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Keywords:

  • Phenacoccus solenopsis;
  • acetamiprid;
  • resistance;
  • biological parameters;
  • stability

Abstract

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 MATERIALS AND METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

BACKGROUND

Acetamiprid is a neonicotinoid insecticide that is effective against both soil and plant insects, including insects of the orders Lepidoptera, Coleoptera, Homoptera and Thysanoptera. In order to estimate the effects associated with insecticide exposure and devise better pest management tactics, a field population of Phenacoccus solenopsis was exposed to acetamiprid in the laboratory. Subsequently, cross-resistance and the effects of acetamiprid on the biological parameters of P. solenopsis were investigated.

RESULTS

Following five rounds of selection with acetamiprid, P. solenopsis developed a 315-fold greater resistance to this chemical compared with an unexposed control population. The selected population also demonstrated very high to moderate cross-resistance to other tested insecticides. Furthermore, acetamiprid resistance remained unstable when the acetamiprid-selected population was not exposed for a further five generations. The acetamiprid-selected population had a relative fitness of 0.22, with significantly lower survival rate, pupal weight, fecundity, percentage hatching, net reproductive rate, intrinsic rate of natural increase, biotic potential and mean relative growth rate, with prolonged male and female nymphal duration, developmental time from egg to female adult and male and female longevity compared with the control population.

CONCLUSION

P. solenopsis biological parameters are greatly affected by acetamiprid, and it is of significant cost for the insects to counter these effects. This study will be a valuable source of information for further understanding of acetamiprid resistance and for assisting the development of resistance management programmes. © 2014 Society of Chemical Industry


1 INTRODUCTION

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 MATERIALS AND METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

Cultivated cotton, Gossipium hirsutum, is the prime cash crop and an important source of foreign exchange earnings for Pakistan[1] and other countries.[2] This crop is attacked by a variety of chewing[3] and sucking pests that remain active for almost the entire growing season and have detrimental effects to crop productivity.[1] Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) is of great concern to cotton producers owing to its polyphagous nature and potential to cause heavy crop losses.[4] P. solenopsis is a soft-bodied insect that damages the crop by sucking the cell sap.[5] Attacked cotton plants remain stunted and produce fewer bolls of smaller size; leaves become distorted, yellow and finally drop off.[6, 7] Boll opening is also adversely affected, and yield reduction ranges from 58 to 73%.[8] The attacked leaves and shoots are malformed as a result of direct feeding damage of P. solenopsis, and honeydew excreted by the pest also favours mouldy growth, which ultimately hinders the process of photosynthesis.[9] As much as 90% damage may result from attack by P. solenopsis.[10]

In Pakistan, from 2005 onwards, P. solenopsis has been recorded as a threatening pest of cultivated cotton.[11-13] During 2007, this insect was responsible for the loss of more than 40% of the cotton crop in Pakistan.[2] It has also been reported as a serious pest in the United States,[14] Thailand and Taiwan[13] and India and China.[15, 16] Different insecticides such as organophosphate, pyrethroid and neonicotinoid have been used for the control of this pest. However, resistance to insecticides is a main problem linked with uninformed and excessive use of the chemical control of insect pests.[17] The widespread application of conventional insecticides such as organophosphate, carbamate and pyrethroid has provided an ideal environment for resistance evolution in Pakistan.[11] Resistance to acetamiprid has been reported in a number of insects, including Plutella xylostella,[18, 19] Bemisia tabaci[20] and Leptinotarsa decemlineata,[21] from different parts of the world. However, to the authors' knowledge, there have been no reports of acetamiprid resistance in P. solenopsis.

Neonicotinoids are the latest major class of insecticides with a novel mode of action. These insecticides are very important in agriculture because they are efficient against a broad spectrum of insect pests.[22] Nicotinergic acetylcholine receptors are the target sites of this class of insecticides, which agonise the receptors, thus hindering nerve impulse transmission as a result of a depolarising effect within the central nervous system of insects.[23] Acetamiprid is a neonicotinoid that is effective against both soil and plant insects, including insects of the orders Lepidoptera, Coleoptera, Homoptera and Thysanoptera. Acetamiprid has contact, systemic and osmotic action[24] and is recommended especially for P. solenopsis.[25]

