Reduced susceptibility to permethrin in diamondback moth populations from vegetable and non-vegetable hosts in southern Australia
Department of Primary Industries, Knoxfield, Private Bag 15, Ferntree Gully Delivery Centre Victoria 3156, Australia Corresponding author: Nancy.Endersby@dpi.vic.gov.au
Diamondback moth (DBM), Plutella xylostella (L.), has attained major pest status in Brassica vegetable crops around the world. In many cases, use of synthetic pyrethroid insecticides for control of other pests, such as Pieris rapae (L.), has disrupted natural enemies and selected for insecticide resistance in DBM, changing the pest status of the moth from minor to major. We estimated levels of resistance to the synthetic pyrethroid, permethrin, using a leaf dip bioassay, for 28 DBM populations collected from brassicaceous weeds, canola, forage turnips and seed turnips and for five DBM populations from Brassica vegetables. Populations were collected in Victoria, Tasmania, southern New South Wales, ACT, South Australia and Western Australia between September 1999 and January 2000. Nineteen of 28 populations from non-vegetable hosts were significantly more tolerant than a susceptible laboratory population (resistance ratios ranged from 2.1 to 6.9). All five populations from vegetable hosts were significantly tolerant (resistance ratios from 3.6 to 13.0). These results indicate that populations of DBM with reduced susceptibility to permethrin may be found in areas that are remote from intensive vegetable growing districts. Brassica vegetables account for only a small proportion of the host plants available for DBM in southern Australia. Large areas of non-vegetable hosts have, in the past, received few applications of insecticides. Reports that growers of forage brassicas and canola are finding it necessary to apply synthetic pyrethroids to their crops with increasing frequency (1–4 applications per crop) suggest that resistant DBM populations are being generated. Alternatively, DBM populations may be moving from the more intensively sprayed vegetable crops onto non-vegetable hosts. Further studies on the insecticide resistance status of DBM populations from a range of host plants in different locations in conjunction with use of molecular markers to study population structure of DBM, may provide evidence of isolation or mixing of populations which will have important consequences for insecticide resistance management.
Keywords
Plutella, insecticide resistance, pyrethroid, canola
Introduction
Diamondback moth (DBM), Plutella xylostella, has damaged Brassica vegetable crops throughout the world and is renowned for developing resistance to insecticides (Talekar & Shelton 1993). In Australia, resistance to synthetic pyrethroid insecticides has been identified in DBM populations from vegetable growing areas in all states (Wilcox 1986, Altmann 1988, Hargreaves 1996, Endersby & Ridland 1997, Baker & Kovaliski 1999).
Canola, brassicaceous weeds and forage brassicas are all hosts for DBM (Talekar & Shelton 1993). Brassica vegetables account for a very small proportion of the host plants available for DBM in southern Australia. 12,226 hectares were planted to Brassica vegetables in 1999 (ABS 2000), whereas the area planted to canola in Australia increased by 79% from 697,000 hectares in 1997–98 to an estimated 1.2 million hectares in 1998–99 (ABS 2000). The biggest increase was in Western Australia, which saw plantings increase by 116% to 536,000 hectares (ABS 2000). There are also vast areas of brassicaceous weeds, particularly in Western Australia where wild radish, Raphanus raphanistrum, has developed resistance to acetolactate synthase (ALS)-inhibiting herbicides (Rieger et al. 1999). In southern Victoria, dairy farmers often grow large areas of forage brassicas such as turnips in late spring to early summer.
In contrast to the intensively sprayed vegetable crops, non-vegetable host plants have received few, if any, insecticide applications until recent times. If we assume there is no regular long-range movement of DBM from vegetable production areas to remote areas of canola and weeds, we would expect DBM found on such non-vegetable host plants to be susceptible to synthetic pyrethroids. The current study, however, documents low levels of resistance to permethrin in DBM populations from canola, forage brassicas and brassicaceous weeds in southern Australia.
