T.J. Dennehy, Xiaohua Li & Monika Wigert
A resistance crisis in 1995 in Arizona cotton prompted the development of an integrated resistance management strategy targeting whiteflies. The strategy recommended use of appropriate sampling and thresholds, once-per-season use of the insect growth regulators, Knack ® (pyriproxyfen), and Applaud ® (buprofezin), and measures to delay use of pyrethroids for as long into the growing season as possible. Results of the use of this strategy in the 1996, 1997, and 1998 seasons have been very promising in terms of whitefly control, overall reductions in insecticide use against whiteflies, and reduced resistance of whiteflies. Statewide monitoring in 1998 demonstrated that, with a few notable exceptions, susceptibility to synergized pyrethroids continued to be high in whiteflies throughout much of the State. This represents a remarkable reversal of the extreme levels of pyrethroid resistance we documented in 1995. However, intermediate levels of resistance to synergized pyrethroids were detected in Somerton, Dome Valley and Casa Grande, Arizona in 1998. While not nearly as resistant as populations were in 1995, these sites with intermediate levels of resistance highlighted the vulnerability of the present success and the necessity for continued educational programs emphasizing limiting pyrethroid use and diversifying the insecticides used against whiteflies as well as other cotton pests. Overall reductions in resistance to synergized pyrethroids were correlated with small but consistent increases in susceptibility to endosulfan from 1995 to 1998. It appears that the reduced insecticide use resulting from implementation of the new strategy has served to preserve efficacy of nonpyrethroid insecticides as well as the pyrethroids insecticides for which it was devised.
Our data show clearly that Arizona cotton growers have managed resistance to synergized pyrethroid insecticides and in doing so have reduced overall costs of whitefly control and enhanced the sustainability of their IPM program.
In 1995, many cotton growers in Central Arizona experienced severe failure of virtually all insecticides registered for control of whiteflies, Bemisia argentifolii Bellows and Perring. In problem areas, they applied as many as 8-
12 spray applications, and spent in excess of $200 per acre on insecticides (Dennehy and Williams, III, 1997).
Most striking was the development of whitefly resistance to pyrethroid insecticide mixtures, pyrethroids mixed with non-pyrethroid ‘synergist’ (Dennehy and Williams 1997). The synergist was commonly the
This is part of the 1999 Arizona Cotton Report, The University of Arizona College of Agriculture, index at http://ag.arizona.edu/pubs/crops/az1123/

organophosphate, Orthene ® (acephate), but was also a variety of carbamate, cyclodiene or formamidine insecticides. Prior to 1995, these so-called ‘synergized pyrethroids’ were the most effective insecticide treatments for controlling whiteflies in Arizona cotton. With the onset of the crisis of 1995, failure of synergized pyrethroid treatments was widespread. In severe cases, treatments provided less than a few days of suppression of adult whiteflies (Simmons and Dennehy 1996).
Statewide monitoring of resistance conducted by the Extension Arthropod Resistance Management Laboratory
(EARML) revealed that highly resistant whitefly populations were present at locations throughout Central Arizona but were less common in the eastern and western regions of the State (Dennehy et al. 1996, 1997). Laboratory studies provided evidence of cross-resistance between Danitol+Orthene and essentially all of the pyrethroid insecticides registered for use in Arizona cotton (Sivasupramaniam et al. 1997).
The 1995 resistance crisis in Arizona cotton prompted the development of an integrated resistance management strategy targeting whiteflies. The strategy incorporated once-per-season use of the insect growth regulators,
Knack ® (pyriproxyfen) and Applaud ® (buprofezin), and measures to delay the use of pyrethroids for as long into the growing season as possible. Results of use of this strategy in the 1996, 1997 and 1998 seasons have been very promising. Insecticide use against whiteflies has declined from an average of • 6 treatments per acre in 1995
(range 2-12) to 1-2 treatments per acre in 1996-98 (Fig. 1).
Whitefly were collected from 16 location throughout the major cotton production areas of Arizona (Figure 2). Of these, 11-12 populations had numbers and/or survivorship that allowed bioassay results to be used. Adult whiteflies were vacuum-collected directly from cotton foliage into plastic vials using a Makita® cordless vacuum
(4071D). The samples were transported in ice chests directly to the Extension Arthropod Resistance Management
Laboratory (EARLM) in Tucson, where they were released into cages containing young cotton plants (DPL-50).