Insecticides are chemical weapons used to control insects by killing them or preventing them from engaging in undesirable or destructive behaviours.[26] Insecticides could affect the physiological make-up of the target pests by causing changes in growth, development and reproduction parameters, or by causing changes in the nutritional contents of the host plants, which may result in enhanced developmental time, decreased survival, fecundity and reproduction or other changes in the behaviour of the target pest.[27] Insecticidal effects on biological parameters of insects potentially have an ecological impact. Previously, attempts have been made to examine the effects of different insecticides on biological parameters of different insects. The effects of imidacloprid on Nilaparvata lugens,[28] tebufenozide on P. xylostella[29] and Spodoptera exigua,[30] thiamethoxam on B. tabaci,[31] trichlorphon on Bactrocera dorsalis,[32] imidacloprid on S. litura[33] and emamectin benzoate on Chrysoperla carnea[34] have been reported.

Bearing in mind the significance of P. solenopsis as an invasive pest in Pakistan and many other countries, this research was carried out with the objective of investigating the effect of acetamiprid on the biological parameters of different populations of P. solenopsis. Moreover, cross-resistance to different insecticides in an acetamiprid-selected population (Aceta-SEL Pop) and the stability of acetamiprid resistance were determined.

2 MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 MATERIALS AND METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

2.1 Insects

Studies on the toxicity of insecticides and the effect of acetamiprid on the biological parameters of P. solenopsis were carried out in the laboratory of the Faculty of Agricultural Sciences and Technology, Bahauddin Zakariya University, Multan, Pakistan. The field population was collected from the cotton field of the Central Cotton Research Institute, Multan. The area (Multan) from where the population was collected is heavily sprayed with insecticides from different classes in order to control various insect pests of cotton.[35] Insects were brought into the laboratory and reared on twigs of China rose, Hibiscus rosasinensis, with leaves in plastic jars (12 × 24 cm). Fresh stems of China rose with leaves were plucked, cleaned and put in jars, whereas dried stems with leaves were removed every 2–3 days. The colony was maintained under controlled conditions at 27 ± 2 °C and 60 ± 5% RH in the laboratory with a 14:10 h light:dark photoperiod.

2.2 Insecticides

The commercial insecticide products used in the bioassay were: chlorpyrifos (Lorsban, 40 EC; Dow Agro Sciences, Pakistan), deltamethrin (Decis Super, 10 EC; Bayer Crop Sciences, Pakistan), acetamiprid (Mospilon, 20 SP; Arysta Life Sciences, Pakistan) and imidacloprid (Confidor, 200 SL; Bayer Crop Sciences, Pakistan).

2.3 Bioassays

Bioassays were performed on three-day-old second-instar nymphs of P. solenopsis populations under laboratory conditions using the leaf-dip method.[35] Fresh china rose leaves were used for the bioassays. Five concentrations of each insecticide were used to determine the LC50 value, and each concentration was replicated 5 times. The leaves were dipped into the test solutions for about 10 s and then allowed to dry for approximately 1 h at room temperature. These leaves were kept in Petri dishes (5 cm in diameter), and in each Petri dish moist filter paper was used to prevent the leaves from dehydration. Five second-instar nymphs were used in each Petri dish, so that 25 nymphs were exposed per insecticide concentration. Control experiments were performed with nymphs exposed to water-treated leaves. The bioassays were kept at a temperature 27 ± 2 °C and 60 ± 5% RH with a photoperiod of 14:10 h light:dark. Mortality was assessed after 48 h of exposure to ogranophosphates and pyrethroids,[18] and 72 h of exposure to neonicotinoids.[35] The nymphs were considered to be dead if they had no leg movement after a gentle touch with a fine and soft needle-like hair brush. The field-collected population (designated as the field population) at G2 was assayed for two representatives of neonicotinoids (acetamiprid and imidacloprid), one organophosphate (chlorpyrifos) and one pyrethroid (deltamethrin).

2.4 Selection of P. solenopsis with acetamiprid

The field-collected population was separated at G2 into two subpopulations after performing bioassays. One subpopulation was not exposed to any insecticide, designated as the UNSEL Pop, and the second subpopulation, designated as the Aceta-SEL Pop, was selected with acetamiprid from G3 to G7. Selection of P. solenopsis with acetamiprid was done by the leaf-dip method as described above by exposing three-day-old second-instar nymphs to different lethal concentrations (45.73, 250, 510.38, 903.15 and 2103.85 µg mL−1) from G3 to G7. The nymphs selected per generation ranged from 100 to 300. Mortality was evaluated 72 h after exposure to insecticide. The survivors of every selection were reared to obtain the next generation.