Materials and methods
DBM eggs, larvae and pupae were collected from 33 locations in southern Australia from September 1999 to January 2000. Populations were reared on cabbage seedling leaves (Brassica oleracea var. capitata cv. Green Coronet) in the laboratory at 25 °C (16h:8h, L:D) for one to three generations. A susceptible laboratory population of DBM was used as a reference in each assay. The susceptible population has been maintained at Knoxfield since it was obtained from the University of Adelaide, Department of Crop Protection, Waite Campus, SA, in 1994. For each DBM population we estimated levels of resistance to permethrin. In all, we tested 28 DBM populations from non-vegetable host plants and five populations from vegetables.
A leaf dip bioassay (after Tabashnik & Cushing 1987) was adopted for testing susceptibility to permethrin. Cabbage leaf discs of 4.5 cm diameter were dipped for 5 s in distilled water solutions of formulated insecticide (Ambush® Emulsifiable Concentrate Insecticide–Crop Care Australasia Pty Ltd) and hung vertically to dry in a fume hood for 2 h. Control discs were dipped in distilled water. No wetting agents were used. Discs were placed into Gelman® 50 mm diameter x 9 mm plastic Petri dishes. Ten larvae were placed on each disc and four replicates of seven concentrations were set up. Mortality was assessed after 48 h at 28ºC. Larvae were considered dead if they did not move when touched with a paintbrush.
(a) Analysis
Concentration-mortality data for each population were analysed using the probit analysis program, POLO-PC (LeOra Software) (Russell et al. 1977). We used the program to estimate the lethal concentration expected to cause 50% mortality (LC50) of each insecticide for each DBM population and the 95% confidence intervals for these concentrations. The slope (+ standard error) of the probit line was also estimated.
The program ran χ2 tests for goodness-of-fit of the data to the probit model. If the model fits, the calculated value of χ2 is less than the χ2 table value for the appropriate degrees of freedom. If the model does not fit (i.e. the χ2 value exceeds the table value), the LC50 value for the particular population may not be reliably estimated and is adjusted with the heterogeneity factor (χ2/df). The index of significance for potency estimation (g) was used to calculate 95% confidence intervals for potency (relative potency is equivalent to tolerance ratio) (Robertson & Preisler 1992).
Parallelism of the probit regression lines implies a constant relative potency at all levels of response (Finney 1971). POLO-PC was used to test equality and parallelism of the slopes of the probit lines for the field population and the laboratory susceptible population. If the slopes are parallel, then overlap of the 95% confidence intervals for the two populations indicates that no significant difference exists between the LC50 values.
Results
All five populations from vegetable hosts were significantly resistant (resistance ratios from 3.6 to 13.0) (Table 2) and 19 of 28 populations from non-vegetable hosts showed a low level of resistance to permethrin (resistance ratios ranged from 2.1 to 6.9) (Table 1).
The resistance ratios of the DBM populations from vegetable crops at Bairnsdale and Nairne (Table 2) are approaching the level at which growers were experiencing control failures in 1993–5 (Endersby & Ridland 1997). Since the first round of tests in 1993–6, a decrease in resistance levels of DBM from vegetable brassicas in Werribee is indicated. This may reflect the reduction in use of synthetic pyrethroid insecticides that occurred after the control failures and/or movement of susceptible moths into the area.
The two highest resistance ratios to permethrin estimated in populations from canola are from Western Australia (Table 3). Three other populations from canola were susceptible to permethrin (Balliang and Balliang East from Victoria and Yeelanna from South Australia).
