Biological Control of Silverleaf Whitefly in Floriculture

1995 Annual Report for AS95-022

Project Type: Research and Education
Funds awarded in 1995: $0.00
Projected End Date: 12/31/1995
ACE Funds: $45,389.00
Region: Southern
State: Georgia
Principal Investigator:
Mary Harris
University of Iowa

Biological Control of Silverleaf Whitefly in Floriculture


Silverleaf whiteflies are a serious pest of many field and floricultural crops and chemical insecticides are relied upon for control. In addition to environmental and worker exposure risks, the effective use of insecticides for whitefly control can be greatly diminished through the development of insecticide resistance by these insects. Use of alternative control methods in addition to chemical insecticides would not only reduce environmental and worker contamination risks, but also would relieve pressure on the insects to develop insecticide resistance. This, in turn, would sustain the efficacy of the insecticides while reducing the amounts of chemicals required to successfully suppress this pest.

One such alternative is the insect-attacking fungus Paecilomyces fumosoroseus (PFR). PFR infects and kills all stages of silverleaf whiteflies and has been developed for commercial use on floricultural crops produced in greenhouses. Plant diseases must also be controlled on such crops, such control usually being achieved through chemical fungicide applications. For a successful IPM program utilizing a beneficial fungus such as PFR for whitefly control compatible fungicides that will not inhibit PFR must be identified and safe intervals determined for their use.

1) Determine fungicides compatible with PFR and application schedules of compatible fungicides for use with PFR.

2) Determine the efficacy of treatment of plant material with PFR for eliminating whitefly infestations during propagation and prior to shipping to producers.

3) Determine the efficacy of PFR in managing whiteflies under commercial production conditions in comparison to the conventional chemical control.

To determine fungicide compatibility with PFR, a laboratory bioassay was utilized in which PFR was grown in Petri dishes and exposed to fungicide-saturated filter paper discs. Aliette, Potassium bicarbonate, Captan, Chipco26019, Cleary’s 3336WP, Daconil, Fore, Kocide, Subdue, Systec, Terrachlor, and Triforine were tested, at the manufacturer’s recommended rate, along with 10% clorox (as an inhibition standard) and distilled water (as a no-inhibition standard). Inhibition of PFR growth was assessed following a 72 h period of exposure to the treated discs.

Potassium bicarbonate, Captan, Daconil, and Triforine were identified as inhibitory to PFR.These fungicides were then included in additional bioassays to determine their comparative strengths of inhibition. Aliette, although not determined to be inhibitory to PFR germination, was included in these further tests because of its widespread and frequent use in floricultural crop production. The same procedures were followed as before, however, widths of the zones surrounding each treated disc in which PFR growth was inhibited was measured. We refer to these areas as the zones of inhibition (ZOI).

Each fungicide in the inhibitory subset were applied to greenhouse-grown lantana plants to develop treatment schedules of the fungicides and PFR that would not diminish the efficacy of PFR. After 0, 3, 7, and 14 days after fungicide treatment, plants were treated with PFR and leaf rinses collected. Samples of these rinses were then plated on artificial media in the laboratory and PFR germination assessed by counting colonies. The effect of fungicide on PFR growth also was assessed by measuring the diameter of a random sample of colonies.

The efficacy of preshipment treatment of rooted cuttings with PFR to eliminate existing whitefly infestations was determined cooperatively. Verbena cuttings (cv. Homestead Purple) were rooted at the Research and Education Center, University of Florida, Apopka, FL greenhouses. Cuttings were allowed to become infested by silverleaf whiteflies and counts of these insects made on individual leaves. Cuttings were then treated with varying rates of PFR, placed in coolers to simulate shipping conditions, and then held for 1 week at low (one half of the cuttings) or high humidity (the remaining half). Whiteflies were counted and % mortality determined.

Additional cuttings on which whitefly numbers had been counted were treated with varying rates of PFR, with and without a wetting agent (Cell-U-Wett; C.S.I., Michigan City, IN) and shipped overnight to the Experiment Station in Griffin, Georgia. The numbers of alive, dead, and emerged whiteflies were then counted on the individual leaves.

Commercial production use of PFR was assessed at the Georgia Experiment Station and at commercial cooperators: Bill’s Greenhouses, McDonough, GA and Camp and Co. Greenhouses, Dacula, GA. Lantana cuttings which had received PFR treatments during propagation or prior to shipment and cuttings which had received no treatment were grown to a saleable size in 10-cm square pots. Weekly whitefly counts were made on plants in the following production treatment groups: 1) PFR preshipment / PFR production, 2) PFR / imidacloprid (Marathon=AE, Olympic Chem. Co., Mainland, PA), 3) none / PFR, 4) none / imidacloprid. When the cuttings were established a single imidacloprid treatment was applied to plants in treatment groups 2 and 4. At this time, a series of 4 applications, a 1-week interval, of PFR were initiated on plants in treatment groups 1 and 3.

