1997 Season Update on Resistance of Arizona Whiteflies to Synergized Pyrethroid and Select Non-Pyrethroid Insecticides
Timothy J. Dennehy, Extension Arthropod Resistance Management Laboratory
A whitefly resistance crisis in Arizona in 1995 prompted the development of a resistance management strategy in 1996 that recommended maximal once per season use of two insect growth regulators, pyriproxyfen (Knack®) and buprofezin (Applaud®), and limited and delayed use of synergized pyrethroid insecticides in cotton. Statewide monitoring of whitefly resistance has shown that implementation of this strategy has substantially reduced whitefly resistance to the synergized pyrethroids and has also resulted in increased susceptibility to key non-pyrethroid insecticides. Having benefited from two years of success with this strategy, the Arizona cotton industry now faces the question of whether it can be sustained as pyriproxyfen and buprofezin gain additional registrations for use against whiteflies in vegetables, melons and glasshouse crops.
Since the late 1980's, whiteflies have caused severe damage to cotton, vegetable, and melon crops grown in the irrigated deserts of the southwestern U.S. (Byrne et al. 1990). In cotton, whitefly densities exceeding 5-10 adults per leaf can result in severe reductions in lint quality due to stickiness caused by excreta (honeydew) and subsequent development of sooty mold fungi (Diehl and Ellsworth 1995). Intensive reliance on a limited array of insecticides to combat whitefly populations resulted in an insecticide resistance crisis in Arizona cotton in 1995. This crisis prompted the formulation and implementation of a successful whitefly resistance management program (Dennehy and Williams 1997). In this paper we provide data from statewide monitoring of whitefly resistance in order to assess the success of the program at the end of its second year.
Materials and Methods
Monitoring of Whitefly Resistance to Insecticides
Whitefly were collected in 1997 from 12 locations throughout the major cotton production areas of Arizona. Adult whiteflies were vacuum-collected directly from cotton foliage into plastic vials using a Makita® cordless vacuum (4071D). Samples were transported in ice chests directly to EARML where they were released into cages containing young cotton plants, Gossypium hirsutum L. (DPL-50). Adult whitefly were maintained in these cages until they were placed in bioassays (<7 days). Susceptibility of each population to the following insecticide treatments was estimated: 1) Curacron® (profenofos) + Lorsban® (chlorpyrifos) (1:1 ratio); 2) Curacron + Vydate® (oxamyl) (1:1 ratio); 3) Gowan Thiodan® (endosulfan); 4) a fixed concentration of 1,000 µg/ml of active ingredient Orthene® (acephate) combined with varying concentrations of Danitol® (fenpropathrin); 5) Ovasyn® (amitraz); and 6) Ovasyn + Gowan Thiodan (1:1 ratio).
A derivative of the Rothamsted leaf disk bioassay method (Rowland et al. 1990) was used. Leaf punches 2.5 cm in diameter were taken from cotton plants (DPL-50) 18 - 26 days old. The leaf disks were dipped for 10 s in formulated insecticide diluted in water. After drying, the disks were placed individually on a base of agar (1.3%) within 20 ml glass scintillation vials. Within 2 h of dipping, 20-30 adult whiteflies were aspirated into each vial. Assays were then held in an incubator at 27 ° C for 48 h, after which they were scored using a binocular dissecting scope. Vials were tapped on the counter 10 times after which whiteflies not exhibiting repetitive movement of more than one appendage were scored as dead. At least five different concentrations and six bioassay replications per concentration were evaluated for each insecticide tested.
Results and Discussion
Table 1 presents 1997 results of whitefly susceptibility to mixtures of Danitol+Orthene. Contrasts of 1995, 1996, and 1997 results from statewide collections show significant increases in overall susceptibility to Danitol+Orthene each of the past two years (Figure 1). Severe resistance to the synergized pyrethroids in 1995 was correlated with substantial (<80%) mortality (Figure 1) in bioassays of 100 µg/ml fenpropathrin (+ 1000 µg/ml acephate).
Based on mortality observed in bioassays of 10 µg/ml fenpropathrin+1000 µg/ml acephate (Table 1), the populations with lowest susceptibility to this insecticide mixture were observed in Somerton and Central Arizona locations, Buckeye, Coolidge, and Maricopa. Producers in these areas should be especially careful to limit pyrethroid use to two applications per season to avoid driving resistance levels any higher. Sivasupramaniam et al. (1997) found that resistance to Danitol+Orthene conferred cross-resistance to all of the pyrethroids evaluated for controlling whiteflies in Arizona. Our strategy continues to emphasize holding the synergized pyrethroids in reserve to be used as a last resort, should they be needed late in the season when the crop is at greatest risk of being contaminated by honeydew.
Our 1996 and 1997 data support the conclusion that the Arizona Whitefly Resistance Management Program has yielded benefits in terms of reductions in resistance to synergized pyrethroids.
Table 2 presents 1997 results of whitefly susceptibility to Thiodan. Contrasts of 1995, 1996, and 1997 results from statewide collections show small increases in overall susceptibility to Thiodan each of the past two years (Figure 2). Populations with the lowest susceptibility to Thiodan originated from Buckeye and Mohave Valley in 1997, based on mortality in bioassays of 10 µg/ml (Table 2). The most susceptible populations were found in Safford, Paloma and Coolidge.
Table 3 presents 1997 results of whitefly susceptibility to Ovasyn. Contrasts of 1995, 1996 and 1997 results from statewide collections revealed a small increase in susceptibility from 1995 to 1996 and a decrease in susceptibility to Ovasyn in 1997 (Figure 3). Ovasyn is an important component of the whitefly control program in Arizona. As such, care should be taken to this insecticide in rotations of products that will avoid its overuse.
There was no change in overall susceptibility to the mixture of Thiodan+Ovasyn from 1996 to 1997 (Table 4). Buckeye, Marana, and Maricopa populations were least susceptible to this mixture and Yuma, Coolidge and Parker populations were the most susceptible.
Arizona whitefly populations differed significantly in susceptibility to Curacron+Lorsban in 1997 (Table 5). In bioassays of 10 µg/ml, mortality was as high as 94-109% for the Somerton and Yuma populations or as low as 13% for Coolidge and Maricopa populations. However, based on statewide means and medians, there was no significant change in susceptibility to Curacron+Lorsban from 1996 to 1997.
Arizona whitefly populations were slightly more susceptible to mixtures of Curacron+Vydate in 1997 than 1996 (Table 6). Additionally, populations evaluated in 1997 differed significantly in susceptibility to this mixture. The most susceptible populations originated from Paloma and Yuma. Populations least susceptible to Curacron+Vydate were found at Marana and Safford.
We thank Cotton Incorporated, the Arizona Cotton Growers Association and the University of Arizona for financial support for this project. We thank P. Else and the staff of the UA Campus Agricultural Center for assistance with maintaining the EARML laboratories and greenhouses.
This is a part of publication
AZ1006: "Cotton: A College
of Agriculture Report," 1998, College of
Agriculture, The University of Arizona, Tucson, Arizona,
85721. Any products, services, or organizations that are
mentioned, shown, or indirectly implied in this
publication do not imply endorsement by The University of
Arizona. The University is an Equal
Opportunity/Affirmative Action Employer.