Table 1. LC50 and LC95 for permethrin tested on DBM populations from southern Australia compared with the standard laboratory population (Waite), 1999–2000 (Het.=heterogeneity, g= index of significance for potency estimation, s.e.=standard error, df=degrees of freedom)
Population |
Host |
Slope ± s. e. |
Het. |
g |
χ2 df=26 |
LC50 |
95% confidence intervals |
LC95 |
95% confidence intervals |
Waite |
Lab |
2.35 ± 0.34 |
0.84 |
0.08 |
21.9 |
8.4 |
5.7–11.1 |
42.2 |
31.1–67.4 |
Werribee VIC |
Cabbage |
1.68 ± 0.20 |
1.49 |
0.09 |
38.8 |
25.5 |
17.3–34.6 |
244.3 |
148.1–580.2 |
Waite |
Lab |
2.16 ± 0.38 |
0.75 |
0.12 |
19.4 |
10.5 |
5.7–15.2 |
60.8 |
42.9–108.3 |
Lindenow VIC |
Cabbage |
1.57 ± 0.18 |
1.08 |
0.06 |
28.0 |
52.5 |
39.9–68.5 |
590.4 |
348.7–1364.8 |
Bairnsdale VIC |
Cabbage |
1.58 ± 0.19 |
0.93 |
0.06 |
24.3 |
125.9 |
97.7–172.7 |
1378.2 |
755.0–3570.6 |
Waite |
Lab |
2.23 ± 0.28 |
1.01 |
0.07 |
27.2 |
12.5 |
8.9–16.2 |
68.4 |
49.5–111.9 |
Arthurton SA |
Canola |
1.90 ± 0.22 |
1.05 |
0.06 |
26.3 |
26.1 |
19.7–32.9 |
191.6 |
131.3–341.4 |
Urania SA |
Canola |
1.84 ± 0.20 |
1.19 |
0.06 |
30.9 |
34.7 |
26.3–44.3 |
271.5 |
178.4–520.8 |
Waite |
Lab |
1.82 ± 0.24 |
0.94 |
0.06 |
24.4 |
6.2 |
4.1–8.3 |
49.7 |
34.8–84.2 |
Minlaton SA |
Canola |
1.84 ± 0.21 |
1.53 |
0.08 |
39.8 |
26.7 |
18.6–35.5 |
208.4 |
132.8–442.3 |
Balliang VIC |
Canola |
1.19 ± 0.21 |
0.82 |
0.12 |
48.3 |
7.8 |
3.2–12.8 |
189.9 |
111.6–509.2 |
Waite |
Lab |
1.82 ± 0.14 |
1.66 |
0.10 |
43.2 |
8.8 |
5.3–12.6 |
70.9 |
45.1–150.9 |
Wauraltee SA |
Canola |
1.44 ± 0.19 |
1.93 |
0.14 |
50.1 |
27.1 |
14.9–41.1 |
376.2 |
191.7–1484.6 |
Yeelanna SA |
Canola |
1.53 ± 0.27 |
1.09 |
0.14 |
28.3 |
7.2 |
2.9–11.4 |
85.8 |
55.6–194.4 |
Waite |
Lab |
2.26 ± 0.31 |
0.96 |
0.07 |
25.1 |
7.7 |
5.4–10.0 |
41.1 |
30.1–65.8 |
Wongan Hills WA |
Canola |
1.58 ± 0.19 |
1.58 |
0.09 |
41.0 |
39.1 |
27.2–53.7 |
433.1 |
241.9–1214.7 |
Burabadji WA |
Canola |
1.99 ± 0.26 |
1.24 |
0.09 |
32.1 |
49.2 |
33.0–67.1 |
328.8 |
212.5–682.8 |
Waite |
Lab |
2.52 ± 0.27 |
1.21 |
0.06 |
31.4 |
13.4 |
10.5–16.6 |
60.0 |
43.8–96.5 |
Balliang East VIC |
Canola |
0.92 ± 0.19 |
1.93 |
0.35 |
50.3 |