2.5 Stability test

Five generations (G8 to G12) of the Aceta-SEL Pop were left unselected in order to investigate the stability of insecticide resistance to various insecticides. A decline in resistance to acetamiprid in the Aceta-SEL Pop was measured by determining DR (average response per generation) value. The R-value was estimated as R = [log (final LC50) − log (initial LC50)]/n, where n is the number of generations.[36]

2.6 Reciprocal crosses

To study the life history traits of heterogeneous populations, mass mating was done between the UNSEL Pop and Aceta-SEL to obtain two lines: Cross1 (Aceta-SEL Pop ♂ × UNSEL Pop ♀) and Cross2 (Aceta-SEL Pop ♀ × UNSEL Pop ♂) populations. In an individual cross, the numbers of adult females and males used were 15 and 3 respectively because the proportion of males in P. solenopsis populations is always less than females.

2.7 Biological parameters

Newly hatched larvae, or crawlers, were collected pseudorandomly from the UNSEL Pop, Aceta-SEL Pop, Cross1 and Cross2 populations to study the effect of acetamiprid on biological parameters. The crawlers were weighed and reared in plastic jars (8 × 16 cm) containing fresh china rose leaves. The old leaves were changed, and data were recorded daily. Survival rate from crawler to second instar, male nymph duration (from crawler to second instar), female nymph duration (from crawler to third instar), pupal weight and pupal duration were recorded. Moreover, the number of pupae, the emergence rate of healthy male adults (%), fecundity, hatchability (%) and developmental time (DT) from egg to adult of male and female were also determined. The longevity and generation time of both sexes were also determined to observe the effect of selection on life history traits.

The net replacement or reproductive rate (R0) was calculated using the following formula as reported by Jia et al.:[30]

  • display math

where Nn is the population quantity of the parental generation and Nn+1 is that of the next generation.

The relative fitness was calculated according to Cao and Han as follows:[29]

  • display math

The intrinsic rate of natural increase (rm) was calculated using the following formula reported by Birch:[37]

  • display math

where DT is the developmental time from egg to male or female adult.

Biotic potential (BP) was calculated using the formula given by Roush and Plapp:[38]

  • display math

where Fn is the fecundity (eggs female−1) and DTr is the developmental time ratio of the female, calculated as the DT of the experimental population divided by the DT of UNSEL Pop.

The mean relative growth rate (MRGR) was calculated according to Radford:[39]

  • display math

where W1 and W2 (mg) are respectively the initial crawler and pupal weights for the male or female, and DT is the developmental time of the male or female.

2.8 Data analysis

The LC50 value of each insecticide was determined by the probit analysis method using the EPA Probit Analysis program.[40] Because of the inherent variability of bioassays, pairwise comparisons of LC50 values were at the 5% significance level (where individual 95% fiducial limits for comparison of any two treatments do not overlap).[41] All of the biological parameters were statistically analysed using Statistix 8.1 software (Analytical Software, Tallahassee, FL).[42] A completely randomised design was used for population comparisons, and means were compared by the least significance difference test (P ≤ 0.05).

3 RESULTS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 MATERIALS AND METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

3.1 Toxicity and resistance of the field population and UNSEL Pop

Four insecticides were tested on the field population at G2. Compared with UNSEL Pop, the field population developed resistance ratios of 9.67 towards acetamiprid, 7.63 towards imidacloprid, 3.11 towards chlorpyrifos and 3.21 towards deltamethrin. Fiducial limits of all the insecticides tested on the field population of P. solenopsis at G2 showed no significant difference among their LC50 values. The LC50 of chlorpyrifos was significantly higher compared with other insecticides tested on UNSEL Pop (95% fiducial limits did not overlap). There was no significant difference in LC50 values of acetamiprid, imidacloprid and deltamethrin (95% fiducial limits overlapped) (Table 1).

