Soft-Bodied Pests!.We suggest you to read the article below before you visit the USDA journal).

ALVIN M. SIMMONS and D. MICHAEL JACKSON

USDA, Agricultural Research Service, U. S. Vegetable Laboratory, 2875 Savannah Highway, Charleston, SC 29414

ABSTRACT This study evaluated the usefulness of an ultrasonic fogging device as a delivery system for low dosages of contact insecticides to manage Bemisia argentifolii Bellows and Perring on greenhouse vegetable seedlings. Insecticide treatments with conventional applicators generally do not reach the bottom leaf surface where whiteflies feed. The objective was to determine if this fogging device could provide good under-leaf coverage and good whitefly control at low insecticide dosages, and was conducted in cooperation with Strauch and Sons, Inc., Bethesda, MD. Six-minute fogging exposures were conducted on whitefly-infested collard (Brassica sativus L.) plants in a plastic-covered greenhouse, and infested plants in a separate greenhouse were used for untreated checks. Residue analysis indicated that a high percentage (76%) of sugar esters from Nicotiana glutinosa L., a biorational insecticide, was delivered to the bottom leaf surface compared with the upper surface (100%). The fogger provided good control (LD90 = 27.0 g ai/ha) of adult whiteflies on test plants with foliar imidacloprid at much lower than the label rate (112.3 g ai/ha). The fogger was operated over various temperatures and humidities, but no influence of vapor pressure deficit was observed on mortality. A wettable powder did not work in the fogger because it clogged the machine. Overall, the fogging device looks promising for managing whiteflies with low dosages of contact insecticide in a greenhouse system, and has a possibility for adaptation for field use.

VEGETABLE AND ROW crops in the United States have sustained much damage from Bemisia argentifolii Bellows and Perring (equals B. tabaci Gennadius strain-B, sweetpotato whitefly [Bellows et al. 1994]) over the past few years. Moreover, B. argentifolii and other whiteflies are problematic in greenhouses across the U.S. Thus, B. argentifolii has been the subject of intensive research efforts by federal, university, and industry personnel (USDA 1997). A high percentage of the adult and immature stages are found on the lower surface of leaves on many vegetables (Simmons 1994) and other hosts (van Lenteren and Noldus 1990, Lynch and Simmons 1993). One of the most common short-term whitefly management strategies has been the use of insecticides. However, most conventional insecticide applicators do not provide good coverage of the under leaf surface. Consequently, inadequate mortality is often the result. Not only are insecticides used to directly manage whiteflies on crops, but efforts are also needed to manage associated viruses and plant disorders associated with whiteflies (Cohen and Marco 1973, Nuessly and Perring 1995, Valdez and Wolfenbarger 1995). Because of high cost and other negative aspects of managing insects with insecticides (Pimentel et al. 1991), effective pest management technologies with reduced rates of toxic compounds in vegetable production would be welcomed. We entered into a cooperative research and development agreement with Strauch and Sons, Inc., Bethesda, MD, to evaluate an ultrasonic fogging device as a delivery system for effectively managing B. argentifolii on vegetables in the greenhouse using low dosage of contact insecticides.

Materials and Methods

Trials were conducted in two greenhouses; each was 6.1 m by 4.6 m and was covered with 4-mil plastic. A fogger unit (model FG-620, Shira Aeroponics Inc., Rehovot, Israel) was situated outside one greenhouse and an opening was constructed through which the duct of the fogger was inserted. According to the manufacture, the device dispenses about 15 liters of water per hour, and produces moisture droplets of about 5 m which behaves as a gas. It is 46 x 71 cm and 48 cm high, and about 32 kg. The duct was 21.5 cm diameter, turned upward at about 45 degrees, and the bottom was located 1.1 m from the floor of the greenhouse. A bench, 0.9 m high, was set up in each greenhouse to hold the test plants. A preliminary trial was conducted to see if insecticide materials were able to reach all surfaces of plants, especially the lower surface of leaves. Sugar esters from Nicotiana glutinosa L.(provided by USDA-ARS, Phytochemical Research Unit, Athens, GA) were used for the residue trial. Teflon disks (3 per location) were placed 1.2, 1.8, and 2.4 m from the fog duct and positioned 0.9 m from the floor so that at each distance, either the top or bottom surfaces were exposed or positioned so that one surface was exposed in a vertical plane. Analyses of the residue on disk samples were conducted by the USDA-ARS, Phytochemical Research Unit in Athens, Georgia using published techniques (Severson et al. 1985).