7.3 |
0.4–17.3 |
438.5 |
157.7–15038.9 |
Waite |
Lab |
2.26 ± 0.28 |
1.49 |
0.10 |
34.0 |
21.0 |
14.3–28.0 |
112.4 |
76.2–216.9 |
Manjimup WA |
Cauliflower |
1.25 ± 0.19 |
1.31 |
0.12 |
38.8 |
108.5 |
72.7–177.7 |
2245.3 |
880.3–14538.9 |
Waite |
Lab |
3.51 ± 1.03 |
0.73 |
0.33 |
19.1 |
11.6 |
5.3–15.1 |
34.2 |
26.3–76.2 |
Nairne SA |
Sprouts |
1.30 ± 0.17 |
1.45 |
0.11 |
37.8 |
124.3 |
82.1–177.6 |
2267.9 |
1092.2–9035.5 |
Waite |
Lab |
2.26 ± 0.28 |
1.49 |
0.10 |
34.0 |
21.0 |
14.3–28.0 |
112.4 |
76.2–216.9 |
Clunes VIC |
Weeds |
2.36 ± 0.41 |
3.10 |
0.39 |
80.7 |
9.7 |
2.3–15.8 |
48.4 |
29.1–234.5 |
Waite |
Lab |
2.37 ± 0.25 |
1.05 |
0.05 |
27.4 |
16.8 |
13.4–20.6 |
83.3 |
60.9–131.1 |
Bunbury WA |
Weeds |
1.99 ± 0.22 |
0.60 |
0.05 |
15.7 |
111.4 |
90.6–141.2 |
746.2 |
482.6–1435.3 |
Waite |
Lab |
3.46 ± 0.48 |
1.16 |
0.09 |
30.1 |
8.1 |
6.1–10.0 |
24.2 |
18.8–36.3 |
Balingup WA |
Weeds |
1.12 ± 0.16 |
1.71 |
0.16 |
44.5 |
55.8 |
34.2–89.8 |
1710.4 |
605.3–17517.9 |
Waite |
Lab |
3.42 ± 0.43 |
0.56 |
0.06 |
14.5 |
12.1 |
10.0–14.2 |
36.6 |
29.2–51.3 |
Deadman's Gully WA |
Weeds |
1.49 ± 0.18 |
0.71 |
0.06 |
18.5 |
82.9 |
64.4–110.1 |
1061.7 |
590.3–2679.7 |
Bridgetown SA |
Weeds |
1.58 ± 0.18 |
1.49 |
0.09 |
38.7 |
57.5 |
42.0–79.4 |
632.7 |
343.2–1831.3 |
Waite |
Lab |
3.43 ± 0.51 |
1.65 |
0.15 |
43.0 |
19.8 |
14.0–25.1 |
59.6 |
43.7–109.0 |
Stratford VIC |
Weeds |
1.21 ± 0.17 |
1.13 |
0.10 |
29.5 |
25.3 |
15.5–35.9 |
584.8 |
297.3–2010.2 |
Springhurst VIC |
Weeds |
1.23 ± 0.17 |
1.69 |
0.13 |
44.0 |
36.0 |
22.1–53.2 |
685.2 |
317.2–3231.3 |
Waite |
Lab |
3.19 ± 0.38 |
1.81 |
0.11 |
39.8 |
9.3 |
6.8–11.9 |
30.4 |
22.0–52.6 |
Canberra ACT |
Weeds |
0.79 ± 0.16 |
1.75 |
0.29 |
45.6 |
45.2 |
19.7–87.2 |
5400.4 |
1001.7–1324537.9 |
Finley NSW |
Weeds |
1.07 ± 0.17 |
1.57 |
0.16 |
40.8 |
37.0 |
20.9–57.5 |
1278.8 |
475.7–1.1665.0 |
Jugiong NSW |
Weeds |
1.18 ± 0.17 |
1.14 |
0.10 |
29.6 |
45.2 |
30.7–63.9 |
1120.0 |
515.3–4661.7 |
Waite |
Lab |
3.59 ± 0.47 |
0.53 |
0.07 |
13.8 |
5.9 |
4.8–7.1 |
16.9 |
13.3–24.2 |
Derrinallum VIC |
Weeds |
1.46 ± 0.20 |
1.27 |
0.11 |
33.0 |
17.8 |
10.2–25.7 |
238.0 |
140.0–615.5 |
Waite |
Lab |
2.47 ± 0.27 |