Adult whitefly were maintained in these cages until they were placed in bioassays (<7 days). Susceptibility of each population was estimated to the following insecticide treatments: 1) a fixed concentration of 1,000 µg/ml of active ingredient Orthene® 75S (acephate) (Valent USA) combined with concentrations of Danitol® 2.4 EC
(fenpropathrin) (Valent USA), (2) Thiodan® 3EC (endosulfan) (FMC Corporation), (3) Thiodan 3 EC+Ovasyn®
(amitraz) (AgrEvo USA Company) in a 1:1 ratio, and (4) Curacron® (profenofos) (Novartis Crop Protection, Inc.)
+Vydate® C-LV (oxamyl) (DuPont Agricultural Products) in a 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 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 three different concentrations and six bioassay replications per concentration were evaluated for each insecticide tested.
Table 1 presents 1998 results of whitefly susceptibility to mixtures of Danitol+Orthene. Contrasts of 1995-1998 results from statewide collections (Fig. 2) show striking increases in overall susceptibly to Danitol+Orthene from
1995 to 1997, and sustained susceptibility during the 1998 season (Fig. 3). Severe resistance to the synergized pyrethriods in 1995 was correlated with <50% mortality in bioassays of 10 µg/ml fenpropathrin (+1,000 µg/ml acephate).
Based on mortality observed in bioassays of 10 µg/ml fenpropathrin + 1,000 µg/ml acephate (Table 1), the populations with lowest susceptibility to this insecticide mixture were observed in Somerton, Dome Valley, and

Casa Grande (Table 1). Producers in these areas should be especially careful to limit pyrethroid use to two applications per season to avoid driving resistance levels 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 1998 data supports the conclusion that the Arizona Whitefly Resistance
Management Program has yielded benefits in terms of reductions in resistance to synergized pyrethroids.
®
Table 2 presents 1998 result of Arizona whitefly susceptibility to Thiodan. Contrasts of 1995-1998 results show small increases in median measures of susceptibly to Thiodan each of the past three years (Fig. 4). However, very low mortality was observed in 1998 bioassays of 10 µg/ml of populations from Maricopa and Parker (Table 2). As in 1997, populations with the higher levels of susceptibility to endosulfan were found a Safford and Coolidge in
1998. Also, in 1998, populations tested from Buckeye and Marana were especially susceptible to endosulfan.
These results indicate that some areas are using endosulfan in a sustainable regime, while others are using it in a manner that has caused resistance levels to increase. Maricopa Agricultural Center whiteflies in 1998 had the lowest mortality that we have ever observed (1.3%) in bioassays of 10 µg/ml endosulfan (Table 2).
®
®
There was no change in overall susceptibility to the mixture of Thiodan+Ovasyn from 1996 to 1998 (Table 3).
®
®
Median and mean values of mortality in diagnostic concentration bioassays of Curacron + Vydate declined in
1998 (Table 6). Whitefly populations were somewhat more susceptible to this insecticide mixture in 1997 and
1996. This was one more indication that some populations of Arizona whiteflies have become less susceptible to specific non-pyrethroid insecticides. Given the low number of whitefly treatments applied in 1998 (Fig. 1), is very likely that treatments for pests other than whiteflies account for these reductions in whitefly susceptibility.
This again, underscores the importance of rotating chemical groups for all the arthropods being controlled in cotton fields.
Four years of statewide monitoring of Arizona whiteflies has demonstrated a remarkable recovery of susceptibility to synergized pyrethroids, following implementation of the resistance management strategy in 1996 (Fig. 3). It has also revealed correlated increases in susceptibility to other non-pyrethroid insecticides, such as endosulfan
(Fig. 4). These field data refute the notion that once insecticides begin to fail in the field that little can be done to reclaim them for future use. Synergized pyrethroid insecticides continue to be used in a limited and strategic fashion in our strategy. However, to combat resistance development, pyrethroid use against whiteflies should continue to be delayed as long as possible in the growing season and should be limited to at most two treatments per season.