Dosage mortality data were collected from whitefly-infested collard (Brassica sativus L.) plants following fogging of various dosages of imidacloprid (Provado 1.6 Flowable, Bayer Corporation, Kansas City, MO) in one greenhouse and on untreated plants in another greenhouse. Six dosages, were tested with three replicates of 56.16, 28.08, and 14.04, 6.07 and 1.80 g ai/ha; and four replicates for dosage 7.08 g ai/ha. Also, a test of only water in the fogger was used to compare with the untreated plants. Collard plants for testing were grown in a separate whitefly-free greenhouse. About 48 h before each trial, test plants were exposed in a greenhouse infested with B. argentifolii. The whitefly colony was established in 1992 from a field of sweetpotato, Ipomoea batatas (L.), and had been maintained on several vegetable hosts, and feral B. argentifolii from sweetpotato were added annually. After 12 h, these whitefly-infested plants were gently moved to the fog treatment and untreated greenhouses. A plant was placed 1.2, 1.8, and 2.4 m from the fog duct; additional plants were set up with the same spacing in the untreated greenhouse. The number of adult whiteflies on selected leaves were determined by direct counts before and 24 hours after each insecticide test. The number of whiteflies on the bottom surface of three leaves (3-5 nodes from top) of each of three plants was determined. The number of whiteflies on all leaves were not counted, although whiteflies moved among leaves and plants. Hence, data for the control (no insecticide greenhouse) reflected whitefly dispersal as well as possible mortality. However, if more whiteflies were recovered at the end of each 24 h than at the beginning, the data were adjusted to assume 100% survival. This occurred only once. The number of adults per sampled three leaves per plant ranged from 124 to 1,413 whiteflies at the beginning of the trials. The duration of the fogging for the mortality trials was 6 minutes, and was selected following preliminary trials. Data on temperature and relative humidity at the start of each trial were recorded, and were used for determining vapor pressure deficit. Dosage-mortality data were analyzed assuming the probit model(LeOra Software 1994). Correlations between environment parameters and mortality were done after correction for natural mortality(Abbott 1925) and transformation using log base 10 (x + 1) (SAS 1994).

Results and Discussion

Sugar ester residues were recovered from Teflon disks after the fog treatment. A high percentage (76%) of the sugar ester was recovered from the bottom exposed surface (3.8 mg/cm2) compared with the top exposed surface (5.0 mg/cm2) of the disks. Similarly, vertically positioned disks received a high amount of residue (4.7 mg/cm2)compared with the top exposed surface. These data show that the fogger device can deliver insecticide to the bottom surface where it is need for whitefly control.

Imidacloprid is one of the newest and much used insecticides by growers for whitefly management. The label rate for whitefly control with the Provado formulation is 112.32 g ai/ha. Preliminary tests with the fogger device using dosages of 84.24, 112.32, and 280.80 g ai/ha had resulted in 100% mortality. These data were not included in the probit analysis because a lower dosage provided 100% mortality.

For higher dosage imidacloprid-treated plants, numerous dead adult whiteflies were observed after 24 h, in addition to finding few live adults on the plants. As the insects were overwhelmed by the insecticide, they dropped from the bottom of an upper leaf and fell on the top of a lower leaf. Good control was obtained using reduced rates compared with the label rate (Fig. 1). The LD90 on the whiteflies was 27.0 g ai/ha (slope 2.77 + 0.52)(Table 1). Although dead adults were observed on leaves at lower dosages, numerous live adults were also observed. Nevertheless, there was noticeable mortality with dosages as low as 7.07 g ai/ha. When no insecticide was used in the fogger, no dead whiteflies were observed.

Possible factors that may affect efficacy of the fogger include moisture, temperature, duration of fog, and dosage. Vapor pressure deficit, which expresses moisture independently of temperature, is biologically more meaningful than relative humidity (Ferro and Chapman 1979). Neither vapor pressure deficit nor relative humidity were correlated with mortality, although temperature was positively correlated (P < 0.01; r = 0.56) with whitefly mortality in the fog dosage trials. The size of an area to be treated may also be a factor, i.e., a larger greenhouse would require more fogging time to effectively control whiteflies compared with a smaller greenhouse, and the maximum greenhouse size limitation is not known.

An additional trial was attempted with a wettable powder insecticide. However, the fogging unit dispensed little fog then stopped fogging. Apparently, there was much settling of the wettable powder and the cylinder units were physically unable to displace the tank mixture which lacked agitation. Thus, wettable powders are apparently not good candidates for use in the fogger.

To better understand the potential for using this fogging device for greenhouse and/or for field conditions, additional data are needed. Nevertheless, this method of application is convenient and may be economical. Also, additional useful data are needed on the effect of test materials on immatures (eggs and nymphs) of the whitefly because only adults were evaluated.