3.28 |
0.17 |
85.3 |
9.8 |
5.8–14.3 |
45.5 |
28.3–118.0 |
Cranbourne VIC |
Weeds |
1.63 ± 0.21 |
1.64 |
0.11 |
42.7 |
20.3 |
11.9–29.3 |
207.0 |
123.1–531.4 |
Waite |
Lab |
2.46 ± 0.28 |
1.94 |
0.11 |
28.7 |
8.5 |
5.6–11.5 |
39.5 |
27.0–75.6 |
Thomastown VIC |
Weeds |
1.44 ± 0.30 |
1.10 |
0.20 |
50.4 |
4.1 |
0.9–7.9 |
56.6 |
36.5–131.5 |
Horsham VIC |
Weeds |
1.17 ± 0.19 |
1.02 |
0.12 |
26.4 |
11.0 |
5.0–17.3 |
278.0 |
153.4–852.0 |
Waite |
Lab |
2.38 ± 0.26 |
1.35 |
0.07 |
35.0 |
11.6 |
8.7–14.8 |
57.0 |
40.7–95.8 |
Loch VIC |
Weed |
1.21 ± 0.36 |
1.20 |
0.46 |
31.3 |
1.5 |
0.004–4.9 |
33.2 |
17.7–108.5 |
Waite |
Lab |
2.56 ± 0.28 |
4.14 |
0.20 |
107.7 |
11.8 |
6.7–17.8 |
52.3 |
31.6–157.6 |
Werribee VIC |
Weeds |
1.25 ± 0.17 |
1.01 |
0.08 |
26.2 |
48.1 |
34.6–65.6 |
996.2 |
497.9–3279.8 |
Ayrford VIC |
Turnip |
2.23 ± 0.27 |
0.79 |
0.06 |
20.7 |
50.4 |
38.9–62.9 |
274.9 |
197.6–450.3 |
Waite |
Lab |
3.37 ± 0.41 |
0.46 |
0.06 |
12.0 |
8.6 |
7.0–10.2 |
26.5 |
21.0–37.3 |
Woolnorth TAS |
Turnip |
1.25 ± 0.18 |
1.17 |
0.10 |
30.3 |
43.6 |
28.1–63.1 |
895.0 |
429.7–3474.8 |
Waite |
Lab |
2.87 ± 0.36 |
0.86 |
0.06 |
22.3 |
6.5 |
5.1–8.0 |
24.5 |
18.8–36.0 |
Curdie Vale VIC |
Turnip |
1.17 ± 0.17 |
1.37 |
0.12 |
35.6 |
39.3 |
25.1–57.5 |
991.6 |
432.4–5105.5 |
Table 2. Resistance ratios to permethrin for DBM populations from vegetable crops in southern Australia, 1999
DBM population |
Host plant |
Date collected |
Resistance Ratio |
95% confidence intervals |
Generation tested | |
Lower |
Upper |
|||||
Werribee VIC |
cabbage |
15-Sep-1999 |
3.6 |
2.5 |
5.4 |
F1 |
Manjimup WA |
cauliflower |
7-Oct-1999 |
5.2* |
3.4 |
8.0 |
F1 |
Lindenow VIC |
cabbage |
8-Sep-1999 |
5.8 |
4.0 |
8.8 |
F1 |
Nairne SA |
Brussels sprouts |
15-Nov-1999 |
10.7* |
6.6 |
17.2 |
F1 |
Bairnsdale VIC |
cabbage |
7-Sep-1999 |
13.0 |
8.8 |
20.8 |
F1 |
*calculated at LC50. Resistance ratios assume parallel slopes for each test. If parallel slopes could not be fitted for a particular assay, then ratio was calculated at LC50. A resistance ratio of 1 indicates that a field population is equivalent in susceptibility to the susceptible laboratory population (Waite).