The intermediate levels of resistance to synergized pyrethroids detected in the whitefly populations from
Somerton, Dome Valley and Casa Grande, Arizona, should serve as a warning to producers. While not nearly as resistant as were highly resistant populations in 1995, the 1998 collections with elevated resistance to Danitol +
Orthene illustrated the fragility of the current success in managing pyrethroid resistance and emphasized the need for continuation of educational programs emphasizing limited pyrethroid use and diversifying the insecticides used in cotton.
As in 1996 and 1997, overall reductions in resistance to synergized pyrethroids in 1998 were associated with increased whitefly susceptibility to Thiodan (Figure 4). This very positive result should be tempered by the

observations that a few populations in 1998 had substantially reduced susceptibility to endosulfan (Table 2) and overall susceptibility to the mixture of profenofos + oxamyl was down slightly statewide (Table 4). Where they occurred, we attribute these reductions in whitefly susceptibility to severe lygus bug problems in 1998 that caused some cotton growers to make repeated insecticide treatments. The Arizona whitefly resistance management program depends on limited use of a broad range of insecticide groups. Loss of any members of this suite put other insecticides at greater risk of resistance. It is for this reason that we will continue to educate producers regarding the need to limit and diversify their insecticide use. Though a fragile success, our data show clearly that
Arizona growers have managed resistance to synergized pyrethroid insecticides and in doing so have reduced their overall costs of whitefly control and improved the sustainability of the cotton IPM program.
We thank Cotton Incorporated, the Arizona Cotton Growers Association and the University of Arizona for financial support for this work. We recognize P. Else and the staff of the UA Campus Agricultural Center for assistance with maintaining the EARML laboratories and greenhouses.
Byrne D.N., T.S. Bellows, Jr., and M.P. Parrella. 1990. Whiteflies in agricultural systems, pp. 227-261. In: D.
Gerling (ed.), Whiteflies: Their Bionomics, Pest Status and Management. Intercept, Andover, U.K.
Dennehy, T.J. and L. Williams, III. 1997. Management of resistance in Bemisia in Arizona cotton. Pesticide
Science 51:398-406.
Dennehy, T.J., Livy Williams, III, June S. Russell, Xiaohua Li and Monika Wigert. 1996. Monitoring and management of whitefly resistance to insecticides in Arizona. Proc. Beltwide Cotton Conferences. National
Cotton Council.
Dennehy, T.J., L. Williams, III, X. Li, M. Wigert and E. Birdwell. 1997. Status of whitefly resistance to insecticides in Arizona cotton. Proc. Beltwide Cotton Conferences. National Cotton Council.
Diehl, J.W. and P.C. Ellsworth. 1995. Whiteflies in Arizona No. 3: Evaluation of sampling plans. University of
Arizona Cooperative Extension. (Bulletin).
Ellsworth, P.W. 1998. Whitefly management in Arizona: looking at the whole system. Proc. Beltwide Cotton
Conferences. Natinal Cotton Council.
Palumbo, J.C. 1994. Insecticidal control of sweetpotato whitefly on spring melons, p. 106. In: T. J. Henneberry and N. C. Toscano (eds.), Silverleaf Whitefly (formerly Sweetpotato Whitefly) 1994 Supplement to the Five-Year
National Research and Action Plan, USDA-ARS No. 125.
Rowland, M., and B. Pye, M. Stribley, B. Hackett, I. Denholm, & R.M. Sawicki. 1990. Laboratory apparatus and techniques for the rearing and insecticidal treatment of whitefly, Bemisia tabaci, under simulated field conditions.
Bull. Entomol. Res., 80:209-216.
Simmons, A.L. and T.J. Dennehy. 1996. Contrasts of three insecticide resistance montoring methods for whitefly. Proc. Beltwide Cotton Conferences. National Cotton Council.
Sivasupramaniam, S., T.J. Dennehy & L. Williams, III. 1997. Management of pyrethriod-resistant whiteflies in
Arizona cotton: selection, cross-resistance and dynamics. Proc. Beltwide Cotton Conf.

10 µg/m l fenpr opathri n
+ 1000 µg/ml acephate
100
80
M o r t a l i t y
60
40
20
0
5 4 %
6 2 %
8 3 %
7 9 %
(N=13)
1995
(N=19)
1996
(N=12)
1997
(N=12)
1998
®
®
40
20
0
100
80
60

Table 1. Mortality (± std. dev.) of Arizona whitefly populations collected in 1998 and tested in leaf disk bioassays with variable concentrations of Danitol
®
2.4 EC (fenpropathrin) combined with a fixed concentration of Orthene
®
75 S (acephate).
Summary results of statewide monitoring conducted in 1995, 1996, and 1997 are contrasted below.