Table 1. Toxicity of field population, UNSEL population and Aceta-SEL population of P. solenopsis to representative neonicotinoid, organophosphate and pyrethroid and stability of resistance
PopulationInsecticideLC50 (95% FL) (µg mL−1)Slope (±SE)χ2dfPnaRRbRRcRRdDRe
  1. a

    Number of nymphs used in the bioassay, including the control.

  2. b

    Resistance ratio = LC50 of field population (G2) or Aceta-SEL Pop (G8) divided by LC50 of UNSEL Pop.

  3. c

    Resistance ratio = LC50 of Aceta-SEL Pop (G8) divided by LC50 of field population (G2).

  4. d

    Resistance ratio = LC50 of Aceta-SEL Pop (G12) divided by LC50 of UNSEL Pop.

  5. e

    Rate of decrease in LC50 = log (final LC50 − initial LC50)/n, where n is the number of generations reared without insecticide exposure.

Field population (G2)Acetamiprid45.73 (8.70–83.79)    0.99(±0.29)0.4640.981509.671  
Field population (G2)Imidacloprid41.50 (4.46–84.32)    0.94(±0.31)1.7340.791507.631  
Field population (G2)Chlorpyrifos75.26 (25.95–122.97)  1.51(±0.36)1.7440.781503.111  
Field population (G2)Deltamethrin21.21 (3.13–40.88)    1.37(±0.39)5.1840.271503.211  
UNSEL PopAcetamiprid4.73 (0.03–12.44)   0.86(±0.32)0.2840.9915011  
UNSEL PopImidacloprid5.44 (1.08–9.92)    0.95(±0.29)0.024115011  
UNSEL PopChlorpyrifos24.19 (17.65–31.78)   2.18(±0.35)2.0340.7315011  
UNSEL PopDeltamethrin6.60 (2.15–11.14)   1.05(±0.29)0.114115011  
Aceta-SEL Pop (G8)Acetamiprid1490.90 (919.55–4051.63)  1.24(±0.31)0.2240.99150315.232.6  
Aceta-SEL Pop (G8)Imidacloprid1181.98 (800.06–2175.63)  1.43(±0.31)0.340.99150217.2828.48  
Aceta-SEL Pop (G8)Chlorpyrifos653.06 (402.79–1774.75) 1.24(±0.31)0.2240.99150278.68  
Aceta-SEL Pop (G8)Deltamethrin654.06 (447.54–1232.51) 1.59(±0.33)0.4840.9815099.130.84  
Aceta-SEL Pop (G12)Acetamiprid468.70 (285.13–1390.82) 1.33(±0.32)0.4640.98150  99.09−0.10
Aceta-SEL Pop (G12)Imidacloprid335.76 (218.62–714.18)  1.32(±0.30)0.4340.98150  61.72−0.11
Aceta-SEL Pop (G12)Chlorpyrifos79.40 (51.231–141.021)1.24(±0.29)0.241150  3.28−0.18
Aceta-SEL Pop (G12)Deltamethrin178.70 (98.54–861.52)   0.94(±0.29)0.0641150  27.08−0.11

3.2 Response of Aceta-SEL Pop to various insecticides and cross-resistance

Selection of P. solenopsis with acetamiprid resulted in a 32.60-fold increase in the resistance ratio compared with the field population at G2, and a 315.20-fold increase compared with UNSEL Pop. There was significant change in the slope for acetamiprid (Fn = 27; df = 1, 6; P = 0.002) in Aceta-SEL Pop from G3 to G7. Selection with acetamiprid also conferred moderate cross-resistance (95% fiducial limits overlapped) to imidacloprid and deltamethrin (28.48- and 30.84-fold) and very low cross-resistance (95% fiducial limits overlapped) to chlorpyrifos (8.68-fold) compared with the field population (Table 1).

3.3 Stability of resistance in the Aceta-SEL population

Over five generations of rearing the Aceta-SEL Pop without further insecticide exposure, the resistance ratio for acetamiprid dropped significantly from 315-fold (G8) to 99-fold (G12). This implies that resistance to acetamiprid remained unstable in P. solenopsis. Resistance to deltamethrin remained stable over the five generations. In contrast, resistance to imidacloprid and chlorpyrifos also declined significantly (Table 1).