Table 3. Resistance ratios to permethrin for DBM populations from canola crops in southern Australia, 1999
DBM population |
Date collected |
Resistance Ratio |
95% confidence intervals |
Generation tested | |
Lower |
Upper |
||||
Balliang East VIC |
12-Oct-1999 |
0.2 |
1.3 |
F1 | |
Yeelanna SA |
07-Oct-1999 |
1.0 |
0.6 |
1.5 |
F1 |
Balliang VIC |
27-Sep-1999 |
1.3* |
0.6 |
2.6 |
F1 |
Arthurton SA |
07-Oct-1999 |
2.2 |
1.6 |
3.1 |
F1 |
Urania SA |
07-Oct-1999 |
3.0 |
2.1 |
4.2 |
F1 |
Wauraltee SA |
07-Oct-1999 |
3.5 |
2.1 |
6.0 |
F1 |
Minlaton SA |
07-Oct-1999 |
4.3 |
2.9 |
6.5 |
F1 |
Wongan Hills WA |
07-Oct-1999 |
5.1* |
3.4 |
7.5 |
F1 |
Burabadji WA |
07-Oct-1999 |
6.7 |
4.8 |
9.6 |
F1 |
*calculated at LC50
Three populations from weeds were collected close to vegetable production areas: Deadman’s Gully WA, Werribee VIC and Cranbourne VIC (Table 4) and are likely to have a history of exposure to insecticides. Some extremely susceptible populations were collected in Victoria away from production areas (Thomastown, Clunes and Loch), but there were many other DBM populations showing low levels of resistance which were collected from weeds in areas remote from vegetable growing regions.
Table 4. Resistance ratios to permethrin for DBM populations from weeds in southern Australia, 1999
DBM population |
Date collected |
Resistance Ratio |
95% confidence intervals |
Generation tested | |
Lower |
Upper |
||||
Loch VIC |
07-Sep-1999 |
0.1* |
0.02 |
0.7 |
F1 |
Clunes VIC |
06-Oct-1999 |
0.5 |
0.3 |
0.7 |
F1 |
Thomastown VIC |
19-Oct-1999 |
0.5* |
0.2 |
1.2 |
F4 |
Stratford VIC |
12-Oct-1999 |
1.3* |
0.3 |
6.2 |
F2 |
Horsham VIC |
03-Nov-1999 |
1.3* |
0.7 |
2.3 |
F3 |
Springhurst VIC |
24-Oct-1999 |
1.8* |
0.4 |
8.7 |
F1 |
Cranbourne VIC |
28-Oct-1999 |
2.1* |
1.4 |
3.1 |
F2 |
Derrinallum VIC |
08-Dec-1999 |
3.0* |
2.0 |
4.6 |
F1 |
Finley NSW |
27-Oct-1999 |
4.0* |
2.7 |
6.0 |
F1 |
Werribee VIC |
16-Nov-1999 |
4.1* |
2.8 |
5.8 |
F2 |
Bridgetown WA |
07-Oct-1999 |
4.8* |
3.5 |
6.4 |
F1 |
Canberra ACT |
25-Oct-1999 |
4.9* |
3.0 |
8.0 |
F1 |
Jugiong NSW |
26-Oct-1999 |
4.9* |
3.4 |
7.1 |
F1 |
Bunbury WA |
07-Oct-1999 |
6.6 |
5.0 |
8.9 |
F1 |
Balingup WA |
07-Oct-1999 |
6.9* |
4.6 |
10.2 |
F1 |
Deadman's Gully WA |
07-Oct-1999 |
6.9* |
5.0 |
9.4 |
F1 |
*calculated at LC50
Populations from forage and seed turnips in Victoria and Tasmania also showed low levels of resistance to permethrin (Table 5).
Table 5. Resistance ratios to permethrin for DBM populations from turnips in southern Australia, 1999–2000
DBM population |
Date collected |
Resistance Ratio |
95% confidence intervals |
Generation tested | |
Lower |
Upper |
||||
Ayrford VIC forage |
08-Dec-1999 |
4.4 |
3.0 |
6.6 |
F2 |
Woolnorth TAS seed |
05-Jan-2000 |
5.1* |
3.4 |
7.5 |
F1 |
Curdie Vale VIC forage |
08-Dec-1999 |
6.0* |
4.1 |
8.9 |
F1 |
*calculated at LC50
The lowest resistance ratio estimated was for a population of DBM from weeds remote from vegetable production areas and the two highest resistance ratios were from DBM collected in commercial vegetable crops (Table 6).