Concentration fenpropathrin (µg/ml) +
1000 µg/ml acephate
Site 0
Buckeye 1.4 (2.1)
0.1
-
1 10 100
35 (11) 70 (7.7) 99 (3.6)
Casa Grande 0 (0)
Coolidge 1.9 (3.2)
Dome Valley 1.5 (2.3)
Marana 3.9 (3.6)
Maricopa
Ag. Ctr.
Mohave
Valley
2.1 (3.6)
5.2 (6.2)
Paloma 3.2 (4.2)
-
-
-
-
-
-
-
24 (10) 61 (7.7) 83 (7.5)
70 (15) 88 (4.5)
4.2 (6.2) 55 (9.1) 84 (7.0)
58 (11) 90 (4.5) 97 (5.8)
49 (12) 71 (11)
100(0)
97 (3.7)
52 (15) 75 (12 ) 94 (5.8)
56 (21) 91 (5.9) 96 (5.8)
Parker 5.0 (4.3)
Safford 4.7 (4.1)
Somerton 5.9 (3.6)
Yuma Valley
Ag
2.0 (4.8) -
40 (27) 51 (9.4) 72 (9.0)
59 (12) 82 (7.6) 98 (5.8)
1998 Summary Statistics
N
MEAN
12
3.1
-
-
-
-
84 (9.5) 99 (1.9) 99.1(2.3)
59 (20) 95 (1.3) 100 (0)
12
54
12
77
12
93
MEDIAN
MINIMUM
VALUE
STD. DEV.
2.7
0 -
54
4.2
79
51
16
97
72
9.2
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
1.9
21
1997 Summary Statistics
12
5.1
-
-
12
42
4.7
1.5
-
-
46
15
12
79
83
44
16
12
95
97
85
5.0
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
3.0
16
1996 Summary Statistics
19
3.5
19
7.2
19
40
2.0
0
6.6
1.3
37
5.5
19
70
62
27
21
19
95
97
71
6.8
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
4.6
4.5
22
1995 Summary Statistics
13
7.6
13
6.2
13
31
3.6
0
9.5
4.4
1.3
5.2
27
2.7
29
13
57
54
7.8
38
13
91
96
66
11

Table 2. Mortality (± std. dev.) of Arizona whitefly populations collected in 1998 and tested in leaf disk bioassays with Thiodan ® 3 EC (endosulfan). Summary results of statewide monitoring conducted in 1995, 1996, and 1997 are contrasted below.
Concentration endosulfan (µg/ml)
Site 0
Buckeye 4.4 (0.3)
Casa Grande 0 (0)
Coolidge 16 (4.7)
Marana 1.3 (3.1)
Maricopa
Ag. Ctr.
1.3 (2.1)
Mohave
Valley
4.2 (5.6)
Paloma 3.7 (3.5)
Parker 5.0 (4.1)
Safford 8.9 (11)
Somerton 3.5 (6.4)
Yuma Valley
Ag. Ctr.
5.9 (9.3)
-
-
-
-
-
71 (18)
68 (13)
1998 Summary Statistics
100 (0)
29 (19) 97 (7.7)
97 (5.5) 100 (0)
65 (11) 100 (0)
100 (0)
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
11
4.9
4.2
0
4.4
-
-
-
-
11
62
68
1.3
29
1997 Summary Statistics
11
99
99
94
1.9
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
N
12
6.5
6.2
0.67
1
-
-
-
-
-
-
-
-
-
-
10
84 (8.1) 100 (0)
44 (18) 99 (3.6)
82 (11) 99 (1.8)
93 (7.7) 99 (1.9)
1.3 (2.0) 97 (5.3)
50 (9.2) 94 (9.7)
12
58
58
27
4.5
17
1996 Summary Statistics
19 19 19
100
12
99
100
99
0.28
19
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
6.2
4.9
0
12
9.8
0
50
47
23
98
100
92
2.1
N
MEAN
6.3
10 19
1995 Summary Statistics
13
4.5
13
8.8
13
40
13
97
MEDIAN
MINIMUM
VALUE
STD. DEV.