3.4 Life history traits

Life history traits of UNSEL Pop, Cross1, Cross2 and Aceta-SEL Pop are given in Table 2. Aceta-SEL Pop showed significant decrease in survival rate from crawler to second instar (Fn = 16.6; df = 3; P <0.001) compared with other tested populations, while Cross1 and Cross2 showed survival rates similar to that of UNSEL Pop. The nymphal durations of males (Fn = 39.9; df = 3; P < 0.001) and females (Fn = 21.8; df = 3; P < 0.001) from Aceta-SEL Pop were significantly prolonged compared with those of UNSEL Pop, Cross1 and Cross2. The nymphal duration of males of Cross1 was significantly higher than that of males of Cross2, while the nymphal durations of females of Cross1 and Cross2 were similar. Development time (DT) from egg to male adult did not differ significantly (Fn = 1.17; df = 3; P = 0.388) in any of the tested populations. DT from egg to female adult of Aceta-SEL Pop was significantly prolonged (Fn = 11.9; df = 3; P = 0.003) compared with the DT of females from UNSEL Pop, Cross1 and Cross2. There was no significant difference in the number of pupae among all tested populations (Fn = 1.70; df = 3; P = 0.243). The pupal weight of Aceta-SEL Pop was significantly lower (Fn = 8.89; df = 3; P = 0.009) compared with the Cross1 and Cross2 populations, but similar to UNSEL Pop. The pupal weights of Cross1 and Cross2 were similar. The pupal durations of Aceta-SEL Pop and UNSEL Pop did not differ significantly (Fn = 23.5; df = 3; P < 0.001), but Cross2 showed significantly higher pupal duration compared with all other tested populations.

Table 2. Biological parameters of UNSEL Pop, Cross1 (Aceta ♂ × UNSEL Pop ♀), Cross2 (Aceta ♀ × UNSEL Pop ♂) and Aceta-SEL populationa
Biological parametersUNSEL Pop (±SE)Cross1 (±SE)Cross2 (±SE)Aceta-SEL Pop (±SE)
  1. a

    Means within a row followed by the same letters are not significantly different at P > 0.05.

Number of crawlers at beginning75606048
Survival rate from crawler to second instar (%)89.33(±1.33) a83.33(±1.67) a86.67(±1.67) a68.75(±3.61) b
Male nymph duration (days)15.33(±0.88) b13.00(±0.00) c10.00(±0.00) d19.33(±0.88) a
Female nymph duration (days)19.67(±0.88) b18.00(±0.58) b17.67(±0.33) b24.00(±0.58) a
DT from egg to adult male (days)24.33(±1.33) a26.33(±0.00) a25.50(±0.72) a27.64(±1.86) a
DT from egg to adult female (days)23.00(±1.15) b21.00(±0.58) b21.67(±0.33) b26.33(±0.33) a
Number of pupae  2.00(±0.58) a2.00(±0.58) a  3.00(±0.58) a  1.33(±0.33) a
Pupal weight (mg)  0.92(±0.03) bc  1.16(±0.12) ab  1.34(±0.11) a  0.68(±0.01) c
Pupal duration (days) 5.67(±0.33) c 8.67(±0.83) b13.64(±0.72) a   4.67(±1.20) c
Emergence rate of healthy male adults (%) 88.89(±11.11) a100.00(±0.00) a 100.00(±0.00) a 100.00(±0.00) a  
Fecundity330.32(±18.46) a  177.55(±6.05) c 258.35(±13.59) b129.02(±33.96) c
Hatchability (%)87.63(±0.37) a  86.70(±4.12) a89.26(±2.07) a46.48(±0.97) b
Longevity of male adult (days)  4.67(±0.67) ab 6.55(±0.05) a4.33(±0.17) b 6.33(±0.67) a
Longevity of adult female (days)10.77(±0.67) c 19.70(±1.83) ab21.37(±0.34) a16.46(±1.34) b
Male generation time (days)28.00(±1.15) a28.03(±3.02) a30.97(±0.61) a31.67(±1.20) a
Female generation time (days)32.77(±0.89) b39.70(±1.30) a42.04(±0.68) a41.80(±1.23) a
Number of next-generation crawlers482133856847691
Net reproductive rate (R0)64.2884.63114.1214.4
Relative fitness11.321.780.22