Table 6. Permethrin resistance ratio categories of DBM populations from vegetable and non-vegetable host plants in southern Australia, 1999–2000
Host plant |
Number of populations in resistance ratio category | |||
A |
B |
C |
D | |
Weeds (Brassicaceae) |
2 |
4 |
10 |
0 |
Canola (Brassica spp.) |
0 |
3 |
6 |
0 |
Brassica vegetables |
0 |
0 |
3 |
2 |
Forage turnips (Brassica rapa) |
0 |
0 |
2 |
0 |
Seed turnips (Brassica rapa) |
0 |
0 |
1 |
0 |
Total |
2 |
7 |
22 |
2 |
A = significantly lower than standard laboratory population (Waite)–susceptible, B = no significant difference from Waite population–susceptible, C = significantly higher than Waite population–low level of resistance, D = significantly higher than Waite population–level approaching control failure
Discussion
In Australia, insecticides are applied to canola and forage brassicas with increasing frequency. Some growers use synthetic pyrethroids early in the crop for control of redlegged earth mite, Halotydeus destructor (Tucker), a practice which could inadvertently select for resistance in DBM. Canola growers in northern Western Australia have started to apply synthetic pyrethroids for control of DBM, particularly in response to very high pest pressure in spring 1999 and winter 2000. Many forage Brassica growers in Victoria applied synthetic pyrethroids to their crops in spring 1999, with a frequency of one to four applications per crop.
Many factors may be responsible for increased severity of DBM outbreaks in vegetable crops, elevation of levels of resistance to synthetic pyrethroids and detection of resistance in populations of DBM remote from areas of intensive insecticide use. In some regions, particularly Western Australia, the increase in area of host plants must be generating higher numbers of moths. Weather conditions favourable to DBM such as a dry winter in 1999 and 2000 could explain the massive numbers of DBM in spring canola and forage brassicas. Spraying for other pests may be inducing insecticide resistance in DBM as well as destroying natural enemies of the pest. We observed high levels of biological control by the ichneumonid parasitoid, Diadegma semiclausum (Hellén), in unsprayed forage crops around Warrnambool, Victoria, in December 1999, but in crops sprayed with synthetic pyrethroids within the same district, the level of biological control was decreased. The low cost of synthetic pyrethroid insecticides make them a viable option for use in broadacre crops, but a continued increase in their use will exacerbate problems with DBM. Enhanced biological control and other non-insecticide control methods will be the only way to reduce DBM to minor pest status in these crops.
There is little published information about long range movement of DBM in southern Australia between different types of host plant, but resistance levels in remote weed crops suggest that moths are moving away from vegetable and canola growing regions. Management of resistance to insecticides based on knowledge of gene flow and mixing or isolation of Australian moth populations is of major importance to the Australian vegetable and canola industries, but few studies of population structure and movement of DBM in Australia have been made.
The current study will be expanded to gain a better understanding of long range moth movement using microsatellite DNA markers. Such hypervariable genetic markers can provide information about population movement at different spatial scales and may reveal source populations for geographic invasions, founder effects and population bottlenecks (Loxdale 2001). For example, microsatellites are being used to study origins of Helicoverpa armigera (Hübner) and H. punctigera (Wallengren) populations in south western Queensland (Graham 2000). Microsatellite studies of the Queensland fruit fly, Bactrocera tryoni, a major quarantine pest in Australia, were able to identify genetically isolated subpopulations that would be suitable targets for an eradication program (Yu et al. 2001). These examples demonstrate that both local and regional information about population movement has important implications for population management and similar information about DBM will have fundamental importance in development of insecticide resistance management strategies.
Acknowledgements
We thank the Department of Agriculture, Western Australia; SARDI; Department of Primary Industries, Water and Environment, Tasmania and others for collecting DBM populations from canola and vegetable crops.
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