In summary, residue analysis indicate that a high percentage of contact insecticide was delivered to the lower surface. The fogger provided good control of whiteflies on test plants in a greenhouse while using much reduced rates of imidacloprid insecticide. This fogger may have applications for greenhouse commodities other than vegetables. For example, commercial plants such as poinsettia are good whitefly hosts, but have low damage thresholds for esthetic reasons. This fogger may help growers manage whiteflies on these and other greenhouse commodities with low level of contact insecticides.

Commercial greenhouses can harbor whiteflies year round. Migration from the nursery and transplanting of infested seedlings can be a meaningful source of field infestation (Simmons and Elsey 1995).

Overall, the fogger looks very promising for use in a greenhouse, and a possibility for adaptation for field use, and could be used to target other insects with low-volume of environmentally friendly insecticides.

Acknowledgments

Thanks for technical assistance is extended to Bushrod Davis and Julie Day. We also thank Larry Chandler and Gary Leibee for their helpful comments on an earlier manuscript draft.

References Cited

Abbott, W. S. 1925. A method of computing the effectiveness of an insecticide. J. Econ. Entomol. 18: 265-267.

Bellows, Jr., T.S., T.M. Perring, R. J. Gill, and D.H. Headrick. 1994. Description of a species of Bemisia (Homoptera: Aleyrodidae). Ann. Entomol. Soc. Am. 81: 195--206.

Cohen, S., and S. Marco. 1973. Reducing the spread of aphid-transmitted viruses in peppers by trapping the aphids on sticky yellow polyethylene sheets. Phytopath. 63: 1207--1209.

Ferro, D. N., and R. B. Chapman. 1979. Effects of different constant humidities and temperatures on two-spotted spider mite egg hatch. Environ. Entomol. 8: 701--705.

LeOra Software. 1994. Polo-PC, probit and logit analysis. LeOra Software, 11199 Shattuck Ave., Berkeley, CA.

Lynch R. E., and A. M. Simmons. 1993. Distribution of immatures and monitoring of adult sweetpotato whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), in peanut, Arachis hypogaea. Environ. Entomol. 22: 375--380.

Nuessly, G. S., and T. M. Perring. 1995. Influence of endosulfan on Bemisia tabaci (Homoptera: Aleyrodidae) populations, parasitism, and lettuce infectious yellows virus in late-summer planted cantaloupe. J. Entomol. Sci. 30: 49--61.

Pimentel, D., L. McLaughlin, A. Zepp, B. Lakitan, T. Kraus, P. Kleinman, F. Vancini, W. J. Roach, E. Graap, W. S. Keeton, and G. Selig. 1991. Environmental and economic effects of reducing pesticide use. BioScience 41: 402--409.

SAS Institute. 1994. SAS/STAT user's guide, version 6, 4th ed. SAS Institute, Cary, NC.

Severson, R. F., R. F. Arrendale, O. T. Chortyk, C. R. Green, F. A. Thome, J. L. Stewart, and A. W. Johnson. 1985. Isolation and characterization of the sucrose esters of the cuticular waxes of green tobacco leaf. J. Agric. Food Chem. 33: 870--875.

Simmons, A. M. 1994. Oviposition on vegetables by Bemisia tabaci (Homoptera: Aleyrodidae): temporal and leaf surface factors. Environ. Entomol. 23: 382--389.

Simmons, A. M., and K. D. Elsey. 1995. Overwintering and cold tolerance of Bemisia argentifolii (Homoptera: Aleyrodidae) in coastal South Carolina. J. Entomol. Sci. 30: 497--506.

Valdez, J. A., and D. A. Wolfenbarger. 1995. Yellow traps and insecticides for control of a strain of sweet potato whitefly and associated virus incidence on pepper. J. Entomol. Sci. 30: 342--348.

van Lenteren, J. C. & L. P. J. J. Noldus. 1990. Whitefly-plant relationships: behavioral and ecological aspects, pp. 47-89. In D. Gerling [ed.], Whiteflies: their bionomics, pest status and management. Intercept, Andover, Hants, U. K.

Footnotes

This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or recommendation for its use by USDA.

Table 1.
Dose response of adult B. argentifolii 24 h after 6-minute fogging in greenhouse with imidacloprid; labeled rate, 112.32 g ai/ha.

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Effective       Dose       Lower       Upper
doses       (g ai/ha)       limitsa limitsa

LD10       3.20           2.03           4.33

LD50       9.31           7.54           11.24

LD90       26.98         21.10         38.65

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a95% confidence limits.

Figures

Fig. 1. Probit mortality ratio of B. argentifolii adults for different log dosages of fogged foliar imidacloprid in greenhouse; the labeled rate is 112.3 g ai/ha.

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