There was no significant difference in emergence rate of healthy male adults (Fn = 0.85; df = 3; P = 0.510) among the populations tested. Fecundity of Aceta-SEL Pop was significantly reduced (Fn = 25.3; df = 3; P < 0.001) compared with that of UNSEL Pop and Cross2, but similar to that of Cross1. The Cross2 population demonstrated significantly higher fecundity compared with Cross1, but significantly lower fecundity compared with UNSEL Pop. The hatchability (%) of Aceta-SEL Pop was significantly lower (Fn = 133; df = 3; P < 0.001) than that of UNSEL Pop and reciprocal crosses. The longevity of male adults was significantly higher (Fn = 4.36; df = 3; P = 0.049) in Aceta-SEL Pop than in Cross2 but similar to that of Cross1 and UNSEL Pop. The Cross1 population also showed significantly greater longevity of male adult compared with the Cross2 population. Female adult longevity of Aceta-SEL Pop was significantly higher (Fn = 15.3; df = 3; P = 0.001) than that of UNSEL Pop and lower than that of Cross2, but similar to that of Cross1. Female adult longevity of Cross1 and of Cross2 were similar but significantly lower than that of UNSEL Pop.

Male generation time did not differ significantly (Fn = 1.21; df = 3; P = 0.366) among the tested populations. Female generation time was significantly higher (Fn = 17; df = 3; P < 0.001) in Aceta-SEL Pop and reciprocal crosses than in UNSEL Pop. Reciprocal crosses showed similar female generation times, which were significantly higher than that of UNSEL Pop. The numbers of next-generation crawlers of Aceta-SEL Pop, Cross1, Cross2 and UNSEL Pop were 691, 3385, 6847 and 4821 respectively. The net reproductive rates of Aceta-SEL Pop, Cross1, Cross2 and UNSEL Pop were 14.40, 84.63, 114.12 and 64.28 respectively. Compared with UNSEL Pop, the relative fitness value of Aceta-SEL Pop, Cross1 and Cross2 populations were estimated to be 0.22, 1.32 and 1.78 respectively.

3.5 Intrinsic rate of natural increase, biotic potential and mean relative growth rate

The intrinsic rate of natural increase (rm) of females in Aceta-SEL Pop was significantly lower (Fn = 27.1; df = 3; P <0.001) compared with UNSEL Pop and reciprocal crosses, while the intrinsic rate of natural increase of females was similar for reciprocal crosses and UNSEL Pop (Fig. 1). The intrinsic rate of natural increase (rm) of males in Aceta-SEL Pop was significantly lower (Fn = 11.5; df = 3; P = 0.004) compared with all other tested populations, while the intrinsic rate of natural increase of males was similar for reciprocal crosses and UNSEL Pop (Fig. 1). The biotic potential was significantly different among all populations tested (Fn = 40.7; df = 3; P < 0.001) (Fig. 2). The MRGR of males of Aceta-SEL Pop was significantly lower (Fn = 18.6; df = 3; P < 0.001) by comparison with UNSEL Pop, Cross1 and Cross2 populations; however, there was no significant difference between Cross1 and Cross2 populations (Fig. 3). The MRGR of females was significantly different among all the populations tested (Fn = 302; df = 3; P < 0.001) (Fig. 3).

image

Figure 1. Means (±SE) of intrinsic rate of natural increase (rm) of males and females among UNSEL Pop, Cross1, Cross2 and Aceta-SEL populations of Phenacoccus solenopsis. Error bars represent standard errors. Bars with different letters are significantly different (LSD test, P ≤ 0.05).

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image

Figure 2. Means (±SE) of biotic potential among UNSEL, Cross1, Cross2 and Aceta-SEL populations of Phenacoccus solenopsis. Error bars represent standard errors. Bars with different letters are significantly different (LSD test, P ≤ 0.05).

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image

Figure 3. Means (±SE) of mean relative growth rate (MRGR) of males and females among UNSEL Pop, Cross1, Cross2 and Aceta-SEL populations of Phenacoccus solenopsis. Error bars represent standard errors. Bars with different letters are significantly different (LSD test, P ≤ 0.05).