Of the 12 fungicides tested in the laboratory the following were not observed to be inhibitory to the germination of PFR: Aliette, Chipco26019, Cleary’s 3336WP, Fore, Kocide, Subdue, Systec, and Terrachlor. Potassium bicarbonate, Captan, Daconil, and Triforine did inhibit PFR germination, although, only the inhibition associated with Daconil was statistically significant. In addition, Daconil consistently and significantly inhibited PFR growth, whereas Sodium bicarbonate and Triforine each inhibited PFR growth in individual tests, but the overall level was insignificant. Aliette neither inhibited germination nor growth of PFR.

Similar results were obtained when fungicide effect on germination and growth of PFR on plants treated in the greenhouse was assessed. Aliette did not affect colony number or width. Neither Potassium bicarbonate nor Triforine reduced PFR germination or growth, on the contrary, a significant increase in colony width was observed 3 days after treatment with Potassium bicarbonate. Only Captan and Daconil significantly reduced numbers of PFR colonies one day after fungicide treatment. Furthermore, these two fungi cides reduced mycelial growth, an effect sustained three and seven days after treatment. These results suggest a safe interval of at least three days after a treatment with Aliette, Potassium bicarbonate, or Triforine prior to an application of PFR. If Daconil must be used, application of PFR should not be made sooner than 14 days after treatment.

The use of Captan in an IPM program, which includes PFR is contraindicated by the greenhouse results. The potential compromise of PFR by the use of Captan was not indicated by measuring zones of inhibition in the laboratory bioassay. However, Captan was included among the fungicides selected from the initial list for further testing because of the detection of inhibition, although at a statistically insignificant level. The possibility of false negatives in the laboratory strongly suggests that fungicide effects should be examined on treated foliage in addition to growth-media in the laboratory.

Treatment of rooted cutting with PFR prior to simulated or actual shipment consistently reduced whitefly infestations, although not all treatment combinations resulted in significant reduction. Results of the simulated shipments indicate 5 g PFR formulated material per liter will reduce whitefly infestations by 52.52% on cuttings held at lower humidity following shipment. Cuttings treated with 5 g PFR per liter again resulted in the highest mortality levels with the wetting agent (86.9%) or without it (86.8%).

Lantana cuttings which were propagated by Camp and Co. Greenhouses were not treated during rooting. There was little or no whitefly pressure during this period. One-half of the rooted cuttings were treated prior to shipment to Bill’s Greenhouses at which time we were unable to find a single whitefly immature among the material. These cuttings were divided into their respective production-treatment groups and followed until sale, 4 weeks after the last PFR treatment. Although whitefly pressure was never great, plants receiving PFR during production consistently had fewer whiteflies than imidacloprid treated plants (these differences were significant on 2 of the 5 count dates).

Both lantana and verbena cuttings were propagated and then grown to saleable size at the Georgia Experiment Station under more intense whitefly pressure than that encountered at our commercial cooperators. Plants which had received PFR applications during rooting had lower mean numbers of whiteflies during early production than did plants which had received no treatment. This difference was particularly evident among the verbena plants. Furthermore, PFR production-treated plants had statistically indistinguishable numbers of whiteflies from those on imidacloprid treated plants.

Impact of Results
The results of this project clearly demonstrate the effective use of a biological control agent in floricultural greenhouse production in the southeastern United States. Specifically, the insect pathogenic fungus, Paecilomyces fumosoroseus (PFR), provided efficacious control of the silverleaf whitefly, Bemesia argentifolii, on lantana and verbena. Levels of control achieved with this fungus were statistically indistinguishable to those obtained through the use of the current chemical insecticide standard, imidacloprid. In addition, this research demonstrates the effectiveness of pre-shipment PFR treatments to eliminate whitefly infestations on propagated plant material. Through early infestation control during crop production, total pesticide use will be reduced.

The results of this study also provide lists of both PFR-compatible and incompatible fungicides to aid growers in fungicide treatment decisions within a pest management program that includes the use of PFR. Furthermore, these results provide growers with safe time intervals for scheduling compatible fungicide and subsequent PFR applications so that the efficacy of PFR will not be compromised.

These results provide floricultural growers with information to successfully utilize an alternative control strategy for one of the industry’s most important insect pests. Alternatives, such as PFR, to chemical control are more likely to be accepted if they can be demonstrated to be efficacious and that their use will not compromise the control of other pests, such as fungal plant pathogens. Through the use of alternatives, chemical pesticides will be used less often which will reduce environmental contamination, worker exposure, and the development of pesticide resistance.

December 1996.