3.6
0.64
3.7
8.1
2.2
5.8
42
17
16
99
86
4.4
1000
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
13
99
100
99
0.28
19
100
100
100
0

Table 3. Mortality (± std. dev.) of Arizona whitefly populations collected in 1998 and tested in leaf disk bioassays with a 1:1 mixture of Thiodan ® 3EC (endosulfan) and Ovasyn ® (amitraz). Summary results of statewide monitoring conducted in 1996 and 1997 are contrasted below.
Concentration endosulfan+amitraz (µg/ml)
Site
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
0
3.8
2.6
0
3.4
1+1
30
27
2.4
23
10+10 100+100 1000+
Buckeye 2.1 (2.4)
Casa Grande 0 (0)
Coolidge 3.1 (3.7)
Marana 0 (0)
3.9 (3.6) Maricopa
Ag. Ctr.
Mohave
Valley
2.1 (3.2)
-
-
-
-
-
-
89 (15)
42 (13)
75 (9.5)
74 (14)
80 (6.5)
48 (20)
100 (0)
99 (1.6)
99 (3.3)
100 (0)
99 (2.1)
99 (2.6)
Paloma
Parker
4.8 (3.0)
7.0 (7.6)
Safford 5.2 (5.8)
Somerton 5.4 (0.6)
Yuma Valley
Ag. Ctr.
14(14)
N 11
-
-
-
-
-
57 (18)
69 (18)
100 (0)
40 (18)
91 (12)
1998 Summary Statistics
11
100 (0)
100 (0)
100 (0)
100 (0)
100 (0)
11
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
4.3
3.9
0
3.9
-
-
-
70
74
40
100
100
99
0.5
N 12
20
1997 Summary Statistics
12 12
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
7.3
5.3
1.3
5.8
-
-
-
71
74
41
99
100
99
0.20
N 19
17
1996 Summary Statistics
18 19 19
72
76
28
21
97
100
86
4.2
-
-
-
-
-
-
-
-
-
-
19
0
-
-
-
-
-
-
-
-
-
-
-
100
100
100

Table 4. Mortality (± std. dev.) of Arizona whitefly populations collected in 1998 and tested in leaf disk bioassays with a 1:1 mixture of Curacron ® 8E (profenofos) and Vydate ® C-LV (oxamyl). Summary results of statewide monitoring conducted in 1996 and 1997 are contrasted below.
Concentration profenofos+oxamyl (µg/ml)
Site
Buckeye 2.9 (3.8)
Casa
Grande
1.4 (2.2)
Coolidge 2.5 (3.1)
Marana 1.3 (2.0)
0 (0) Maricopa
Ag. Ctr.
Mohave
Valley
12 (11)
Paloma 2.1 (2.4)
Parker 3.7 (2.9)
Safford 5.2 (3.6)
Somerton 0 (0)
Yuma
Valley
Ag. Ctr.
7.1 (8.6)
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
0
11
3.5
2.5
0
3.5
10+10
-
-
-
-
-
-
-
-
-
-
-
32+32 100+100 1000
+1000
15 (11)
0.7 (1.6)
4.7 (7.8)
1.3 (2.1)
7.3 (5.5)
14 (13)
7.1 (6.5)
14 (9.6)
11 (5.5)
10 (7.4)
19 (11)
1998 Summary Statistics
-
-
-
11
9.5
10
0.7
90 (4.5)
51 (7.6)
50 (13)
48 (11)
72 (18)
70 (16)
67 (17)
72 (13)
63 (16)
52 (17)
89 (15)
11
66
67
48
15
100 (0)
100 (0)
100 (0)
100 (0)
99 (2.7)
100 (0)
100 (0)
96 (4.4)
100 (0)
100 (0)
100 (0)
11
100
100
96
1.2
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
11
4.8
4.1
1.4
3.8
0
5.8
1997 Summary Statistics
-
-
-
11
43
36
23
11
94
96
76
21
1996 Summary Statistics
10+10
7.5
32+32 100+100
-
-
-
-
-
N
MEAN
MEDIAN
MINIMUM
VALUE
STD. DEV.
19
5.7
4.0
0
5.1
19
7.2
6.8
0
5.5
19
27
23
7.3
19
19
86
92
55
14
320
+320
19
99
100
99
0.31