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4 DISCUSSION

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 MATERIALS AND METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

Acetamiprid binds to nicotinic acetylcholine receptors (nAChRs), has high toxicity against many insect pests and is mainly used to control the sucking insect pests of cotton, vegetables and ornamental plants.[20] Acetamiprid resistance has previously been reported in different insect pests from Pakistan.[18-20] There was no previous report of insecticide resistance in P. solenopsis to acetamiprid. At the time of collection, the field population showed low levels of resistance to acetamiprid and imidacloprid, with resistance ratios of nine- and sevenfold, respectively, compared with the control. The same population also demonstrated very low resistance to chlorpyrifos and deltamethrin, with resistance ratios of threefold for each (Table 1). After five rounds of selection with acetamiprid, resistance to this compound increased to 315.20-fold that of UNSEL Pop, suggesting that selection had a marked effect on resistance. The high level of acetamiprid resistance in Aceta-SEL Pop after five rounds of selection could be due to enhanced detoxification of the insecticide by metabolic enzymes. Although this hypothesis has not been tested in the present study, research being carried out in the authors' laboratory suggests that the presence of monooxygenase and esterases is the major mechanism of resistance in P. solenopsis from Pakistan (Afzal MBS, unpublished data).

There are no previous reports of cross-resistance to acetamiprid in P. solenopsis, but cross-resistance to acetamiprid has been reported in many other insects, such as in an acetamiprid-selected strain of B. tabaci which suggested very high cross-resistance to thiamethoxam (>500-fold).[43] A 110-fold acetamiprid-resistant strain of P. xylostella (L.) displayed low cross-resistance to cartap and phenthoate.[18] A 118-fold acetamiprid-resistant B. tabaci strain has been reported to develop very low cross-resistance to endosulfan (fivefold) and bifenthrin (fourfold).[20] In the present study, Aceta-SEL Pop (315-fold) showed moderate cross-resistance to imidacloprid (28.48-fold) and deltamethrin (30.84-fold), and very low cross-resistance to chlorpyrifos (8.68-fold) compared with the field population. In general, cross-resistance to compounds within the same chemical group may generally be observed, although without supporting experimentation it remains hard to predict in advance.[44] The cross-resistance among insecticides having different structures and modes of action (intergroup cross-resistance) is extremely unpredictable,[45] but the cross-resistance to unrelated insecticides could be due to a common mechanism affecting the insecticides or to genetically linked independent mechanisms. Cross-resistance among insecticides from different chemical groups could also be possible when an isoenzyme from insects acts on different types of insecticide.[46]

Knowledge of the stability of resistance after exposure to insecticides has important practical implications. This knowledge, along with regular resistance monitoring, is crucial for developing effective insecticide resistance management strategies. For example, if resistance is unstable, then removal of the insecticides from spray schedules could bring the resistance level down, and thus the efficacy of a chemical can be prolonged.[20] Rapid decrease in insecticide resistance has been reported for insect populations selected for resistance in the laboratory or field.[47] A high fitness cost and incomplete resistance might be associated with rapid reversion of insecticide resistance.[20] In the present study, resistance to acetamiprid also remained unstable in Aceta-SEL Pop in the absence of insecticide exposure. Similarly to the present results, unstable acetamiprid resistance has also been reported in highly resistant P. xylostella[48] and B. tabaci.[20] The present finding on the rate of resistance decline suggests that the fitness cost of maintaining the resistance gene(s) might be high. The present study suggests that the acetamiprid selection pressure to the field strain was not intense in the field, and, because of this, deleterious mutations may have appeared in the strain.[49] It has been shown that, in the absence of selection, the average fitness of individuals could be reduced by the accumulation of deleterious mutations.[50] The reversion of resistance is also likely to be due to the presence of heterozygotes in the selected population.[20] The decline in resistance in the Aceta-SEL population indicates that, if the insecticides are removed from the spray schedule, the resistance may drop in the field.

Insecticides can alter the biology of resistant insects. Many studies suggest that resistant insects show fitness costs.[51, 52] Studying the relative fitness costs of the resistant population is the basis for understanding and resolving the problem of resistance.[53] It is usually believed that biological characteristics (prolonged growth period and declined fecundity) change the relative fitness cost. The present study shows that, under continuous selection pressure, Aceta-SEL Pop had a significantly lower survival rate, longer male and female nymphal duration, longer developmental time from egg to female adult, reduced fecundity and low percentage hatching. Therefore, acetamiprid resistance in the P. solenopsis population corresponded to a significant decrease in many biological fitness parameters, which indicates a trade-off in distribution of resources among resistance and fitness costs.

Decreased relative fitness associated with insecticide resistance has been demonstrated for many insects, including S. litura,[33] P. xylostella,[29, 54] B. dorsalis,[32] N. lugens,[28] S. exigua,[30] B. tabaci[31] and L. decemlineata.[55] The present study also indicates that the acetamiprid resistance could decrease relative fitness in P. solenopsis. Aceta-SEL Pop expressed a relative fitness of 0.22 compared with UNSEL Pop. This finding suggests that Aceta-SEL Pop would not increase as rapidly as UNSEL Pop if acetamiprid selection were discontinued. Although fitness cost determination is important in the homozygous resistant population, it is impossible to ignore the fitness in heterozygotes because, in the early stages of resistance evolution, heterozygous individuals mostly act as carriers of resistant gene alleles. In the present study, fitness costs are not only evident in Aceta-SEL Pop but also in reciprocal crosses (Cross1 and Cross2). Results illustrated that heterozygous genotypes, Cross1 and Cross2, exhibited advantageous traits, with a relative fitness of 1.32 and 1.78, respectively, compared with UNSEL Pop. This phenomenon could be the result of fitness reduction associated with extensive inbreeding in Aceta-SEL Pop and UNSEL Pop. The crossing between Aceta-SEL Pop and UNSEL Pop may result in recovery of advantageous traits in heterozygous progeny (reciprocal crosses). Another possibility is that, in hybrids, the level of enhancement in vigour was significantly higher than the level of decrease in fitness as a result of resistance development. Therefore, examination of biological parameters in reciprocal crosses and resistant populations is crucial to the formulation of a resistance management strategy.[30] The present study provides valuable information on biological fitness parameters in UNSEL, reciprocal crosses and Aceta-SEL populations.

The intrinsic rate of natural increase (rm) provides an estimate of growth potential of insect populations,[56] which provides considerable insight, aside from individual life history parameters. The net reproductive rate (R0) is not, however, the only component needed to assess the potential of population growth, because the intrinsic rate of natural increase depends on fecundity, percentage hatching, growth and adult eclosion.[57, 58] Therefore, variations in the above life history qualities could influence the rate of P. solenopsis population increase. The intrinsic rate of natural increase (rm) of males and females in Aceta-SEL Pop was significantly less compared with that in UNSEL Pop and reciprocal crosses (Fig. 1). There is a positive relation between intrinsic rate of natural increase and mean relative growth rate.[33, 59-62] It was previously reported that the intrinsic rate of natural increase in imidacloprid-resistant S. litura,[33] in pyrethroid- and organophosphate-resistant C. carnea,[59] in deltamethrin- and indoxacarb-resistant H. virescens[61] and in spinosad-resistant P. xylostella[62] demonstrated a positive association with mean relative growth rate. In the present study, a positive relationship was found with the mean relative growth rate, resulting in a subsequent decrease in intrinsic rate of natural increase in Aceta-SEL Pop of P. solenopsis. The lower rate of population increase appeared to be mainly due to decreased fecundity and low percentage egg hatching, and also the longer developmental time from egg to adult emergence. In insect populations with non-overlapping generations, such developmental asynchrony could lead to non-random (assortative) mating among resistant insects and, through interaction with season length, increase or slow down the evolution of resistance.[63]

Management of insecticide resistance depends on fitness costs, such that the removal of selection pressure will lead to decrease in the number of resistance alleles.[64] Incomplete resistance and fitness costs can delay the insecticide resistance developed within an insect population.[65] Resistant individuals also suffer some impairment in performance in the presence of insecticides in comparison with their performance in the absence of insecticides owing to the phenomenon of incomplete resistance.[60] To the best of the authors' knowledge, this is the first report of cross-resistance and fitness cost to acetamiprid in P. solenopsis worldwide. Based on the findings of the present research it can also be concluded that, for good management of P. solenopsis under field conditions, only those insecticides that have lower stability and correlate with a high reversion rate of insecticide resistance should be used. Consequently, rotation of insecticides can be implemented in resistance management strategies.

ACKNOWLEDGEMENTS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 MATERIALS AND METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

The authors are extremely grateful to local offices of international pesticide companies for supplying the pesticide used in the bioassays.

REFERENCES

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 MATERIALS AND METHODS
  5. 3 RESULTS
  6. 4 DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
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